Kinetics of the Pact Process

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PII: SO043-1354(97)00170-X

Wa[.Res,Vol.32,No. 1,pp. 107-I14, 1998@ 1998ElsevierScienceLtd.Allrightsreserved

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KINETICS OF THE PACT PROCESS

C. COSTA and M. C. M~RQUEZ*ChemicalEngineeringDepartment, Faculty of ChemicalScienees,Universityof Salamanca,Plaza de 10S

Caidos 1-5, 37008Salamanca, Spain

(ReceivedApril 1996;acceptedin revisedform April 1997)

Abstract-Kinetics of textiledyeremovalfrom a dyeingwastewaterwasresearched.Treatmentwascarriedout in an activated sludge lab plant with powdered activated carbon (PACT). A kinetic model thatinvolveddye adsorption on biomassand carbon and dyedegradation by microorganismswas developed.The dedueedmeehanismof dye removal from the mathematical model showedthat the PACT processmust he consideredbasicallyas a biologicalprocess,becausedye removalis 99°/0 dueto microorganisms.@ 1998ElsevierScienceLtd.Allrightsreserved

Key words-powdered activated carbon, kinetics, dye removal, activated sludge, wastewater treatment

NOMENCLATUREb = Freundlichadsorptionparameter (dimensionless)

C,= activated carbon concentration in the completelymixed zone (mg Iitre-’)

C,,.= activatedcarbon concentrationin the effluent(mgIitre-’)

C.,,= activatedcarbon concentrationin the recycle(mglitre-i)

E = dye degraded by microorganisms(mg)k,= Contois kineticconstant (dye mg biomassmg-’)/Z = Contois kinetic constant (organics mg biomass

mg-’)m = Freundlichadsorptionparameter (dimensionless)

rreb = dye adsorbedon biomass(dyemg biomassmg-’)m. = dye adsorbed on carbon (dye mg carbon mg-l)Q = feed flow (litres h-’)Q,= recycleflow (litres h-i)Q.= flOW of sludgedrain (Iitres h-’)S, = dye concentration in the effluent(mg litre-’)S[ = organicsconcentrationin the effluent(mglitre-’)SO=dye concentration in the influent (mg litre-’)S6= organicsconcentrationin the influent(mgIitre-’)T = hydraulicresidencetime in the completelymixed

zone (h)t= process time (h)V = volume of the completelymixed zone (Iitres)u = dye specificdegradation rate (h-l)

U’= organics specificdegradation rate (h-’)u.., = dye maximumspecificdegradation rate (h-’)v& = organicsmaximumspecificdegradationrate (h-l)

X = biomass concentration in the completelymixedzone (mg litre-’)

X = biomassconcentrationin the effluent(mgIitre-’)X,= biomassconcentration in the recycle(mg litre-’)

Greeksymbol& = cell residencetime (d)

INTRODUCTIONIn the conventional activated sludge process usedin wastewater treatment, a high number of kinetic

———*Author to whom all correspondenceshould be addressed

models for degradation phenomena has beendeveloped (Gaudy and Srinivasaraghaven, 1974;Vavilin, 1982; Benefield and Molz, 1983;Specchia and Genon, 1984). In the enhancedprocess with powdered activated carbon (PACT)(Hutton and Robertaccio, 1975), adsorptionphenomena must be also enclosed (Shen and Zhao,1992).Therefore, the higher complexity of PACTkinetics causes the lack of mathematical models forthe study of the process. The scarce mathematicalmodels formulated consider separately modelling ofboth phenomena.

Thus, Benedek et al. (1985) suppose that rawwastewater contains four fractions of organics:soluble biodegradable fraction, particulatebiodegradable fraction, soluble unbiodegradablefraction and particulate unbiodegradable fraction.These authors predict the decrease of unbiodegrad-able dissolvedfraction by adsorption on the activatedcarbon. Biodegradable particulate fraction is re-moved by adsorption on bacterial floes and solublebiodegradablefraction by bacterial metabolism.Thegeneration of unbiodegradableparticulate fraction isdue to cell decomposition.

Benedek et al., establish several equations todescribe the process: utilization of the solublebiodegradablesubstrate is described by Monod-kin-etics, and adsorption processesare modelled by theLangmuir isotherm. This model was constructed fora raw wastewaterwith an important suspendedsolidsconcentration and, therefore, it must be applied fora wastewater with an important biodegradableparticulate fraction.

