NITRIFICATION IN POWDERED-ACTIVATED CARBON-ACTIVATED SLUDGE PROCESS By Adam S. Ng' and Michael K. Stenstrom, 2 Member, ASCE

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1 NITRIFICATION IN PODRD-ACTIVATD CARBON-ACTIVATD SLUDG PROCSS By Adm S. Ng' nd Michel K. Stenstrom, Member, ASC ABSTRACT: Powdered ctivted crbon (PAC) hs been dded to ctivted sludge processes over the pst 10 yers to improve process performnce in vriety of wys, including mmoni removl. Improved mmoni removl is surprising benefit of PAC since it is not dsorbed. Investigtors hve speculted tht PAC dsorbs inhibitory compounds or provides medi for nitrifier growth. To scertin the mechnism of nitrifiction enhncement, series of experiments were performed with dsorbble (niline, phenol) nd nondsorbble (ethnol) inhibitors. xperimentl results show tht dsorption of nitrifiction inhibitors cn drmticlly improve nitrifiction rtes in uncclimted ctivted sludge cultures. INTRODUCTION Previous investigtors (,13,15,3,30) hve provided evidence tht the ddition of powdered-ctivted crbon (PAC) to nitrifying ctivted sludge (AS) cn improve nitrifiction rtes. Plusible, but unsubstntited, mechnisms tht hve been proposed to explin PAC-enhnced nitrifiction include dsorption of compounds toxic to nitrifiers (13,15,3), enhnced nitrifier growth on the crbon's surfce (,30), nd bioregenertion. This reserch ws conducted to substntite mechnisms ofpac enhncement, nd experiments were conducted to evlute dsorption, ttched growth nd cclimtion mechnisms. Bioregenertion Bioregenertion is term used to describe the synergism which is often observed in powdered-ctivted crbon-ctivted sludge (PAC-AS) processes. Vrious reserchers (4-6,11) hve noted tht in specific cses, PAC-AS processes cn remove n orgnic compound more efficiently thn would be expected from either biodegrdtion or dsorption lone. Proposed theories of bioregenertion require tht compound be dsorbed onto the crbon's surfce where microorgnisms reside, nd tht dsorption results in higher substrte concentrtions thn would be expected in the bulk solution. This increse in concentrtion stimultes biologicl growth nd replenishes the crbon surfce for further dsorption. Once dsorbed, these compounds re in contct with the biomss for length of time equl to the system's cell retention time. This mechnism's pplicbility to nitrifiction is questionble, since dsorption of mmoni t the ph nd concentrtions found in wstewter is negligible ; however, bioregenertion my be importnt in the cclimtion of heterotrophic orgnisms to certin slowly biodegrdble inhibitors of nitrifiction. Adsorption of 'Res. ngr., Argonne Nt. Lb., Argonne, IL. 'Prof., Dept. of Civ. ngrg., Univ. of Cliforni, Los Angeles, CA Note. Discussion open until My 1, To extend the closing dte one month, written request must be filed with the ASC Mnger of Journls. The mnuscript for this pper ws submitted for review nd possible publiction on September 15, This pper is prt of the Journl of nvironmentl ngineering, Vol. 113, No. 6, December, ASC, ISSN /87/ /$ Pper No

2 the inhibitor would increse the time vilble for heterotrophic cclimtion nd, ultimtely, for the removl of nitrifier inhibition. nhnced Nitrifier Surfce Growth Up until 1955, it ws generlly considered tht prticulte mterils such s clcium crbonte, which ws used s buffer in erly culture experiments, were necessry for nitrifier growth. It ws postulted tht the solid surfce in the growth medi provided obligte surfce sites to which nitrifying orgnisms dsorbed nd multiplied. Although this "obligte surfce" theory ws lter disproved by workers who successfully grew Nitrobcter (1) nd Nitrosomons (9) in cler medi, the role of suspended solids in nitrifiction remins uncler. A stimultory effect of suspended prticles on nitrifiction rtes hs been reported by number of investigtors in studies with surfce wters nd soils. These