INVESTIGATION OF RATIONAL EFFLUENT AND STREAM STANDARDS FOR TROPICAL COUNTRIES. M.B. Pescod Professor of Environmental Engineering

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1 AIT / /. &. # Asian Institute f Technlgy Bangkk Thailand LIBRARY lr;i-.:;;'.,:^l Reference Centra fr Cmmunity Watsr Supply research reprt INVESTIGATIN F RATINAL EFFLUENT AND STREAM STANDARDS FR TRPICAL UNTRIES by M.B. Pescd Prfessr f Envirnmental Engineering Asian Institute f Technlgy Bangkk, Thailand May 1974 xq5 -^ i (0

2 }< / IN REPRT N. FE (Annual Reprt) GRANT N. DA-RDRF-S G179 DA PRJECT/TASK AREA/WRK UNIT N. 2M061102B52B FE INVESTIGATIN F RATINAL EFFLUENT AND STREAM STANDARDS FR TRPICAL UNTRIES by M.B. Pescd Prfessr f Envirnmental Engineering Asian Institute f Technlgy Bangkk, Thailand May irv-n'jg.r.::.-, - " " > ; " ] i f d\"-t?!. -; U.S. ARMY RESEARCH AND DEVELPMENT GRUP FAR EAST AP SAN FRANCIS 96343

3 ABSTRACT Water quality standards were reviewed and tentative stream standards prpsed fr use in develping cuntries f Sutheast Asia n the basis f legitimate water uses and adaptatin f available data t lcal cnditins. A survey f stream standards and water uses applied in the Sutheast Asian regin indicated that few cuntries had adpted standards and practically n attempt had been made t adjust t suit lcal cnditins. Experimental studies suggested that xidatin pnd effluent wuld have a beneficial effect n the xygen balance f a stream under trpical cnditins prvided that the algal cncentratin was nt mre than 1 x 10 5 cells/ml after dilutin in the stream. xidatin pnds were assessed as being mre attractive than either trickling filter r activated sludge treatment plants fr ppulatins less than 175,000 and land rental csts f U.S.$0.10 per square meter per year r less. ii

4 Table f Cntents I DESIRABLE SURFACE WATER QUALITY STANDARDS Page 1 Fishing 1 Irrigatin 3 Ptable Water Supply 3 Minr Uses f a Stream 6 II CURRENT WATER QUALITY STANDARDS IN ASIA 7 Ptable Water Supply 7 Irrigatin 9 Fishing 10 Waste Dispsal 10 Summary 16 III EFFECTS F IDATIN PND EFFLUENT N TRPICAL STREAMS 17 Experimental Prcedure 18 Experimental Results 18 xygen Prductin Rates 27 Summary 30 IV EVALUATIN F WASTEWATER TREATMENT ALTERNATIVES 31 Degree f Treatment Required 31 Wastewater Treatment Systems 32 Cmparative Cst f Wastewater Treatment Systems 35 V NCLUSINS AND REMMENDATINS 37 iii

5 Table f Cntents (Cnt'd) VI REFERENCES Page 39 VII APPENDI I 40 VIII ACKNWLEDGEMENTS 51 iv

6 List f Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 xygen Prductin in Light Bttles Incubated at the Surface f a Turbid Stream xygen Prductin in Light Bttles Incubated at a Depth f 0.25m in a Turbid Stream Algal Phtsynthetic Rate as a Functin f Algal Ppulatin at the Surface and 0.25 m Depth in a Turbid Stream Net xygen Prductin Rate in the Euphtic Zne f a Stream as Functin f Algal Ppulatin Relatinship between Stream D and Cncentratin f rganic Wastes Discharged int a Stream Relatinship between Treatment Plant Capacity and Cst v

7 List f Tables Table 1 Prpsed Interim Stream Standard fr Page 2 Fishing Table 2 Prpsed Interim Stream Standard fr 4 Irrigatin Table 3 Prpsed Stream Standard fr Ptable Water 5 Supply Table 4 Summary f Questinnaires Sent t and Respnses 8 frm Different Cuntries in Suth East Asia Table 5 Standards fr Undergrund Irrigatin Water in 11 the Arid Zne Table 6 Quality Standards fr Streams Used fr Fishing 12 in the Philippines Table 7 Effluent Standards in Thailand 13 Table 8 Quality Standards fr Streams Used fr Navigatin 14 and Wastes Dispsal in the Philippines Table 9 Effluent Standards in the Republic f China 15 Table 10 Characteristics f AIT xidatin Pnd Effluent 17 Table 11 Algal Respiratin and Phtsynthetic Rates at 22 the Surface f a Turbid Stream Table 12 Algal Respiratin and Phtsynthetic Rates at 23 a Depth f 0.25m belw the Surface f a Turbid Stream Table 13 Minimum and Maximum D Levels at the Surface f 25 a Turbid Receiving Water with Initial D f 6.5 mg/1, Caused by Algae at Different Cncentratins Table 14 Minimum and Maximum D Levels at a Depth f m in a Turbid Receiving Stream with Initial D f 6.5 mg/1, Caused by Different Algal Cncentratins Table 15 Net Algal Respiratin Rate at 0.5m Depth in 28 Turbid Stream Table 16 Net Algal Respiratin Rate at 0.75m Depth in 28 Turbid Stream vi

8 I DESIRABLE SURFACE WATER QUALITY STANDARDS The mst imprtant uses f surface waters in the develping cuntries f Asia are fishing, irrigatin and ptable water supply. Maintenance f prper quality standards fr 1 these uses is an imprtant respnsibility f develpment planners in all cuntries f the Asian regin. Neglect f this culd neutralize future develpment gains if sciety has t pay mre than necessary fr its basic water needs. Develping cuntries must nt pursue develpment prgrams withut cnsidering their impact n future uses f watercurses. n the ther hand, these cuntries cannt affrd t spend limited resurces n intensive wastewater treatment, waste discharge mnitring and receiving water mdelling which are necessary fr evaluating the ptimum allcatin f water resurces t present and expected future users. ne f the simplest ways f achieving a reasnable allcatin in practice is by impsing a stream standard t meet the quality requirement f the mst sensitive use f a particular watercurse. In this way, cntrl can be exercised using a minimum number f sampling statins. A ratinal stream standard must neither be t lax, and destry a pssible future use f the river, nr t strict, and require mre waste treatment than necessary. A lax standard frges the future benefits t be derived frm ptential uses f the watercurse, while a very strict standard frges the alternative investment pprtunities f the capital which wuld be used fr unnecessary waste reductin. A cmprehensive literature review f the critical parameters that affect uses f watercurses fr fishing, irrigatin, ptable water supply, pwer, navigatin and sprt r recreatin was made. The survey included published research cnducted in trpical develping cuntries as well as in temperate cuntries f the develped wrld. This literature survey allwed a preliminary set f standards t be prpsed fr different water uses. Emphasis was placed n the three mst imprtant uses f surface waters in trpical cuntries f Asia, namely fishing, irrigatin and ptable water supply. Fishing The fishing industry is an imprtant sectr in develping cuntries as a principal surce f badly needed prtein in the diet. The main parameters that affect fish catch are disslved xygen cncentratin, presence f txic cmpunds such as cyanide and heavy metals, temperature, the presence f nn-bidegradable substances which are cncentrated in the fd chain (such as DDT), and substances which impart an undesirable dr and taste t the fish. The recmmended levels f stream standards are shwn in Table 1. 1

