TEMPERATURE MODELING AND CONTROL FOR BIOLOGICAL WASTEWATER TREATMENT DESIGN

Transcription

1 TEMPERATURE MODELING AND CONTROL FOR BIOLOGICAL WASTEWATER TREATMENT DESIGN Christ L. Cruikshnk, P.E. & Dvid G. Gilles, P.E. ADVENT-ENVIRON 201 Summit Vie Dr Suite 300 Brentood, Tennessee ABSTRACT modeling during preliminry engineering tsks of biologicl steter tretment system design ensures engineers evlute nd ccount for the cooling nd/or heting requirements necessry for proper opertion of tretment system. This pper presents methods nd prctices for modeling nd controlling temperture, cse studies of severl industril biologicl steter systems, nd presenttion of equipment used for cooling or heting steter. The model presented hs been used in the design of biologicl rectors including nitrifiction / denitrifiction systems. An equilibrium model s developed in the 1970 s (Comprehensive Model For Aerted Biologicl Systems, Argmn nd Adms, 1977) to predict verge monthly tempertures of in-ground bsins for the biologicl steter tretment rector in both inter nd summer conditions. This model ccounted for het sources from solr rdition, biochemicl rections, nd mechnicl inputs, nd het losses from evportion nd conduction through lls nd ter surfces. The originl model considered erted bsins ith mechnicl surfce ertion or submerged diffused ertion. With industril steter tretment processes becoming more complex, the model hs been dpted nd expnded to be prt of more holistic pproch to process design. These processes typiclly include equliztion, ctivted sludge (both noxic nd erobic), secondry clrifiction nd possibly tertiry polishing. The optiml temperture rnge in biologicl oxidtion is nrro thus requiring tighter control by designers. By considering upstrem nd recycle processes in the entire steter tretment system, more ccurte estimtion of steter tempertures cn be determined. This llos designers to consider cooling nd/or heting options erlier in the evlution process to ensure costs for ll equipment re included. The temperture of industril steter strems cn be highly vrible depending on the mnufcturing process nd site loction. This pper includes cse study of conventionl ctivted sludge plnt s ell s chemicl fcility ith complex nitrifiction / denitrifiction system ith both the predictive nd ctul temperture dt nlysis. In ddition, there ill be discussion of both heting nd cooling requirements t to other fcilities in the Chemicl Process Industries. The types of cooling equipment vilble for use to the CPI industry ill lso be provided ith their dvntges nd disdvntges. Keyords: Modeling, biologicl tretment processes, steter temperture, steter cooling, steter heting 120

2 INTRODUCTION Biologicl Tretment Systems re sensitive to temperture s is every other living entity. Conventionl mesophilic bcteri hs been shon to perform most optimlly hen the rector steter temperture is mintined beteen 78 nd 95 F (26 nd 35 C). Nitrifying bcteri hve n even tighter rnge of optiml rector temperture beteen 85 nd 92 F (29 nd 33 C). Mny industril processes tht generte steter from such unit opertions s distilltion bottoms, stripping, tnk clenouts, quenching cn produce elevted tempertures tht exceed the idel environment for robust biologicl tretment. These rmer steters cn cuse the biomss to operte t much loer efficiency ultimtely loering the effluent dischrge qulity from the fcility. Elevted tempertures lso effect oxygen trnsfer in the rector further ffecting the performnce of the steter tretment plnt. During development of process design prmeters nd preliminry engineering of mechnicl systems for ne or upgrded steter tretment fcilities, n engineer should evlute influent temperture vritions, tmospheric conditions, nd mechnicl inputs tht my effect the biologicl system. This evlution should be completed using dt from different sesons of the yer tking into ccount locl ether dt, the storge volume of the tnks or bsins, mterils of construction for these systems, s ell s the energy (het) input to the overll process through ertion nd/or mixing. The Evolution of Modeling Industril steter tretment is still reltively young science. Eckenfelder (1966) published Industril Wter Pollution Control, one of the first texts tht explored methods of reducing industril ter pollution by treting the process steter prior to dischrge. The Clen Wter Act, encted in 1972, required industry to comply ith ne federl stndrds. Biologiclbsed tretment plnts ere chosen by mny industries ith strong orgnic ste strems becuse they ere, nd still re, economiclly ttrctive nd ese of opertion. Reserchers nd engineers first begn looking t the effect temperture hd on erted lgoons nd ctivted sludge systems becuse of economic issues. Systems tended to be over designed due to conservtive estimtes nd sfety fctors. Dvis, Shih, nd Sebest (1973) ttempted to predict tempertures in ertion bsins ith mechnicl ertors. Novotny nd Krenkel (1974) mde n erly ttempt t riting het blnce for erted bsins. Argmn nd Adms, (1977) developed nd tested comprehensive eqution for erted biologicl systems built on this erlier ork nd combined other fctors to crete non-itertive model for predicting erted bsin tempertures. 121