investigtors mintin tht the enhncement mechnism involves either the prticle's bility to provide physicl support medium for growth of nitrifiers (19,9) or substrte (mmonium ion) concentrtion t the surfce of the prticles (). Conflicting evidence hs been reported in which no detectble effects on nitrifiction rtes due to suspended prticles were observed (1). Other investigtors (1,19) ssert tht while suspended solids were not essentil for nitrifier growth, ttchment to prticles nd enhncement occurred if the solids were present. As prt of this experimentl investigtion, rector ws operted using bentonite cly s site for nitrifier ttchment. Bentonite cly ws dded to n ctivted sludge rector, identicl to the rectors used in this study, to mtch the externl surfce re provided by the crbon in PAC rectors. xperiments were performed for severl compounds, both dsorble nd nondsorbble over two-yer period, testing both cute nd chronic inhibition. The results re reported in detil by Ng, et l. (4) nd show tht dding bentonite cly ws not beneficil to nitrifiction. Adsorption of Compounds Toxic to Nitrifiers Mny orgnic compounds re known to inhibit nitrifiction in pure (31,35,37) nd ctivted sludge cultures (30,34,36). Some of these compounds re biologiclly resistnt to degrdtion while others re resistnt nd highly dsorbble by ctivted crbon. Therefore, it is likely tht the presence of ctivted crbon cn protect nitrifiers from dsorbble, inhibitory compounds. In summry, vrious mechnisms cn be suggested to explin the role of dsorption in PAC-enhnced nitrifiction. The mjor purpose of this study is to further define the role of dsorption in PAC-enhnced nitrifiction, nd to ccess these effects independently of biologicl cclimtion. mphsis is plced on the dsorption of toxics mechnisms becuse recent work in one lbortory (4) suggest tht this is dominnt mechnism in PAC-enhnced nitrifiction. XPRIMNTAL MTHODS A series of btch ssy experiments using vrible ctivted crbon doses ws performed to study PAC-AS nitrifiction enhncement in the presence of known nitrifiction inhibitors with different dsorptive chrcteristics. Selection of inhibitors for experimenttion ws bsed upon n 186 TABL 1. Adsorption nd Nitrifiction Inhibitory Chrcteristics of Compounds Tested in Crbon Dose xperiments Compound (1) Freundlich Prmeters () Aniline K = 1. mg/g Q = mg/l n-' = 0.5 b = 0.34 Phenol K = 1.0 mg/g Q = mg/l n- ' = 0.54 b = Cynide dt unvilble mximum dsorption 0.65 mg/l =.0 mg/g t Co = 0.0 mg/l thyl Alcohol K = 0 mg/g mximum dsorption,500 mg/l = 0 mg/g t Co = 1,000 mg/l Freundlich dsorption dt for phenol nd ethyl lcohol from Dobbs nd Cohen (7). Other dsorption dt for ethyl lcohol from Perrich (7). Cynide dsorption dt from Hoffmn (15). Other inhibition dt from Tomlinson, Boon, nd Trotmn (35). intensive survey of ll known nitrifiction-inhibiting compounds nd their crbon dsorption properties. Potentil compounds were chrcterized either s dsorbble-inhibitory (AI) or nondsorbble inhibitory (NAI). Other considertions for compound selection included industril significnce nd solubility. Tble 1 shows the compounds selected for experimenttion. Included re the compound's nitrifiction inhibition nd dsorptive chrcteristics. ith the exception of the dsorption prmeters for niline, which were experimentlly determined, dt from Tble 1 were extrcted from the literture nd hence re subject to interprettion due to differing sets of conditions under which the vlues were obtined. Nonetheless, the dt do provide n indiction of the reltive dsorptive nd inhibitory properties of the compounds chosen for evlution. For Al compounds, concentrtions were selected to produce significntly more thn 75% inhibition. For NAI compounds, inhibitory concentrtions tested were chosen to yield pproximtely 75% inhibition in control ssys without PAC ddition. This ws to ensure tht nitrifiction would continue in ny