H A. Handbook of UNDERDRAWS OF ENDURING PUBLISHED BY PUBLIC WORKS PUBLICATIONS

Transcription

1 H A Handbk f UNDERDRAWS OF ENDURING PUBLISHED BY PUBLIC WORKS PUBLICATIONS VITRIFIED CLAY

2 Frewrd LIBRARY Internatinal Reference Certtlt fr Cmrr-'nltv V-'^r Suppfy TRICKLING FILTERS were in use in 3,506 municipal ' wastewater treatment plants in the United States in 1962, as reprted in a statistical summary issued by the U. S. Public Health Service in 1964, the latest cmpilatin available. Of these plants, 2,135 emplyed standard and 1,371, high rate filters. The ttal ppulatin served by these facilities at that time was 23,006,271. While a single stage filter was used in many cities as the sle secndary treatment prcess, in ver 2,600 plants they were emplyed in multiple stages r in cnjunctin with ther bilgical prcesses as intermediate r final steps. With the increasing need fr advanced waste treatment, research is being directed tward taking advantage f the ptential versatility f niters in develping prcesses that will exceed rdinary secndary treatment in perfrmance. The grwth in ppularity f trickling filters is illustrated by peridic census studies. In 1940, there were 1,486 plants in use, nly a few f which were high rate. In 1945, 1,581 plants were reprted, f which 122 were high rate; mst f the cnstructin in this war perid was fr military purpses, municipal cnstructin f wastewater treatment plants being severely curtailed. The 1957 reprt shwed the existence f 1,870 standard rate and 812 high rate niters. Frm 1957 t 1962, there was an increase f 265 standard rate and 559 high rate filters. Nne f these figures includes institutinal r industrial data. Fr example, mst secndary treatment devices fr service areas n majr turnpikes emply this methd. The first use f trickling filters f recrd was in 1889, when the Lawrence Experiment Statin f the Massachusetts State Bard f Health put tw filters int service using gravel media. Their use led t an understanding f ne f the essential features f this methd f treatment the slw mvement f the liquid in films ver the surface f the stnes while in cntact with air. Even thugh these first filters were develped in the United States, English engineers develped the methd and the means f applying the wastewater as a spray t the surface f the filter; and, highly imprtant, the use f "false bttms" t supprt the media and, at the same time, t eliminate clgging at the filter flr level. The first trickling filter installatin fr a city in this cuntry was recmmended fr Atlanta, Ga., in 1903, but it was nt cnstructed until Hwever, a small experimental unit had been installed in 1901 in Madisn, Wise., and frm this and frm test and pilt plants valuable design infrmatin was btained. The plant fr Clumbus, Ohi, put int peratin in 1908, prvided 10 acres f filters. Als, in 1908, a ne-acre plant was built fr Reading, Pa., and anther small plant fr Washingtn, Pa. Trickling filters at Mt. Vernn, N. Y., were put int peratin in 1910, and the large plant at Baltimre, Md., with 14 acres f filters, was cmpleted in Other installatins were made fr the municipalities f Yrk, Allentwn and Meadville, Pa., Rme, N. Y., and Nrth Plainfield, N. J. This grwth f use f trickling filters prduced perating data n which many imprvements have been based. One f the early bservatins related t the necessity f having a "false flr" t prevent the suspended matter in the wastewater frm accumulating n the flr f the filter and interfering with drainage and ventilatin. The use f an inverted half-rund pipe was prbably mst cmmn. Hwever, at Baltimre, a series f channels were designed with perfrated tile cvers. During the past twenty-five years, the use f clay filter bttm blcks r underdrains has grwn and it is a very rare installatin where such blcks are nt used. The manufacturers making filter underdrains, united in the Trickling Filter Flr Institute, engage in research, imprvement f manufacture, develpment f specificatins, and advertising t cntribute t the mre effective use f trickling filters. This bklet, the fifth editin f the "Handbk f Trickling Filter Design," is a prduct f the Trickling Filter Flr Institute in cperatin with PUBLIC WORKS Magazine. Its purpse is t furnish technical data and prvide infrmatin whereby trickling filters will d a still better and mre ecnmical jb in waste treatment. Cpyright 1970, Public Wrks Jurnal Crpratin, 200 Suth Brad Street, Ridgewd, New Jersey

3 Wastewater Treatment with Trickling Filters C ONVENTIONAL wastewater treatment prcedures invlve slids remval and stabilizatin f the rganic cmpunds disslved in the liquid fractin. The latter step is usually accmplished by bilgical xidatin, in which a culture f micrrganisms "feeds" upn the rganics. The devices used fr bilgical xidatin establish aerbic cnditins t develp the culture and keep it active fr maximum effectiveness. This means prviding an envirnment f xygen r air. Cnsequently, as a bilgical treatment medium, a trickling filter is essentially an aeratin device and prvides very little mechanical remval f suspended slids as the term "filter" might imply. The adaptability f trickling filters t varying rganic and hydraulic ladings has resulted in their wide usage. Many variatins in design have been develped capable f prducing almst any desired end results. All f these variatins, hwever, appear t retain the ability t perate in cld weather r in ht, r with verlads, with nly reasnably skilled supervisin. Certain cnditins are necessary fr mst effective peratin. These include: i) Prper pretreatment f the waste t remve slids, usually attained by passage thrugh a sedimentatin tank; 2) reasnably unifrm applicatin f the settled waste t the surface f the filter, as by a rtary distributr r spray nzzles; 3) use f prperly sized filter media, free frm fines and substantially unifrm in grading; 4) prvisin f underdrainage facilities fr the prmpt and cmplete remval f the material passed thrugh the filter bed; and 5) assurance f adequate ventilatin s that each unit f the filter receives the air needed fr the xidatin prcess. Filters are classified accrding t the rganic lading fr which they are designed, as high rate r standard (r lw rate); and by depth, as deep r shallw. The amunt f rganic lading, in punds f BOD applied per cubic yard, 1000 cu. ft. r acre-ft f media determines whether the filter is t be termed high rate r standard. Thugh there is n definite Table 1 Abbreviatins 5-day Bichemical Oxygen Demand BOD Suspended Slids SS Gallns per minute gpm Gallns per day gpd Millin gallns per day mgd Gallns per persn per day. gpcd Acre-ft = 43,560 cu. ft at Millin gallns per acre per day mgad Milligrams per liter mg/l standard, a generally used lading fr a standard filter apprximates, as a maximum, 600 lbs. f BOD per acre-ft, 13.7 lbs. per 1000 cu. ft. r lb. per cu. yd. Thugh many standard filters are carrying heavier ladings, they were nt generally designed t d s initially. The Manual f Practice, Water Pllutin Cntrl Federatin lists the standard rate filter range as 5 t 25 lbs. per 1,000 cu. ft. High rate filters are usually designed fr an applied lading, based n settled sewage strength, f 2,000 t 3,000 lbs. per acre-ft r 1% t 1% punds f BOD per cu. yd. Hwever, the WPCF Manual f Practice gives the range as 25 t 300 lbs. per 1,000 cu. ft., which is 1,090 t 13,100 lbs. per acre-ft. Just as in the case f rganic lading, there is n clear line t distinguish deep frm shallw filters. A generally used standard in respect t depth cnsiders a filter in which the media is less than 4.5 ft. in depth as a shallw filter; and ne with media mre than 4.5 feet in depth as a deep filter. The depth is the average distance frm the tp f the underdrain flr t the tp surface f the filter media. Definitins and Abbreviatins Fr the purpse f this text, the abbreviatins shwn in Table 1 will be used. Definitins f ther terms used are as fllws: The rganic lad n a filter is the BOD cntent in punds that is applied t the filter. It is usually the BOD cntent f the raw sewage less the BOD that is remved by the pretreatment devices such as screens and clarifiers. Fr instance, if the sewage flw is 1 millin gallns per day and the BOD f the raw sewage is 240 mg/l, the ttal BOD will be 2,000 punds. If 30 percent is remved by primary treatment devices, the rganic lad applied t the filter will be 2,000 x 0.70 = 1,400 punds BOD. The vlume r hydraulic lading n a filter is the amunt f sewage r waste applied t it, cmputed n Curtesy W. S. Dickey Clay Mfg. C. ALL elements f a trickling filter are shwn in this cutaway view. A is the influent line; B, the drainage flr; C, a vent; D, the media; E, distributr feed clumn; and F, the rtary distributr. The varius functins are described in detail in the text.

4 the basis f millins f gallns per acre f filter surface per day. If the sewage flw is 1 mgd and this is applied t a filter which has a surface area f 0.10 acre, the vlume lading will be 10 millin gallns per acre per day. A single-stage filter is ne in which treatment is accmplished by ne r mre passes f the sewage thrugh the filter. This term is usually restricted t high rate filters using recirculatin. If there are tw r mre filters in a single-stage plant, these filters are perated in parallel. In a tw-stage filter system there are tw filters in series, with the effluent frm the first r primary filter being passed thrugh the secndary filter. Recirculatin is usually emplyed n bth, niters. Large tw-stage treatment plants may have fur, six r mre niters, each pair f them wrking as a team. Recirculatin is the return f a prtin f the sewage r waste which has already passed thrugh the filter, and smetimes als has been settled, fr anther passage thrugh the filter. This return may be directly t the filter r it may be t the inlet f the primary settling tank. The primary clarifier, primary tank r primary settling tank is the sedimentatin device ahead f the filter which pretreats and prepares the waste fr applicatin t the filters. An intermediate clarifier r tank is the sedimentatin device between any tw filters. A secndary clarifier r secndary r final settling tank is the sedimentatin device fllwing the final filter. This usually prvides the final treatment fr the waste except fr chlrinatin. In cnsidering trickling filters, the filter r niters and the fllwing clarifier shuld always be cnsidered as a unit in bth design and peratin. There is n place in sewage treatment fr a trickling filter withut a final clarifier in which t settle the filter effluent. (This des nt apply t the first stage filter f a tw-stage filter system.) Other Units f Express/n Hydraulic lad, including recirculated flw if used, is in gallns f flw per square ft f surface area f the filter. The standard rate filter range will be 25 t 100; the high rate range will be 200 t 1,000. The rate in millin gallns per acre per day multiplied by 23 will give gallns per square ft per day. Thus a rate f 20 mgad equals 460 gsfd. The rate in punds f BOD per acre ft per day multiplied by Curtesy Kppers C. TYPICAL distributr in peratin at Salisbury, Md. wastewater treatment plant. equals the punds f BOD per 1,000 cu. ft. per day. Cnversin frm punds f BOD per cubic yard per day t punds f BOD per 1,000 cu. ft. per day is accmplished by multiplying by 37. Thus 2,420 punds f BOD per acre ft, multiplied by represents 55.6 punds per 1,000 cu. ft., as des the equivalent 1% lbs. per cu. yd. multiplied by 37. Fr high rate niters, the recirculated flw is nt cnsidered in cntributing an additinal BOD lad. Percentage f recirculatin equals recirculated flw divided by waste flw multiplied by 100. The standard rate filter range is 0 t 50 and the high rate range is 25 t 300. Cmparisn f Filter Types There is n essential difference in cnstructin between the high rate and the standard r lw rate filter. The same media, the same underdrains and the same ventilatin facilities are prvided in either case. The distributrs are essentially the same, except that thse used fr high rate filters must be designed fr larger vlume rates f applicatin. The ptential BOD remval appears t exist in every filter; the rate and methd f applicatin f the sewage t the filter determines hw and t what extent this ptential is utilized. The high rate filter, because it is laded much mre heavily the BOD lading may be three t six times as great as that n the standard rate filter- is crrespndingly smaller and usually csts less t build. There are differences in the quality f the effluents frm high rate and standard niters. The latter prduce a higher degree f nitrificatin, s that strengths f effluents frm the tw types f niters, stated in milligrams per liter f BOD, cannt be directly cmpared. This is especially true f single-stage high rate filters; tw-stage high rate niters may prduce sme degree f nitrificatin. Hwever, lcal cnditins usually gvern in determining the degree f treatment needed and the actual reductin in the rganic lad discharged t the stream is a mst imprtant factr. If lcal cnditins dictate the selectin f a plant t prduce an effluent lw in BOD, well nitrified and highly stable, full cnsideratin shuld be given t a deep filter and a lw rate f lading. If a less cmpletely stabilized effluent is permissible, with a slightly higher BOD cntent, a single-stage high rate filter may be selected because it is usually lwer in initial cst. If the waste is very strng and a gd effluent is necessary, a tw-stage filter may be preferable, since this cmbines many f the cst-saving features f the single-stage high rate filter with an effluent that may apprach that f the standard filter in BOD cntent. Pre-Treatmenf Standard practice is t prvide pre-treatment ahead f filters by means f settling tanks, either rund r rectangular, equipped with mechanical apparatus fr remving the sludge and preceded by a cmminutr and prperly cleaned screens. Standard practice usually prvides fr a detentin perid f abut tw hurs in the primary settling tank fr design flw plus recirculatin, in the case f a high rate filter. Hwever, there is cnsiderable variatin in requirements amng the several states and the applicable regulatins shuld be determined and fllwed. Surface verflw rates, tank depths and ther factrs are als subject t state regulatins and