9 Table 1. PRPSED INTERIM STREAM STANDARDS FR FISHING Quality Parameter C0 S PH NH 3 Heavy metals Cpper Arsenic Lead Selenium Cyanides Phenls Disslved slids Detergents Disslved xygen Suggested Level f Stream Standard 12 mg/ less than 1 mg/1 less than 1 mg/1 less than 0.02 mg/1 less than 1 mg/1 less than 0.1 mg/1 less than 0.1 mg/1 less than mg/1 less than 0.02 mg/1 less than 1,000 mg/1 less than 0.2 mg/1 2 mg/1 r abve 2

10 Fishing as a sprt is relatively unknwn in trpical develping cuntries with respect t the freshwater habitat. Neither d game fish, in the western sense, exist and s the water quality requirements fr this use are nt cnsidered. Irrigatin The main water quality parameter fr irrigatin water is the cncentratin f salts which causes plasmlysis f plant cells. The lethal cncentratin f salts varies frm plant t plant, and is dependent n cntact perid and ambient temperature. The secnd prblem with salts, especially thse cntaining sdium, is the displacement f calcium in frm the sil, causing permeability t decrease. A gd drainage system and prper irrigatin management lwers the cntact perid between the plant and saline water s that a well-drained field may tlerate a lwer irrigatin water quality. High temperature increases water cnsumptin f plants, and this demands a better irrigatin water quality than wuld be required in a cld climate. Besides thse parameters mentined, an excessive cncentratin f metal ins retards plant grwth and smetimes causes death, even thugh the same metal ins are essential micr nutrients in very lw cncentratins. The recmmended stream standards fr irrigatin use are shwn in Table 2. Ptable Water Supply The critical aspects f a ptable water supply are the presence f txic materials and pathgenic rganisms. There is great uncertainty cncerning the txic levels t man f cmpunds in water and the standards impsed by the Wrld Health rganizatin (WH, 1971) and the United States Public Health Service (USPHS, 1962) are necessarily cnservative. The literature review revealed that the WH (1971) standard fr clifrms in raw water is very strict fr trpical develping cuntries. Research by PRASAD & KAUSHIK (1964) shwed that very few water surces culd meet the criterin, althugh many water surces have been used fr a lng time by the lcal ppulatin,, It is suggested that, apart frm during epidemics, stream standards intrduced t prtect a surface water fr direct use as a ptable water supply shuld be related t its natural state, befre receiving waste discharges. Stream standards fr metallic cntaminants are taken directly frm the WH (1971) Internatinal Standards fr Drinking Water. The recmmended list f standards is given in Table 3. 3

11 Table 2. PRPSED INTERIM STREAM STANDARDS FR IRRIGATIN Quality Parameter Suggested Level f Stream Standard Ttal disslved slids (TDS) Sdium adsrptin rati (SAR) Nt mre than 400 mg/1 where there are pr drainage, saline sil and inadequate water supply. (EC e less than 0.75 millimhs per cm. at 25 C.) Nt mre than 1,000 mg/1 where there are gd drainage and prper irrigatin management. (EC e less than 1.75 millimhs per cm. at 25 C.) Nt mre than 2,000 mg/1 where there are salt-resistant crps, gd drainage, prper water management and lw sdium adsrptin rati (SAR) f water. (EC e less than 2.25 millimhs per cm. at 25 C.) Nt mre than 10 where there is pr drainage. Nt mre than 18 where there is gd drainage. Brn Disslved xygen Pesticides DDT Endrin B.H.C. Methyl parathin Malathin Nt mre than 1.25 mg/1 where there are sensitive crps. Nt mre than 4 mg/1 where there are tlerant crps. Greater than 2 mg/1. A level f 2 mg/1 shuld nt ccur fr mre than 8 hurs ut f any 24-hur perid mg/ mg/ mg/ mg/ mg/1 4

12 Table 3. PRPSED STREAM STANDARD FR PTABLE WATER SUPPLY Quality Parameter Mst prbable number f clifrms (MPN) ph Disslved xygen Arsenic Lead Chrmium (hexavalent) Cyanide Phenlic substances Chlrides Ttal disslved slids Suggested Level f Stream Standard Effluent quality similar t the natural state f surface water greater than 2 mg/1 less than 0.05 mg/1 less than 0.05 mg/1 less than 0.05 mg/1 less than 0.2 mg/1 less than mg/1 less than 1,000 mg/1 less than 4,000 mg/1 5

13 Hwever, it is strngly recmmended that ptable water supply criteria nt be accepted as stream standards except where n alternative water supply can be prvided fr a sizeable ppulatin. If this water use is widely adpted as the majr cntrlling use fr which a surface water must be managed, it will seriusly restrict ther uses which might be f high reginal benefit and increase the csts f water quality cntrl ver thse impsed by less quality-demanding uses. It will generally be mre ecnmical fr water quality management authrities t prvide alternative ptable water supplies t cmmunities cntiguus t- surface waters rather than maintain a quality in the surface water which wuld be safe fr drinking. Minr Uses f Streams Surface waters may als used as industrial cling and prcess waters, and fr cmmunicatin, bathing and recreatin. The majr requirement fr these water uses with which a public authrity in a develping cuntry shuld cncern itself is a disslved xygen level abve zer, simply t maintain aerbic cnditins. Industry shuld generally prvide its wn water treatment prcesses and will nrmally lcate in an advantageus psitin t minimize csts. Recreatin and water sprts demanding high quality water are luxury uses which shuld nt be cntrlling in water quality management at the present stage f develpment f mst trpical cuntries in Asia. In general, peple experiencing a significant degree f pverty are nt very sensitive t pr surface water quality (unless extreme), and this wuld nt restrict their recreatinal use f a bdy f water. 6

