THE EFFECT AND REGULATION OF WASTEWATER RESIDENCE TIME IN SEWERS

Transcription

1 October 1969 Report No. EVE THE EFFECT AND REGULATION OF WASTEWATER RESIDENCE TIME IN SEWERS Davd R. O'Toole and Donald Dean Adran, Project Investgator Partally Funded by Offce of Water Resources Research Grant WR-BO 11-MASS and Federal Water Polluton Control Admnstraton Tranng Grant 5T1-WP ENVIRONMENTAL ENGINEERING DEPARTMENT OF CIVIL ENGINEERING UNIVERSITY OF MASSACHUSETTS AMHERST, MASSACHUSETTS

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3 TKE EFFECT AND REGULATION OF WASTEWATER RESIDENCE TIME IN SEWERS by! Davd R. O'Toole and Donald Dean Adran, Project Investgator October 1969

4 THE AUTHORS w Davd Rchard O'Toole receved hs B.S. degree n 0 Cvl Engneerng from the Unversty of Massachusetts n June 1968, After a summer workng for a consultng engneerng frm he enroled as a graduate student n the Envronmental Engneerng Program, Department of Cvl Engneerng, Unversty of Massachusetts. Upon recept of hs Master's degree n September 1969, he was "commssoned by the U. S. Publc Health Servce to work as an Assstant Santary Engneer wth the Indan Health Servce n Portland, Oregon. Donald Dean Adran receved hs B.A. degree n Lberal Arts and hs B.S. degree n Cvl Engneerng from the Unversty of Notre Dame n 1957 and 1958, respectvely. He enrolled n the graduate program n Santary Engneerng at the Unversty of Calforna at Berkeley and was awarded the M.S. degree n The Ph.D. was obtaned from Stanford Unversty n Cvl Engneerng n Experence has been obtaned wth the Calforna Health Department, Vanderblt Unversty and the Unversty of Massachusetts. He s presently an Assocate Professor of Cvl Engneerng.

5 ACKNOWLEDGMENT ( Ths report wth mnor modfcatons embodes a thess by Mr. O'Toole enttled "The Effect and Regulaton of ; Wastewater Resdence Tme n Sewers," and submtted by Mr. O'Toole n partal fulfllment of the requrements for the degree of Master of Scence n Envronmental Engneerng, Department of Cvl Engneerng, Unversty of Massachusetts, September Recognzed s the cooperaton receved from both the Northampton and Amherst Departments of Publc Works., wth a specal thanks to Mr. Francs (Pat) Ryan and Joseph Gessng, respectvely Cty Engneer and Sewer Foreman of Northampton, and Mr'. Stanley Publc Works Zomek, Superntendent of the Department of n Amherst. Unon Carbde Corporaton s recognzed for furnshng the frcton-reducng polymer, POLYOX, used n these experments. Helpful comments were receved from Dr. John McCullough, Research Manager, both Chemst, and Mr. A. K. Ingberman, Product Marketng assocated wth Unon Carbde Corporaton. Fnancal support for ths study was provded by the Offce' of Water Resources Research Grant WR-B011-MASS, and by Federal Water Grant Number 5T1-WP Polluton Control Admnstraton Tranng 111

6 ABSTRACT As populaton denstes ncrease, there wll be a gradual trend to "larger and more centralzed sewage treatment facltes whch n turn wll necesstate longer and larger collecton systems. Snce a collecton system generally accounts for 40 to 70 percent' of the entre sewerage system costs, proper collecton system desgn wll be most mportant. Therefore, t s mperatve that nvestgatons be made to establsh proper desgn crtera. Ths study was desgned to determne what sewage qualty changes take place n transt srtce certan changes n qualty could lead to sewer corroson, and to determne what effect the addton of frcton-reducng polymers have on the flow velocty. It was found that a notceable lag n oxygen uptake rate exsts n fresh sewage; whereas sewage whch had been retaned n a pumpng staton for 8-12 hours exhbts an ntally hgher oxygen uptake rate durng the frst few hours. The addton of a frcton-reducng polymer produced a marked ncrease n the sewage flow velocty, The polymer was more effectve when added to a hgh velocty flow than when added to a low velocty flow. The polymer showed no toxc effect on bologcal reactons n the sewage; n fact, the results ndcated that t was used as a food source as other organcs n the sewage were depleted. A

7 From ntal calculatons.t appears that when a sewer s over- " -. loaded by nfrequent short perod peak flows, t may be more economcal to manage these peak flows ay the addton of a frcton-reducng polymer rather than by the constructon of an addtonal sewer lne. It s recommended that polymer usage be consdered when sewer lnes are subject to nfrequent surcharges. It s also recommended that polymer usage be encouraged n long lnes whch are subject to surcharge, as the cost of polymer dosage s a functon of the flow rate and not a functon of the length of the lne. The cost of polymers s hgh enough that ther routne contnual use s not recommended for regulatng qualty changes n sewage by decreasng ts transt tme.

8 TABLE OF CONTENTS Ttle ;... Acceptance Acknowledgment Abstract..'.. v Table of Contents Lst of Fgures v x Lst of Tables x Part 1. Introducton t ] 1.1 Statement of Problem Justfcaton for the Study 4 "U3 Statement of Objectves 5 Part 2. Lterature Revew Sewage Qualty Frcton-Reducng Agents 13 Part 3. Theoretcal Consderatons Sewage Qualty Changes Sewer Corroson Bologcal Reactons Factors n Sulfde Generaton Temperature of Wastewater Strength of Wastewater 31 v

9 Velocty of Flow Age of Wastewater ph of Wastewater Effect of Sulfate Concentraton Sulfde Control Proper Desgn Control of BOD Control of Temperature Control of ph.., Chlornaton Aeraton Sodum Ntrate Frcton-Reducng Addtves Type of Addtve Used Scour Effect 45 Part 4. Expermental Procedure Sample Collecton, Preservaton, and Storage Temperature of Sewage Analyss Performed to Determne Qualty Changes Addton of Frcton-Reducng Addtves 51 Part 5. Expermental Results and Dscusson Sewage Qualty Changes 60 S Oxygen Uptake 60 vn