On the other hand, Specchia et al. (1988)formulateda mathematical model describingadsorp-tion-degradation kinetics of a textile dye in anactivated sludge system with powdered activated

WR 32/1 E 107

108 C. Costa and M. C. Marquez

carbon (PACT). They established separatelyequations for the reactor and the settler. Like inBenedek et al., adsorption phenomena on biomassand carbon are explained by the Langmuir adsorp-tion isotherm. Theseauthors justify the low bacterialdegradation of the dye by a dependence betweenspecificdegradation rate and substrate concentrationin agreement with a Monod-type equation.

The limited information about the PACT kineticsand the absence of the proven experimental valuesto verify the mathematical models leads one tothink that new research about this subject isessential.The present work is a study of adsorption–degradation kineticsof a textile dye (Acid Orange 7,C.I. 15510) in an activated sludge process withpowdered activated carbon (PACT). Experimentaldata were obtained in a lab plant, and dye fractionsadsorbed on biomass and activated carbon andremoved by microorganismswere determined. Kin-etics of each phenomenon was investigated toformulate a mathematical model, and it was provedthat the PACT process is basically a biologicalprocess.

MATERIALANDMETHODS

ExperimentalequipmentExperiments were carried out in a 5.5-litre activated

sludge lab plant. Operating conditions were set at thefollowing values: temperature of 20 f 3°C (Sedlak andBooman, 1986; Chandra et al., 1987) and oxygenconcentration of 5.8–8.8mg litre-’ (Gaudy and Srini-vasaraghaven, 1974;Randall and Buth, 1984).Syntheticwastewater (COD= 250f 40mg-0~ Iitre-’; COD/BOD = 1.44and pH = 7.3)was treated at a flowrate of 1.0

litre h-’, witha hydraulicresidencetime of 3.75h. This timeis considered suitable for the treatment of this kind ofwastewater (CCE, 1982; Benedek et al., 1985). Dyeconcentration(AcidOrange7, C.I. 15510)was20mglitre-],which is similar to a textile dyeingwastewater (Crespi andHuertas, 1987;Specchia er al., 1988;Escalas and Crespi,1989).

Concentrationof PAC in the biologicalreactor was 0.5glitre-l, and the particle size was 81 ~ 7 pm. These valueswere obtained in batch system.

Experimentalmethods and composition of the syntheticwastewaterhavebeendescribedin detail in a previouspaper(Marquez and Costa, 1996).

AnalyticalmethodsInfluent and effluentdye concentration was analyaed by

spectrophotometryat 484.0nm. In order to avoid turbiditydue to microorganisms, samples were first filtered oncellulosenitrate membranes of 0.45-ym pore size.

Biomassconcentration was determined by weighingthedried solid after filtering the sample on Milliporemembranes of 0.45pm (Rodier, 1981). This method isconsidered better than standard determination of volatilesuspendedsolids (APHA er al., 1986).COD was analysedby standard method (APHA er al., 1986).

RESULTSANDDISCUSSION

Tests were run at different values of biomassconcentration x in the range 4004000 mg Iitre-i,corresponding to standard conditions at large-scaleplants (De Lora and Mirc5, 1978; Collado, 1991). Thedifferent values of biomass concentration wereachieved from different cell residence times 0,.

Experimental data are listed in Tables 1–3, wheredt is the time between two subsequent samples.Table 1 presents process variables for conventionalprocess of activated sludge without activated carbon.

Table 1. Experimentaldata obtainedfrom conventionalactivatedsludgeprocesa

Q so se(litresh-l) (mg Le-l) (mg litre-f) (mg Iitre-]) ($;

0.884 641 19.0 19.00.936 1237 19.5 19.5 241.007 1329 21.0 19.5 241.068 1460 20.0 19.5 240.943 1373 19.5 19.0 240.989 1366 20.0 20.0 720.958 S51 20.5 19.0 241.109 1599 19.5 19.5 241.109 1779 17.5 17.5 481.080 2077 19.0 18.5 721.056 1550 19.0 1s.5 241.085 1686 18.5 1s.0 241.085 1696 19.0 1s.0 241.085 1691 17,5 17.5 240.955 1777 17.0 17.5 720.875 1630 17.0 16.5 240.979 1418 16.5 16.0 241.000 1827 16.5 15.5 24I.000 2387 17,5 17.0 961.044 2381 17.5 16.0 241.081 2756 19.0 17.0 240.958 1723 17.5 16.5 1200.968 1574 16.0 15.0 480.968 1596 15.O 13.0 241.167 2019 15.0 13,5 961.167 1764 17.0 16.5 240.979 1316 12.5 14,0 480.979 1370 12.0 11.5 960.979 1840 12.5 10.0 720.928 1960 13.5 12.0 96