3 Argmn & Adms Model The Model is bsed on this simple premise: T = Ti + Eqution 1 ρ c Q Where, T = Wter temperture in completely-mixed ertion bsin, C T i = Influent ste temperture, C Σ = Summtion of net het gin by ter in the ertion bsin, Cl/dy ρ = Wter density, Kg/m 3 c p = Wter specific het cpcity, Cl/Kg- C Q = Flo rte, m 3 /dy When defining Σ in n ertion bsin, one hs to consider eight energy components: Solr Mechnicl Biologicl Long ve rdition Evportion Air conduction Aertion Wll Conduction Ech of these sources of het cn be defined mthemticlly: Solr et p 2 s = s o( C c ) A, Eqution 2 s,o = Averge dily bsorbed solr rdition rte under cler sky conditions, Cl/m 2 -dy C c = Averge cloud cover, tenths A = Surfce Are of the bsin, m 2 s,o is function of the ltitude here the site is geogrphiclly locted nd the position of the sun on given dy. It cn be clculted by knoing ht dy of the yer (1 to 365) it is nd ht the ltitudinl position is. C c must be interpreted from ether dt vilble on the CD-ROM: Interntionl Sttion Meteorologicl Climte Summry under the Sky Cover. There re designtions of: 122

4 Cler, 0/10 sky cover Scttered, 1/10 to 5/10 sky cover Broken, 6/10 to 9/10 sky cover Overcst/Obscured, 10/10 sky cover These designtions re used nd reported by the Ntionl Wether Service in this y. To figure the verge cloud cover, it is best to use eighted verge of the number of dys during ech month correspond to ech designtion. For uniformity, e use: Cler = 0/10 Scttered = 3/10 Broken = 7.5/10 Overcst/Obscured = 10/10 Mechnicl et 6 m = P Eqution x 10 6 is the conversion fctor to convert horsepoer to clories per dy P = Aertion poer, hp Biologicl et 6 b = S r Eqution 4 S r = Orgnic Removl Rte This eqution is bsed on the folloing ssumptions: Free energy of oxidtion of orgnics is 3,300 Cl/g COD oxidized Free energy of conversion of substrte to pyruvte is 100 Cl/g COD Free energy of conversion of pyruvte to cellulr mtter is +930 Cl/g COD Net cellulr yield is 0.25 g VSS/g COD removed COD of cellulr mtter is 1.42 g COD/g VSS Using these vlues the net energy relese from biodegrdtion of orgnics is estimted to be beteen 2,700 nd 3,000 cl/g COD in helthy system. oever, this vlue cn vry. According to Argmn & Adms model, the b term is very sensitive to the net cellulr yield. A highly endogenous system (high sludge ge) ill convert more of the vilble energy to het. Under the ssumed free energy vlues, the net energy relese my rnge from 370 to 3,300 Cl/g COD depending on the net VSS production. 123

5 Rdition et L = [ (1 β ) ( T T ) (1 β ) T ] A Eqution 5 β = Atmospheric rdition fctor, dimensionless (rnge from 0.75 to 0.95) T = Ambient ir temperture, C Evportion et 6 f T 0.96 e = [ (1 ) ( T T )] e V A Eqution f = Reltive humidity, percent V = Wind Velocity t tree top, m/sec This eqution cme first from Novotny nd Krenkel (1973). The controlling fctors for this element of the het budget is surfce re nd ind speed. There re fctors to be considered hen modeling ind speed. Are there buildings surrounding the tnks? Is the ind dt locl? Does the ind generlly come from one direction? Is there significnt freebord on the tnk/bsin? Air Conduction et c = ( T T ) V A Eqution 7 This is het tht is removed due to interfce ith the ir nd ter. Aertion et = NFV [300( T T ) e T N = Number of ertors F = Aertor spry verticl cross-sectionl re, m 2 F (1 ) + 175e T When surfce ertion occurs, there is cooling cused by both evportion nd conduction of the spry ter. Cross-sectionl spry ptterns cn be obtined from the ertion equipment vendor for different horsepoer spryers. Wll Conduction et ( T T )] Eqution 8 = A U ( T T ) Eqution 9 124