10 5.1.2 ph Varaton Wth Tme Effects of Polymer on Sewage Degradaton Effects of Polymer Addton on Sewage Velocty Economc Consderatons Part 6.. Conclusons 37 Part 7. Recommendatons 39 Bblography 90 Appendx 95 vn

11 LIST OF FIGURES 1. Rheogram for 10 and IQQ'ppm of Polyethylene Oxde of 4-mllon Molecular Weght Frcton Reducton Versus Concentraton for POLYOX WSR Correlaton of BOD and Sulfde Concentratons Relatve Rates of Sulfde Producton at Varous ph Values Molecular Structure of Polyethylene Oxde Profle of Sewer Number Profle of Sewer Number Profle of Sewer Number BOD Curve for Test Seres Number 1 on Unversty of Massachusetts Sewage BOD Curve for Test Seres Number 2 on Unversty of Massachusetts Sewage BOD Curve for Test Seres Number 3 on Unversty of Massachusetts Sewage BOD Curve for Test Seres Number 1 on South Amherst Sewage BOD Curve for Test Seres Number 4 on Unversty of Massachusetts Sewage ph of Unversty of Massachusetts Sewage at Varous Polymer Concentratons Versus Tme ph of South Amherst Sewage at Varous Polymer Concentratons Versus Tme 77

12 16. Comparson of Oxygen Uptake' Rate at Varous Polymer Concentratons of Unversty of Massachusetts Sewage Wth Tme Comparson of Velocty Increase Wth polymer Concentraton for Sewer Number Comparson of Velocty Increase Wth Polymer Concentraton for Sewer Number 2 83

13 LIST OF TABLES 1. Varous Hgh Molecular Weght Polymer Addtves Polymer Concentraton and Capacty Increase for a 6-Inch Sewer Lne Polymer Concentraton and Capacty Increase for a 24-Inch Sewer Lne.! Cost of Addtve Estmated Maxmum Rates of Sulfde Generaton Requred Velocty to Prevent Sulfde Buld-up Test Seres Number 1 on Unversty of Massachusetts Sewage Test Seres Number 2 on Unversty of Massachusetts Sewage Test Seres Number 3 on Unversty of Massachusetts Sewage Test Seres Number 1 on South Amherst Sewage Test Seres Number 4 on Unversty of Massachusetts Sewage. - : Comparson of Oxygen Uptake Rates at Varous Polymer Concentratons of Unversty of Massachusetts wth Tme 79

14 PART 1. INTRODUCTION 1.1 Statement 'Of the Problem As the Envronmental Engneer projects requrements for future waste collecton and treatment systems, he must seek answers to several questons ncludng: What economes of scale may be acheved by a sngle centralzed waste treatment plant as contrasted several smaller waste treatment plants? How large should the man nterceptors of a waste wth collecton system be to serve adequately n the future? What economc tradeoffs are avalable between the economes of scale of a large centralzed waste treatment plant and a larger, more costly waste collecton system ' to serve ths' plant? A noted authorty n Envronmental Engneerng has predcted the trend wll be toward large centralzed waste treatment plants sayng, " When one weghs the arguments for and aganst the nstallaton of larger and larger treatment plants, the apparent trend toward centralzaton at large treatment plants would seem to be justfed." (1) However, a larger treatment plant requres that the area to be served be proportonately larger. The larger servce area necesstates longer transport dstances for the sewage, thus provdng a greater

15 opportunty for natural degradaton n transt. The declne n the sewage's dssolved oxygen content pror to arrval at the waste treatment plant may necesstate aeraton pror to treatment. addton, there s the opportunty, n extreme cases, for structural damage through corroson of the waste transport system. Generally, as the resdence or transt tme of a sewage the dssolved oxygen content (D.O.') of the sewage decreases to a In ncreases, pont where there s no dssolved oxygen present; under ths latter condton, the sewage becomes anaerobc (septc). Under anaerobc condtons, one of the man products resultng from sewage degradaton s the generaton of hydrogen sulfde (H^S) gas. Ths hydrogen sulfde s utlzed n a seres of reactons by mcroorgansms whch may result n structural damage to concrete sewers through corroson.' Gaudy (1) has nvestgated the problem of sewer corroson. hs work, he has concluded that sewer ppe corroson may occur n long runs of ppe wth low veloctes. To prevent anaerobc cond tons from occurrng the dssolved oxygen content (D.O.) of the sewage must be greater than 0.0 mg/1 (2). From As one can see from the above statements, ncreasng the velocty decreases the resdence tme of the sewage n the sewer and reduces the opportunty for anaerobc condtons to develop. Thus, by avodng the generaton of hydrogen sulfde gas, sewer corroson s. precluded.

16 The rate at whch the dssolved oxygen s removed from the sewage depends on such factors as the age, composton and strength of the waste. The standard for measurng the strength of the waste s the Bochemcal Oxygen Demand (BOD) after fve days at 20 C. when one consders that the transport tme of sewage s generally less than one day, the applcablty of the fve day BOD test to pre detng qualty changes occurrng durng transt.s questoned. It would seem that a test measurng the oxygen uptake at certan ntervals for a perod of less than one day would be more sutable. s strengthened by the fact that some researchers Ths belef (3) have reported a hgh oxygen uptake rate durng the frst few hours whle others (4) have stated that there may be an ntal lage perod durng whch = lttle oxygen uptake occurs. The njecton of hgh molecular weght ( 4,000,000), straght chan polymers nto the sewer method of ncreasng the velocty of flow. lne has recently been consdered as a Ths ncrease n velocty s brought about by the frcton-reducng effect of the polymer. date there s only one case where polymers have been appled to a sewer (5). In ths case under certan condtons, polymers were found to ncrease the flow by more than twce the flow of the nonpolymered soluton. < Most of the work wth polymers has been carred out by. the U. S. Navy, wth specfc applcaton to naval vehcles beng able to provde.sudden, bursts of speed (6). To