5 '- s"^v **i^% Curtesy P.F.T. Div. Rex Cliainbelt, Inc. AERIAL view f Winstn-Salem's treatment plant and fur 200 ft. diam. filters. these may vary materially amng the states. In the treatment f cannery wastes and creamery prducts, sme engineers use a fine screen and a small equalizing tank fr pre-treatment instead f the usual primary tank. This is based n studies and bservatins which indicate the "micrbial frest" n the filter media can treat any material nt large enugh t interfere with the applicatin f the sewage t the filter. Intermediate and Pst-Treatment Intermediate clarifiers have been used in a relatively small number f plants. They are f mst value where the sewage r industrial waste is very strng. Detentin perids are usually smewhat less than fr primary clarifiers. The secndary clarifier is the key factr in the design f filters fr best peratin. It is nt the functin f the filter t remve suspended matter, but t change this t a relatively stable frm. This material, and the spent gelatinus film which slughs ff the filter media, cntinuusly in the high rate filter and frm time t time in the standard filter, must be remved by the clarifier. As with primary tanks, the detentin perid, verflw rate, weir verflw rate and tank depth f the secndary clarifier are subject t state health department regulatins. In general, detentin perids f 2 t 3 hurs shuld be prvided fr design flw plus recirculatin, if the secndary clarifier is invlved in the recirculatin prcess. Where recirculatin is used, the necessary vlume may be taken frm a launder installed in the secndary tank. Effects f Temperature Trickling filters perate better and prduce better effluents in warm, weather perids. Hwever, thugh they are affected in sme respects by lw temperatures, extreme cld is necessary t create any serius adverse effect. In the Nrth Central States, it is fairly standard practice t prvide cvers, either f prestressed cncrete r f wd, fr filter units, fr experience has shwn that perating results are imprved and trubles frm icing are eliminated. Cld weather effects may be f tw kinds: 1) the surface f the filter may becme iced; and 2) the efficiency f the filter in respect t BOD remval may be impaired. In ice frmatin, t which snwfall may cntribute, wind appears t be an imprtant factr as it drives spray t the dwnwind side where depsits may build up. Hwever, difficulties are restricted t the really cld areas. N such trubles were reprted in the peratin f uncvered filters at mre than 300 Army psts during Wrld War II, thugh sme f these were lcated in lw temperature areas. It is gd practice t prvide bleed valves in the distributr supprt psts f rtary distributrs and in the distributin system, f spray nzzle installatins s as t prevent freezing and cnsequent damage if the flw f sewage stps r becmes s small that the dsing siphns wrk nly at lng intervals. In sme plants, a small pipe is tapped int the feed pipe at the distributr base and is left pen all winter. T determine the effect f temperature n the efficiency f BOD remval by lw rate trickling filters, the recrds f nine military sewage treatment plants were studied by a Sub-Cmmittee n Waste Dispsal f the Natinal Research Cuncil. These were all standard rate plants. Results btained in July, August and September f 1943 were cmpared with results in December, 1943, and January and February, With adjustment fr differences in lading during the tw perids, there was a small but significant drp in efficiency during the clder mnths during which the air temperature averaged 38.1 F lwer than the summer mnths. The subcmmittee reprts that: "With military sewage f the usual cncentratin, this difference in efficiency results in effluents with a BOD abut 20 percent larger in winter than in summer." On this basis, it was estimated that a filter shuld be abut 43 per cent larger if it is t perate as well at an average temperature f arund 40 F as the nrmal filter will perate during the warm summer mnths. The army designs f Wrld War II specified that filters in the nrthern states shuld be 25 percent larger than thse in suthern areas. Under the best f cnditins, sme time is required t build up the bilgically active film necessary n the filter media. The time required depends n the temperature and prbably n a variety f lcal cnditins. Usually 3 t 4 weeks are required. If the plant is started late in the fall, when air and sewage temperatures are lw, an even lnger perid will be required befre adequate treatment results can be expected. A change frm high rate t lw rate peratin, r vice versa, will prbably require 4 t 6 weeks t develp the prper bilgical grwth. Changes in rate f lading are als likely t require cnsiderable perids befre results truly reflect the change. Applicability Trickling filters are applicable t weak r strng dmestic wastewaters and t many types f industrial wastes. By utilizing recirculatin, whereby the incming sewage is reduced in strength, even exceedingly strng wastes can be treated satisfactrily and ecnmically. Recirculatin may be applied t either high rate r standard filters. High rate and standard filters may be cmbined in a single treatment plant, a prcedure especially adaptable t the prductin f an excellent effluent. In such cases, the high rate filter is used as the primary filter t remve as much as pssible f the rganic lad; and the effluent frm the high rate filter is applied t the standard filter. Final treatment, fllwing the high rate filter, may als be prvided by an activated sludge plant. In either case, the high rate filter markedly reduces the rganic lad n the secndary unit and delivers t it a waste f unifrm strength.

6 Media, Drainage and Ventilatin HE five essential elements in a Ttrickling filter are the base supprt; the underdrain blcks which carry ff effluent and prvide ventilatin; the cntaining wall; the media t supprt the grwth f micrrganisms; and the distributr fr applying settled wastewater t the filter. Filter Flr and Center Channels The flr shuld prvide a firm and unifrm base n which t build the remainder f the filter. It shuld be f cncrete, pured n a wellcmpacted earth base. Since the lad carried by the flr is nt great, a cncrete thickness f 4 t 6 inches is ample. Light reinfrcement is desirable fr nt nly must the flr be unifrm initially t facilitate gd drainages, but it must retain that unifrmity fr the life f the plant The flr may be pured in ne run, r there may be jints. If used, jints shuld run in the directin f the flr slpe, and reinfrcing dwels shuld be carried thrugh the jints. Flr slpe tward the drain channels shuld be nt less than 2 in., fr any semidiameter, and is usually abut 0.3 ft. per 100 ft. There are varius designs, but the mst cmmn practice invlves bisecting the filter flr with a center channel tward which the tw halves f the filter slpe. Precise analysis f the flw in the center channel is difficult because f the interference with the flw due t the discharges frm the many underdrains entering at right angles t the main flw. Generally, satisfactry capacity will be prvided by a crss-sectin and a slpe that will, as determined by the usual flw frmulas r charts, carry the maximum pssible flw that may ccur with the liquid surface in the channel slightly belw the bttm f the underdrain channels. Design f center channels differs smewhat fr shallw high rate filters and fr standard rate deep filters. In a shallw filter, the feed pipe fr the distributr must generally pass under the filter flr and up thrugh the distributr supprt. In the standard rate deep filter, the feed pipe is usually passed thrugh the bdy f the media. This is largely because f the relative sizes required fr the feed. In a standard rate filter 100 ft. in diameter with a rate f surface applicatin f 2 mgad, the flw t the filter will average 0.36 mgd, which can be carried by an 8-inch pipe (r even a 6-in.). In a high rate filter, f the same size, using a surface rate f applicatin f 20 mgad, the feed pipe will have t carry an average f 3.6 mgd and a 16-in. r 18-in. pipe is required. If it is necessary t cnserve head, an even larger pipe may be used. The larger pipe wuld blck ff an undesirably large prtin f the filter if the pipe were carried thrugh the media instead f beneath the flr. The center channel in the standard rate filter can be smaller because it carries a lesser vlume f wastewater. As a result, it is ften passed thrugh the distributr supprt, thugh smetimes split r carried arund it. In the high rate filter, the center channel must be larger and cnsequently cannt be carried thrugh the distributr supprt. It is cmmn practice t ffset the channel frm the center enugh t clear the supprt clumn. A few large filters have been built with tw channels and fewer have used peripheral channels, the flr slping t the circumference. Center channels may be rectangular in crss sectin r the bttm may be semi-circular. In either case, channel cvers are necessary t supprt the superimpsed media. These cvers will be described under a later heading. T facilitate cleaning and inspectin, the center channel may be extended thrugh the filter wall at bth ends, terminating at the upper end in a small pen bx cvered with a grating. The bx shuld be f sufficient size t permit entrance. The value f ample ventilatin is prved by the excellent perating recrds f many filters where vitrified clay underdrain blcks have been used. Sme engineers add vertical vent pipes arund the inner circumference f the walls at the periphery f the filter and thers als extend the drainage ducts thrugh the filter walls. This is advantageus if prvisin fr flushing f the channels is desired, as when industrial wastes are treated. The abslute necessity fr adequate drainage and ventilatin in a DICKEY high-rate blcks were used in the trickling filter at Springdale, Ark. 5

7 STANDARD rate filter underdrain blcks made by the member firms f the Trickling Filter Flr Institute. All are made t cnfrm t ASTM specificatins given in this Handbk and are f crrsin resistant vitrified clay. trickling filter shuld be recgnized. Cmplete systems f vitrified clay blcks are laid directly n the filter flr. These units are nt affected by the actin f wastewater and thus are as lng lived as the filter itself. The units f these systems are quadrangular prisms having penings in the upper face which cllect the wastewater passing thrugh the filter. Ample cnduit capacity is prvided fr rapidly discharging the effluent, and fr admitting air int the filter fr ventilatin and aeratin. The blcks are laid n a thin layer f grut r f dry mrtar, rapidly and precisely, s that the drainage channels are autmatically aligned. The strength f the blcks is greatly in excess f that required under any cnditin f service. ASTM specificatins fr these blcks are given n pages 9 and 10. Underdrain Blcks Underdrain blcks differ in design primarily accrding t use in standard r high rate filters. The latter are deeper and have channels f greater capacity fr carrying ff the filter effluent. Actually, the standard blcks have the ability t handle any applicatin rate up t 50 mgad n a 200-ft. diameter filter, but the high rate blcks prvide additinal space fr air penetratin int the filter and are preferred by sme engineers, especially fr industrial waste applicatins. The blcks described belw cmply with all requirements f ASTM Specificatin C159. All blcks are f vitrified clay which experience has shwn t be resistant t all elements in either dmestic r industrial wastewater; t have excellent flw characteristics because f the smth surface; and t resist attachment f grwths which might tend t clg the ventilating penings r flw channels. Standard Blcks DICKEY, made by the W. S. Dickey Clay Mfg. C., are 18 in. lng, 11% in. wide and 4Vi in. deep, with 18 tp penings. POMONA, made by Pmna Pipe Prducts, are 13% in. lng, 12 in. wide and 5% in. deep. TRANSLOT, made by Can-Tex Industries, Inc., a divisin f Harsc Crp., are 11% in. wide, 18 in. lng and 4LVA in. deep. High Rate Blcks TRANSLOT "Hi-Rate" blcks are 18 in. lng, 11% in. wide and 7 ] /i in. deep. They have three flw channels and five transverse tp slts. These are made by Can-Tex Industries, Inc., a divisin f Harsc Crp. POMONA "Rapid-Fl" high rate blcks are 13 V 2 in. lng, 11% in. wide and in. deep, with three slt penings 1% in. wide made by Pmna Pipe Prducts. DICKEY, made by W. S. Dickey Clay Mfg. C. are 18 in. lng, 11% in. wide and 7M> in. deep with tw flw channels and six transverse tp slts. It is imprtant t insure in the specificatins the use f gd materials and installatin methds. A specificatin which will assure gd underdrainage cnstructin fllws (the latest ASTM standard revisin number shuld be used). Underdrains The cntractr will furnish and install vitrified clay underdrains as shwn n the plans. These shall be laid in a dry mrtar bed, n the flr f the filter befre H HIGH-RATE filter blcks made by members f the TFFI. These high-rate blcks feature deeper channels by tw inches r mre than thse used in the standard-rate filters. In all ther respects, the blcks are quite similar. the stne is placed. Underdrains must cmply with standard specificatins ASTM C159, and shall be equal and similar t thse manufactured by members f the TFFI. The mrtar shall cnsist f sand and cement, 1 cement t 6 sand. After the underdrains are laid and befre the stne is placed, the dry mrtar bed shall be wetted by sprinkling. Blcks must be laid in true alignment, with crss jints staggered in lngitudinal rws at right angles t the center drains. Angles, Cver Blcks, Ventilatin and Laying Fr use in circular filters, blcks cut t 22y 2, 30, 45 and 67M> are available t match apprximately the curvature f the filter walls, and crss-sectins at the pier, effluent channel and perimeter wall.