14 II CURRENT WATER QUALITY STANDARDS IN ASIA As mst f the available publicatins n water quality parameters discussed in Sectin I cvered research carried ut in temperate cnditins f develped cuntries, a questinnaire was sent t different gvernment and research agencies in Suth East Asia cncerned with fishery, irrigatin, pwer develpment and water supply. Alng with the questinnaire, the interim prject reprt (PESD, 1973) was sent t all the agencies, t infrm them f the prject and make them aware f the apprach t water quality standards being taken. The questinnaire inquired if there were lcal adaptatins f existing ptable water standards (say frm the WH r USPHS standards) t suit natinal r reginal cnditins, as well as requesting infrmatin n the management alternatives cnsidered when a stream's water quality fell belw the standard. With regard t irrigatin, the questinnaire inquired abut the predminant crps f the cuntry, the tlerance f thse crps t varius types f cntaminants in the water, and the present stream standards fr irrigatin water. Fr fisheries, it inquired abut the mst cmmn freshwater fish in the cuntry, research cnducted n the tlerance f the different fish species t txic cmpunds cmmnly fund in watercurses and the present stream standards fr fishing. A sample questinnaire is prvided in Appendix I. The number f questinnaires sent ut and the respnses received frm each cuntry are shwn in Table 4. Apprximately 50% f the cntacted agencies had respnded within seven mnths f the questinnaire being sent ut. Many wrte f the absence f any stream standards r research related t the tlerance f crps and fish in the cuntry. All agencies replying indicated that they were pleased t receive the interim prject reprt and wuld welcme a cpy f the final reprt when prepared. Ptable Water Supply Pakistan, Malaysia, Hng Kng and Thailand have adpted the WH (1971) drinking water standards withut mdificatin, while the Philippines has adpted the USPHS (1962) ptable water standards. N research has been cnducted in any cuntry n the pssibility f mdifying the standards t suit lcal cnditins. The Republic f China has mdified drinking water standards which allw fr a higher cncentratin f arsenic and lead at 0.1 mg/1, cmpared with the 0.05 mg/1 f the WH standard. Hwever, the phenlic substances cncentratin is mre stringent, at mg/1, as well as the disslved slids cncentratin at 1000 mg/1, cmpared with mg/1 and 1,500 mg/1, 'respectively, recmmended by WH (1971). A wider range f ph is allwable in Taiwan, frm 6.0 t 9.0. The chlride cn- 7

15 Table 4. SUMMARY F QUESTINNAIRES SENT -T AND RESPNSES FRM DIFFERENT UNTRIES IN SUTH EAST ASIA Cuntry Questinnaires Respnses Hi/TTa Hngkng.«.. India Indnesia Khmer Republic Malaysia Pakistan Philippines Republic f China Singapre Sri Lanka Thailand Vietnam '

16 centratin, althugh set at 300 mg/1, is flexible when excessive chlride is present in a natural water and when n alternative surce is available. Vietnam fllws the WH (1971) standards with mdificatin f the chlride cncentratin, frm 200 mg/1 t a maximum f 650 mg/1. Similarly the ph range is extended, frm a minimum f 6.5 t a maximum f 9.2. Irrigatin Malaysia's nly criterin is a ttal disslved slids limit f less than 600 mg/1. Singapre and Pakistan have n stated standards. The Philippines has n fficial standards but the Agricultural Research Center f the Natinal Institute f Science and Technlgy recmmends the fllwing guidelines: TDS Sdium in max. Brn Disslved xygen mg/1 60 mg/ mg/1 3 mg/1 PARKER (1971) recmmended the fllwing standards fr Thailand: D.. BD D Ammnia - N Zinc Cpper Irn Ttal Heavy Metals Arsenic Cyanide Phenls nt less than 3.0 mg/1 mre " 6.0 mg/1 10 mg/1 2.0 mg/1.5 mg/1.5 mg/1 1.5 mg/1 2.0 mg/1.1 mg/ mg/1.005 mg/1 9

17 ph nt mre than Temperature " " " 35 C Clr and Turbidity shuld relate t natural cnditins. These standards fr irrigatin water were als recmmended fr fishing because water use fr irrigatin and fishing culd nt be differentiated in Thailand. Fr arid cnditins, the Central Arid Zne Research Institute has been cnducting studies n varius water qualities applied t different crps. Acceptable standards fr different types f crps and sil are shwn in Table 5. The crp yields at these maximum salinity levels are apprximately 507 f thse irrigated with "sweet" water. Hwever, these salinity cncentratins in the irrigatin water are acceptable in that there will be n deteriratin f sil quality, and hence culd be used fr irrigatin if n ther surce is available. Fishing Hng Kng des nt have any stream standards fr fishing but the Agriculture and Fisheries Department regularly mnitrs the disslved xygen level and ph f streams. Singapre and Malaysia d nt have any stream standards t prtect fisheries. Standards fr Thailand are similar t the standards fr irrigatin. The Philippines standards fr fishing as prmulgated by the Natinal Water and Air Pllutin Cntrl Cmmissin are shwn in Table 6. Waste Dispsal Thailand uses an effluent standard but des nt enfrce any stream standards. There are tw standards available in Thai literature, the first ne as recmmended by PARKER (1971) t the Ministry f Public Health, and the effluent standard enfrced by the Ministry f Industry. These standards are given in Table 7. The Philippines stream standards as prmulgated by the Natinal Water and Air Pllutin Cntrl Cmmissin are given in Table 8. The Republic f China enfrces the standards fr effluent discharges as shwn in Table 9. 10

18 Table 5. STANDARDS F UNDERGRUND IRRIGATIN WATER IN THE ARID ZNE Crp i Texture f Sub sil Light Medium Heavy Wheat Crp every year Crp alternate years 3,000-2,500 8,000 1,700 6,000 Barley Crp every year Crp alternate years 5,500-3,000 10,000 2,500 8,000 Mustard Crp every year Crp alternate years 3,000 5,500 2,000 3,000 1,300 2,500 Chillies Crp every year 1,700 1, Cumin and Criander Crp every year 1, Qualifying ntes a) The dminant salt in the undergrund waters used fr irrigatin is sdium chlride and S.A.R. ranges between 8 and 30. ften, the waters als cntain appreciable quantities f nitrates and phsphates. b) The sils are generally calcareus with a fairly pervius cncretinary strata at depth f cm and lw water table. c) Yields f crps, btained with abve waters are abut 50% f thse btained with sweet water. Hwever, they are cnsidered remunerative enugh fr sci-ecnmic situatin prevailing in the arid zne. 11