17 A. G. Fablua (6), of the Naval Ordnance Test Staton at Pasadena, Calforna, has expermented wth several dfferent types of polymers. He concluded that polyethylene oxde, a water-soluble, hgh molecular weght materal of completely lnear constructon, was the most effectve frcton-reducng agent known. Ths polyethylene oxde s sold by Unon Carbde Corporaton under the commercal name, POLYOX. In a regonal wastewater collecton system, the resdence tme may be suffcent to permt sewer corroson caused by sewage qualty changes. However, t may be possble to reduce ths resdence tme wth the addton of a frcton-reducng polymer. Therefore, n order for the Envronmental Engneer to properly desgn regonal wastewater collecton and treatment facltes, he must be aware of the sewage qualty changes takng pjace n the collecton system; he must know f there s a crtcal resdence tme after whch sewer corroson wll occur; he must avod desgns whch enhance corroson; he should,.know the measures whch can be.taken to regulate these qualty changes; and he should know f t would be benefcal to ntroduce a polymer soluton nto the sewer lne. 1.2 Justfcaton for the Study Snce a collecton system generally accounts for 40 to 70 percent of the entre sewerage system costs, t wll be extremely mportant to desgn the most economc collecton system. When

18 consderng a regonal collecton system, the length of the lnes and { consequently the resdence tme of the sewage wll ncrease sgnfcantly; consequently, t wll be mportant to determne f there s a crtcal resdence tme after whch the collecton system becomes damaged by sewage qualty changes. Also, t would be mportant to know f a frcton-reducng polymer could: regulate ths qualty change by ncreasng the velocty of the sewage, be used as a more economcal method of ncreasng the present collecton system capacty rather than constructng parallel sewer lnes. To date, there has been lttle research conducted whch would provde answers to these questons. as: 1.3 Statement of Objectves Ths thess wll be drected toward answerng such questons How rapd are the qualty changes whch take place n the waste collecton system? Wll polymers exhbt smlar frcton reducng abltes n open channel flow that have been observed n closed condut flow? -- Can the tme n transt of the wastewater be reduced by the ntroducton of frcton'reducng polymers nto the flow?

19 When wll t be economcally feasble to ncrease the capacty of a sewer through the use of polymers nstead of constructng a parallel larger capacty sewer?

20 PART 2. LITERATURE REVIEW ' I V 2.1 Sewage Qualty The lterature'.provdes lttle nformaton on the qualty changes of sewage whle n transt. Consderable work has been done and publshed wth reference to the standard 5-day Bochemcal Oxygen Demand (BOD) Test. However, ths test s not exactly applcable to ths research because t does not gve the rate of oxygen consumpton for a perod of less than one day. The 5-day BOD test s wdely used to determne the strength of wastes n water and sewage (7); n effect, t measures the amount of bodegradable organc materal present n a wastewater by the quantty of oxygen consumed by mcroorgansms n decomposng the waste (8). By takng dssolved oxygen measurements at one day ntervals or less, the rate at whch the organc materal s stablzed can be determned. However, ths method of determnng the rate of stablzaton becomes tedous when the DO determnatons are more frequent than one day; the reasons beng: several dlutons are requred necesstatng a large number of bottles, a large constant temperature storage space s requred for the bottles, and great care must be taken n preparng the reagents to mantan the precson of the test. Much has been wrtten about the classcal BOD curve. s obtaned by plottng the BOD exerted versus tme. order reacton. Ths curve at one day ntervals or less When the ponts are plotted, they approxmate a frst Streeter and P6eftps'(9) stated that the rate of

21 bochemcal oxdaton s proportonal to the concentraton of unoxdzed substrate measured n terms of oxdzablty. Ths results n a frst order reacton whch can be expressed n equaton form as: Y = L(l - ltf kt ). Eq. (1) where y s the BOD exerted at'.tme t, L s the ultmate BOD and k s the rate constant for the reacton. Several methods have been proposed to calculate the parameters, L and k, for the BOD curve. Thereault's tabular method (10), Among the most wdely used methods are the Thomas slope technque (11), the Moore moment method (12), Navone's method usng smultaneous equatons (13), and several graphcal methods by Far (14), Wegand (15) and Fujmoto (16). From these methods a k value can be determned; however, some authors (3, 4) beleve that the k value vares durng the frst several hours dependng on the characterstcs of the sewage. Ths varaton s not mportant when one s concerned only wth the strength of the waste, but for ths proposed research, the amount of oxygen consumed durng the frst several hours s mportant. Far has stated (4): In unseeded samples (.e., samples that harbor ntally an nadequate flora and fauna to actvate the BOD reactons), lag perods are experenced before the reacton proceeds normally. By contrast, wastewaters that have undergone partal anaerobc decomposton or contan reducng chemcal substances may exert an mmedate demand (sometmes called a chemcal demand) at the begnnng of the BOD run.

22 In contrast, Schroepfer (3) has conducted research on raw sewage from the Mnneapols - Sant Paul Santary Dstrct; he reported, "a very hgh rate of deoxygenaton occurs n the frst day and then decreases to usual values thereafter." Thus, k s tme dependent wth values n the frst 4 to 12 hours beng as hgh as 1.0 to 2.0 per day, gradually lowerng to 0.2 to 0.3 per day after 1 to 3 days. The lag phase n the frst part of the BOD curve can be expressed n equaton form as (12): ' or, -k(t-t_) y = ID -,10 ] Eq. (2a) y = L(l -c!0" kt ) Eq. (2b) kt n whch t s the lag perod and c Both the slope method and moment methods can be extended to account for ths lag perod (17). Researchers do not beleve the calculaton of lag factors are worthwhle. Ruchhoft (18) has summarzed ths feelng as follows: The calculaton of the lag factor, as emboded n ether the modfed slope method or the present moment method, has never seemed to be worthwhle n ths work. Ths postve "lag s a real thng especally when low temperature BOD observatons are made. Under such crcumstances, t s preferred smply to examne the data and apply t to the data after recovery from the lag s ndcated. For ths proposed research t wll be mportant to know the amount and rate of oxygen consumpton durng a perod of less than one day. In other words, how, long would t take for all the dssolved oxygen to be removed from the sewage? Once all the dssolved oxygen has