8 Als shrt sectins f standard blcks are prvided t fill ut runs and eliminate the necessity fr cutting blcks n the jb. Vent blcks t supprt pipe riser vents t the surface f the filters are furnished; and als wall reducers fr carrying ventilatin ducts thrugh the filter walls fr use where flushing r added ventilatin is deemed desirable. In rdering blcks fr a filter, it is necessary nly t prvide the manufacturer with a plan f the filter, shwing essential dimensins. Shipments f blcks are then made with the crrect number f straight blcks, angle blcks, shrt sectins, specials and cver blcks. Guidance in laying is als prvided by the manufacturers t insure gd practice with the minimum labr requirement. Standard cver blcks fr the center drainage channel are nrmally limited t 30 in. in length. At least 3 in. f bearing shuld be allwed at either end. If it is necessary t prvide a center channel wider than 24 in., either a central supprt shuld be used r a special channel cver designed. Filter Design Defalts Filter depths range all f the way frm 3 ft t 10 ft. Recently experimental wrk has been carried n with filters up t 24 ft. deep; and sme deeper test units have been built. Lw rate filters, hwever, are usually designed with minimum depths f 5 t 6 ft., and with a maximum f 9 t 10 ft.; mst are abut 6 ft deep, which depth is mre r less standard. High rate filters vary quite widely. The Bifilter is designed with depths all the way frm 3 t 6 ft., but 4 t 5 ft. is prbably mst used. The Aerfilter is nrmally nt less than 6 ft. deep and may be deeper. The Accel filter is nrmally 6 ft. The depth functin depends much n lcal tpgraphy and n cst factrs. N matter what the depth r what the vlume lading, the underdrains previusly described have ample lad, drainage and ventilatin capacity. That is, they are strng enugh t carry the weight f the deepest filter, with a very wide margin f safety. The drainage channels are sufficiently large t handle prperly the flws frm, a filter 200 ft in diameter perating at a vlumetric rate f 50 millin gallns per acre per day. The tp penings are ample in size t prvide the necessary air t insure gd perating results. Much experimental wrk has been dne in attempts t imprve filter peratin by blwing air thrugh the media; even, xygen has been used. N results have been btained which indicate that added ventilatin, ver and abve that prvided by the standard vitrified clay underdrains, is helpful in better peratin. Wall Cnstructin Mst walls fr filters have been f reinfrced cncrete. When used, such walls are reinfrced circumferentially, using the hp frmula fr design, with sme vertical reinfrcing extending int the fting. Walls f this type are usually 8 t 12 in. thick. It has been prpsed t cnstruct filters withut walls, allwing the filter media t seek its angle f repse. It will generally be fund that such cnstructin is nt ecnmical. The additinal stne required fr the peripheral unused filter media, plus the cst f extending the filter flr ut t the te f the slpe f the stne, will usually exceed the cst f cnstructing walls t retain the stne. Mrever, experience has shwn that this marginal area f stne, which is partly r ccasinally wetted is liable t be a prducer f filter flies. Filter Media Crushed stne is the mst widely used media fr trickling filters. Crushed slag is als used, especially in areas where it has a price advantage ver stne. Runded gravel is smetimes used but sme reprts indicate it des nt give as gd results as d the angular aggregates resulting frm crushing. Hwever, in ther plants well-graded gravel has prduced excellent effluents, althugh an apprximately cubical shape appears preferable. Federal specificatins SS-S-448 require media "as nearly equidimensinal as pssible. Pieces that are elngated r flat, r bth, shall nt exceed 10 percent by weight. These shapes are defined as having length exceeding width, and width exceeding thickness in rati f mre than 1.5 t 1.0, dimensins being maximum fr the size f pieces under cnsideratin." The media shuld be free f fine particles, grease and il and shuld be prperly screened and washed t remve dust and dirt befre placement in the niters. Even with washing, enugh sand and fine stne particles may remain t cause truble when filter peratin is begun. Unless precautins are taken t prevent it, fine inrganic material will be carried t the secndary clarifier and thence t the digester, where it may cause further perating difficulties. In extreme cases, the mixture f sludge and dust may clg the sludge lines. It is gd practice fr the designer t make prvisin, either temprary r permanent, t permit the filter effluent t be bypassed arund the secndary clarifier until after a sufficiently lng perating perid t assure that all fine particles have been remved. Slag media shuld be free frm all free irn. Media Specificatins There are differences f pinin in regard t stne and slag media size. The mdem trend is tward use f larger aggregates 2V2-inch minimum. Unifrmity is als imprtant, and particles smaller than the lwest limit shuld nt exceed 10 percent by weight f the ttal. Federal Specificatins SS-S-448 state that the filter media shall be as unifrm in size as pssible and, when tested with standard labratry sieves having square penings, POMONA Rapid-Fl flr blck in a 93-ft. dia. high rate filter at Lynehburg, Va.

9 Table 1 Federal Specificatins fr Filter Media Grading Sizes (Specificatin SS-S-448, Int. Amendment-1) Nminal Size (square ODeninps ^'f ^* P i j^«u in inches) 3y 2 t 2V 2 3V 2 t 2 3 t 2 3 t IV2 21/2 t IV2 4 in Amunt Passing Sieve (square penings) percent by weight 3'/ 2 in in shall cnfrm t the gradings shwn in Table 1 f this text. Any f the first three sizes given are preferable t the smaller sizes, fr dmestic wastewater. Cmmercial aggregate specificatins are nw generally based n the use f screens having square penings. Federal SS-S-448 has been cited. ASTM specificatins C88, C136 and D75 apply t aggregates and include grading, sundness, wear and ther factrs. In recent years, ther materials have been develped by certain manufacturers. Amng these are plastic sheets (plystyrene r plyvinyl chlride) and redwd slats. The plastic sheets are assembled in "hneycmb" mdules small and light enugh t be placed by hand. Crrugated r alternate crrugated and flat sheets are used. The mdules have an effective surface area f 27 sq. ft. per cu. ft and have 2V2 in in V/2 in. 1 in, greater than 90 percent vid space. The weight is 2 t 3 lbs per cu. ft. Advantages are light weight, ease f placement and superir ventilatin qualities. These permit the cnstructin f deep filters up t 40 ft PVC mdules are made by Dw Chemical C., B. F. Gdrich Industrial Prducts C. and Ethyl Crp. The redwd slats are als available in mdule frm. These are rugh sawed t prvide purchase fr bilgical grwth. They are % in. tihick, V/z in. wide and either 35^ r 47V2 in. lng. Spacing is 0.69 in. between edges. They are furnished by Del- Pak Crp. Placing the Media N generally acceptable methd f placing the media in the filter has been develped which will insure that pckets f fines d nt ccasinally ccur in filters. If these are deep in the filter, they may re- CHANNEL blck made by Can-Tex Industries being placed in ne f the filters which serves the Central Sewage System f the Trinity Valley Authrity near Dallas. duce filter capacity and effluent quality. Hwever, shallw surface pnding is nt always due t such fines and ften des nt affect the peratin f the filter. In view f the excellent perating results btained frm s many filters, it is prbable that usual methds f cntrlling fines are adequate. Hwever, it shuld nt be frgtten. The methds used in placing media in the Frt Wrth filters represent very gd practice. The specificatins required that, in the placing f the filter media, breakage and segregatin and the entrance f fine particles int the filter "shall be prevented. Media shall be placed in the filter using a belt cnveyr, by bxes, r by ther apprved methds, which methd des nt require traffic f any type ver the tp f the stne placed in the bed, whether n the stne r n mats placed n the stne." It was als required that "immediately prir t final placing, the media shall be passed ver a suitable screen with nt less than 1-inch square penings." Distributrs Rtary distributrs, which are made fr all sizes f filters frm the smallest t the largest are generally used fr distributing the sewage ver the stne. Spray nzzles are als available fr rectangular installatins and special disc distributrs fr sme types f small filters. The rtary distributr has the advantage imprtant in mst plants f requiring very little head fr its peratin. Rtary distributrs are made by the fllwing firms: Ralph B. Carter C.; Drr-Oliver Inc.; Eimc Crp.; General Filter C.; Infilc Div. f Fuller C.; Water Pllutin Cntrl Div. f Keene Crp.; Lakeside Engineering C.; Link-Belt Div. f FMC Crp.; Pacific Flush Tank Div. f Rex Chainbelt Inc.; Smith and Lveless; Walker Prcess Equipment; and Yemans Brs. Nzzles fr rectangular filters are furnished by Pacific Flush Tank Div. Special disc distributrs are made by Lakeside and Yemans. Any f these manufacturers will supply the detailed drawings cvering the installatin f equipment

10 SCOPE 1. These specificatins cver vitrified clay filter blck made frm clay r shale r mixtures theref. TYPES 2. (a) Tw types f filter blck are cvered: L A nemege filter, blck suma ite. fr-use' ki- cnstructing a -single-curse trickling,. ^^^^^^^isi^^^afiriri^^t ductjs iftspugh; ;$& ls»^b^srtin i jhfe ; i in the P. ft A tw^'isce filter Hck sys ifr, that provmes d^nctge and fi pil's r i fi$r fbnss esf^^ ^. dr< filter bed sna fr passage:ti{(sirint afta aefa#n grilles fr passage f li<..jsuitabje. fpr use in cnstrutig^ a tw-curse trie &tin. VS%en placed r agsemimd, a curse f the fr cnve.y^ne f liquids tjpr bed. Ah assembly f the y^'er unijs frms sfrm, and air int, the filteiimg': ihedia. (b) When in psitin f use bth types f blck shall cmpletely cver the flr f the trickling filter bed. (c) The purchaser shuld specify whether blck shall be type I-S (standard-rate), type I-H (highrate), r type II. COMPRESSIVE STRENGTH 3. (a) The average cmpressive strength f five full-size filter blcks shall be nt less than 600 psi, based n grss area f the blck, with the lad applied in the same directin as in service. N individual blck shall have a cmpressive strength less than 500 psi. (b) Bth cmpnent units f type II systems, tested individually, shall cnfrm t the cmpressive strength requirement f Paragraph (a). ABSORPTION 4. The average water absrptin f five filter blck, by 1-hr submersin in biling water, shall nt exceed 6 per cent f the dry weight f the blck. ACID RESISTANCE 5. Filter blck shall be acceptable nly when the percentage f acid-sluble matter fr specimens representing such blck des nt exceed 0.25 per cent by weight. SHAPE 6. (a) Type I filter blck shall be square r rectangular in plan, as laid. When placed in parallel rws n the subflr f the filter bed, cells in a single curse f type I filter blck shall frm cntinuus drainage ducts, and the apertures r slts shall prvide fr drainage and aeratin f filter media. (b) Units fr the lwer r drainage curse and units fr the upper r aeratin curse f type II filter blck shall be square r rectangular in plan, as laid. When placed in parallel rws n the subflr f the filter bed, the lwer units shall frm cntinuus drainage channels. Upper units, when laid in parallel rws n lwer units, shall prvide fr drainage and aeratin f filter media. PERMISSIBLE VARIATIONS IN DIMENSIONS 7. (a) Length, width, r height f a type I filter blck shall nt vary by mre than l/ 4 in. per ft. ver r under the nminal dimensin stated by the manufacturer. (b) Lwer drainage channels and upper aeratin grilles f type II blck shall nt vary by mre than % in. per ft. ver r under the nminal dimensin stated by the manufacturer. APERTURES 8. (a) Apertures in the tp f type I filter blck shall be nt mre than 1 '/ 2 in. in width, but may vary in length. Aperture area shall be nt less than 2.0 per cent f the tp area f the filter blck when in psitin f use. (b) Apertures in the tp, r aeratin curse f type II filter blck shall be nt mre An 1'/» in. in width, but may vary in length,.stfpar unbstructed aperture area shall be nt; MSs than 30 per cent f the tp area f the bl6 ; <*K when in psitin f use. Table I. Crss-Sectinal Drainage Areas Minimum Effec/iV&A&e.a, Sq. in. per fl Pffldth ^SHELL AND WEB THICKNESSES 9.. (a^-.fexferir w@ss- (sheffe) f type I shifiri be nt less than S/16 in. in thi inffefet webs shall be rit less than thickness. 20 blck, and f in. in (b) Exterir walls (shells) and interir webs and struts f type II filter blck cmprising the lwer r drainage curse shall nt be less than 9/16 in. in thickness. Exterir walls (shells) and lngitudinal webs f type II filter blck cmprising the tp r aeratin curse shall be nt less than l'/g m - in thickness. Stiffening webs fr stiffening the lngitudinal webs, shall be nt less than 9/16 in. in thickness. DRAINAGE CHANNELS 10. (a) Effective crss-sectinal drainage area shall be defined and measured as the sectinal area lying belw the lwest level f the aperture slts. Aggregate effective crss-sectinal area f drainage channels in a filter blck shall be nt less than shwn in Table I. (b) The bttm f the drainage channels f any filter blck shall be curved r narrwed in width t frm a trugh in the bttm f each channel t accelerate flw f effluent and t minimize stranding f slids in such channels. (c) The height f drainage channels in any filter blck shall be nt less than 2! / 2 in., measured frm the lwest level f the aperture slts. (d) There shall be nt less than tw parallel drainage channels per ft f width f a type I filter blck. WORKMANSHIP AND FINISH 11. (a) Filter blck may be either glazed r unglazed. If glazed filter blck are specified, at least 90 per cent f the inner and uter surfaces f the blck shall be cvered with glaze, excepting the wire-cut ends where it may be entirely absent. FOLD FLAP OUT