19 Table 6. * QUALITY STANDARDS FR CLASS C WATERS IN THE PHILIPPINES Items Clifrra--MPN/100 ml, max. Turbidity-Units, max. Clr--Units, max. dr--threshld dr Number, max. Temperature-- F max. Flating Slids--max. Txic Substances--mg/l, max. Fluride--mg/l, max. Ttal Slids--mg/l, max. Hardness--mg/l, max. Chlrides--mg/1, max. ph Range Phenlic Substances--mg/l, max. il--mg/l, max. BD (5-day)--mg/1, max. D.0.--mg/l, min. Ammnia--mg/1, max. Specific Cnductivity-mh x i - 25 C, max. C0 2 --mg/1, max. Specificatins Nne Nne alne r in cmbinatin with ther substances r wastes in sufficient amunts as will render the water injurius t fish r impair the waters fr any ther best usage established fr this class * Class C Usage f Waters: Fishing and any ther usage except fr* bathing r as a surce f dmestic water supply. Cnditins related t Best Usage The waters will be suitable fr fishing : and fr ther uses requiring waters f lwer quality. 12

20 Table 7. EFFLUENT STANDARDS IN THAILAND Characteristic Recmmended values f Ministry f Public Health Ministry f Industry requirements fr industrial wastes B..D. C..D. Suspended slids Heavy metals (ttal) Arsenic Zinc Cpper Irn Cyanide Ammnia Nitrgen Sulphide il and grease Tar Phenls Pesticides Detergents Ttal dis. slids PH Permanganate values Chlrine Temp nne visible C (depending n dilutin) nil nil 1.0 nil C 13

21 Table 8. QUALITY STANDARDS FR CLASS E WATERS IN THE PHILIPPINES Items 1. Flating Slids and Sludge Depsits 2. il--mg/l, max 3. Disslved xygen mg/1, min. 4. Clr units, max. 5. dr Threshld dr Number, max. 6. ph min. Specificatins Nne in sufficient amunts as t cause a public nuisance * Class E Usage f Waters: Navigatin and waste dispsal and any ther uses except fr agricultural and industrial water supply, fishing, bathing r surce f dmestic water supply. Cnditins Related t Waters will be suitable fr naviga- Best Usage : tin where navigable waters are invlved and will be used fr waste dispsal t the extent that it will nt cause a nuisance and that the waters can accmmdate the waste dispsal within the limits f the prescribed specificatin fr this class. This class will nt be assigned t waters which can be prperly assigned t a higher class. 14

22 Table 9. EFFLUENT STANDARDS IN THE REPUBLIC F CHINA Item Level BD Suspended Slids Cyanides Lead Chrmium (hexavalent) Cadmium Phenlic Substances Chlrides PH Clifrm /100 mg/1 mg/1 mg/1 mg/1 mg/1 mg/1 mg/1 mg/1 ml 15

23 Summary Frm the questinnaires returned it is evident that very few Asian cuntries have set up any stream standards fr the management f their watercurses. Where the need fr a stream standard has been urgent, such as in ptable water supply, the WH (1971) r USPHS (1962) standards have been adpted withut any attempt being made t mdify the standards t suit lcal cnditins. Frm thse cuntries which have cnducted research t mdify the WH and USPHS standards it is apparent that mst f the changes made have been cnservative. Althugh fishing is ne f the mst imprtant uses f Asian streams, research n the parameters likely t affect the fishing industry has been very limited. In mst cuntries, the mst that has been dne is a mnitring f disslved xygen level in streams. Similarly fr irrigatin waters, mnitring f salinity has been the prime cnsideratin withut any attempt being made t use the infrmatin t cntrl use f the surce. Hwever, research at the Central Arid Zne Research Institute in India has shwn that a wide range f water salinity is tlerable by varius types f crps. In fact, when irrigatin water is scarce a trade-ff can be made between lwering the crp yield and prviding an alternative water surce. Mst cuntries surveyed adpted effluent standards fr wastewater discharges withut any allwance fr streamflw dilutin. With this apprach, a stream culd be plluted althugh all dischargers might have cmplied with the effluent standards. n the ther hand, anther stream's assimilative capacity might nt be utilized t its fullest extent and sme vital industry r industries might be frced t invest in unnecessary wastewater treatment. It is evident that mre ratinal evaluatin f the quality demands f majr water uses and the adptin f realistic stream standards assciated with particular reaches f surface streams will allw the greatest benefit frm water resurces in develping cuntries. 16

24 Ill EFFECTS F IDATIN PND EFFLUENT N TRPICAL STREAMS The presence f txic substances such as cyanide, chrmium, mercury, arsenic, etc, in a stream will nrmally be due t industrial discharges which will vary frm ne industry t anther. A mre cmmn effect f waste discharges n stream water quality is the depletin f disslved xygen level due t the discharge f municipal sewage and ther rganic wastes. In trpical develping cuntries xidatin pnds have ften been suggested as the ideal treatment methd fr handling sewage and rganic wastes due t their lw cst f installatin, ease f peratin and high efficiency. xidatin pnds can effect a high degree f treatment but invariably discharge significant numbers f algae in their effluents. These algae will exert BD in the standard BD test, due t incubatin in the dark, and xidatin pnd effluents regularly shw BD levels f 50 r 60 mg/1, related mainly t the algae; filtered effluent BD levels are usually 20 mg/1 r less fr a prperly designed xidatin pnd system. n discharge f xidatin pnd effluent t a surface stream in the trpics, grwth cnditins fr the algae are different frm thse in an incubated BD bttle and the actual effects n the xygen balance in the stream are nt bvius. During daylight hurs the algae will phtsynthesize, depending upn their lcatin ver the depth f stream and the light transmissin, and in the dark will respire. Unless light inhibitin ccurs there will nrmally be a net xygen prductin ver a full 24 hur perid. Hwever, in the trpics, surface waters are generally highly turbid and this will seriusly affect the ttal xygen prductin by suspended algae. The experimental prtin f the wrk was designed t evaluate the effects f xidatin pnd effluent discharges n the xygen balance f trpical surface streams. Mst experiments were cnducted using the Asian Institute f Technlgy first-stage xidatin pnd effluent, whse characteristics were as shwn in Table 10. This pnd effluent was used as a cncentrated algal suspensin and diluted with surface water t give a range f algal cncentratins fr study. Table 10. CHARACTERISTICS F AIT IDATIN PND EFFLUENT Unfiltered D Filtered D Ttal Kjedahl Nitrgen Suspended Slids Algal Ppulatin Algal Species Temperature mg/ mg/ mg/ mg/1 1.0 x 10 5 t 3.0 x 10 5 per ml mstly euglena and chlrella 17 trpical cnditins '(20 ^ 30 C)