23 10 been removed, the sewage becomes anaerobc, a condton whch could lead to serous problems such as sewer corroson. Anaerobc condtons are one of the factors whch cause the generaton of hydrogen sulfde. Hydrogen sulfde does not cause corroson tself, but s ndrectly responsble for crown corroson n concrete sewers. Sewer corroson s a problem n many areas of the Unted States; t occurs n regons where domestc wastewater temperatures are hgh and detenton tmes are long (19). The transport system can be consdered part of the overall treatment process for the sewage nasmuch as the sewage s contnuously undergong bologcal changes whle n transt. If there s suffcent reaeraton n the sewer to mantan aerobc condtons, the opportuntes for sewer corroson are remote, provded sewer veloctes are hgh enough to preclude the buld up of sludge deposts whch may become anaerobc (sluggsh veloctes wll be dscussed n detal later). Ttefact that sewage does urdergo bologcal changes whle n transport has caused researchers (20) at the Unversty of Delaware to nvestgate the possblty of so-called "n-lne treatment." They clam that a secton of the sewer lne can be adapted nto a two-phase bologcal reactor. Ths reactor would consst of an aeraton secton followed by another aerated secton to whch s added actvated sludge. Ths sludge s obtaned at the head end of the plant. To show the advantages of ths n-lne treatment, the researchers completed a case study comparng the performance of a

24 typcal, conventonal, actvated sludge plant that treats the effluent from a communty of 100,000 havng a 5-day BOD of 200 mg/1. From ths,.the researchers predct that the n-lne treatment scheme would have a BOD removal >of 96 percent as compared wth 90 percent removal from the conventonal plant. secondary treatment aeraton tank. Also, the -former method elmnates the Probably the most sgnfcant results of the study are the cost comparsons -- the n-lne treat ment method would cost 30 percent or $700,000 less than the conventonal 1 plant. The major concluson from ths research was: "Ppelne sewage treatment s not only feasble, but promses to offer greater BOD reducton at lowerjntal captal costs than conventonal actvated sludge plants" (20)'. The whole dea of n-lne treatment,has been summed up.by ' Gaudy (1) as follows:, Indeed, the trend toward larger and larger collecton systems has fostered a new concern n the polluton control feld,.e., a so-called -Cn-Hne treatment. Concernng bologcal nteractons n the collecton system, we may onthe one hand attempt to create condtons whch would be deal for the desred type of bologcal actvty, thereby provdng some type of pre-treatment before the waste.enters the treatment plant, wth the very mportant provson that the bologcal actvty does not tend to deterorate the materals of whch the reactor s made. On the other hand, we can take the tact that the conductng materal performs so vtal a functon that we can not afford to take the rsk of deteroratng t n any.way and we should provde an nlne treatment whch arrests bologcal actvty whch may be deleterous to the materals. In ether case t s obvous that we must gve prme consderaton to the nteractons whch take place between the mcrobes and materals of constructon. To measure the oxygen uptake of the sewage whle n transt, t would be mportant to smulate as closely as possble the actual '

25 12 condtons n the.sewer. Such factors as the amount of mxng, the temperature of the waste, and actual strength of the sewage should.*, be consdered. Wth the standard BOD bottle method, these factor's are not consdered. In the standard BOD test, a very small sample s dluted down decreasng the probablty of a representatve sample (21), temperature s controlled at 20 C, there s no mxng, and nutrent salts and sometmes a seed s added to ad n the bologcal actvty. In ths research the sewage should be at the same temperature t s n the sewers. It should experence the same type of mxng as t does n the sewers, and no nutrent salts should be added whch would enhance the bologcal actvty whch would otherwse exast n the sewer. A resprometrc method of measurng the bochemcal oxygen demand appears to be deal for ths research because t can control to some extent all the parameters whch are consdered mportant for ths work. A resprometer s an nstrument desgned to measure small quanttes of gas, ether adsorbed or evolved n bochemcal reactons. The apparatus conssts of a constant temperature water. bath equpped wth a shakng mechansm to whch several small flasks are connected, each wth a monometer for ndcatng changes n gas volume. Ths resprometrc method s based on the theory that only oxygen and carbon doxde are nvolved n the gas exchange; and also the as

26 13 effect of the latter s completely elmnated by usng an alkal absorbent placed n a separate compartment wthn the reacton flask. Ths belef has been substantated by Wooldrdge and Standfast (22). They aerated sewage for consderable perods of tme under condtons whch enabled them to analyze the ar by standard gas analyss methods. Ther fndngs showed negatve tests for hydrogen sulfde, hydrogen, ntrogen, and methane and they concluded that under ordnary condtons for the BOD test, there s no reason to suspect any other gases besdes carbon doxde and oxygen to be nvolved. When the results obtaned from the bottle method are compared to the manometrc method, t s found that the manometrc results are much hgher. Also, dluton of the sample to be used n the resprometer has an effect on the results. Dluton lowers the calculated demand and ths becomes more notceable wth hgh dlutons (23). As can be seen, manometrc technques provde a method of observng changes n the system wthout destroyng allows, for a contnuous BOD curve to be obtaned. the sample whch 2.2 Frcton-Reducng Agents As early as 1906, Blatch (24) demonstrated that water flowng wth very low concentratons of sold partcles had lower head losses than wth water flowng alone. In 1927, Gregory (25) found that at low clay concentratons, head losses were somewhat suppressed. More recent nvestgatons (26, 27) have proven conclusvely that clay-water suspenson head losses are defntely lower than plan water head losses.