given ssy nd tht ny benefit, due to PAC ddition, would be detected. Source of Nitrifying Activted Sludge Nitrifying mixed liquor for experimenttion ws drwn from continuous flow bench scle ctivted sludge plnt fed synthetic substrte nd operting t men cell retention time of nine dys nd hydrulic retention time of eight hours. The rector ws constructed of 1.5 cm (0.5 in.) plexiglss, with working volume of 1. L in the ertion section nd 1.5 L in the solid-liquid seprtor. Severl holes were provided in the lid of the rector : lrger holes for ph probe nd ccess for mintennce nd smller holes for influent, bse ddition, nd ir lines. A port hole on the 187 Lngmuir Prmeters (3) Concentrtion Required for 75% Inhibition (4)

3 side of the ertion section ws used to withdrw mixed liquor for experimenttion nd for control of cell retention time. Air, which ws dded through diffuser stones locted ner the bottom of the mixed liquor ertion section, provided oxygen for microbil growth s well s turbulence for mixing. Airflow rtes, which rnged from 0.14 to 0.8 m3/hr., were monitored by rotmeters. Rector ph ws mintined, using sturted solution of sodium bicrbonte, t rnge of 7-7. by mens of ph control meter (Horizons, Inc., Model ). Synthetic Feed Composition Due to the lrge quntity of substrte required, dilution system ws used whereby concentrted feed ws utomticlly diluted before being pumped into the rector. The liquid level in the mixing reservoir ws electroniclly sensed by two flot switches which controlled both the concentrte feed pumps to the reservoir nd n externl solenoid vlve for the flow of dilution tp wter. The diluted substrte ws pumped directly from the mixing reservoir, contined in refrigertor t 4 C, into the rector using seprte pump system. The feed ws composed of glucose, mmoni, nd other nutrients required to support the growth of heterotrophs nd nitrifiers. The CC1-MgC1 solution ws seprtely pumped into the mixing reservoir to prevent the formtion of clcium phosphte precipittes. The concentrte ws diluted pproximtely 50 times during ech cycle. The totl stedy stte influent mmoni-n concentrtion ws clculted nd mesured to be 50 mg/l. Additionl detils describing the rector, substrte dilution system, nd synthetic feed re vilble elsewhere (4,5). XPRIMNTAL PROCDUR Btch Inhibition xperiments The generl procedure for ll experiments consisted of the following steps : 1. PAC (estvco Nuchr SA-15) ws dried t 150 C for minimum of three hours nd stored in dessictor until use.. Approprite mounts of PAC were nlyticlly weighed nd plced dry into designted empty 500 ml rylenmeyer flsks. 3. Rector effluent mmoni-n nd nitrte-n concentrtions were mesured. 4. A mesured volume of mixed liquor ws withdrwn from the rector nd divided eqully into the flsks, resulting in ctivted crbon dosges of 500, 1,000,,000, nd 4,000 mg PAC/L of mixed liquor dded. For NAI compounds evluted, only PAC concentrtions brcketing the highest (4,000 mg/l) nd lowest (500 mg/l) concentrtions were tested. In ll experiments, minimum of two flsks were retined s controls nd did not receive ny PAC. 5. An exogenous source of mmoni-n in the form of solution of mmonium chloride ws pipetted into ech flsk to bring the NH4+-N concentrtion from less thn <0.1 to mg-n/l ml of the test compound from concentrted stock solution ws pipetted into the test flsks to give the desired clculted concentrtion. It 188 should be noted tht the chnges in volumes brought bout by the dditions of PAC, mmoni-n, nd the inhibitory compound were incorported into the clcultion of the finl concentrtions used. 7. All flsks were plced under mnifold nd erted throughout the experiment. Air ws supplied through disposble, plstic qurium diffuser stones t flowrte of 0.1 m 3/hr. 8 One or two minutes fter the strt of ertion (designted s time = 0 hours), two seprte 5-10-ml portions of the mixed liquor were withdrwn from ech flsk using volumetric pipettes nd then dded into 100-ml volumetric flsk, hlf filled with distilled wter preserved. The flsks were then diluted to volume, cpped, shken, nd stored for the nlysis of mmoni nd nitrte t the end of the experiment. 