Kineticsof the PACT process

Table2.ExperimentaldataobtainedfromPACTprocesswithoutmicroorganismscolonizingthe adsorbent

Q(Iitresh-’) (mgire-’) (mgi?;re-’) (mg&-’) ;;1.000 1406 19.5 19.51.000 1379 21.0 20.0 2401.043 1352 20.5 20.0 240.909 1151 19.5 19.0 120.886 400 21.5 20.5 1200,958 668 20.5 19.0 241.023 936 20.0 19.5 240.969 958 20.0 18.0 720.958 980 18.5 17.5 241000 898 20.5 18.5 240.951 898 20.0 19.0 241.024 2254 21.0 19.00.963 3040 m.o 19.0 960,956 3589 18.0 16.51.122 3989 19.0 18.0 721.029 3685 21.5 20.5 480.977 3381 20.5 19.5 241.000 3209 21.0 20,5 1681,000 3356 22.0 21.5 1441.000 3522 21.5 20.5 481.023 3500 18.0 17.0 288

Table 2 showsvariables in PACT process before the The behaviour of COD removal was the same incolonization of carbon surface by microorganisms the conventional activated sludgeprocess and in theand Table 3 after the colonization was reached. PACT, and it was correctly explained by Contois

Table 3. Experimentaldata obtainedfrom PACT processwith microorganismscolonizingthe adsorbent

0.(d) (Iitre!$h-’) (mg I;re-’) (mg ;;re-’) (mg Le-’)

6 1.021 989 21.0 0.56 1.000 989 18.5 0.56 .0.954 1068 21.0 0.56 1.068 967 20.5 0.56 1.021 1007 18.0 0.56 1.048 993 20.0 0.2

12 0.900 1760 20.5 0.212 1.023 1760 21.5 0.212 1.023 2227 22.0 0.512 0.956 1807 21.5 0.512 1.000 1925 20.5 0.512 0.933 2084 20.5 0.512 0.904 1842 19.5 0.512 0.939 1513 19.5 0.56 0.952 753 20.5 0.56 1.024 753 20.5 1.06 1.024 975 20.0 0.56 0.951 721 20.5 0.36 0.960 900 19.5 0.56 0.963 1471 20.5 0.56 I,ooo 882 18.5 0.5612 1.000 2000 20.0 0.3612 0.979 2495 20.5 0.36-12 I.000 2300 20.0 0.56-12 1.087 2206 18.5 0.56-12 0.881 1833 21.5 0.56-12 0.833 1819 19.5 0.3612 1.281 1457 21.5 I.o612 0,909 1429 23.0 0.5

12 0.889 2700 20.5 0.312 0.958 2700 21.O 0.312 0.958 2300 20.5 0.312 0.958 1891 20.0 0.312 I.000 1800 22.0 0.312 0.958 1700 21.0 0.512 1,021 2026 20.0 0.312 0.909 1422 21.0 0.312 0.619 1350 18.5 0,312 0.651 2144 20.5 0.512 0.500 1280 21.5 0.312 0.704 1002 21.0 0.3

kinetics where the organics specific degradation rate has been discussed in a previous article—Marquezwas calculated from theamount oforganics degraded and Costa, 1996). Therefore, a mathematical modelby microorganisms for biomass unit, reactor volume was developed which takes into account dyeunit and hydraulic residence time unit: (& —S~)/XT adsorption on activated carbon and biomass and dye(Vavilin, 1982). Similar kinetic parameters were degradation by microorganisms. Variables of theobtained from both systems (Fig. 1), and it led to the model are shown in Fig. 2.conclusion that COD removal is not influenced by the Because the activated sludge process is a nonadsorbent. steady-state process, a differential balance, which

However, the behaviour of dye removal was enclosestime-changeof consideredparameters, mustdifferentin the conventionalactivated sludgeprocess be assumed. Since recycle flow only affects mixingand in the PACT, because dye removal is strongly conditions and sludge settleability, an overallinfluencedby the adsorbent (the mechanismthrough material balance to the system reactor-settler waswhich PACT enhances the activated sludge process applied:

I/v’(h)

60- Iw- 17,1 + 0.67.3 XISe r= 0.918

Z’, -1.8. MN Ixgmic9mg ~fw-’20-

AIN. 18.6+ 0.643me r-o.sss o

;; -1.7. Id OlgMiamgti-alg-l

o IO 20 so 40 50 64 70

(blomaaa mg orgsnlcs mg-l)