11 (b) The lwer bearing surface f filter blck shall nt vary frm a plane by mre than 3/16 in. per ft f the greatest length r width f the blck. (c) All filter blck shall be well burned and substantially free f laminatins and fractures. All blck shall be free f pen cracks exceeding V/2 in. in length (such cracks shall nt extend cmpletely thrugh a cnnecting prtin f the web r shell), and shall be free f ther cracks exceeding 3 in. in length that d nt extend thrugh the webs r shells. All blck shall be free f chips larger than 2 by 1 by 3/16 in. and f blisters whse diameters exceed 2 in. and which prject mre than!/ 8 in. abve the nrmal surface. MARKING 12. All filter blck shall bear the name, initials, r trade-mark f the manufacturer and shall be marked t shw cmpliance with ASTM Specificatins C 159. These marks shall be indented n the exterir f the blck and shall be legible. REJECTION 13. If test specimens fail t cnfrm t requirements fr cmpressive strength, absrptin, r acid resistance, the manufacturer may srt the shipment, and new samples shall be selected by the purchaser frm the retained lt and tested at the expense f the manufacturer. In case the secnd set f test specimens fails t meet requirements, the entire lt shall be rejected. EXPENSE OF TESTS 14. Except as specified in Sectin 13, expense f inspectin and testing shall be brne by the purchaser. SAMPLING AND TESTING 15. (a) The purchaser r his authrized representative shall be accrded prper facilities fr sampling and inspectin f filter blck, bth at the place f manufacture and at the site f the wrk. At least ten days frm the time f sampling shall be allwed fr cmpletin f tests. (b) Filter blck shall be sampled and tested in accrdance with the Methds f Sampling and Testing Structural Clay Tile (ASTM Designatin: C 112), except that resistance f blck t the actin f acids shall be determined in accrdance with the applicable prtins f Sectins 14 t 18 f the Methds f Testing Clay Pipe (ASTM Designatin: C 301). GUARANTEE OF VITRIFIED CLAY UNDERDRAW FLOOR BLOCKS Warrant and Guarantee t that all vitrified clay underdrain blcks manufactured by the abve said cmpany and furnished fr installatin in under cntract dated between..and. are prf against impairment f service r failure due t rust, crrsin, decay, chemical decmpsitin and/r disintegratin caused by any actin f acids, alkalis, sewage, sewer gases, grund waters r industrial wastes, excepting Hydrfluric Acid nly. Als guaranteed against destructin by rats, rdents r termites. The undersigned bind themselves fr a perid f FIFTY YEARS after date heref t furnish t the abve named free f cst, vitrified clay underdrain blcks in the same quantity and sizes equal t any f the vitrified clay underdrain blcks manufactured and furnished by abve named manufacturer under said cntract which may have been damaged, destryed r impaired in service by reasn f any f the abve named causes. It is expressly understd that the cnditins f This Guarantee d nt prvide fr Furnishing f Blcks which have been brken r impaired by reasn f Imprper Handling, Placement r Cnstructin. The. By -Title-. Cmpany m Signed this- _day f RICKLING FILTER LOOR INSTITUTE

12 Standard Rate Filters THE first trickling filters were f the standard rate type, designed t use the cncept f lw rates f applicatin, up t 600 lbs. f BOD per acre-ft, n relatively deep beds f rck r crushed stne. The ppular methd f distributin f wastewater ver the media was by means f fixed nzzles, with distributing maniflds buried in the media. In the 1930's, the trend switched t circular beds with rtating distributrs which cnsisted f arms extending ver the diameter f the filter suspended frm a central influent clumn. In this same decade, the cncept f cntinuus applicatin was develped, which resulted in recycling the effluent in a predetermined rati t influent n an average flw basis. The recirculatin principle als helped maintain applicatin t the beds, regardless f influent flw cnditins. This device became knwn as a high rate filter because the verall dsage is much higher than that f standard niters. Bth high rate and standard rate filters are in use tday, thugh the trend is tward higher ladings and high rate filter cnstructin. Many studies have been made n trickling filter perfrmance, the mst ntable f which was that f the Cmmittee n Sewage Treatment at Military Installatins f the Natinal Reset rch Cuncil. This was dne in and was reprted in the Sewage Wrks Jurnal, September, Trickling filters are cnstructed in varying depths t carry varying rganic ladings with equally varying hydraulic ladings. Thus, there are prblems in determining what may be expected f a prpsed filter under varying lading cnditins. The Natinal Research Cuncil Study (hereafter referred t as NRC) established empirical frmulas which can be used t determine quite clsely what may be expected under given cnditins f lading. Other frmulas have als been derived, but these relate generally t high rate filters. What might be termed an average depth fr standard filters is 6 ft. but actual depths f installatins in use vary frm 5 t 10 ft. It has been cntended that the greater depths results in increased nitrificatin f the effluent. The NRC study referred t abve stated that "n significant degree f nitrificatin ccurred in plants with a daily BOD lad in excess f 500 punds per equivalent acre-ft." Sme plants "did nt prduce a significant amunt f nitrate despite lw lading." The BOD lading standards fr standard filters are by n means fixed. The upper limit is generally abut 600 lbs. f BOD per acreft per day, but a lading f the rder f 400 lbs. is mre cmmn. On 15 New Jersey plants studied by Frman and Shaw, applied BOD ladings averaged 367 punds f BOD per acre-ft. The average lading n 10 Army plants as reprted by the NRC Cmmittee was 377 lbs. In general, primary cnsideratin in designing and in determining the lad n standard filters shuld be based n lcal cnditins and n State Health Department r ther regulatry agency standards. Heavier ladings are pssible in warm climates than in cld climates. Vlumetric Lading The vlumetric lading n standard filters averages abut 300,000 gallns per day per acre-ft; r, fr a 6-ft. deep filter, a surface lading rate f abut 1.8 mgad. There is, hwever, n fixed standard. Based n a BOD strength f raw sewage f 240 mg/l, a flw f 1 mgd, a remval f 30 percent in the primary settling tank, and an rganic lading f 400 punds f BOD per acre-ft, there wuld be required 3.5 acrefeet f filter. In this case the vlumetric lading rate wuld be abut 285,000 gallns per acre-ft per day, r abut 1.71 mgd per acre f surface. As in this example, the rganic lading will ften gvern the vlumetric lading. The median vlumetric lading n the 10 Army plants referred t was 1.13 mgad, a rather lw value. The media used in standard trickling filters shuld cmply with the requirements given elsewhere in this text Specificatins shuld include resistance t freezing (nt needed in far suthern states); size range; percentage f fines; screening befre placement; methd f placement in the filter; and ther items cntained in the sample specificatins given. Fr underdrainage and ventilatin, specially designed blcks, als described in anther chapter, shuld be used. Specificatins fr such blcks are included elsewhere. Center channels in standard filters are small in cmparisn with thse required fr high rate filters, and standard channel cver blcks shuld be used. Vent blcks fr penings thrugh the wall, and cver blcks fr vent risers, are available frm the same surces as are underdrain blcks and channel cvers. Rtary distributrs are nrmally LARGE areas ccupied by standard rate filters cmpared with thse used by high rate units have favred cnstructin f the latter type in mdern design practice. 11

13 used fr applying the sewage t the filter. Mst f these are revlved by the reactin t the discharge thrugh the rifices which are placed n ppsite sides f the ppsing arms. Mst distributrs will perate satisfactrily with a minimum discharge that is 40 percent f the maximum, that is, they will handle flws up t 2% times the minimum, but 4 ft. f head shuld nrmally be prvided with this rati. The flw f wastewater des nt always fall within the perating limits f the distributr, and a dsing tank is usually prvided. The high water level f the dsing tank may be 2 t AVz ft. abve the level f the rifices in the distributr arms. Hwever, where lcal cnditins require that every pssible inch f fall be cnserved, special designs are available and the manufacturer's engineers shuld be cnsulted. Distributrs having 4 arms, arranged s that 2 arms serve fr lw flws and 4 arms fr high flws permit peratin with flws up t abut 5 times the minimum. The same result can be btained by having tw channels in each arm, an upper and a lwer ne. In all cases where high flws may ccur, the hydraulics f the siphn, the feed pipe and the distributr shuld be calculated carefully t insure that the frictin head is nt s great as t cause the siphn chamber t verflw. Head Lsses The head lsses between high water level in the dsing tank and the tp f the filter media include: a) The entrance lss in feet due t discharge int the siphn piping which may be assumed at 4v 2 /2g, where v is the velcity in the filter feed pipe; b) the dsing tank lss r the drp in level in the dsing tank required t fill the distributr arms which equals the vlume f the arms in cubic feet divided by the area f the dsing tank in sq. ft.; c) the frictin lss in the feed pipe which is cmputed frm pipe flw tables with allwances fr bends and fr increase in velcity head if the feed pipe is smaller than the siphn; d) the distributr lss which shuld be btained frm the manufacturer; 12 in. is the usual permissible minimum; and e) the distance frm the center line f the distributr arm t the tp f the media which is usually 6 in. The dsing tank shuld prvide abut 2 minutes detentin at twice the average rate f flw. This allws frequent dsing and keeps the size f the dsing tank within practical 12 limits. Since every ft f drawdwn in the dsing tank represents s much head lss, and since the tp layer in the dsing tank is distributed mst effectively ver the filter surface, it is custmary t maintain a drawdwn f 10 in. but nt ver 12 in. The ttal lss f head frm the high water level f the dsing siphn can be determined frm these data. Where dsing tanks are nt emplyed and the wastewater is pumped, the distributr must be designed t take the flw as it is received. Pretreatment Pretreatment is nrmally by settling; a perid f 2 t 2.5 hurs shuld be prvided, with prper allwance fr future-grwth. Since recirculatin is rarely used with standard rate filters, thugh it can be, settling tank capacity is based n the expected flw f wastewater at the date selected fr design. Retentin perid, verflw rate, depth f tank, weir design and sme ther factrs are subject t State Bard f Health r ther regulatry agency requirements and may differ materially frm state t state. In design, a BOD remval f 30 percent t 35 percent in the primary settling tank is used, depending n verflw rate and retentin perid. The specific figure t be used will generally depend n State Bard f Health r ther regulatry agency requirements. In general, the abve applies als t secndary settling tanks fllwing the filter. Filter Perfrmance Thugh standard filters are quite unifrmly stable in. day-t-day perfrmance, there are variatins in effluent quality frm filter t filter and even ccasinally frm mnth t mnth, fr which adequate explanatins are nt available. The frmula which fllws is based n the curve f best fit fr 10 Army plants studied by the NRC Cmmittee previusly mentined. It shws what a standard filter may be expected t d n the average. There will be variatins frm such average, especially seasnal variatins. The 10 Army plants prduced effluents ranging frm 14 t 89 mg/l, BOD with an average f 38.0 mg/l. Design Cmputatins The frmula shwn belw was derived by the NRC subcmmittee previusly referred t. It measures the prbable efficiency f the filter and the fllwing clarifier. It will nt always indicate what a particular filter is ding, but it will clsely apprach what may reasnably be expected frm a well designed, well cnstructed and prperly perated filter. The efficiency f such a filter shuld be: E = where E is the efficiency f the filter and fllwing clarifier in terms f the percent remval f applied BOD; and L is the rganic lading in punds f BOD per acre-ft that is applied t the filter. Since, in a filter f the type discussed in this chapter, there is n recirculatin, L is the applied BOD in punds divided by the filter vlume in acre feet The prcedure in using the frmula is shwn in the fllwing illustrative examples. With a flw f 1 mgd, a raw sewage BOD f 240 mg/l and 30 percent remval in the primary clarifier, the BOD applied t the filter is 240 x 8.33 x 0.70, r 1400 lbs. Assume an rganic lading f 400 lbs. f BOD per acre-ft, which will be the value f L in the frmula. There will be required 1400 H- 400, r 3.5 acre-feet f media. With a filter depth f 6 ft., the area f the filter will be acre. The efficiency f remval by the filter and fllwing clarifier by the frmula will be 85.5 percent. The effluent frm the secndary clarifier will cntain 1400 ( ) r 203 punds f BOD which is the equivalent f slightly mre than 24 mg/l. The verall reductin thrugh the plant will be 90 percent. The filter in the abve example has been laded at what is prbably the best standard lading. The effect f using a smewhat higher lading will nw be shwn. Assume a lading f 600 punds f BOD per acre-ft which will be L in the frmula. With a depth f 6 ft, the filter area will be 0.39 acre and the filter vlume will be 2.33 acre-ft. E then is 82.8 percent and the effluent will cntain 29 mg/l f BOD. The verall reductin thrugh the plant will be abut 88 percent. The standard filter, prperly laded, gives very fine results in BOD remval, with the prductin f a stable effluent. The vlume f media and the area f filter reauired is large. If these factrs are f imprtance frm the viewpints f space required r f cst, it is better t use a high rate filter than t verlad a standard filter.