9. Step 8 ws repeted for time =, 4, 6, nd 8 hours. 10. very hour throughout the ertion period, the ph ws checked nd mnully djusted, if necessry, to the rnge of using 0.1 N NOH. In the finl experiment (with 10 mg/l niline), nitrite-n, nd liquid phse inhibitor concentrtion were determined t vrious times during the ertion period. Aniline Adsorption Isotherm An niline dsorption isotherm ws conducted, becuse niline ws considered to be n "idel" inhibitor to study PAC-enhnced nitrifiction. The effects of niline on ctivted sludge nitrifiction re well documented (18,34), nd it is known to be both dsorbble nd biodegrdble, lthough the extent of its dsorptive properties hve not been well estblished. Aniline t 100 mg/l ws contcted with PAC over concentrtion rnge of g/l. Smple bottles were gitted for nine hours t room temperture (7-9 C). Smples were nlyzed by gs liquid chromtogrphy following centrifugtion. ANALYTICAL MTHODS Inorgnic Nitrogen Specific ion electrodes for mmoni (Orion Model 95-10) nd nitrte (Orion Model 93-07) in conjunction with n Orion Ionnlyzer (Model 407A) were used to directly mesure mmoni-n nd nitrte-n concentrtions in influent, mixed liquor, effluent, nd ssy smples. The probes were clibrted t lest once, using lbortory prepred stndrds, prior to nd during ech nlyticl run. If necessry, mmoni nd nitrte smples were preserved with IM HCl (0.1 ml/0.i L smple) or IM boric cid (0.1 ml/0.1 L), respectively. Smple volume nlyzed ws 100 ml or liquots diluted to 100 ml. Nitrite-N ws determined by wet chemicl technique described in Stndrd Methods (1975). All nitrite smples were preserved with 4 mg HgC1 /0.1 L smple, filtered (0.45 micron), nd diluted with distilled wter to cover the pplicble rnge of the method ( mg nitrite-n/l). Photometric determintions were ccomplished using Busch nd Lomb Spectronic 0 (1 cm light pth) t 543 nm. Stndrd curves were obtined for ech nlyticl run using serilly diluted nitrite stndrds (NNO1. 189

4 Determintion of Aniline Liquid phse niline concentrtions for the dsorption isotherm nd single btch experiment were mesured by direct queous injection using Vrin 6000 gs chromtogrph nd Hewlett Pckrd 3300 integrtor. The cpillry column used ws SP 100 (Supelco, Inc.) operted isothermlly t 110 C. Optiml opertionl conditions for the nlysis were found to be s follows : Injector temperture = 00 C ; Detector (FID) temperture = 300 C ; Crrier gs = Helium t.0 ml/min ; Detector mke-up gs = 30 ml/min ; Air nd Hydrogen t 300 nd 30 ml/min, respectively ; Spitless injection t 1.0 microliter smple size with purge vent opened fter 0.9 min. t 100 ml helium/min. Reproducibility for given injection ws found to be within 5% for pek re response nd 7% for pek height response. All determintions were bsed on pek re response. A preliminry test ws conducted to determine whether niline could be recovered from the liquid phse of mixed liquor nd ccurtely nlyzed for by gs chromtogrphy. The nture of the substrtes used for rector feed suggested tht there would be little interference in the nlysis of niline by direct queous injection. The procedure used for the test ws s follows : L of mixed liquor (MLSS = 150 mg/l) ws withdrwn from the rector nd divided eqully into three stirred flsks.. An pproprite mount of n niline stock solution ws quntittively dded into ech flsk producing clculted niline concentrtions of 1.0, 5.0, nd 10.0 mg/l. 3. After mixing the contents of ech flsk for two minutes, 10 ml of mixed liquor ws withdrwn from ech flsk nd centrifuged for three minutes or until cler superntnt ws visible. The resulting superntnt ws then pipetted into 7.7 ml glss vils with Teflon