25 Experimental Prcedure The pnd effluent was diluted with stream water in the ratis 1:1, 5:1, 10:1, 50:1 and 100:1. The stream water used had turbidity between 56 and 133 JTU in its natural state but was filtered t remve algae prir t dilutin, t prevent cntaminatin f the xidatin pnd effluent algal ppulatin. Replicate samples f diluted pnd effluent were placed in light BD bttles and incubated at depths f 0.0 m, 0.25 m, 0.5 m, and 0.75 m belw the stream surface. Filtered and unfiltered pnd effluent, at similar dilutins,were placed in dark BD bttles and incubated at a depth f 0.75 m belw the stream surface. The duratin f each run was 36 hurs and samples were tested fr disslved xygen (D) nce every three hurs. Experimental Results The disslved xygen levels in the light BD bttles incubated at the surface and at a depth f 0.25 m were pltted against time as shwn in Fig. 1 and 2. Disslved xygen was fund t vary accrding t the sunlight availability and the algal cncentratin. At higher algal cncentratins, higher maximum D was btained, due t the phtsynthetic prcess. The peak was reached with all levels f algal cncentratin at 5:00 P.M., declining steadily during the night time. The decline is due t algal respiratin which uses up D in the water in the absence f light. It was bserved that the secnd day D cncentratins were higher than the initial levels f D. The D values in all cases never reached zer. In Fig. 1 an algal cncentratin less than 2.0 x 10 3 per ml at the high dilutin rati, gave a D.. level in the bttle which was essentially independent f the sunlight intensity. At this level f algal cncentratin at the surface, the disslved xygen level was nt affected by the algal phtsynthetic rate r the respiratin rate and in practice wuld depend nly n the surface diffusin f xygen affected by the hydraulic prperties f the stream. At the depth f 0.25 m, the effect f algal phtsynthesis and respiratin is mre evident at all levels f algal cncentratin. It shuld be nted in Fig. 2 that althugh the highest D level was btained with the highest cncentratin f algae, the lwest D level als ccurred with this algal cncentratin. It culd be cncluded that althugh algae aid the xygenatin f a stream during daytime, their effect during the nighttime culd be detrimental t aquatic life when they cmpete in the utilizatin f disslved xygen. Hwever, it is safe t say that within the nrmal range f effluent dilutin (as used in the experiments) there is n danger f net xygen depletin in a shallw stream (up t 0.25 m deep) due t algal grwth at algal cncentratins belw 1.5 x 10 5 per ml. The experimental pints fr dark and light perids were cnnected 18

26 MJtt- ^r^ t C\J CM - - CD E Q. CM «= E *= a. CM -1 r--, 1 r-l.m-u '»t --f--"p- '-» m <3- C\J t l/ - l 6ai tla6ax pa'ajssj^ CM- CM- t 3 Q. a. l * * * g in h-;.<j in CM cn-<n! E Q. I I I! * +i i i! <3 Q Q E3 +., ;CM u. CM. t if) 19

27 1 /-6lU ' Ue6*Q P3A 0SS!Q 20

28 by straight lines fitted using the methd f least squares. The psitive slpe which ccurs frm 6:00 AM t 5:00 PM is the algal phtsynthetic rate, reduced by the bacterial respiratin rate. The negative slpe which ccurs between 5:00 PM and 6:00 AM the next day, is the sum f the algal respiratin and bacterial respiratin rates. Tables 11 and 12 shw the different phtsynthetic and respiratin rates. The bacterial respiratin rate was btained frm dark BD bttles cntaining the filtered pnd effluent placed at depths similar t the light bttles cntaining diluted effluent. The phtsynthetic rates at the tw depth levels were pltted against algal cncentratins as shwn in Fig. 3. It is apparent that the algal phtsynthetic rate is prprtinal t the algal cncentratin. The increase is linear abve an algal ppulatin f 9.0 x 10 3 per ml and belw this pint the algal phtsynthetic rate was higher at the surface than at a depth f 0.25 m. Cmparing the values in Tables 11 and 12 it is seen that the algal respiratin rate during the night at the surface was slightly higher than the bacterial respiratin rate. At the depth f 0.25 m the algal respiratin rate was three times the bacterial respiratin rate. This means that, due t the presence f algae, the D at the surface is used up at a rate twice what it wuld be with bacteria alne, while at a depth f 0.25 m the rate f D use is fur times that with bacteria alne. During daytime the phtsynthetic rate at the surface was 3.8 t 22.7 times the respiratin rate, while at the depth f 0.25 m the phtsynthetic rate was 4.6 t 17.0 times the respiratin rate. Althugh the verall phtsynthetic prductin f xygen during daytime was greater than the respiratin rate during nighttime, the xygen generated during phtsynthesis wuld be partly released t the atmsphere nce the D level in the stream exceeded the saturatin value. Recvery f disslved xygen frm the atmsphere during nighttime respiratin is cntrlled by the hydraulic turbulence f the stream, which is nrmally lw in meandering trpical streams under lw flw cnditins. Depletin f xygen during the night by high cncentratins f algae is the critical aspect in the stream's xygen balance. Fr unplluted streams, a typical D level in the trpics is 6.5 mg/1. Using the phtsynthetic rate and the algal and bacterial respiratin rates determined in the experiments, the effects f varius algal ppulatins n the D levels f a trpical stream may be predicted as in Tables 13 and 14. These shw that algal cncentratins f 1.5 x 10 5 r less are beneficial t a shallw turbid stream because the rate f phtsynthetic prductin f xygen is greater than the bacterial respiratin rate. At the same time the lwest level f D due t algal respiratin is likely t be 3.6 mg/1, which allws sufficient D fr ther aquatic life such as fish and freshwater crustaceans t thrive. This assumes 21

29 v > Algae Phtsynthetic Algae Resp. Rate N v r-l CM CM m i-i i m Algae Phtsynthetic Rate (mg/l-hr) Algae Resp. Rate (mg/l-hr) negible Bacteria Resp. Rate (mg/l-hr) , , "0,032 CM Algae Phtsynthetic Bact.Resp. Rate (mg/l-hr) 3,916 2, , Bact + Algae Resp. Rate (mg/l-hr) Algal Ppulatin (per ml) in I in i-h r-l m vt!-l r- CM 1.8 x 10* m r-l 1-4 N m I-l CM I-I i-i x> t H Stream Water t Phd Eff, Ra'i.s I-I I-I r-i»-i 10 : 1 10 : 1 10 : 1 50 : : 1 22