14 A Nmgraphic Slutin fr Design THE PURPOSE f the nmgraph I presented here is primarily t reduce the cmputatin time ir a preliminary investigatin f the size f units fr a trickling filter cmplete treatment sewage plant. While nmgraphic slutins cannt ffer the accuracy f mathematical cmputatin, they can be used t give quickly an estimate f general unit sizes fr a prpsed plant. The nmgraph fr the design f trickling filters is essentially crrect in terms f its representatin f the NRC frmula, but great precisin is lst in scale distributins. Cnsequently, it is suggested that this nmgraph be used primarily fr initial estimates r grss estimates f trickling filter sizes. Anther use is fr the cmputatin f the theretical percentage f BOD remval in any filter, given the filter diameter, the depth, recirculatin factr, and the punds f BOD applied per day. Cmputatin f filter dimensins can be made by selecting 'the desired percent f BOD remval and the punds f BOD t be applied t the filter per day. The filter bed diameter, the depth, and the recirculatin rati can then be determined. The design limits are nt indicated n the nmgraphs. It is suggested that thse utilizing this graph familiarize themselves with the apprpriate state limitatins. Fr using the nmgraph fr filter design nly three values must be knwn: 1) The percent BOD remval desired thrugh the filter; 2) the punds f BOD which will be applied t the filter per day; and 3) the ttal gallns f waste (in mgd) applied t the filter. (A valid assumptin is that 65 percent f the daily raw sewage BOD will be applied t the filter.) Knwing these values, it is then pssible t examine and select cmparative sizes, depths, rates f applicatin and recirculatin, if required. Step 1. Thrugh the scale "% BOD Remval" lcate the % f BOD t be remved frm the filter (within the latitude ranges as indicated n these scales). Step Z. Extend a line thrugh value n "% BOD Remval" scale thrugh intersectin f R-R and T-T until it strikes a pint n "Punds f BOD Applied per Acre- Ft" scale. Step 3. Jin knwn values f "Punds f BOD Applied t Filter Per Day with the pint lcated in Step 2 and nte intersectin f Scale R-R. Step 4. Prject this pint n Scale R-R alng the parallel guide lines t a pint f intersectin n scale T-T. Step 5. The pint lcated in Step 4 (abve) becmes the pivt pint fr relating depth and area figures t meet design requirements as fund in Step 2. With a straight edge pass a line thrugh pivt pint n scale T-T and read values frm scales "Effective Depth (Ft)" and "Filter Bed Diameter (Ft)". Any cmbinatin f Effective Depth and Bed Diameter will meet design criteria in terms f rganic lading. Nte: The cncept f effective depth is related t recirculatin in high rate filtratin. In a standard rate filter, the effective depth is equal t the actual depth and recirculatin is zer. The effect f recirculatin, r the recirculatin factr F where: 1 -t R/I F = ( R/I) - allws, in the NRC frmula fr trickling filters, fr a mathematical cmbinatin f depth and F, which is labeled "effective depth." Cnsequently the recirculatin factr F has the effect f increasing the depth. The actual depth can then be fund thrugh a nmgraphic slutin shwn n the chart which relates the effective depth thrugh a recirculatin rati R/I. Step 6. Practical selectin f depth and area-diameter values will be dependent upn design criteria and ecnmic cnsideratins t be evaluated by the designer. Step 7. The "Hydraulic Lading Rate" may be checked by passing a line thrugh "Filter Bed Diameter" as selected in Step 6 and the "Daily Flw t the Plant (mgd)" t the "Hydraulic Lading Rate (mgad)" scale. Area and flw cmbinatins shuld fall int the categry f high r standard rate depending upn criteria chsen. If this des nt ccur, adjustment f filter diameter and depth shuld be made t accmmdate the hydraulic lading rate. When an existing filter is in peratin the theretical efficiency can be easily cmputed with this nmgraph. All the fllwing factrs shuld be knwn frm the installatin: 1) Filter diameter; 2) actual depth; 3) recirculatin rati; and 4) punds f BOD applied t the filter. Step 1. If recirculatin is used, jin values f "Actual Depth" and "Recirculatin Rati" t determine the "Effective Depth." Step 2. Jin value f "Effective Depth", as fund in Step 1, with values f "Filter Bed Diameter" and nte pint f intersectin with scale T-T. Step 3. Prject pint at T-T alng parallel lines t intersectin with scale R-R. Step 4. Thrugh R-R and value f "Punds f BOD Applied t Filter" lcate value f "Punds f BOD Applied per Acre-Ft." Step 5. Thrugh value fund in Step 4 and pint f intersectin f scales R-R and T-T jin a line t intersectin with "% BOD Remval" scales. This value yields the theretical "% BOD Remval" fr the filter f given size. The red lines indicate an example wrked ut fr high rate filters; the green, fr a standard filter. Frm an article by Walter R. Lynn, PUBLIC WORKS, August

15 POUNDS OF B.O.D. APPLIED PEF l\) Ol -f> O) <T> -W0D(0O O O O O O O O OO J I I ili ilil n r Ol POUNDS OF B.O.D APPLIED PE Ol CUB 70 I I 70 Q CO O Q n Cn 15 1 Tl m m m H X _ J I I Ol r Ol < > 00 P 1 1',-J 1 1 0) "b 1 1 Ol Ol ' 1 1 I'l'l r ID CO "b 1 1 ->l ' Ol POUNDS OF B.O.D. APPLIE 1 c 14

16 - FOOT _, 0E> U> O ' *"" fr3o DESIGN OF TRICKLING FILTERS (U.M.R.B. CRITERIA) Based upn N.R.C. frmula: 100 P ADF Where: P = W = A = D = F = % B.O.D. remval lbs. f B.O.D. applied area f filter in acres depth f bed in feet recirculatin factr I n O I'TM'I -ICON O O O m 1 1 ' 1 ' w 1 n O 1 TO FILTER PER DAY 15

17 The Design f High Rate Filters HIGH RATE filter plant serving Salina, Kansas, with 115-ft. distributrs is shwn abve. At right is a 70- ff. diameter distributr dsing at the rate f 25 mgad at Suffern, N. Y. WALLACE J. BENZIE. P. E. Directr, Sanitary Sewers and Engineering, Genesee Cunty Drain Cmmissin, Flint, Michigan HIGH RATE trickling filters are thse in which sme prtin f the filter discharge r final effluent is returned t the influent f the primary sedimentatin tank where it jins the raw sewage; r t the filter influent where it jins the primary effluent. Plants can be designed t give any degree f treatment between sedimentatin and cmplete treatment. Filters are usually dsed at 5 t 10 times the rates emplyed n standard filters, and with careful design, they can prduce an effluent as lw in suspended slids and BOD as a standard filter but with little, if any, evidence f nitrificatin. The much heavier rganic ladings used with high rate filters have resulted in the use f a variety f terms t designate such lading. The rather cmmnly used basis, with standard filters, f punds f BOD 16 per acre-ft, have been applied t high rate filters, and ladings f BOD f 2,540 t mre than 3,000 punds per acre-ft have been cmmn. Sme engineers have reduced this lading t a cubic yard basis, dividing the acre-ft ladings by 1,613. There has als been sme use f 1,000 cu. ft. as a lading basis, and it has been recmmended by a cmmittee f the Water Pllutin Cntrl Federatin that such usage be standardized in sewage treatment practice. Since 1,000 cu. ft. equals 37 cu. yds., the cubic yard lading is multiplied by 37 t btain the 1,000-cu. ft. lading. Thus, 3,000 lbs. per acre-ft is the same as 1.86 lbs. per cu. yd. r 68.8 lbs. per 1,000 cu. ft. There may be a slight saving in time in making design cmputatins by either the cu. yd. r the 1,000-cu. ft. basis, but mst frmulas nw in general use, fr determining filter efficiency are based n the acre-ft. There are three principal types f high rate filter in cmmn use tday, based n rate f feeding, recirculatin r lading. In ne type f filter peratin, a prtin f the trickling filter effluent