lined cps nd stored t 4 C until nlysis. These steps were necessry to preserve the stbility of niline nd were stisfctory for subsequent nlysis by gs chromtogrphy (8). Recovery of niline from controls verged 89.8% of theoreticl recovery. Recovery rnged from low of 86% t 1.0 mg/l to 96.4% t 10 mg/l. It is likely tht the difference between clculted nd mesured vlues were due to either inherent experimentl error nd/or dsorption of niline onto the biomss. The ltter explntion is resonble, since ll mesured vlues were less thn the corresponding clculted vlues. The lrgest difference (14%) noted ws substntilly less thn the reported 75% recovery of niline from wstewters using the sme column nd methylene chloride extrction step (8). RSULTS AND ANALYSIS Aniline Adsorption Isotherm The equilibrium dt for the dsorption of niline on PAC fitted both the Freundlich (r = 0.98) nd the Lngmuir (r = 0.97) dsorption model resonbly well. Results show tht pproximtely 1. mg niline/g PAC will be dsorbed t n equilibrium niline concentrtion of 1.0 mg/l. Clculted dsorption prmeters from the experimentl dt re shown in Tble z 0 f FIG. 1. NH 4 +-N versus Time for Vrible Crbon Dose (10 mg/l Aniline 1-85) F Q r TIM (HOURS) TIM (HOURS) FIG.. NO;N versus Time for Vrible Crbon Dose (10 mg/l Aniline) Btch Inhibition xperiments Ammoni oxidtion nd nitrte production dt generted from three seprte experimentl runs using 10 mg/l niline (Figs. 1-), 0.7 mg/l cynide (Figs. 3-4), nd 500 mg/l ethnol (Figs. 5-6) s inhibitors show the effect of PAC concentrtion on nitrifiction rtes. Dt presented in these figures re representtive of the nture of dt generted from other experimentl runs, which re not shown here. In order to interpret nd quntify observed nitrifiction rtes for ll experiments, zero-order kinetics were used to estimte nitrifiction rte constnts. Zero-order nitrifiction kinetics hve been observed nd used to describe nitrifiction under vrious conditions (10,17,0,5,34,36). Reported KS vlues for mmoni oxidtion in ctivted sludge typiclly rnge from mg-n/l (3,8,3) ; therefore, for mmoni concentrtions used in this study the kinetic expression for mmoni oxidtion in btch ssy cn be expressed s : d NH4 - N - -K (1) dt No 191 PAC 1000 m91lpac

5 Z Z O f TIM (HOURS) FIG. 3. NH 4';.N versus Time for Vrible Crbon Dose (0.7 mg/l Cynide) TIM" (HOURS) FIG. 5. NH;N versus Time for Vrible Crbon Dose (500 mg/l thnol) r x z TIM (HOURS) FIG. 4. NO;N versus Time for Vrible Crbon Dose (0.7 mg/l Cynide) Assuming tht mmoni oxidtion is rte limiting, the nitrte production rte cn be similrly expressed : The observed zero-order mmoni oxidtion nd ssocited nitrte production rte constnts K were clculted under nonlimiting substrte conditions by simple liner regression nd re presented for ll experimentl runs in Tble. Also included in Tble re coefficients of determintion 71 which indictes the ccurcy of fit between the dt nd the liner regression eqution. The term zero-order here pplies to rection rte with respect to mmoni. The rection constnt K is dependent upon nitrifier biomss concentrtion. In our erly delibertions bout experimentl design, we considered wys of estimting nitrifier biomss concentrtion. None of the vilble methods were sufficiently precise nd inexpensive to be useful to us. Therefore we devised n inhibitor constnt (4), which is the rtio of rection rtes obtined in experiments to the rection constnt obtined in controls, without inhibitor, but t the sme 19 FIG. 6. N03-N versus Time for Vrible Crbon Dose (500 mg/l thnol) biomss concentrtion. The inhibition constnts re independent of biomss concentrtion. In generl, nitrte production rte constnts were observed to rnge from % ofthe corresponding mmoni oxidtion rte constnts. The discrepncy in rte constnts ws most evident if mmoni ws oxidized reltively rpidly. This suggested n initil lg period for nitrite oxidtion since