Fig. 1. Determination of kinetic parameters from Contoismodel for COD removal:(a) conventionalactivated sludge

process, (b) PACT process.

adsorbedon biomassand carbonin effluent.i

The mathematical expressions of eqn (1)follows:

variation of dye mass V dSJdt

dye dissolvedin influent QSo

dye dissolvedin effluent QS,

dye adsorbed on biomass in influent

dye adsorbed on carbon in intluent

dye adsorbed on biomass in effluent

(Q– Q.)~e~b+ Q.~r~b

dye adsorbed on carbon in effluent

(Q– Q.)C.W.+ Q.cc.rm.

are as

dye degraded by microorganisms dE/dt (7)

Q.% x, S@,cc~ eX*V.8. ~-,~G uaAcmR

IQg, xx, & .%,

Qw.a.sa ● c~sFig. 2. Flow diagram from the mathematical model.

Kineticsof the PACT process 111

where mb and m, are the weight fractions ofdye adsorbed on biomass and carbon, respec-tively, and dE is the dye removed by biologicalphenomena.

Since activated sludge is a dynamic process,biomassand carbon concentrationsare changingandfluctuations must be considered with regard toadsorbed dye:

variation of biomass

~d(Xm,)/dt] = V(dX/dl)m~+ VX(dmb/dt) (8)

variation of carbon

~d(C.m.)/dt] = V(dC,/dt)m.+ VC~(dm,/d~)(9)

The overall balance in eqn (1) can be expressed asfollows:

Q(S, – S,) – [(Q– Q.)X. + Q.X~]mb

– [(Q– Q.)G,. + Q.Glm

=dE/dt + V(dSJdt) + V(dX/dt)m~+ VX(dmt./dt)

+ V(dC,/dt)m,+ VC,(drnJdt) (lo)

If closed time intervals are taken for experimentaldata, then eqn (10)wouldbe written in terms of finitedifferences:

Q&– &) At– [(Q– Q.K + Qv/&]nhAt

–[(Q – Q.)&+ Q&,Jm At

= AE+ V(ASe+ Axmb+ ~ A% + AC,mc

+~ Am,) (11)

Sublinedvariablesare the averagevaluesdeterminedfor two subsequent samples. Expression (11) is thesame that Specchia et al. (1988) formulated bysetting the balance to reactor and settler separately.This expressionpermits one to obtain the amountsof dye adsorbed on biomass (rob) and carbon (m.),and the amount of dye degraded by microorganisms(AE).

‘Ifthe presenceof activatedcarbon does not modifyadsorption properties of the biomass, the citedparameters can be determined from three differentsituations: (1) results from the system runningwithout activated carbon can be used to obtain thevariable mb (conventional process of activatedsludge), (2) results from the system running withactivated carbon before the colonization of carbonsurfaceby microorganismscan be used to obtain m,,and (3)resultsfromthe systemrunningwithactivatedcarbon after the colonization is reached can be usedto obtain AE.

In all cases, X, and C,,,were not considered in themathematicsmodel, becauseit was assumedthat theirvalues are practically zero. The validity of thisassumption was verifiedexperimentally.

Biological system without activated carbon(determi-nationoft??b)

For these conditions, variables related to carbonadsorption and microbial degradation (m, and AE)are zero in eqn (11). Sincethere is not an importantvariation on substrate concentration in the feed, Ambcan be neglected.In this way, the expressionof mb is

mb=[Q(& – ~) At– V ASJ/(V AX+ Q.JJ At) (12)

Obtained values agree with a Freundlich adsorptionisotherm (mb= b~~, m e 1),typical of a non-favour-able adsorption (Lowell, 1979). This isothermexplains adsorption phenomena for liquid com-pounds (McCabe et al., 1991;Rao et al., 1994).Forthis system,valuesof 0.00269and 0.76wereobtainedfor b and m, respectively:

mb= 0.00269SZ76 (13)

Biological system with activated carbon not colonizedby microorganisms (determination of mJ

In the system with non-colonized activated carbon,bacterial degradation (AE) is assumed zero and Am.and ACCare neglectedineqn(11) because,by periodicadditions to complete the carbon loss at the sludgedrain, the carbon concentration remained constant.In this case, the amount of dye adsorbed on activatedcarbon can be obtained by substituting eqn (13) forbiomass adsorption isotherm in eqn (11):

m. = [Q@ – ~) At – VAS.