18 I RECIRCULATION design arrangements available are varied. In the diagram abve, larger circles represent filters; small circles are clarifiers; and heavy lines, recirculatin paths. BOD reductins are smewhat dependent n layut emplyed. is recirculated t the primary settling tank influent. It is smetimes desirable t add a secnd stage filter t this system where it is necessary t prvide fr additinal treatment. Lading rates and recirculatin ratis can be mdified t prvide the desirable effluent quality. Organic ladings are in the rder f 2,500 t 3,000 lbs. f BOD per acre ft, r 1.5 t 1.9 lbs. per cu. yd. based n the strength f the primary tank effluent. The vlume ladings range frm 10 t 30 millin gallns per acre per day (mgad). The unsettled filter effluent may als be recirculated directly back t the filter influent with n intermediate settling prvided. This recirculatin principle can be used n either standard r high rate filters. In additin, primary effluent may be distributed ver a maximum area f the filter at ne time with recirculatin nly at times necessary t maintain the flw requirements t prvide fr satisfactry distributin f the waste nt the filter media. Recmmended rganic ladings range frm 3,000 t 3,200 lbs. f BOD per acre ft r 1.9 t 2.0 lbs. per cu. yd. based upn the applied sewage. Additinal treatment may be prvided by utilizing a secnd stage filter. Because f the heavier rganic lad applied t a high rate filter per unit f filter vlume, the effluent frm a single stage high rate filter will be higher in BOD than that frm a lightly laded filter. T vercme this factr, the effluent frm the filter may be mixed with the incming raw sewage and passed again thrugh the filter; r tw filters, in series. These prcedures may result in treatment cmparable t that frm a filter that is laded less heavily, while the smaller units required in the high rate prcess cmpared with standard filters may reduce cnstructin csts materially. Flwsheets Flwsheets mst frequently used are shwn in the accmpanying diagram. The chice f flwsheet is dependent upn several factrs, including the strength f the raw sewage, the quality f effluent desired and, t sme extent, n the size f the plant. In a plant where multiple filter unite are required because f size, lcal requirements r ther reasns, it is frequently desirable t arrange fr tw-stage peratin with small additinal cst. A single-stage plant is differentiated frm a tw-stage plant by the fact that in the latter the sewage is passed successively thrugh a primary filter and thrugh a secndary filter. Recirculatin may be applied t bth filters. Thus, a single-stage filter is ne in which the filters, if there are mre than ne, are perated in parallel. Sewage may be, and usually is, recirculated thrugh singlestage filters, the effluent frm each filter being passed t a secndary settling tank after r befre withdrawing the prtin that is t be recirculated. In a tw-stage filter, the effluent frm the primary filter r filters, after withdrawing that prtin t be recirculated and returning it t and thrugh the primary filter, passes t a secnd filter; and the effluent frm the secnd filter, less recirculatin, is discharged t the final settling tank. Recirculatin In all high rate trickling filter plants, sme frm r pattern f recirculatin is always used. The rati f the vlume f flw recirculated t the vlume f average raw sewage flw is defined as the recirculatin rati R. Where mre than ne recirculatin means is used in a plant, the ratis are distinguished as Rj, R2, etc. The beneficial effects f recirculatin have been demnstrated. Return f recirculated filter effluent t the raw sewage has prved beneficial in reducing drs particularly in initial stages f plant peratin when the sewage flw is frequently much belw design cnditins and ften septic. Recirculated flw is als effective in reducing scum, particularly when treating strnger sewage. There are many variatins in the arrangements fr recirculatin. In a single-stage filter, the mst cmmn arrangement is t return a prtin f the filter effluent t the inlet f the primary settling tank. In the tw-stage plants, the mst used plan is t return a prtin f the primary filter effluent fr anther passage thrugh the primary filter; and a prtin f the secndary filter effluent fr anther passage thrugh the secndary filter. Hwever, excessive recirculatin is nt necessary. Very gd effluents can be btained with recirculatin ratis f 0.5 t 1. Ladings n high rate filters must be cnsidered frm tw bases: The rganic lading in punds f BOD per acre-ft, cubic yard, 1,000 cubic feet, r square ft f surface area; and the rate at which sewage is applied t the surface, usually in mgad. The rganic lading is based n the strength f the sewage applied t the filter. In design, it is cmmn practice t allw 30 t 35 percent remval f BOD in the pri- 17

19 mary settling tank and t assume that 65 t 70 percent f the raw sewage BOD will be applied t the filter. Much heavier ladings than thse stated abve are applied t the primary filters f the tw-stage plants. In a tw-stage plant, the same vlume f filter media is used as fr a single-stage plant, with the media divided, usually equally, between tw filters. The entire rganic lad, after primary settling, is applied t the primary filter. This may result in ladings f 3 t 4 lbs. per cu. yd. r 4,800 t 6,400 lbs. per acre ft. In effect, the primary filter becmes what has been called in the past, a rughing filter, with the principal functin f remving BOD and preparing the sewage fr the final passage thrugh the secnd filter. Vlume ladings range frm 10 t arund 30 mgad, based n the surface applicatin rate. Assuming a sewage flw f ne mgad, a recirculatin rati f 0.5 t 1, in a tw-stage filter, the BOD f the raw sewage is 240 mg/l and the remval in the primary settling tank is 30 percent, the rganic lad n the filter will be 8.33 x 240 x 0.70 = 1,400 lbs. If the unit rganic lading in the filter is 1.75 lbs. per cu. yd., 800 cu. yds. f media will be required, divided int tw filters, each cntaining 400 cu. yds. If the filters are 5 ft. deep, the surface area f each will be 2,160 sq. ft. r abut 0.05 acre. Since the daily flw f sewage plus recirculatin effluent will amunt t 1.5 mgad, the surface lad n the filter will be abut 30 mgad. If a single-stage filter were used, all f the 800 cu. yds. f filter media wuld be in a single filter, and a vlume lading rate wuld be half as great, r 15 mgad. Few plants are designed fr rates much abve 30 mgad and 50 mgad is abut the maximum that has been used. Predicting Perfrmance When laying ut new high rate filter plants, designers need t estimate r predict their perfrmance. This can be dne with reasnable cnfidence thrugh the applicatin f certain frmulas develped frm analysis f perating data frm existing high rate plants. It shuld nt be cncluded that sewage treatment has becme an exact science with precise perfrmance predictable by frmulas. Such frmulas d, hwever, serve as a tl r guide t the experienced designer. One such basis is the accmpanying Ten States Standard Curve. This shws that with BOD applied t the filter at the rate f 40 lbs. per 1,000 cu. tt. there will be 28 lbs. per 1,000 cu. ft. remved. Other expected remvals are readily btainable frm this curve. Anther methd f predicting perfrmance preferred by sme designers is n the basis f the NRC frmulas develped specifically frm trickling filter installatins serving military bases in Wrld War II. In these frmulas, weight is given t the rganic lading placed n the filter, and als t the rati f recirculatin, since it was fund that these factrs affect the quality f the effluent. The vlume f filter media is stated in acre-feet. The rganic lading is a functin f the BOD lading, the acre-ft vlume f the filter, and the recirculatin factr. Since, in recirculatin, part f the rganic material is remved in each passage thrugh the filter, a recirculatin factr F is derived frm the recirculatin rati by means f the fllwing frmula, where R is the recirculatin rati: F = 1 + R...(1) ( R) 2 Fr a 1 t 1 recirculatin rati, F is 1.65 and fr a rati f 0.5 t 1, F is The percent efficiency f remval f BOD in a trickling filter and its fllwing clarifier is expressed by frmula (3) belw, where E is the efficiency in percent f remval f rganic matter and L is the rganic lading. L is derived by frmula (2) belw and, fr filters with recirculatin, is based n pund f BOD applied, filter vlume and recirculatin. With V representing filter vl- ume in acre-feet, F the recirculatin factr and W the punds f applied BOD: W L =...(2) V X F Thus, if applied BOD is 1,400 punds, V is 0.5 and F is 1.65, L = The filter efficiency frmula is: Tji V L... (3) and where L = 1,700, the efficiency f the filter and accmpanying clarifier will be 73.7; that is 73.7 percent f the applied BOD will be remved in the filter and final clarifier (nt cnsidering the remval in the primary clarifier.) Thus, fr the example cited, remval in the filter and fllwing clarifier will be 1,400 X r 1,032 punds. BOD remaining in the effluent will be 1, = 368 punds. Overall remval in the plant, including that in the primary clarifier will be 81.6 percent. The abve frmula applies t any single stage high rate filter n matter what the rate f lading. It als applies t the primary filter f a tw stage plant; but it des nt apply t the secnd stage filter f such an installatin. In predicting the perfrmance f the secnd stage f a tw-stage plant, the reduced treatability f the rganic waste must be taken int accunt. A frmulatin is given in an article, "Design Cnsideratins fr Bilgical Filters," PUBLIC WORKS, February, Cmpilatins f data n the peratin f tw- BOD REMOVAL curve used in Ten States Standards fr filters and final settling. m r BOD Applied - Punds per 1000 Cu. Ft. B Pint cient d This Insuff Be yn 50 18

20 stage filters are available. Tw ntable references are "Sewage Treatment at Military Installatins," Sewage Wrks Jurnal, Vl. 18, N. 5 (1946) and R. S. Rankin, Transactins ASCE, Vl. 120, p. 823 (1955). Clarifiers Primary and secndary clarifiers will nrmally be designed in accrdance with the State Bard f Health requirements; cmmnly recmmended detentin is abut 2 hurs in primary settling tanks, with verflw rates f 650 t 800 gallns per sq. ft per day; and the same fr secndary clarifiers. Depending n recirculatin practices, clariiier capacity may be based n raw sewage flw; as when recirculatin is merely t maintain distributr peratin during lw flw perids; r n raw sewage flw plus recirculatin, when strng sewage r ther factrs dictate recirculatin t prvide dilutin. The size f the primary clarifier is usually selected n the basis f average 24-hur raw sewage flw plus recirculated flw returned t the clarifier influent. Treatment Results Single-stage high rate filters will nrmally prduce effluents f the rder f 40 t 45 mg/l BOD when laded in the range f 2,400 t 2,800 punds f BOD per acre-ft. Twstage filters, with the same lading will nrmally prduce effluents in the range f 24 t 28 mg/l BOD. The Sandusky, Michigan, treatment plant has a filter vlume f 9,850 cu. ft. and was designed with a recirculatin rati f 1:1. This plant has shwn reductins in BOD thrugh the filter f mre than 80 percent when laded at 54 punds f BOD per 1,000 cu. ft. This plant is quite typical in its perating results, f a number f plants f this type existing in Michigan tday. The perfrmance f trickling filters is affected by several factrs including media size, depth f media, ventilatin, hydraulic rate f applicatin, rganic lading and temperature variatin. In a paper published in the WPCF Jurnal, Vl. 35, N. 4, pp. 445 t 455, the effects f temperature variatin n trickling filter perfrmance at several selected plants in Michigan were reprted. The cnclusins f this study are: 1) There is a significant difference in trickling filter efficiencies between the summer and winter mnths. 2) Recirculatin f wastes thrugh a filter has a marked cling effect n the waste during the winter mnths; and by lwering the temperature f the wastes applied t the filter, the efficiency f the trickling filters is lwered. 3) Lwer air temperature has a mre significant effect n reducing filter efficiencies in plants that recirculate than thse that d nt practice recirculatin. 4) In plants that recirculate, the winter filter efficiency was 21 percent less than the summer efficiency. 5) Filters having a BOD lading f mre than 10 lbs. per 1,000 cu. ft. f filter media shwed n greater seasnal variatin in efficiency than when laded at less than 10 lbs. 6) Filter efficiency was affected in thse plants nt practicing recirculatin when the air and sewage temperatures were equal. 7) In the design f trickling filter plants in the nrthern part f the United States, cnsideratin shuld be given t the marked change in efficiencies which ccur between the summer and winter seasns. T btain the predicted perfrmance in an perating plant, intelligence must be used in the selectin f materials and equipment ging int the plant and in btaining a qualified peratr t run it. Such items as filter media, filter underdrains and rtary distributrs are very imprtant and specificatins shuld leave nthing t chance. Crrectly sized pumps, particularly fr recirculated flws, are als very imprtant. Experience has demnstrated that prperly designed, cnstructed and perated high rate trickling filter plants have little difficulty in btaining results equal t the predicted perfrmance. ODD DISTRIBUTOR with multiple arms is a feature f the Aer-filter in a Gaffney, S. C. installatin. Belw are shwn the secndary units f a tw-stage filter plant that prvides treatment fr the Air Frce Academy. 19