27 14 Vanon (28) used a laboratory flume to'measure the frcton factors for varous channel slopes, and dfferent concentratons of aspended loads. He found that sedment-laden water had less 'resstance than water wthout sedment; therefore, sedment-laden water flowed faster than clear water. The reducton n resstance vared wth the gran sze and total sedment load. Wth certan suspended loads the frcton factor was decreased by 20 percent. Later, Vanon and Nomcos (29) showed that suspended solutons could reduce the frcton factor up to 28 percent. Wood pulp and synthetc fber solutons have also been used to reduce resstance. Daly and Buglarello (30) found solutons of ths \ type to decrease the frcton.factor wth ncreasng solds concentra- \ \ ton, fber length and ppe dameter. \ More recently, nterest has centered on the addton of small \! amounts of soluble hgh-molecular weght polymers n causng drag reducton.. \ have caused sgnfcant drag reducton. Very small concentratons of these polymers ( ppm) The applcaton of polymers for frcton reducton has gone unnotced for so many years because of the methods used n measurng frcton reducton. Hbyt (6) stated: The reason the drag-reducng effect of polymers escaped notce for so long probably les n the characterstcs of the nstrument used tradtonally to measure the frcton created by the flow of fluds, the vscometer. Snce all vscometers operate n lamnar flow, measurement of polymer-soluton vscosty as shown n Fgure 1 ndcates only that these solutons

28 15 10' E u 10 3 _ 1/1 C/l at s- +-> (S) ret 10' O A ['J 100 ppm 10 ppm dstlled water 10' 10' 10* 10' Wall Shear Rate, Sec -1 Fgure 1. Rheogram for 10 and 100 parts per mllon of Polyethylene Oxde of 4-mllon Molecular Weght (6)

29 16 have a somewhat hgher vscosty than the solvent. On the bass of ths data only, one would "logcally predct that a slght frcton ncrease, not a remarkable decrease, would occur durng the turbulent flow of fluds contnng polymers. However, Hoyt (6) stated that when a soluton of one of three polymers (polyethylene oxde) was pumped n turbulent flow through a ppe (.e., at a Reynolds number greater than 3,000) frcton reductons such as those shown n Fgure 2 were obtaned. The U. S. Navy has done much work-wth polymers as frctonreducng agents. They frst became nterested n ths subject n 1961 when Westco Research made known ther fndngs. Ths frm s an olfeld equpment concern. Ther workers found that less power was requred to pump hgh-pressure water through ppelnes when they added Guar.gum to suspended sand partcles n the water (6). j Based on these ntal fndngs, the U. S. Navy set up an extensve screenng program to determne useful hgh-molecular weght polymer addtves. A. G. Fabla (6) of the Naval Ordnance Test Staton has dentfed polyethylene oxde as the most effectve known frcton reducng agent. It s sold under the commercal name POLYOX.. POLYOX s a hgh-molecular-weght materal, havng a straght chan structure consstng of oxygen, hydrogen and carbon. The U. S. Navy has dentfed several other promsng chemcal addtves, such as the polyacrylamdes, polysulfonates, and hydroxyethyl cellulose. See Table 1..

30 Polymer Concentraton n Water, ppm Fgure 2. Frcton Reducton versus Concentraton for POLYOX WSR-301 (6) 120 (0.19-CM. ID Ppe; Temperature 21.1 C; Re = 14,000)

31 TABLE 1. Varous Hgh Molecular Weght Polymer Addtves (34) Trade Name Chemcal Identfcaton Manufacturer Molecular Weght ADM-7097 Archur Danels M.dland Co. Mnneapols, Mnn. 24 x 10 6 B-1459 Polysaccharde gum (fermentaton) Commercal Solvents Corp. New York, New York B-1459 Elanco Products Co. Indanapols, Ind. Carbopol Carboxyl vnyl polymer B. F. Goodrch "Chemcal Co Cleveland, Oho DMC - 74SCP Sodum carboxymethyl cellulose Hercules Powder CO; Wlmngton, Del. (5 to 50) x 10 Dextran Alpha glucosyl polysaccharde Pharmaca, Uppsala, Sweden (5 to 40) x 10 ( ET-597 ET-720 Sulfonated polyynyl toluene Sodum polystyrene sulfonate Dow Chemcal Co. Mdland, Mch. 8. x 10 6 (8 to 9) x 10 6 Jaguar Modfed guar gum Sten, Hall, and Co New York, New York Kelco Gel HV Kelgn HV Sodum algnate Kelco Co., SanQego, Calf,

32 Table 1 (contnued) Trade-. Name Kelzan Kelzan H Chem cal Identlf cat on Xanthon gum Manufacturer Kelco Co., SanQego, Calf Molecular Weght Methocel - 90HG Methyl cellulose Dow Chemcal Co., Mdland, Mch. Polyhall - 27 Non-onc polyacrylamde Stefn, Hall, ant Co. New York, New York Polyox ' Ethylene oxde polymer Unon Carbde Chemcals Co. New York, New York to 5 x Reten MX Catonc polymer Hercules-- Powder Co. Wlmngton, Del. Separan - NP20 Non-onc polyacrylamde Dow Chemcal Co. ~ Mdland, Mch. 2 x 10' Westco - J2 The Western Co. Dallas, Texas 2.2 x TO'

33 20 Wth reference to polymer addton, Hoyt (31) concludes that ths frcton-reducton effect wll manfest tself when the follow-! 1 " ng condtons preval: -- The flow s turbulent. -- The polymer used has a molecular weght of 50,000 or more. The polymer s straght-chaned. -- The polymer s soluble. Wlknson, et_ jal_ (32) tested two dfferent polymers on a 6-nch test lne. The polymer solutons were capable of ncreasng the flow by more than twce the flow of the non-polymered sewage. Under certan condtons the flow was ncreased by about 2.4 tmes. Table 2 shows these test results. It should be noted that the settleable solds content for the sewage used was 9 mg/1 -- much less than the 120 mg/1 (33) expected n a typcal sewage. The names of the addtves used were not gven. They also added polymers to a 24-ch sewer lne whch was beng surcharged. The lne conssted of a 24-nch concrete sewer ppe havng a grade of 0.32 percent. It was 4,100 feet long and dscharged nto a 30-nch outfall lne. The lne receved flow from a 36-nch nterceptor. Durng the testng the lne was experencng surcharges of sx feet or more. Two dfferent add tves were used (ther names were not gven). In each of the tests, polymer addton stopped when the desred results were obtaned -- the necessary reducton n head n the lne to elmnate the surcharged condton. Table 3 shows these results. In ths partcular case,