30 w CJ < fa >x> Algae Phtsynthetic, Algae Resp.; Rate I l-l m d I fa- m Algae Phtsynthetic Rate : (mg/l-hr) ; m r-l CM m CN fa «* Algae Resp. Rate (mg/l-hr) 991* < w Q H Bacteria Resp. Rate (mg/l-hr) u M H H Q M PQ g H «JJ fa CN r-l Algae Phtsynthetic Bact. Resp. Rate (mg/l-hr) Bact + Algae Resp. Rate (mg/l-hr) r-l ;c r-l ; t fa J 3 -. Algal Ppulatin '(per ml) 10 r-l m -r-4...'._. r-i. 1= rh i-l _. w r-i x-, CM;- '"It s Stream Water t Pnd Eff. Ratis I-I l-l I-I m 10 : 1 I * 05 23

31 <. Q _ c MB 3 Pp «p w *- > < I 3 H tfj gend: a> _i >» Q. c ibati Incu u H k. he su I?.-L. ace 1 t % *- lw J» E in CM i t V N \ \ \ \ \ \ t \ N \ \. V \ ^ \ \ \\ %\ CVI m - )/tm - 8JDH iiaqjusimd P&IV A. \\ \ \- «i~ - %. ~"~ - I 2 p * E w 0> a. </> 15 c iti Ppu _ Alga H- c " c 3 u_ t ate a: hetic *» syn Pht «_ < 1 r u_ c M»» Q. E m CM TJ C CD rfaci 3 </) CD x: +- < 24

32 Z I-I H H w u u H 52 pa t* w <: PQ Q W < 6C e U v fc H * Min. D.. Level 12 hi Phtsynthesis Max. D.. Level 12 hr Phtsynthesis Percent D.. Depleted (% ) D.. Depleted After 12 hr (mg/l) Min. D.. Level After 12 hr Respiratin (mg/l) Algae Phtsynthetic Rate (mg/l-hr) Algae Respiratin Rate (mg/l-hr) Algal Ppulatin (per ml) Stream Water t Pnd Eff. Ratis ,244 in r-i u I-I TH t-i u i-l I-I CM i-i CM * r ,890 0,049 r-l i-l I-H c 1-1 CM u-i negligible c : 1 c crj u c at u cfl u 9) a. 3 c U-l 0) 01 M 00 0> -a 60 U 01 > g t U 01 > e 01 C CU t-l 3 y u Q e M H 3 U c y 25

33 rj < M H M Z PS H W H z w w a a r-l M g H <; z m CM 6M H CM W Z H Z W u z u H z w 6M 6M H <! W < < PQ w <! H <! rj 63 > W r-l E 6M Q H Min. D.. Level 12 hr Phtsynthesis Max. D."!" Level 12 hr Phtsynthesis Percent D.. Depleted a) D.. Depleted After 12 hr (mg/1) Min. D.. Level kfter 12 hr Respiratin (mg/1) Algae Phtsynthesis Rate (mg/l-hr) Algae Respiratin Rate (mg/l-hr) Algal Ppulatin (per ml) Stream Water t Pnd Eff. Ratis in t-i m i-i i-h i-i r-l m , t-i : , c i-l CM 50 : 1 3 U c c r^ 4-1 u a 4-1 u 01 CL a <4M u >> I-l 01 > > 1-M fr 01 > r-l B 41 u 4-1 c 4) & u a u u B a B > 01» 1-4 R r-l c r-l s r-l c r-l 4J (0 u a 4-1 u 01 ex a 4J u 4-1 c 4) 1-4 a a 4J i-l r-l H 14M 1-1 B CD U 4J C 4) > r-l a u u Q B a B H c r-l s, 01 M 01 a B 4-J 4-J l 01 i-( 41 U r-l + 26

34 that xygen tensins abve saturatin wuld be released t the atmsphere, which might nt be true in practice as super saturatin is ften encuntered. Hwever, it wuld be a cnservative apprach t ensure that the stream D standard was maintained. Phtsynthetic xygen prductin at depths f 0.5 m and 0.75 m was insignificant cmpared with that at the surface and at a depth f 0.25 m. The diurnal D variatins were similar t the variatins in the dark BD bttles. Tables 15 and 16 shw respiratin rates at the depths f 0.5 and 0.75 meters. In the range f 1.0 x 10 3 t 2.7 x 10 4 algal cells per ml, the algal respiratin rate was negligible cmpared with the bacterial respiratin rate, but fr an algal cncentratin f 1.0 x 10 5 the algal respiratin was 3.6 times that f bacterial respiratin. With lw phtsynthetic rates at depths greater than 0.5 m and high rates f respiratin, with mre than 1.0 x 10 s algae per ml, the algae are likely t deplete stream D in the case f deeper streams. Hwever, algae exhibit helitrpic effects which wuld limit the grwth f algae at lwer depths in deep streams, s that the critical algal cncentratin f 1.0 x 10 5 per ml is nt likely t ccur unless the riginal dilutin f the xidatin pnd effluent prduces such a level. ' Samples f final effluent taken ver a perid f several mnths in 1972 frm xidatin pnds at the U.S. Air Base in Krat, Thailand shwed a maximum algal cncentratin f 0.26 x 10 5 cells per ml. At the Applied Scientific Research Crpratin, Bangkk the effluent frm the xidatin pnd has cntained a maximum f 0.29 x 10 s algal cells per ml, while at the Asian Institute f Technlgy the final effluent frm the secnd stage xidatin pnd has shwn a maximum algal cntent f 0.99 x 10 5 cells per ml. If these are taken as typical xidatin pnd effluents in trpical cuntries, then after dilutin upn discharge t a surface stream it is nt pssible t achieve the critical level f algal cncentratin which wuld cause prblems in deep streams. xygen Prductin Rate Net xygen prductin rates averaged ver the euphtic zne f the stream were calculated by subtracting the xygen cncentratin in the dark bttles frm that in the light bttles after ne day f incubatin, and multiplying the rate by the area f the bttle. Prductin rates were calculated at varius depths f the surface layer in which phtsynthesis ccurred and averaged. Fig. 4 shws the expnential increase f xygen prductin rate with increase in algal ppulatin at varius dilutins. 27