mesured nitrte concentrtions were ll within 14% of expected vlues (bsed on rection stoichiometry nd excluding heterotrophic mmoni uptke nd endogenous decy) t the end of ech experiment. If it hd been possible to use continuous experiments, one would expect more greement between mmoni oxidtion nd nitrte production rtes. Tble shows tht for experiments performed using dsorbble inhibitors (i.e., phenol nd niline), PAC ddition resulted in enhnced nitrifiction rtes. For nondsorbble inhibitors (i.e., ethnol nd cynide), little or no significnt nitrifiction enhncement ws observed. The results ofthe experiment with ethnol re in drmtic contrst to experiments using dsorbble inhibitors, where the degree of nitrifiction enhncement is relted to PAC dosge. 193

6 TABL. NH 3-N nd N03-N Rection Constnts nd r for Crbon Dose xperiments Spiked Compound Type (1) () (3) control NA1 (4) control Ad (5) 4,000 Ae (6),000 A' (7) 'AI = Adsorbble inhibitor. bnai = Nondsorbble inhibitor. `Control NA = Control with no ctivted crbon or inhibitor dded. dcontrol A = Control with inhibitor dded but no ctivted crbon. 14,000 A = Inhibitor nd 4,000 mg/l crbon dded.,000 A = Inhibitor nd,000 mg/l crbon dded. 8 1,000 A = Inhibitor nd 1,000 mg/l crbon dded A = Inhibitor nd 500 mg/l crbon dded. 'Crbon doses not evluted in these experimentl runs. 1,000 A9 (8) 500 A" (9) Aniline Al' r mg/l -K(NH3) (1-85) K(N0 3) Phenol AI r mg/l -K(NH3) (1-85) r K(N0 3) Cynide NAI r ! mg/l -K(NH3) (1-85) r K(N0 3) Cynide NAI r mg/l -K(NH3) (-85) rt K(N0 3 ) thnol NAI r mg/l -K(NH3) (-85) r K(N03 ) Aniline AI r mg/l -K(NH 3) (-85) r K(N03) z 0 FIG. 7. NH4N versus Time for Vrible Crbon Dose (10 mg/l Aniline -85) In the finl btch experiment with 10 mg/l niline (Figs. 7-10), liquid phse niline concentrtions were mesured by gs liquid chromtogrphy t specific smpling periods throughout the experiment. Results show tht nitrifiction inhibition is inversely relted to PAC concentrtion. Since the mount of niline dsorbed is directly relted to PAC concentrtion, it cn be shown tht nitrifiction inhibition is directly relted to liquid phse niline concentrtion. It ppers tht for ssys with no PAC ddition, pproximtely mg/l of niline ws either metbolized nd/or dsorbed onto the biologicl mss. Figure 10 lso indictes tht ner equilibrium conditions were rpidly estblished, with 80%-95% of the totl dsorption occurring within 30 min. Using the previously determined dsorption isotherm prmeters for niline, the expected liquid phse niline concentrtion t ech PAC dosge cn be determined nd compred to the corresponding mesured concentrtion. The following eqution, derived from the Freundlich dsorption model, ws used to clculte expected liquid phse niline concentrtions : KfG (3) In using ethnol, the lest dsorbble inhibitor of ll compounds evluted, there ws no significnt difference mong nitrifiction rtes in ll test ssys. Dissolved oxygen concentrtion ws mesured during nd fter the ethnol experiment. This ws done to insure tht oxygen limiting conditions were not creted through incresed heterotrophic ethnol oxidtion. All DO levels were mesured to be greter thn 5 mg/l. Dt (see Tble ) from the experiments involving cynide showed enhncement due to PAC ddition. The degree of enhncement ws significntly less thn those observed for dsorbble compounds, nd suggests tht the degree of nitrifiction enhncement is relted to dsorptivity of the inhibitor. 194 Z FIG. 8. NO3-N versus Time for Vrible Crbon Dose (10 mg/l Aniline -85) 195