6 A = Effective ll re, m 2 U = et trnsfer coefficient, Cl/m 2 -dy- C The effective ll re is ll tnk surfces in contct ith ter, both lls nd bottom. The vlue for U cn be found in text books. Generlly e use 620,000 for steel nd 20,000 for concrete (i.e. concrete retins more het thn steel surfce). The Eqution These equtions shon bove ere combined nd mnipulted by Argmn nd Adms to rrive t the folloing eqution: 6 Q T f NFV ( Ti T ) + (1 10 ( C c ) s, o) ( β 1) ( β 1) T e (1 )(1.145A + ) + T T A A A = Q T NFV T UA (0.0686e ) A V + ( e ) + A A A APPLYING TE ARGAMAN & ADAMS MODEL 1.8S + A The model s tested using on ctivted sludge systems ith surfce ertors nd in-ground lgoons ith surfce ertion nd diffused ertion. The model s firly ccurte in predicting the tempertures using ctul plnt dt. To be n effective design tool, prior to plnt dt being vilble, the model hd to be dpted to predict theoreticl bsin tempertures. Much thought hd be given to the nticipted flo nd substrte loding, s ell s, ccurte ether dt, tnk sizing informtion, nd ertion requirements. The theoreticl model is lso used to predict orst cse summer nd inter tempertures nd ssist in determining the mount of het tht must be tken y (or dded). The temperture model cn be used to evlute the different tretment technologies or cooling options. During the design phse, the steter process engineer should be given the process effluent temperture s it leves the process or primry tretment re nd enters the bttery limits of the steter tretment plnt. Since the ertion bsin is only one prt of the entire tretment trin, the entire system must be modeled from the equliztion bsin to the clrifier from here the recycled ctivted sludge returns. As technology hs chnged nd more is knon bout biologicl processes, the vribles to this model hve incresed. Mny industril steter tretment plnts moved their tretment systems bove ground in the 80 s nd 90 s. Additionlly, the dvnces nd use of high rte biologicl systems incresed the het input for the rectors. These tnks re bigger nd deeper ith diffused ertion nd re often situted in the middle of the production fcility here nturlly occurring cooling is diminished by surrounding structures. In ddition, more fcilities re using the nitrogen cycle to crete nitrifiction/denitrifiction systems ith even tighter temperture control being required. m r 125

7 In order to discuss these more modern fcilities, to cse studies re shon belo. CASE STUDIES (ith permission) CAMUS Wter Technologies Muskegon, MI To illustrte fcility ith conventionl ctivted sludge, e use plnt operted by ENVIRON knon s CAMUS Wter Technologies (CAMUS), locted in Muskegon, Michign. This plnt verges just over 1 MGD (3,980 m 3 /d) nd hs men COD loding of bout 23,700 pounds per dy (10,800 kg/d). The ertion bsin is 100 ft in dimeter (30.5 m) nd hs side ll depth of 33 feet (10 m). Prior to the ertion bsin there is n equliztion tnk nd primry clrifier; secondry clrifier follos ertion. For clrity, e modeled the 15 th dy of every month. Belo is Figure 1 shoing the ctul vs. predicted tempertures of the CAMUS ertion bsin using recorded plnt dt. Figure 1 Actul vs. Predicted Aertion Bsin s (F) /14/2005 1/13/2006 2/12/2006 3/14/2006 4/13/2006 5/13/2006 6/12/2006 7/12/2006 8/11/2006 9/10/ /10/ /9/ /9/2006 1/8/2007 Dte Aertion Bsin Actul 2006 Aertion Bsin Modeled 2006 The Argmn & Adms model did very ell predicting the ctul ertion bsin tempertures. There re to vribles tht cn be djusted to obtin the best fit. One is the ind fctor. Wind dt is usully given t tree top level. If the ertion bsin is locted in the middle of the production fcility, the ind ptterns cn be significntly different. The other fctor is biochemicl rection. ighly endogenous systems tend to crete more het ( Argmn/Adms 1977). Once the model s clibrted to ccount for those fctors, e used historicl dt nd plnt verges to compre the theoreticl ith the ctul plnt dt. 126