34 21 TABLE 2. Polymer Concentraton and Capacty Increase for a 6-Inch Sewer Lne (32) Add t ve Sewage Sewage Fl ow Flow.Concentraton Concentraton Temperature Rate Increase Addtve ppm ml/1 of S.S. F GPM % FCIxOF No. No. No TOO , No No No * Flow Capacty Increase Tmes Orgnal Flow

35 22 TABLE 3. Polymer Concentraton and Capacty Increase for»a 24-Inch Sewer Lne (32) Addtve Addtve Sewage Sewage Cone. Temperature Cone. ppm ml/1 of S.S. Flow Increase th Flow Capacty Increase Tmes Orgnal Flow No No No No No No No No

36 23 larger flow ncreases would have been realzed f the concentratons were ncreased.. The cost of addng the two addtves used on the 24-nch surcharged sewer has been fgured at concentratons of 100 and 150 ppm (parts per mllon). per hour per njecton. The cost s gven as materals cost n dollars See Table 4 for costs at varous wastewater flow rates. Unon Carbde, the manufacturer of POLYOX, has quoted prces rangng from $1.50 to $0.60 per pound, dependng on the g^ade of POLYOX used, when purchased n 20,000 pound carloads. In concluson, the lterature has provded nformaton on a varety of chemcal addtves used as frcton-reducng agents; however, the majorty of ths work was carred out n the laboratory under deal condtons, that s, the major parameters such as flow rate, solds content, ppe roughness and mean velocty were controlled. Even n the reported feld study (32), the settleable solds content was much less than the expected value n a typcal wastewater. What appears to be mssng s chemcal addtve data on gravty sewers carryng a normal wastewater. to provdng ths data. Part of ths research s drected

37 24 TABLE 4. Cost of Addtve (32) CFS Flow MGD 100 ppm Addtve No ppm Addtve No ppm 1 50 ppm ' Ths chart ndcates the materal cost n dollars per hour for njectng the slurry at TOO and 150 ppm polymer concentraton at varous wastewater flow rates.

38 25 PART 3. THEORETICAL CONSIDERATIONS 3.1 Sewage Qualty Changes Of concern In ths research s the removal of dssolved oxygen from the sewage. If the rate of reaeraton exceeds the rate of deoxygenaton, the chance that sewage quantty changes wll have an adverse effect on the transport system s remote. In theory an equaton can be wrtten to determne the dssolved oxygen content at any tme (t) knowng the characterstcs of the waste; however, n practce, predctng the dssolved oxygen concentraton n a sewer lne at tme (t) s dffcult. Ths dffculty can be seen by the fact that no researcher has developed any type of relatonshp whch would predct the dssolved oxygen content n a typcal sewer system. Much of.the dffculty s due to the several factors nvolved. Among these factors are the velocty of flow, the strength of the waste, the sulfde content of the waste, temperature of the waste, depth of flow n the sewer lne, toxcty of the waste, temperature of the waste, the number of force mans n the system, the amount of turbulence exstng p the sewer lne, the age and ph of the waste, and the amount of ventlaton n the sewer. Most of the work conducted on oxygen balance for wastewater has been done on rvers and streams, manly to consder how much sewage can be dscharged nto a stream before the assmlatve capacty s l ; exceeded.

39 26 In ther study of the polluton and natural purfcaton of the Oho Rver, Streeter and Phelps (9) developed the followng equaton K,L -K,t -K 9 t m --Kot -r 1 / <- N / rw r \ * <- r / -> \ - C - K _ K (e -e ) - (C - G Q ) e Eq. (3) where t = flow tme n days, C = oxygen content of water at tme t, (mg/1), C = Q Oxygen content of water at tme 0, (mg/1), C = Oxygen solublty of water n contact wth ar, (mg/1), K-, = constant of oxygen consumpton, (d~ ), l<2 = constant of oxygen reaeraton, (d~ (} Utlzng the above equaton, the dssolved oxygen content of the stream could be predcted at tme t. Ths equaton assumes as constant K,. Ths assumpton s vald because the BOD of a stream s generally low; also the tme perod s \ Usually several days, a perod long enough for K-, to stablze tself at a constant value. K^, the reaeraton constant, s calculated on the bass that the percent oxygen n the ar above the stream remans the same. Ths, too, has been shown to be a vald assumpton (9). When ths equaton s appled to a sewer lne, problems result, The K, value of raw sewage has been shown to vary consderably durng

40 27 the frst day (3, 4). The strength of a raw sewage s much greater than that of a stream; ths fact coupled wth a varable K- ntroduces consderable error n predctng a dssolved oxygen content. vary dependng on the depth of flow, and the amount of mxng. K~ wll there s a decrease n the oxygen content of the ar n a sewer as Also, the dssolved oxygen n the sewage s consumed and reaeraton depletes the supply of oxygen n the ar space above the sewage. For the reasons ponted out above, the Streeter-Phelps equaton does not lend tself to sewer lne applcaton. A practcal method of evaluatng oxygen uptake can be obtaned by usng a resprometer. Ths, too, does not fully smulate the cond ton n a sewer lne, but t does have certan mechansms (.e., shakng apparatus, constant temperature bath) whch can be employed to resemble sewer condtons. By usng ths nstrument, the tme requred for a waste to utlze the ntal dssolved oxygen could be determned. Knowledge of ths tme would not permt the soluton to all sewer lne problems, but t would be a pece of nformaton would brng engneers to better Understand what s gong on n the that sewer and how mportant s the concept of mantanng a certan dssolved oxygen level n the sewer lne. 3.2 Sewer Corroson Ths secton concerns tself wth corroson brought about by the generaton of hydrogen sulfde. Corroson of a concrete sewer s