7 TABL 3. Aniline Predictions J Z F F z PAC Dose xpected Liquid Mesured (mg/l) Phse Aniline (mg/l) Aniline (mg/l) (1) () (3) , , not detected 4, not detected J Z 10 J_ 6 TIM (HOURS) FIG. 9. NOz-N versus Time for Vrible Crbon Dose (10 mg/l Aniline -85) in which C; = initil niline concentrtion, 10 mg/l ; Cf= finl equilibrium liquid phse niline concentrtion, mg/l ; l/n = experimentlly determined Freundlich prmeter (0.5) ; Kf = 1. mg/g ; nd G = PAC dose, g/l. The eqution is implicit, requiring tril nd error to determine the expected liquid phse niline concentrtions. Results re shown in Tble 3 for n initil niline concentrtion of 10 mg/l. The detection limit for the gs chromtogrphy method ws 0.3 mg niline/l. The resons for the difference between expected nd mesured vlues re specultive. Different mixing conditions exist between ctivted sludge cultures nd isotherm experiments. quilibrium conditions my not hve been obtined in the 30 minute period used for this experiment. Also there my hve been competition for crbon sites. Mrtin nd lwugo (37) reported tht suspended solids, prticulrly biologicl solids, could interfere with the dsorption process, both in terms of cpcity nd rte. They found tht orgnic suspended solids t 500 mg/l or higher concentrtions interfered with the dsorption process for single solutes nd tht dsorption ws significntly reduced. By interpoltion of Fig. 10 (to be discussed) for the finl niline experiment on -85, it cn be estimted tht the PAC concentrtion t J Control, No PAC 500 nn/lpac 1000 e/lpac 000 A 4000 mg/lpac I I 0 ~ I 1 1 / I I TIM (HOURS) FIG. 10. Liquid Phse Aniline Concentrtion versus Time (-85) 196 which 75% nitrifiction occurred ws pproximtely 80 mg/l PAC. The corresponding liquid phse niline concentrtion for this PAC dose, bsed on the isotherm prmeters, is 7. mg/l. For comprtive purposes, Tomlinson (35) observed, under similr experimentl conditions, tht the concentrtion of niline cusing 75% nitrifiction inhibition in nitrifying ctivted sludge is 7.7 mg/l. Thus, the foregoing discussion suggests, t lest for niline, tht nitrifiction inhibition is cused by liquid phse inhibitor concentrtion, s opposed to totl inhibitor concentrtion in the PAC-AS process. To form quntittive bsis for compring the degree of nitrifiction enhncement mong different experiments, n inhibition coefficient (1) ws clculted to express the degree of inhibition observed t ech PAC dosge for ech inhibitor tested. I is defined s the rtio of clculted rection rte constnt K in the presence of the dded compound tok in the control (i.e., no dded compound). Clculted 1's for mmoni oxidtion rte constnts for ll experiments re presented in Tble 4. Inspection revels tht for nondsorbble compounds such s cynide, nitrifiction enhncement bove tht in the control ssys with no PAC dditions ws 16 nd 30%, respectively, for 0.7 nd 1.4 mg CN/L, t the highest PAC dose tested (4,000 mg/l). For ethnol, the lest dsorbble of the compounds tested, the degree of enhncement ws only 3% t the highest PAC dose tested. In contrst, nitrifiction enhncements of 75%, 97%, nd 94% over the corresponding controls with no PAC ddition were observed for niline, niline, nd phenol, respectively, t 4,000 mg/l PAC. These results provide strong evidence tht dsorption is the mjor mechnism of nitrifiction enhncement in ctivted sludges. Figure 11 shows inhibition constnts, bsed upon mmoni oxidtion rte, plotted ginst PAC dosge for dsorbble nd nondsorbble inhibitors. There is generl reltionship, dependent on inhibitor dsorptivity, between the degree of nitrifiction enhncement nd PAC concentrtion. For dsorbble inhibitors, t the initil concentrtions used, the reltionship cn be chrcterized by n S-type curve. This indictes tht nitrifiction is mrginlly enhnced t low PAC doses nd tht the degree of enhncement increses stedily with incresing PAC dose until plteu is reched. For nondsorbble inhibitors, the enhncement/pac reltionship is chrcterized by reltively horizontl curve showing little or no enhncement. Thus, it ppers tht enhncement depends upon the lowering of inhibitor concentrtion, through dsorption, to some threshold vlue before nitrifiction cn proceed t resonble rtes. Similr results were obtined for inhibition constnts bsed upon nitrte production. 197