8 Figure 2 Actul vs. Theoreticl Aertion Bsin s (F) /14/2005 1/13/2006 2/12/2006 3/14/2006 4/13/2006 5/13/2006 6/12/2006 7/12/2006 8/11/2006 9/10/ /10/ /9/ /9/2006 1/8/2007 Dte Aertion Bsin Actul 2006 Aertion Bsin Theoreticl (istoricl Averges) The theoreticl temperture model s bsed on 40 yers of meteorologicl dt nd more ccurtely portrys the likely bsin tempertures for ny given yer. The tble belo shos the differences beteen the dily dry bulb tempertures nd the corresponding differences in bsin tempertures. Tble 1 istoricl nd 2006 Reported Dry Bulb nd Bsin s Dte Actul Dry Bulb (1) istoricl Averge (2) Difference in Actul Aertion Bsin Theoreticl Aertion Bsin Bsed on istoricl Averges Difference in Aertion Bsin 01/15/ /15/ /15/ /15/ /15/ /15/ /15/ /15/ /15/ /15/ /15/ /15/ (1) Actul Wether Dt from 1 block from Muskegon Stte Prk, North Muskegon, MI Wether Monitoring Sttion KMINORT2 (2) istoricl Wether Dt From Interntionl Sttion Meteorologicl Climte Survey: NOAA; Grnd Rpids WSO 127

9 SOLUTIA INC Dectur, AL Soluti Inc opertes chemicl mnufcturing fcility in Dectur, Albm. In 2004, the steter s treted using nitrifiction / denitrifiction plnt. The min tretment trin included equliztion, nitrifiction (erobic), denitrifiction (noxic), nd secondry clrifiction. All of the fcilities re in ground bsins. Oxygen to the ertion bsin s supplied using corse bubble diffusers. In order to ensure dequte nitrte removl the recycle beteen the ertion nd noxic bsins is bout 5 times the forrd flo. The plnt treted bout 2 MGD (7640 m 3 /d) ith n verge COD loding of 26,650 pounds per dy (12,115 kg/d) nd TKN loding of 4,650 pounds per dy ( 2,115 kg/d). To control temperture in the inter time, the opertors inject stem to rm the feed to the denitrifiction bsin. The model is ble to ccount for this by clculting the BTUs (KJ) rejected in the system nd replcing them ith stem. There ere some ssumptions mde during the modeling: The stem is superheted The control volume is the pipe here the stem is injected The volume is constnt The stem is completely condensed upon entering the ter phse. Figure 3 Actul vs. Predicted Nitrifiction Bsin s TEMPERATURE (F) /25/2003 1/14/2004 3/4/2004 4/23/2004 6/12/2004 8/1/2004 9/20/ /9/ /29/2004 2/17/2005 DATE Nitrifiction Bsin Actul Nitrifiction Bsin Modeled After clibrting the model to ccount for ind differences, stem ddition, nd biologicl differences, the historicl model s pplied. 128

10 Figure 4 Actul vs. Theoreticl Aertion Bsin s TEMPERATURE (F) /25/2003 1/14/2004 3/4/2004 4/23/2004 6/12/2004 8/1/2004 9/20/ /9/ /29/2004 2/17/2005 DATE Nitrifiction Bsin Actul Nitrifiction Bsin Theoreticl The dily temperture dt nd the historicl verge temperture dt ere not very prt for most months. The nitrifiction bsin tempertures generted by the theoreticl model using verges for flo, COD nd TKN lodings bsed on the 2004 dt do not devite significntly from the ctul dt. Dte Actul Dry Bulb (1) istoricl Averge (2) Difference in Actul Nitrifiction Bsin Theoreticl Nitrifiction Bsin Bsed on istoricl Averges Difference in Aertion Bsin 01/15/ /15/ /15/ /15/ /15/ /15/ /15/ /15/ /15/ /15/ /15/ /15/ (1) Actul Wether Dt from Soluti Dectur DCS Record of Dry bulb temperture (2) istoricl Wether Dt From Interntionl Sttion Meteorologicl Climte Survey: NOAA; untsville, WSO 129