41 28 brought about by a complex seres of bologcal and chemcal reactons. The fnal chemcal reacton n the seres s: Ca(OH)? + H? SO A = CaSO, + 2HJ3 C- C- M". (from concrete) (from bologcal (easly reactons) spa!led) Eq. (4)' As seen n the reacton, the sulfurc acd (H^SOJ reacts wth the lme (Ca(OH)n) n the concrete to form calcum sulfate. Gaudy (1) stated: The formaton of CaSO* adds to the njurous effect snce t may be consderably hydrated, and nternal stresses caused by the ncrease n volume enhance spall ng, thus exposng fresh surfaces to attack." Bologcal Reactons Ths reacton descrbed above has several ntermedate steps whch are brought about by a seres of bacteral populatons. The organc matter and other substances contaned n the sewage satsfy the nutrtonal requrement for a group of bactera classfed as heterotrophs;.e., a food source. these organsms need organc materal as When the sewage becomes anaerobc or when veloctes are such that sludge deposts collect along the bottom of the sewer lne, these organsms reduce gulfates to sulfdes, one of whch s hydrogen sulfde. Under certan condtons (.e., vertcal drops,

42 29 turbulence) some of the hydrogen sulfde s strpped from the flowng sewage and vented to the atmosphere, whle some s adsorbed n'a layer of condensed water on the walls of the ppe (1). The hydrogen sulfde dssolved n ths water layer forms a weak acd whch can attack some free lme n the concrete. Ths reacton proceeds slowly and the corroson t may cause s? bt consdered sgnfcant. The major concern s the presence of a source of reduced sulfur n the layer, coupled wth dssolved oxygen transferred from the ar above the water lne. Ths combnaton provdes an enrched envronment for the growth of autotrophc bactera whch can utlze carbon doxde (C0 2 ) as ther sole carbon source. These autotrophs obtan the energy needed to make use of ths carbon by the oxdaton of hydrogen sulfde (1). Gaudy has summarzed these.bologcal reactons as follows: The type of autotrophc bactera nvolved n concrete corroson are of the genus Thobacllus. Two types 'of thobacll contrbute to corroson. The non-acdurc speces, Thobacl lus novel 1 us and Thobacllus thoparus.. can oxdze reduced sulfur compounds to sulfur and sulfate, and can ncrease the hydrogen on concentraton to 10'^M. Below a ph of 5, these organsms de off/(i). However, the lowerng of the ph to 5 provdes for the growth of a second type of thobacll; the acdurc speces, Thobacllus I thooxdans and Thobacllus concretvorus; snce they can reduce the ph to 1.00 or below, these speces can create severely corrosve condtons. The reactons whch take place n the condensed water layer on the ppe wall are as follows: (1)

43 30 2H 2 S ^ 2S + 2H 2 0 ' Eq. (5) 2S H 2 0 ->2H 2 S0 4 Eq. (6) The sulfurc acd then reacts wth the lme n Equaton (4), producng calcum sulphate whch s easly spalled from the concrete ppe. corroson pathway s now complete. The Factors n Sulfde Generaton Several factors nfluence the amount of hydrogen sulfde whch s actually generated n the sewer lne. These factors are temperature, strengths age, and ph of the wastewater and the velocty of flow Temperature of the Wastewater The amount of sulfde produced depends on the bologcal actvty of certan organsms and temperature has a drect bearng on ther actvty. ' Baumgartner (35) measured the'sulfde generaton n bottles at four dfferent temperatures. The rate of sulfde generaton was found to ncrease 7 percent per degree rse n temperature up to 30 C. Between 30 and 37 C the rates were smlar. Pomeroy and Bowl us (36) have found that the rate of sulfde generaton ncreases progressvely up to 38 C. there s an apprecable lag at 42 C before the maxmum rate of generaton s reached, and ths

44 31 s not as great as at 38 C. The lag s even more pronounced at 46 C and 52 C but eventually maxmum rates are attaned whch exceed those of lower temperatures. Table 5 lsts the maxmum rates of sulfde generaton at varous temperatures. The fgures n ths table are ntended only to show the effects of temperature n a general way. Feld tests (35) have shown that sewage wth.a temperature of 15 C or less s devod of sulfdes except for small amounts of nert metallc sulfdes. Wth reference to gravty sewers, Pomeroy and Bowl us (36) conclude that "for any specfed flow condtons and sewage strength n a free-flowng sewer, there s a mnmum temperature below whch sulfde buld-up wll not occur."! In force mans where the sewage s not n contact wth the atmosphere, a dfferent stuaton exsts. Under these condtons the relatonshp between sulfdes and temperatures follows more closely the results of bottle experments (36). I Strength of Wastewater I Other factors remanng constant, more sulfdes wll be generated as the strength of the waste ncreases. Ths s because of the hgh concentraton of bacteral nutrents n a stronger wastewater. nutrents utlzed by the sulfde-producng mcrobes are not known; however, generalzatons can be made and feld tests have shown that The exact

45 32 TABLE 5. Estmated Maxmum Rates of Sulfde Generaton (36) Temperature ( C) Maxmum Rate of Sulfde Increases (ppm per hr) :

46 33 sulfde generaton n sewers s related to the BOD of the waste. These feld tests were conducted n a sewer lne over a perod, of several hours; Fgure 3 shows the results of ths work. As can be seen from the fgure, sulfde generaton practcally stops when the BOD s below 80 mg/1. Ths s due to aeraton at the surface whch destroyed the sulfdes as fast as they were formed, provded the strength of the waste, Is below ths lmtng value. Ths value of 80 mg/1 as beng the crtcal value before sulfdes would be present was also reported by Shreve (37) n Florda. As shown by ths feld testng, sulfde problems may occur only durng certan perods of the day. Bowl us and Banta (38) also found that sulfdes may be present only at certan tmes. In stuatons where sewage may enter a flled ppe wth no oxygen or n longer force mans, sulfdes may be present n sewage havng a very low BOD value, below 80 mg/1. ' Ths work on the strength of wastewater causng sulfde problems can be summed up n the followng statement: 'There s a "lmtng wastewater strength below whch no sulfde buld-up wll occur for any specfed flow condton and temperature (36) Velocty of Flow The sewage flow velocty s an mportant factor n the control of hydrogen sulfde because accumulaton of slmes and debrs below the water surface are a drect result of low veloctes. These slmes and debrs provde a most sutable habtat for sulfde-producng bactera (37)