21 Trickling Filters fr Industrial Waste Treatment THE wastewaters frm industry are, f curse, extremely varied in character, even within the same industry r manufacturer's peratins since much depends n the type f prcessing emplyed. Many, hwever, are rganic, such as fd wastes and thse frm distilling and pharmaceutical peratins. These are subject t bilgical xidatin and cnsequently may be rganically stabilized thrugh treatment n trickling filters. Sme may be deficient in elements required t sustain micrbial grwth r naturally lacking in rganisms necessary t start grwth n the filter. Hwever, these cnditins can be crrected by adding the required nutrients and bilgical seed. Sme wastes, such as phenls, are txic t micrrganisms, but techniques are available t handle even these wastes n trickling filters. The selectin f the type f filter, whether high r standard rate and its design, depends generally n the same factrs as are encuntered in municipal dmestic wastewater treatment plant design: The amunt f BOD t be remved; preliminary treatment; the lad factr; the recirculatin rati; and site cnditins. One usually different factr is the vlume f flw. Whereas municipal sewage flws are calculated in millins f gallns per day, the flw f industrial wastes will frequently be cunted in the thusands f gallns per day. This wuld perhaps lead t the belief that trickling filters fr industrial wastes can be small. This is nt necessarily the case. A nrmal municipal sewage will 'have a BOD f 250 t 400 mg/l r 2085 t 3336 punds per MG., but it is nt uncmmn fr an industrial waste t have a BOD f 4000 mg/l, and in sme instances as high as 30,000 mg/l. This means 33,600 t 250,000 lbs. f BOD per MG. The trickling niters fr such high rganic ladings, even thugh the daily vlumes are small, must be very large. Preliminary treatment may fllw any ne f a number f accepted patterns including anaerbic digestin f very strng wastes; plant management t mdify peratins prducing strng and difficult t treat wastes; recvery; and blending. The methd selected will depend n lcal cnditins. The aim shuld be t reduce very high BOD cntents and t adjust the wastes s they are amenable t aerbic treatment. Withut prejudice fr ne type r anther, it may be cnceded that the high rate trickling filter will ften have advantages in the treatment f industrial wastes, due t lesser space requirements and ften lwer cst. There is a tendency, where feasible, t cmbine industrial and dmestic wastes fr treatment by a nearby urban cmplex, with the industries prviding their share f the treatment csts. In designing such facilities, careful cnsideratin has t be given t the influence f the industrial waste lad. Sme industries, such as canning, are seasnal in peratin, which discurages the use f bilgical xidatin measures fr handling the industrial waste alne, because time is required t start a trickling filter r an activated sludge plant. The High Rate Filter The higher the initial BOD cncentratin, the greater the lading factr per unit f bed vlume r area may be. Als, as the BOD cncentratin increases, better results frm the standpint f percentage f reductin are btained by increasing the rati f recirculatin. The vlume f recirculated filter effluent has usually little effect n the size f the filter itself but principally affects the design f the distributr unit. The flws frm industrial plants are usually relatively small s that increased recirculatin rates used d nt require the use f large capacity pumps r result in high pwer demands. Fr instance, if the waste flw is 100,000 gpd and it has been determined that tw-stage high rate filtratin will prduce a satisfactry reductin in BOD when the rate f recirculatin in each stage f filtratin is 3 times the vlume f raw waste, recirculatin will be 300,000 gpd in each stage. Fr this, in each stage, a centrifugal pump will be required with a capacity f 208 gpm against a ttal head f 15 ft. This may be btained with a 3-in. pump, requiring a lv2-hp mtr. As these pumps run cntinuusly, the ttal daily usage f electrical pwer with the tw pumps in peratin will be nly 54.0 kwh. T cite an example: The wastes frm the prductin f alchl frm sugar cane mlasses have BOD cncentratins ranging frm 22,000 t 33,000 mg/l. It is cmmn practice, when treating the effluent frm the pre-digestin step used in handling these wastes, t emply recirculatin rates f 5 t 7 times the vlume f digester effluent Even in a large distillery f this type, a waste flw f 225,000 gpd is abut the upper limit, s that with a tw-stage plant, using 3.5 times recirculatin in each stage, the pump size fr each phase will be 547 gpm and the pump will require nly 3 hp t perate it. Specific Examples Cannery Wastes. Trickling filters are satisfactry fr these wastes because the BOD is exerted rapidly due t the high sugar cntent and its quick decmpsitin. Fr wastes averaging arund 1000 mg/l BOD ladings f 5 t 6 lbs. f BOD per cu. yd. may be used. Tests n crn cannery wastes have indicated that ladings as high as 22 lbs. per cu. yd. may be used n primary high rate niters, preceded by sedimentatin. With such ladings, reductins f apprximately 50 percent f the BOD have been achieved, with the raw wastes having an average BOD f 8400 mg/l. Sauerkraut packing wastes have been treated n niters using blast furnace slag as the medium. In the test perid, raw waste ladings n the filters ranged frm 575 t 6800 lbs. BOD per acre-ft per 9-hur day. The dsing rates ranged frm 9 t 30 mgad, and the rates f recirculatin frm 3.3 t 19. The characteristics f the raw wastes were ph 4.0; acidity 1100 mg/l as CaCO 3 ; chlrides 4500 t 6500 mg/l; ttal slids 12,000 t 13,000 mg/l; BOD 2000 t 2400 mg/l. The filter used in the pilt plant was 10 ft. in diameter and 6 ft. deep with slag media. With adjustment f the ph f the wastes t a minimum f 6.6, a BOD reductin f 85 percent was btained. Cncentratins f 20

22 chlrides up t 5000 t 6000 mg/l seemed t have little effect n the bilgical efficiency f the filter. There was n evident relatin between the rati f recirculatin and the reductin in BOD. Dairy Wastes. These cmprise the waste waters frm creameries, where milk is separated; bttling plants; milk cndensing plants; pwdered milk plants; and cheese, butter, ice cream and ther milk prducts plants. The BOD ranges frm apprximately 100,000 mg/l fr whle milk wastes t 30,000 mg/l fr whey. The wastes respnd t treatment n trickling niters after preliminary treatment t remve carse slids. In treatment, it is cmmn practice t include a step f aeratin t inhibit anaerbic actin during the retentin perid and this als aids in BOD reductin. The screened and aerated wastes are applied frm a batch hlding tank t either lw rate filters r t high rate niters with recirculatin. High rate filter lading is 1.0 lb. f BOD per cu. yd. Reductins f 90 t 95 percent f the BOD have been btained. Single-stage recirculating type high rate filters are satisfactry where a final effluent with 70 t 100 mg/l BOD is acceptable. With twstage high rate filters perating in series, with recirculatin in each stage, reductins f 90 t 95 percent have been btained with primary BOD ladings f 1.5 t 2.0 lbs. per cu. yd. and secndary filter ladings f 0.75 lb. per cu. yd. Fermentatin Industry Wastes. These cmprise the wastes frm breweries; distilleries fr the prductin f whiskey, brandy and alchl; and the prductin f alchl frm sugar cane mlasses. Such wastes are nrmally high in BOD and lw in suspended slids, with high ttal slids cntent and lw PH. Distillery Wastes. At ne plant the design lading is 0.75 lb. f BOD per cu. yd. Dsage rate is 8.0 mgad with recirculatin rati f 4 times raw waste vlume. At anther the raw waste averaged 685 mg/l BOD; the high-rate filter lading is 1.1 lb. per cu. yd. and the dsage rate is 30.0 mgad. A recirculatin rati f 11 t 1 prduced an ver-all BOD reductin f 77 percent. Yeast Factry Wastes. Experiments n yeast wastes by Dr. Willem Rudlfs and E. H. Trubnick at Rutgers University, included predigestin, sedimentatin and the applicatin f digester effluent t trickling filters perated in series with recirculatin. It was fund that ladings up t 11,000 t 12,000 punds f BOD per acre-ft per day culd be applied t the filters withut material deteriratin f the nrmal 50 percent remval. Slaughterhuse and Meat Packing Wastes. These wastes vary widely in BOD, ranging frm 2200 t as high as 9100 mg/l. Treatment n trickling filters shuld be preceded by screens, sedimentatin and means fr remving grease. Sedimentatin reduces the BOD 30 t 35 percent. Lw rate niters may remve up t 80 percent f the remaining BOD. In several cases twstage filtratin, with the primary filter s cnstructed that it may be washed, fllwed by sedimentatin and a secnd step f filtratin has been used with an verall plant reductin f 94 percent f the BOD. Phenlic Wastes. Wastewaters cntaining as much as 800 mg/l phenl can be treated n trickling filters t reduce the phenl cntent t less than 1 mg/l thrugh careful cntrl in acclimating the bilgical grwth t the txic nature f the wastes. Dilutin frm ther waste effluents can be used t make the filter effluent acceptable fr discharge. Wastewaters cntaining ther txic rganics such as frmaldehyde, rganic acids and wastes frm petrleum refining have als been successfully treated. Many hydrcarbns, particularly thse f high mlecular weight, are resistant t bidegradability, and experimentatin with specific chemicals is advisable. Pulp and Paper. Sme degree f success has been achieved with these Curtesy Drr-Oliver, Inc. DESIGNED after extensive pilt plant tests, this high-rate trickling filter plant was built by Shell Oil C. fr treatment f refinery wastewater prir t discharge. Textile Wastes. These are treatable with trickling niters, thugh plastic media is preferred. Efficiencies up t 50 percent have been attained, which indicates their use as preliminary treatment devices, fllwed by ther bilgical devices. Pharmaceutical Wastes. Wastes frm plants prducing penicillin, auremycin, streptmycin and ther antibitics have been fund t respnd t treatment n trickling filters. The usual plant cmprises strage and equalizatin tanks, grease fltatin unit, aeratin, sedimentatin and high rate trickling filters fllwed by final sedimentatin. Filters are designed t handle 4300 punds f BOD per acre-ft, with 6-ft depth f stne, and an average recirculatin rati f 3 t 1. wastes, using plastic media. With ladings as high as 800 t 1,000 lbs. per cu. ft., 45 percent reductin f BOD has been btained. Their use in this industry appears t be primarily fr rughing filters. Experience with rck filters has been disappinting with rapid clgging f media a principal prblem. Metal Finishing Wastes. Wastes cntaining cyanide have been applied t trickling filters but due t the extremely lw final cncentratins permitted in streams, niters have nt been used in perating plants. In many cases regulatry agencies require a maximum f 1.0 mg/l in final effluents; thers require 0.1 mg/l; and in ne case the allwed maximum is "substantially nne". 21

23 Design Cnsideratins fr Bilgical Filters EDWARD C. ARCHER and LLOYD R. ROBINSON. JR. Mr. Archer is a Sanitary Engineer with TLM Assciates and Dr. Rbinsn, frmerly Assciate Prfessr f Sanitary Engineering at Mississippi State University is with Camp, Dresser and McKee. SEVERAL mathematical mdels exist fr predicting perfrmance f bilgical filters. All, at best, give an apprximatin f what can be expected frm an actual installatin. These empirical frmulae give a gd indicatin f the perfrmance f the bilgical prcess, althugh they are by n means exact. Field data discussed later in this article will demnstrate the clse crrelatin between calculated and actual results. In the design f the bilgical prcess the engineer is cnfrnted with tw bjectives. First, the prcess must stabilize the waste t a pint that it can flw int the receiving stream withut causing a nuisance, and secnd, the prcess must be as ecnmical as pssible. Until recently very little study had been given t the cst f the varius filter units. In a study undertaken at Mississippi State University, the Natinal Research Cuncil (NRC) frmulae 1 were analyzed using the Internatinal Business Machine 1620 cmputer fr the purpse f develping charts t aid the engineer in the design f the bilgical filter prcess. Numerus variables enter int the prcess and must be cnsidered by the designer. Sme f these, such as strength f waste, vlume f flw, and cnditin f receiving stream are nt nrmally cntrlled by the engineer. Therefre, t prvide a gd design the engineer must balance the remaining variables in an attempt t minimize cst while prviding the treatment required. Tw f the majr items affecting cst are vlume f media and recirculatin. The design charts develped by cmputer analysis give the designer a cmparisn between these tw variables. Knwing pwer and material csts fr a specific lcale, the engineer may then minimize cst fr the prcess. 22 T prvide design charts fr a tw-stage bilgical filter prcess, the effect f varius vlume ratis as well as varius recirculatin ratis fr each stage had t be cnsidered. With the aid f data frm the cmputer and the riginal NRC study, the ptimum cnditin was determined and used as a basis fr the charts. Field data were cllected at the Mississippi State University treatment plant t demnstrate the effectiveness f the NRC frmulae in predicting perfrmance. Five different perating schemes fr the plant were bserved and cmpared with calculated results. Develpment f NRC Frmulae The study entitled "Sewage Treatment at Military Installatins" 2 was cnducted by a subcmmittee n sewage treatment frmed by the Cmmittee n Sanitary Engineering. The net result f the study was the frmulatin f equatins t predict the perfrmance f filters. The data cllected were cmpiled, and a graph f efficiency versus filter lading was pltted. A curve f best fit was applied, and the resulting equatin became knwn as the NRC frmula. The secnd equatin, used fr twstage peratin, is simply a revisin f the first with the reduced treatability f the rganic waste taken int accunt. The equatins are as fllws: First r Single Stage: Secnd Stage: 1 + 0,,0085J- W \-yj~ E x \vf F R (1 + R/10) 2 (1) (2) (3) = Fractinal efficiency f BOD remval f the first-stage filter. W = BOD lading f settled raw sewage in punds per day. This des nt include the BOD f the recirculated waste. F = A factr reflecting increased remval due t recirculatin. R = Recirculatin rati n vlume f recirculated flw divided by the vlume f influent flw. V = Vlume f filter media in acre feet. E 2 = Fractinal efficiency f BOD remval f the secnd stage filter. W = BOD lading applied t secnd stage prcess in punds per day and des nt include the BOD f the recirculated waste. Develpment f Charts As mentined previusly, the engineer must cnsider several variables when designing filters. Sme f these variables have a direct effect n the design but nrmally cannt be cntrlled by the engineer. The required verall efficiency f tihe plant must be determined frm the cnditins f the receiving stream, the vlume f flw, and the strength f the influent waste. Effluent BOD delivered t the receiving stream is f a majr cncern t federal, state, and lcal stream pllutin cntrl fficials and acceptable permissible values shuld be determined in cnsultatin with them. Sme f 1ihe variables which the engineer may cntrl are the size f the units, recirculatin and flw diagram. The fllwing is a list f variables that must be determined: 1) Amunt f treatment required. 2) The strength f BOD f the influent sewage. 3) The quantity f flw and the hydraulic lad f the filter. 4) The vlume and size f the filter media. 5) Recirculatin. 6) Temperature f the sewage. These factrs shall be briefly discussed with emphasis n their effect when using the NRC frmulae. The BOD lad is the parameter used t determine the efficiency f the prcess when using the NRC