35 Table 15. NET ALGAL RESPIRATIN RATE AT 0.5 M DEPTH IN TURBID STREAM (I) (ID (III) (IV) (V) (VI) Stream Water t Pnd Eff. Ratis Algal Ppulatin (per ml) Algae+ Bact Net Resp. Rate at 0.5 m depth (mg/l-hr) klgae + Bact Resp. Rate (in dark bttle) (mg/l-hr) Bacteria Resp. Rate (mg/l-hr) Net Algae Resp. Rate at 0.5 m depth (mg/l-hr) 10 : x negligible 50 : x negligible 100 : x negligible Table 16. NET ALGAL RESPIRATIN RATE AT 0.75 M DEPTH IN TURBID STREAM (I) (ID (III) (IV) (V) (VI) Stream Water t Pnd Eff. Ratis Algal Ppulatin (per ml) Algae+ Bact Net Resp. Rate at 0.5 m depth (mg/l-hr) Algae + Bact Resp. Rate (in dark bttle) (mg/l-hr) Bacteria Resp. Rate (mg/l-hr) Net Algae Resp. Rate at 0.75 m depth (mg/l-hr) 1 : x : x negligible 10 : x : x negligible 28

36 <. <0 <p 3 a > <J) c E w 0> Q. UJ 9 C _i ^. U. - (0 0> CD 0> c Nl JZ C Q. UJ c * i? < a: c 5 <= 3? c 0) 2 2 *- 5 0) J 0> tf> t CJ ADp-w/6-94y uiinpid ua6fa 4d(y 29

37 Summary f Experimental Findings The effluent frm an xidatin pnd system treating municipal sewage nrmally cntains nly a small amunt f bidegradable rganic matter which can deplete the stream disslved xygen. Hwever, the effluent als cntains algae which cmpete with bacteria and ther aquatic life in the stream fr disslved xygen during the night when underging respiratin. During the daytime, algae near the surface prduce xygen by phtsynthesis and smetimes can alleviate the disslved xygen deficit in the stream caused by bacterial breakdwn f rganic matter. Althugh the algae prduce mre xygen during phtsynthesis than they cnsume during respiratin, excess xygen prduced may be released t the atmsphere when the disslved xygen cncentratin reaches saturatin. N xygen is prduced by algae at depths greater than;0.5 m in turbid trpical streams as a result f the absence f light. "The experimental studies suggest that if the algal cncentratin iii a shallw stream: is greater than 2 x 10 5 cells per ml, the stream cduld-turn anaerbic befre sunrise even thugh it might be saturated with xygen befre sunset. T maintain a disslved xygen cncentratin Q,f. x,at least 2.5 mg/1 at all times in a turbid trpical stream with depth nbt^mre than 0.5 m, the algal cncentratin shuld be less than 1.5 x l^jper ml. In the case f turbid streams with depth greater than 0.5 m, an algal cncentratin f 1.0 x 10 5 cells/ml r mre wuld deplete disslved xygen because f the high respiratin demands fr xygen f algae lcated belw the limit f light penetratin. ' '.-' The algal cncentratin in final effluents frm xidatin pnd systems treating dmestic wastes in Thailand has nt been fund t exceed 1 x 10" 5 cells per ml. A minimum f dilutin n discharge t a surface stream, even as lw as 1:1, wuld result in an algal cncentratin "which wuld nt be expected t be a drain n the stream's xygen resurces. At this algal cncentratin, the phtsynthetic xygen 1 prduced <by algae in the upper 0.5 m f depth wuld be beneficial t the/stream's ;$xygen balance. ' -. T ensure that an xidatin pnd effluent wuld nt adversely affect a trpical stream, it wuld be necessary t prvide enugh dilutin s that, the algal cncentratin in the stream wuld nt exceed 1 x 10 5 cells/ml. Multiple stages f pnds wuld minimize the algal cncentratin in the final effluent frm an xidatin pnd system and shuld be encturaged in design. L.....^ ' t.? *>«,:,'.' 30

38 IV EVALUATIN F WASTEWATER TREATMENT ALTERNATIVES The experimental results have suggested that the effluent frm an xidatin pnd system can be discharged int a turbid stream in the trpics withut having a detrimental effect n the xygen balance prvided that the algal ppulatin is kept belw 1 x 10 5 cells per ml after dilutin. Remval f nutrients frm effluents t prevent subsequent algal grwth in streams, as ften practised in temperate cuntries, is unrealistic in trpical Asian cuntries cnsidering that mst streams are very turbid in their natural state and carry lts f nutrients frm surface runff Algal remval will nt serve any purpse in enhancing the aesthetic value f a stream and ther imprtant stream uses will nt be affected by the presence f algae. Thus it wuld seem that the main cnsideratin in the chice f wastewater treatment system in trpical regins shuld be ecnmic rather than technlgical sphisticatin. xidatin pnds are acceptable rganic waste treatment devices but they must cmpete with alternative secndary prcesses n a cst basis. Degree f Treatment Required CHU (1970) estimated the strength f typical dmestic sewage in Asia at 376 mg/1 f B0D 5 at a flw rate f 114 lpcd, giving a per capita discharge f 42,86 g f B0D 5 per day. Althugh sewage is nt discharged unifrmly thrughut the day, in the case f xidatin pnds with lng detentin time cmpared with the interval between peak sewage discharges, the pnds will even ut flw and strength fluctuatins. Thus, assuming a unifrm effluent discharge thrughut the day, the BD cncentratin at the pint f discharge can be expressed as: P(l-e) a + Q + P(.H4) U ; where, L is the instantaneus BD f the stream at the a site f discharge, mg/1, L e P Q is the initial BD f the stream befre the discharge pint, mg/1, is the degree f treatment, is the ppulatin served in thusands, is the design stream flw rate in m 3 /day STREETER and PHELPS (1925) derived the expressin fr the minimum D in the stream as: 31

39 L a Dc = r{f[l-.(f-l) D «/I, l (. 1) (2) where, D is the maximum disslved xygen deficit f the stream, mg/1, D a r k is the xygen deficit in the stream just befre the pint f discharge, is the rate f stream reaeratin, per day, is the xygen uptake rate by micrrganisms in the stream, per day. QAIMKHANI (1968) fund the xygen uptake fr trpical streams at 0.15 per day and SERMPL (1968) btained the stream reaeratin rate f 0.25 per day which was practically cnstant ver a wide variatin f flw rates. The Streeter-Phelps xygen sag mdel is nly a rugh apprximatin f the mechanism f xygen uptake and reaeratin in a stream as it des nt cnsider algal phtsynthesis and respiratin, nitrificatin, xygen uptake f the stream benths, and variatin f reaeratin cefficient alng the stream. Hwever, it is quite simple fr direct applicatin in the field withut the need fr sphisticated cmputing equipment. Fig in FAIR, GEYER and KUN (1968) gives the relatinships between BD cncentratin at a discharge pint, the initial xygen deficit and the maximum xygen deficit dwnstream frm the discharge pint. Using these curves with the initial xygen deficit and a recmmended stream standard fr D.., the necessary value f the initial ultimate B0D 5 in the stream (L a ) can be determined. This will allw the degree f treatment t be evaluated by: (L a - L n ) (Q P) P (42.86) ^ ' Wastewater Treatment Systems Alternative rganic wastewater treatment systems t xidatin pnds are trickling filters and the activated sludge prcess. Hwever, FRANKEL (1968) fund the incremental cst f secndary treatment using trickling filters and activated sludge t be very small within the range f 30% t 80% remval f B0D 5 frm sewage. It therefre seems unecnmical t cnstruct an activated sludge r trickling filter plant and 32