47 T O O Fgure 3. BOD, mg/1 Correlaton of BOO and Sulfde Concentratons"!36)" (reachng treatment plant). CO

48 35 Debrs and other types of bottom deposts can be prevented by havng flow veloctes of three ft. per sec. or more (36). The rate at whch sulfdes are destroyed at the surface has also been shown to * depend on velocty. Streeter, et_ a]_ (39) have found that absorpton of oxygen by a stream vares as the 1.75 power of the velocty. Of less mportance s the evoluton of hydrogen sulfde but ths may also vary n a smlar way. From ths, t can be concluded that'velocty s a major factor n determnng whether the rate of sulfde subtractons from the stream by evoluton and oxdaton wll keep up wth the rate of sulfde generaton by the slmes (36). Ths prncple may be stated as follows: For any sewage strength and temperature combnaton, there ' s a lmtng velocty above whch sulfde buld-up wll not occur. Ths prncple apples only to free-flowng (gravty) sewers (36). Snce the lmtng velocty s Dependent on both temperature and sewage strength, t has been found convenent to combne these two condtons nto a sngle factor called "effectve BOD." Based on the standard BOD at 20 C, the BOD for any other temperature s calculated on the assumpton that bologcal actvty wll ncrease 7 perfcent per degree (geometrcally). Ths s expressed n a formula (36) as: where Effectve BOD = Standard BOD x (1.07) 1 -" 20 (36).' t = temperature n degrees centgrade Table 6 shows the mnmum velocty needed to prevent sulfde buld-up

49 36 TABLE 6. Requred Velocty to Prevent Sulfde Buld-up (36) Effectve BOD Velocty (mg/1) (ft. per sec.)

50 Age of Wastewater The effect wastewater age has on the sulfde problem depends on how much reaeraton Is takng place to destroy these sulfdes. If there s suffcent reaeraton, wastewater age has no effect; however, when reaeraton s not adequate there wll be a contnual buld-up of sulfdes (36) ph of Wastewater I The ph of wastewater has been shown to have an effect on the total sulfdes generated. Fgure 4 shows the relatve rates of sulfde producton at varous ph values. The porton of the sulfdes n the forn of hydrogen sulfde s also greatly nfluenced by the ph. At a ph of 6.0, 83 percent of the dssolved sulfde content s hydrogen sulfde. When the ph s 7.0 only 33 percent, and when the ph s 8.0 the porton of hydrogen sulfde's only fve percent (36). Shreve (37) has reported that t s not economcally feasble to control the ph because sulfde-producng organsms thrve over a wde range of ph values Effect of Sulfate Concentraton 1 Sulfdes n sewage arse chefly through the reducton of sulfates However, from work on sulfate concentratons versus sulfde generaton no drect relatonshp has been found between the amount of sulfdes

51 38 CD <U X (U M- o c: a; o _ cu D- Fgure 4. Relatve Rates of Sulfde Producton at Varous ph Values (36)

52 39 generated at certan sulfate concentratons (36); ths was shown by an experment n whch bottles of sewage contanng 220 ppm of sulfate were ncubated along wth bottles of the same sewage enrched wth sodum and magnesum sulfates to 390 ppm and 560 ppm of sulfate. At the end of three days all bottles had 8 to 9 ppm of sulfde (36). Shreve (3?) also reports that sulfate concentratons above 50 mg/1 have lttle effect on the rate of sulfde producton over a perod of several days Sulfde Control Proper Desgn The concept of sulfde generaton should be consdered n the desgn of any sewerage system. It should be taken nto account n the desgn of such features as velocty,'force mans, vertcal drops, and constructon materals and coatngs (36). For a wastewater of constant strength and temperature, the controllng factor as to whether su.lfde buld-up wll occur or not depends on the velocty. Based on the estmated peak BOD. and temperatures, the mnmum I safe velocty canbe found n Table 6. An allowance of 25 percent should be made to the value found n Table 6 as a factor of safety. When dealng wth ndustral wastes, ths allowance should be ncreased to 50 percent. Sulfdes may be a problem n the early part of a desgn perod but wll dsappear as the flow reaches the desgn flow.

53 40 Sulfde generaton can be expected n sewers flowng completely full except where the sewage s qute weak or t s farly well aerated. Because of ths, force mans should be kept at a mnmum. In some cases t may be benefcal to lft the sewage vertcally from one gravty sewer to another, at the expense of deeper-excavatons, n order that possble sulfde problems may be elmnated (36). Often sewage has a hgh velocty just before enterng the treatment plant. In cases where t has a hgh sulfde concentraton, these large veloctes should be avoded. Vertcal drops n manholes should also be avoded when possble because ths type of desgn leads to excessve release or hydrogen sulfde.! As dscussed earler, unprotected concrete has been attacked severely to the extent of causng damage to structures. Vtrfed. clay lners have been used for protecton. More recently Derated concrete has been found to resst sewer corroson. consttuents n ths Derated concrete One of the major s fluorde whch has an nhbtng effect on the formaton of elemental sulfur whch s used by certan organsms to brng about acd condtons (1) Control of BOD Snce the BOD of the waste! s proportonal to the rate of sulfde f generaton, a reducton n the BOD would reduce the rate 'at whch sulfde s produced. dluton.. Ths reducton can be brought about by pretreatment and (or) ;

54 Control of Temperature The rate of sulfde generaton ;can be reduced by lowerng the temperature. Ths s not consdered a practcal soluton and s not practced, except ncdentally when an unpolluted water s combned wth a polluted wastewater reducng the effectve BOD. ' Control of ph Snce sulfde-producng organsms can operate over a wde ph range, t s not feasble to control ph. The control of ph may be consdered when dealng wth ndustral wastes. An ndustral waste whch would lower the ph of the sewage would cause a hgh porton of the'sulfde to exst as hydrogen sulfde (36); Chlornaton sulfde producton. Chlornaton s one of the prncpal methods used to control It has, been used successfully over the past twenty years and should contnue to be an mportant method n the future. Chlornaton acts n three ways (36): t destroys sulfdes by chemcal reacton; t causes mld oxdzng compounds to be produced n the sewage whch temporarly retards bologcal actvty, thus permttng sewage to accumulate a lttle dssolved oxygen; and j t destroys the sulfde-producng organsms, thus preventng sulfde generaton.