–t?rb(Qw~At + k’AX)]/(Qw!&At) (14)

The amount of dye adsorbed on activated carbon(m.) could also be fitted to a Freundlich adsorptionisotherm:

mC= 0.0257SZ’7 (15)

For the S. values measured, adsorption on PAC ishigher than adsorption on biomass, as shown by theadsorption isothermsdescribedby eqns (13)and (15).

Biological system with activated carbon colonized bymicroorganisms (determination of AE)

In the systemwith colonizedactivated carbon, t??b

and m. values obtained from eqns (13) and (15)permit the determination of AE (see Table 4). Thesameassumptionsmade for the previoustwo systemswere also considered valid for this system. Hence,eqn (11) takes the form:

AE = Q@ – &) At – VASe

–(Q.&At + VAx)m – Q.Q% At (16)

If mb,m. and AE/K values are checked (Table 5),it can be seen that the highest factor is AE/~.Therefore, the main step in dye removal for a PACTprocess is microbial degradation; it is much higherthan adsorption on activated carbon and it is higherthan adsorption on biomass. Quantitatively, theamount of dye degraded by microorganisms per

Kineticsof the PACT process 113

Table 5. Comparativeanalysisbetweendye adsorptionon biomassand carbonand dye microbialdegradation

(dyemgbiomassmg-’)

0.001590.001590.001590.001590001210.000790.001210.001590.001590.001590.001590.001590.002160.002160.001280.001280.001590.001590.001280.000940.001280.001590.001590.001280.001880.002160.000940.000940.000940.000940.001280.001280.000940.000940.001280.001280.00094

(dye mgc~~onmg-’)

0.01990.01990.01990.01990.01740.01420.01740.01990.01990.01990.01990.01990.02310.02310.01790.01790.01990.01990.01790.01530.01790.01990.01990.01790.02160.02310.01540.01540.01540.01540.01790.01790.01540.01540.01790.01790.0154

AE/~(dyemgbiomassmg-’)

0.4651.7281.4291.4071.3720.5400.5190.7400.5070.4540.6460.4930.6092.1813.8411,6411.4911.1260.9271.2510.1940.8190.2250.6691.2111.9290.1660.3731.0840.7831.6781.5171,8832.0330.8121.4651.046

biomass unit (AE/XJis about 100times higher thandye adsorbedon activatedcarbon per adsorbentmassunit (m,) and this is about 10 times higher than theamount of dye adsorbed on biomassper biomassunit(W,).

The PACT process must be assumed basically abiological process, because 99Y0 of dye removal isdue to microbialdegradation. Thus, the degradationkinetics of the pollutant can be determined fromthe dye specific degradation rate. This variable isdefined as the amount of dye degraded bymicroorganisms per biomass unit, reactor volumeunit and hydraulic residence time unit: AE/@”T)(Vavilin, 1982).

A kinetic study shows the Contois model as thebest to fit the experimental data. A linearizedrepresentationof this modelallowsthe determinationof model parameters (Fig. 3). In this case,Vm..= 0.442h-] and k, = 7.84 x 10-7 substrate mgbiomass mg-’.

This kind of kinetics includes the “saturation”phenomenon by a high substrate concentration in thereactor and the decrease of the degradation rate withthe increase of biomass concentration. The firstphenomenon is related to the limitation of substratedegradation at a high value of substrate concen-tration, due to vital requirements of microorganisms

(Vavilin,1982).The decreaseof the degradation ratefor a high value of biomass concentration has beenexplained in a previous work (Marquez and Costa,1996)and it is produced by the inclusion of PAC

I Iv(h)

sv - Vmax~k=X +Se

ltv.226+ O.oosmme r-o.w

‘m - ❑

00 mm 4mo woo am mom!

(blOSSWSSmg dye mg-l)

Fig. 3. Determination of kinetic parameters from theContois model for dye removal in the PACT process.

particles into bacterial floe and, hence, adsorptionproperties of carbon are lost.

CONCLUSIONS

The removal of biodegradable organics is notinfluenced by the adsorbent in the carbon-activatedsludge system and it is similar to the conventionalprocess without carbon. Therefore, PAC addition tothe biological reactor does not improve the processefficiency.

However, in the carbon-activated sludge system,the removal of organics particularly resistant tobiodegradation (xenobiotic organics such as textiledyes) is closely associated with the presence of theadsorbent. A mathematical model explaining thebehaviour of experimental data must includeadsorption on carbon and biomass and degradationby microorganisms processes.

The higher amount of degraded xenobioticorganics with regard to the amount of adsorbedxenobiotic organics leads to the conclusion that thePACT process must be basically a biological processwhere degradation is explained by the Contois kineticmodel.

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Kinetics of the Pact Process

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