40 perate it at less than 80% remval efficiency. n the ther hand, the ptimum stream standard will take advantage f the assimilative capacity f a stream t its fullest extent withut causing extensive damage t present users f the stream. Hence, if the necessary degree f treatment is less than 807a a split flw system wuld meet the lwer treatment requirement and yet maintain the cst effectiveness f these secndary treatment prcesses. The mixture f untreated and treated waste streams wuld give an equivalent remval efficiency t that calculated using Eq. 3. In.practice it wuld be necessary t prvide primary treatment t the whle, waste flw. By means f a mass balance the fractipn.f ttal wastewater flw which must receive secndary treatment wuld be given by: P. = e/ f ; (4) where, P is the fractin f the wastewater flw v that must be treated, >'.'i!l > * -;.. f is the,treatment plant ptimal remval f - - s '"-bipd.egradable waste, and e is as defined befre.. "", v ^ --' The prtin tti&p. is allpwed t by-pass, secndary treatment is then Uv equal t: v -; ".-. ' < ' ' '"-''*, : '^b"'^ ^ " Kf \ ; (5) '" ' '-"-' 'ViC'-i where, P. " "is...the fractin that has t; be by-passed. The relatinships between initial xygen deficit in the stream, the initial rganic lad in the stream after mixing f the discharged waste, the., degree f'waste treatment required and the stream D'standard as calculated by the Streeter-Phelps dxyge'h sag mdel using parameter values previusly stated are shwn in Fig. 5.\ It is clear that as the standard becmes mre stringent, that is, the.xygen deficit becmes smaller'",---the 5 required efficiency f remval, f BD increases cnsiderably.? Similarly-^---applying Eq. 4 it can be sftwn, that the required prtin f wastewater which must receive s^ecnda-ry treatment increases -cnsiderably with increase in the stream dissqlved-pxygen standard, until all" the..wastewater must be treated when the., remval, required is equal t the ptimat "remval efficiency f-the unity, e "= N fk\ The ptimal remval efficiency is apprximately-8q7_ fr tricfcld.ng'filte't's, 907, fr activated sludge and fr xidatin pttdsv-.. '"""--. '.'">Cs ;

41 r / k = _ / * / 16 a. a> a JC 03 (0 I E / ' P '/ / 0^/ / / > / / ' > / / A^ / Prpsed D.. Standard >» 9 cvi '.0 8 / / * / /JVIre,0-* lv. c 6 ' /stringent >^ Standard Lax c <u u c 4 Standard 2 M^x " i i xygen Deficit Fig. 5 Relatinship between Stream D and Cncentratin f rganic Wastes Discharged int a Stream 34

42 Cmparative Csts f Wastewater Treatment Systems The ttal cst f the activated sludge prcess is primarily dependent n the cnstructin cst, peratin and maintenance csts, which are relatively cnstant within any particular cuntry. Ttal csts f trickling filter and xidatin pnd treatment plants are dependent n the land cst which makes cst evaluatin highly dependent upn lcatin. Due t the lack f reginal cnstructin cst data, and csts f peratin and maintenance fr activated sludge and trickling filter plants, ttal annual csts were estimated n the basis' f data given by SMITH (1968) and updated using the 1973 ENR cst index-r" ( ). The csts fr xidatin pnds were determined using" actual cst data frm the Asian Institute f Technlgy and...the Applied Scientific Research Crpratin f Thailand updated t 1973 using the ENR cst index adjustment. Fig. 6 shws the estimated ttal annual c-sts (including capital depreciatin, peratin and maintenance) fr trickling filters and 6xidatin pnd's at different land values. This figure als shws the ttal annual csts fr activated sludge and primary treatment prcesses. '-'!--. Fr purpses f cmparisn,.land cst, is allwed fr as a rental rate per square meter. This vercmes the prblem f land value appreciatin ver a perid f time and takes int accunt the financial benefit frgne as a result f nt -tasing the land fr alternative purpses during the life f the treatment plant. Fig. 6 indicates that the main cnstraints in the chicef a wastewater treatment system are the ppulatin served and the cst jf rental fr the land used. Fr ppulatins less than 175,000. (20,000 'itf*/day!) and when land is very cheap, xidatin pnds are mst attractive, fllwed by trickling filters. Hwever, when the ppulatin served becmes very large, the land required becmes large s that even at lw land, rental csts, the trickling filter becmes mre attractive. At very high land rental csts, as cmmnly fund in urban areas, and fr large ppulatins the activated sludge prcess becmes cmpetitive with trickling filters. The decisin n the chice f wastewater treatment prcess fr military installatins in trpical develping cuntries will thus depend principally n the value attached t the land used. Fr ppulatins less than 175,000, which wuld include mst military bases, and land rental csts less than U.S.$ 0.10/m 2 per year, xidatin pnds will be the mst ecnmic system prviding secndary treatment. The need fr secndary...treatment, and the...prprtin..f. t.he...wast _JE.LQW-.ejquiring secndary treatment will be strngly influenced by the stream standard fr disslved xygen applied. xidatin pnd effluent will ht adversely affect the xygen balance f a stream prvided that the cncentratin f algal cells after; dilu'tih' in the stream dbe ; s! ht exceed 1 x 10 B cells/ml. 35

43 10 8 Legend : Activated Sludge Trickling Filter at $ 0.01/m land rental/yr. "» at $ 0.05/m land rental/yr.»» at 0.10/m land rental/yr. xidatin Pnds at $ 0.05/m land rentaj/yr.» " at $ 0.10/n?land rental/yr. Primary Treatment r $.I/m land rental/yr. ±*lt er 0.05/m land rental/yr". 2**a> -$.I/m land rental/yr. - ^ *, 0.01/m land rental/yr. 2 I xidatin Pnds.~ ± 0.05/m land rental/yr Treatment Plant Capacity (m/day) Fig. 6 Relatinship between Treatment Plant Capacity and Cst 36