11 MODEL DISCUSSION It my seem from these cse histories tht temperture modeling cn be stright forrd nd simple. This is not lys the cse. There re lys vribles tht re unknon nd biologicl systems do not lys behve identiclly from one dy to the next. The designer should be re of these possible donflls nd evlute ny results generted from the model ith cre nd objectivity. WASTEWATER COOLING If the temperture modeling evlution discussed bove shos tht cooling is required, the steter engineer must evlute severl options nd individul fctors for ech sitution. Mny industril steter strems re mde up of vrious process steter strems t different tempertures (e.g. boiler blo don, snitry shing of tnks, quench ter, etc), nd thorough evlution of cooling either the combined influent strem or source strem(s) should be performed. A fe of the prmeters tht cn be evluted include: Desired temperture reduction during both inter nd summer sesonl vritions t the site (24 hour bsis); Mss, temperture, nd flo blnces of individul strems; Solids, both totl nd dissolved, in the steter strem(s); Source, quntity, nd temperture of cooling ter; Mterils of construction for equipment bsed on influent steter prmeters; eight restrictions or re constrints; Avilbility of mintennce orkforce nd ese of mintennce; Unit spres, both instlled nd uninstlled; Poer nd chemicl costs; nd, Other site fctors such s environmentl permits for ir dischrges, loction of cooling equipment reltive to other plnt process units, locl residences nd prevlent ind direction, etc. Wsteter engineers typiclly use het exchngers, cooling toers, surfce ertors, spry coolers, nd closed circuit coolers hen removing het is necessry for the biologicl process. Ech piece of equipment hs its dvntges nd disdvntges nd the economics nd site circumstnces must be evluted for ech system. Of primry concern in mny industries tody is the bility to minimize stripping VOC s from the steter to the tmosphere. For this reson, spry coolers nd cooling toers, though mny times cceptble, re considered less desirble. ENVIRON hs successfully used closed circuit coolers nd non-contct het exchngers for mny of our industril clients. When evluting het exchngers, the concentrtion of solids, both totl nd dissolved re importnt. Typicl het exchngers such s plte nd frme nd shell nd tube cn be significnt mintennce problem ith steter strems ith high concentrtions of solids. Spirl het exchngers, though more expensive, hve been used successfully for severl pplictions due to their bility to process more solids ithout plugging. The source, mount, nd temperture of vilble cooling ter must lso be evluted ith het exchngers. The 130

12 mount of cooling cn vry depending on the source of ter (e.g. ground ter, chilled ter, cooling toer ter, locl et bulb temperture, etc), nd cn be evluted ith norml engineering equtions nd evlutions. A closed circuit type cooling toer (CCCT) is n option tht hs been used in severl pplictions for industril fcilities, including refineries, coke-mking plnts, nd methnol production fcility. The dvntge of closed circuit cooling toers is tht the steter strem does not come into contct ith the recirculting cooling ter tht sprys over tube bundles. There cn be mintennce issues due to the build up of slts during evportive cooling. This requires both blo don nd mke up cooling ter strem. The mount of cooling is bsed on the et bulb temperture hich is function of the reltive humidity nd mbient temperture. A CCCT cn typiclly reduce the steter strem temperture to ithin 6 8 o F (pproch) of the locl et bulb temperture. WASTEWATER EATING The modeling of biologicl systems sometimes shos tht heting cn be required in colder climtes nd during periods of mbient ether conditions in the trnsition zone tht my cuse biologicl tretment process to slo don nd not perform efficiently. Lo tempertures cn hve significnt ffect on specific groth rtes for biologicl orgnisms. ENVIRON hs evluted mny situtions here the ddition of het my be required to mintin the desired temperture. For these situtions, direct stem ddition into equliztion tnks nd influent steter strems hs been used to ugment the biologicl systems nd other fctors of het sources. Equipment such s het exchngers hve been used for mny yers to rise strem tempertures. Mny of the sme fctors discussed bove in the cooling scenrio re lso pplicble hen heting is required. Sometimes the model shos tht simple conservtion of het in biologicl rector either ith covers on tnks nd the use of certin mterils nd insultion on lls is sufficient. Similr to cooling requirements, n engineer should evlute severl fctors hen evluting the heting. CONCLUSION This pper is n overvie of the temperture modeling process ith some rel orld exmples to vlidte the ccurcy of the Argmn & Adms model even ith complex systems. In summry, biologicl steter tretment systems re like ny other living entity nd require proper nutrients, oxygen, p, nd temperture. Tretment efficiency nd plnt upsets cn occur if the biomss gets too rm or too cold. modeling during the design phse of ne steter tretment plnt or upgrde is essentil to determine if cooling or heting is needed. If therml controls re required, there re severl ys to chieve the proper temperture nd cre should be tken to select the best technology for the service. 131

13 REFERENCES Eckenfelder, W.W., Industril Wter Pollution Control, McGr ill, Dvis, D.L. Shih, C.S. nd Sebest, E.C., Prediction in Activted Sludge Bsins Using Mechnicl Aertors, Presented t the 27 th Annul Purdue Industril Wste Conference, Novotny, V. nd Krenkel, P., "Evportion nd et Blnce in Aerted Bsins," Americn Institute of Chemicl Engineers, Wter 73, lso 74th Ntionl Meeting, Argmn, Y nd Adms, Jr., C.E., Comprehensive Model For Aerted Biologicl Systems, Progressive Wter Technologies, Vol 9, Pergmon Press,