8 TABL 4. Nitrifiction Inhibition Coefficients, I Bsed on Ammoni Rection Constnts for Crbon Dose xperiments Aniline 10 mg/l (-85) Aniline 10 mg/l (1-85) Phenol 0 mg/l (1-85) Cynide 1.4 mg/l (-85) Cynide 0.7 mg/l (1-85) thnol,500 mg/l (-85) U u U F Adsorption Control A 4,000 A,000 A 1,000 A 500 A Prmeters (No PAC) mg/l mg/l mg/l mg/l (4) (5) (6) (7) (8) 'AI = Adsorbble nitrifiction inhibitor. INAI = Nondsorbble nitrifiction inhibitor. `Adsorption prmeters re Freunlich prmeters K nd Un for niline, phenol, nd ethnol (bsed on Ce = 1.0 mg/l) ; for cynide, the dsorption prmeter represents the mximum dsorption observed t n initil concentrtion of 0 mg/l. dcrbon doses not evluted in these experiments. It is importnt to note tht in these experiments, uncclimted ctivted sludge nd virgin PAC were used. For dsorbble inhibitors, the beneficil effects of PAC were observed lmost immeditely fter the strt of the experiments (i.e., 1- hr). These observtions further support the dsorp- Phenol (0 nn,;/0 Aniline (10 n,i/l) Aniline (10 mg/l) Cynide (0.7 ng/l) thnol (500 mg/l) Cynide (1.4 ng/u ( - 85) 0.00 ) CARBON DOS (rng/l) FIG. 11. Inhibition Coefficient, I (NIi4N)versus Powdered Activted Crbon Dosge 198 d 0.0 d d 0.37 d d tion of inhibitory compounds for nitrifiction enhncement. Other theories cnnot ccount for the results since there ws insufficient time for the following : (1) nhnced nitrifier growth on the PAC surfce, () biologicl cclimtion ; or (3) concentrtion of trce nutrients on the virgin PAC. CONCLUSIONS Bsed on the results in this study, the following conclusions re presented: 1. The ddition of PAC in the proper mounts cn completely nullify the toxic effects of n dsorbble nitrifiction inhibitor. For dsorbble inhibitors, the ddition of PAC resulted in nitrifiction enhncements of 75%-97%. For reltively nondsorbble inhibitors, nitrifiction enhncements of only 3%-30% were observed t the sme PAC dosge. These results provide convincing evidence in support of the theory tht PAC cn dsorb inhibitory compounds, thereby enhncing nitrifiction rtes.. Results of nitrifiction enhncement in this study cnnot be ccounted for by ny of the following mechnisms : (1) nhnced growth of nitrifiers on the PAC surfces ; () incresed trce nutrient or substrte concentrtion on the PAC surfces ; or (3) heterotrophic cclimtion nd subsequent bioregenertion. 3. There ppers to be n optiml dose of PAC required to negte the effects of n inhibitor, given tht its concentrtion nd dsorptive properties re known priori. ACKNOLDGMNTS This work ws supported in prt by NSF Grnt C At the time the reserch ws being performed, the writers were postgrdute engineer nd ssocite professor, respectively, Dept. of Civil ngineering, University of Cliforni, Los Angeles, CA APPNDIX I. RFRNCS 1. Aleem, M. I. H., nd Alexnder, M. (1960). "Nutrition nd physiology of nitrobcter gilis." Applied Microbiology, 8, 80.. Bettens, L. (1979). "Powdered ctivted crbon in n ctivted sludge unit." ffluent nd ter Tretment J., 9, Chrley, R. C., Hooper, D. G., nd A. G. McLee. (1980). "Nitrifiction kinetics in ctivted sludge t vrious tempertures nd dissolved oxygen concentrtions." ter Res., 14, Crme, L. (1979). "Activted sludge enhncement : A vible lterntive to tertiry crbon dsorption." Report, PA , U.S. nvir. Protect. Agency (PA), shington, D.C., Delle, F. B., nd Chin,. S. K. (1977). "Biologicl regenertion of powdered ctivted crbon dded to ctivted sludge units." ter Res., 11, Dejohn, P. B., nd Blck, J. P. (1978). "Tretment of oil refinery wstewters with ctivted crbon." Proc., Second Open Forum on Mgmt. of Petroleum Refinery stewter, Report No. PA , U.S. nvir. Protect. Agency (PA), shington, D.C., Dobbs, R. A., nd Cohen, J. M. (1980). "Crbon dsorption isotherms for toxic orgnics." Report No. PA-600/8-80-3, U.S. nvir. Protect. Agency, 199

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