24 equatins. Since the NRC study included nly plants with 8,000 punds per acre-ft per day r less, this lad is usually cnsidered the upper limit f the equatins. The suspended slids are nt cnsidered in the equatins but are assumed t be reduced with, and apprximately as, the BOD is reduced. This assumptin is based n the results f the NRC study and is supprted by field data cllected in the Mississippi State study. Althugh the actual vlume f flw des nt enter int the NRC equatins, it is imprtant in the design f the prcess because it determines the rganic and hydraulic lads applied t the filter. Maximum hydraulic lads, established by state and lcal fficials, are usually fur millin gallns per acre per day (mgad) fr standard-rate filters and 30 mgad fr high-rate filters. The vlume f the niter media, adjusted fr recirculatin, is the cntrlling parameter fr the required efficiency. The cnditins f design are usually such that the designer knws the influent BOD and the efficiency desired and must determine the recirculatin and vlume f media required. The design charts f this study are based n this cnditin. The size f the media is f imprtance fr it determines the vid spaces available fr air flw, waste flw and slime grwth. General media size is frm Wz t 4 inches with the upper ranges used fr high-rate filters and the lwer ranges fr standard-rate filters. 3 Recirculatin is a very imprtant cnsideratin because it can imprve treatment withut increasing the filter vlume. Recirculatin is seldm used n standard-rate filters and almst always used n highrate filters. 4 Recirculatin has several advantages. These include: 1) Return f the rganic waste fr mre than ne pass thrugh the filter. 2) Dilutin f incming waste fr a reduced lad t the filter. This helps t achieve a mre unifrm lad nt the filter. 3) Cntinuus seeding f the raw sewage and filter with active micrrganisms fr an increased rate f bichemical stabilizatin. Gallar and Gtaas 5 have cnducted a statistical ptimizatin analysis that demnstrated recirculatin abve 4:1 is generally unecnmical. If recirculatin belw 4:1 saves sufficient vlume f media t ffset the additinal cst and peratin f the units, then recirculatin shuld be used. A factr "F" is incrprated int the NRC equatins fr increased efficiency due t recirculatin. Equatin (3) describes "F", and it simply indicates the average number f times the waste passes thrugh the filter. Temperature is imprtant in any bilgical prcess. It was riginally thught that air temperature had a prfund effect n the bilgical filter, but experience has shwn that the temperature f the sewage is the influential factr. 6 Shuld extreme temperature variatins prevail in an area under cnsideratin, the NRC equatins shuld be used with cautin. In the study f the NRC equatins, it became apparent that the vlume f the media was varying directly with the BOD and flw f the influent sewage when the efficiency and recirculatin rati were cnstant. The linear relatinship was utilized in the develpment f unit charts fr design. Because f this linear relatinship the vlume f filter media required varies directly with the lad applied t the filter when the efficiency and recirculatin are cnstant. As an example, if the flw remains cnstant and the BOD dubles, then the lad t the filter dubles and the filter vlume dubles. The same wuld ccur if the BOD remained cnstant and the flw dubles. Similarly, if the flw and BOD duble then the lad t the filter and the vlume f the filter increase fur times. In rder t determine the ptimum vlume distributin between filters in a tw-stage system, the NRC equatins were used t study effects that changing that rati wuld have n verall efficiency. Using a fixed ttal vlume f 0.16 acre-ft, a flw f 100 gpm, and a BOD lad f 100 mg/l, the efficiencies were calculated fr a series f vlume distributins. The results, pltted in Figure 1, shw that the maximum efficiency ccurs at a vlume f 0.07 acre-ft in the first filter and 0.09 acre-ft in the secnd filter. Hwever, it is bvius that the efficiency at the pint where the vlumes are equally divided is nly very slightly belw the maximum efficiency. Therefre, the mst ecnmical design with ptimum efficiency wuld be with the vlumes divided equally between the tw filters and the recirculatin equal fr bth stages. Prviding recirculatin is cnstant, a vlume f media divided between tw stages will yield a higher efficiency than a single stage with an equal ttal vlume. Therefre, when high efficiencies are required, the tw-stage filter is nrmally utilized. In the medium range f efficiencies the engineer must cmpare the increase in cst f media vlume fr a single-stage filter with the increase in cst f multiple units t arrive at a minimum cst. When recirculatin is prpsed fr a filter system, direct recirculatin versus recirculatin thrugh the clarifiers may be cnsidered. Recirculatin thrugh a clarifier increases the ttal flw and cnsequently, will increase the size f the unit. Direct recirculatin increases nly the flw thrugh the filters and des nt alter the required size f the clarifiers. Many recirculatin studies have been cnducted which indicate that the quantity and nt the system f recirculatin is the imprtant factr. 3 Culp 7 cncluded frm a study f a field installatin that direct recirculatin is as bene- FIGURE 1. Optimizatin curve fr tw-stage filter with a fixed ttal vlume. U u V <r a 92 b m c 88 S I i 84 UJ u u. li_ I FIRST STAGE VOLUME - Acre Feet SECOND STAGE VOLUME - Acre Feet 23

25 ficial as recirculatin frm behind the clarifiers. Design Charts In mst design, prblems the influent strength is calculated frm available data, and the maximum effluent strength is prmulgated by federal, state, and lcal authrities. With these tw parameters knwn, the required efficiency f the plant may be determined. Since calculated results using the NRC frmulae include the filter and subsequent settling, the primary clarifier is the nly ther unit entering int the verall plant efficiency. Depending upn its design, the primary clarifier can remve 20 t 40 percent f the BOD applied. A general assumptin used by many engineers is that 35 percent f the applied BOD is remved. After assuming a value fr remval in the primary stage f the prcess, the lad t the filter can be determined, and the vlume f media, crrected fr recirculatin, can be calculated. The design prcedure described abve was used as a frmat fr the develpment f the charts. Figures 2 and 3 are charts fr the design f a single-stage plant and Figures 4 and 5 are fr a tw-stage plant.05 SINGLE - STAGE FILTER Lad t Filter: BOD = 100 mg/l Flw= loogpm Figures 2 and 4 give the filter vlume required fr any desired efficiency f the filter stages and are based n the lad actually applied t the filters. Figures 3 and 5 give the filter vlume required fr any desired efficiency f the entire plant and are based n the raw sewage lad applied t the plant. Incrprated in Figures 3 and 5 is the assumptin that 35 percent BOD remval is accmplished by the primary clarifier. The charts are based n a BOD lad f 100 mg/l and a flw f 100 gpm. Because f the linear relatinship between lad and vlume, a simple cnversin can be applied t arrive at a specific cnditin. If, fr example, the BOD f the influent is 200 mg/l and the flw is 500 gpm, then ne f the charts shuld be used t btain a value which must be multiplied by (200 -* 100) x (500 -*-100) r 10. It shuld als be nted that the charts fr the twstage prcess are based n an equal vlume existing in each filter and equal recirculatin existing fr each stage. The charts were pltted using the utput frm cmputer prgrams. Basically the apprach was very simple. Using the NRC equatins, / /. / / / J/i / / / //// I f t-fff tft.5 a set efficiency, a unit flw, a unit BOD and a very small vlume increment, the cmputer made repetitius calculatins until the ttal vlume yielded the required efficiency. The cmputer typed ut the vlume required fr the cnditins set. A semi-lg plt was then made f efficiency versus ttal vlume. The resulting smth curve plts are the design charts. Discuss/n In the design f the bilgical filter prcess using the NRC equatins, the nly three parameters the engineer may vary t achieve the required efficiency with minimum cst are the vlume f media, the recirculatin rati and the number f stages. The charts prduced by this study give the designer the required vlumes fr efficiencies varying frm 75 t 95 percent fr plant design and 40 t 90 percent fr the design f the stages. The recirculatin rati was varied frm 0 t 4:1, and charts are prvided fr bth single and tw-stage peratin. With the required efficiency knwn, the decrease in vlume with the increase in recirculatin can be easily determined using the design charts. Knwing the cnstructin SINGLE - STAGE FILTER PLANT Ld t Plant: BOD= 100 ma/l Flw = loogpm / III 'Ik 'If III < UJ S 13 _l > /, / / / / / / 2T / / / /// / / ^-4-1 *//i/ Figure // EFFICIENCY - Percent f BOD Reductin by Filter and Final Clarifier ^ / V N 0 ne > 90 r eet V OLUME-Acr b >.01 Y / i / / // 4 // /// /A /// /" -4:1 / / / i 3:1 V / / y //t // r / ne / / ' y(// /N// /^ 0 5:1 igure EFFICIENCY - Percent f BOD Reductin by Entire Plant 24

26 and electrical pwer cst fr the particular lcale the engineer can then cmpare the cst f recirculatin versus decrease in vlume. These tw parameters cnstitute the majr csts f the filter and aid the engineer in ptimizing cst and efficiency. The charts d nt give the designer a cut and dried slutin fr a particular cnditin. They simply cmpare tw f the majr factrs affecting cst and leave the actual ptimizatin up t the engineer. In using the charts fr the design f a tw-stage plant, it must be understd that the curves are based n the ttal vlume being equally divided between the tw filters and the recirculatin rati being equal fr bth stages. Hwever, in the study fr the develpment f the charts it became evident that within practical limits the vlume rati between the units has a very limited effect n the verall efficiency. If the vlume f media in the first filter is apprximately fur times greater than the vlume in the secnd filter, the decrease in efficiency frm the ptimum is slightly greater than ne percent Similarly, if the secnd filter is apprximately fur times larger than the first, the efficiency decreases slightly less than ne percent frm the ptimum. This is an imprtant factr when increasing the capacity f an existing plant because the additinal filter may vary frm ne-furth t fur times the size f the riginal filter and still be very clse t the ptimum design. Field Observatins The field bservatins were perfrmed t demnstrate the effectiveness f the NRC frmulae and the design charts t predict the efficiency f bth a single-stage and a tw-stage filter plant under varius perating cnditins. This is by n means a prf f the NRC frmulae but rather a demnstratin f the validity f the frmulae and design charts fr the perating cnditins encuntered. The field data were cllected at the Mississippi State University Treatment Plant. The plant is a twstage filter plant with direct recirculatin arund each filter. The filters are 50 ft. in diameter and have an average media depth f 4 ft with media size ranging frm 2 t ZVz in. The primary, intermediate and final clarifiers are designed n the basis f the influent flw plus recirculatin because prvisins were made fr recirculated flw t pass thrugh any f the clarifiers. Three sampling perids f 24 hurs' duratin were made n the peratin f the plant The first run was made with n recirculatin. The secnd run was made with a 1.1 recirculatin rati. The last run was identical t the first except that the intermediate clarifier was bypassed. In Run III recirculatin was nt emplyed, and the effluent frm the first filter passed directly nt the secndary filter. During the first tw runs the intermediate clarifier was in peratin; therefre, these field data culd be used t study bth single and twstage filter peratin. Prir t each run the plant was allwed t perate under the specific cnditins fr a minimum f fur weeks in an attempt t reach a steady state. Samples were cllected every hur n the hur fr the 24-hur perid with a ttal f 25 samples being taken. The reading n the cumulative flwmeter f the plant was recrded at the beginning and end f the run t determine the ttal number f gallns passing thrugh the plant..05 TWO-STAGE FILTER Lad t First Stage Filters. BOD >= 100 mg/l Flw* 100 gpm M TWO-STAGE FILTER PLANT Ld t Plant : BOD= I00 mg /L Flw = I00 gpm ' EFFICIENCY Percent f BOD Reductin by Filter nd Final Clarifier EFFICIENCY - Percent f BOD Reductin by Entire Plant 25