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A kinetic study of a membrane anaerobic reactor (MAR) for treatment of sewage sludge

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A kinetic study of a membrane anaerobic reactor (MAR) for treatment of sewage sludge
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  A kinetic study of a membrane anaerobic reactor (MAR) fortreatment of sewage sludge A.G. Liew Abdullah, A. Idris, F.R. Ahmadun, B.S. Baharin, F. Emby,M.J. Megat Mohd Noor, A.H. Nour University Putra Malaysia, 43400 UPM Serdang, Selangor, MalaysiaFax  þ 603 86567359; email: ghaniey@eng.upm.edu.my Received 22 February 2005; accepted 15 March 2005 Abstract The application of kinetic models (Monod, Contois and Chen & Hashimoto) and overall microbial kineticon the membrane anaerobic reactor (MAR) for treatment of sewage sludge was investigated. The systemconsists of a cross-flow ultrafiltration membrane and six steady states were attained over a range of mixedliquor suspended solids of 12,760–21,800 mg/l. The results of all six steady states were successfully fitted above98 %  for three known kinetics. The growth yield coefficient,  Y  , was found to be 0.74 gVSS/gCOD while thespecific microorganism decay rate was 0.20 d  1 . The  k  values were in the range of 0.350–0.519 gCOD/gVSS.dand  m max  values were between 0.259 and 0.384 d  1 . The COD removal efficiency was 96.5–99 % with HRT of 7.8days. The methane gas yield was between 0.19 l/g COD/d to 0.54 l/g COD/d when the organic loading rateincreased from 0.1 kg COD/m 3 /d to 10 kg COD/m 3 /d. The system efficiency was greatly influenced by SRT andOLRs. Membrane flux rate deterioration was observed from 62.1 l/m 2 /h to 6.9 l/m 2 /h due to membrane fouling. Keywords : Kinetics; Sewage sludge; Anaerobic; Membrane; Ultrafiltration 1. Introduction Anaerobic biological processes havereceived high interest in wastewater treatmentowing to attractive advantages of energy sav-ing, biogas recovery and lower sludge produc-tion. With the advancement of membranetechnology, application of membrane filtrationin the treatment of wastewater can contributeto developing an efficient wastewater treatmentprocess that is capable of retaining biomassconcentration within the reactor and producinghigh quality effluent. As compared with theconventional treatment methods, it can offeramong others, clear final effluent, less usage of chemicals, low energy consumption, small foot-print, and low maintenance cost. Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005.European Desalination Society. 0011-9164/05/$– See front matter    2005 Elsevier B.V. All rights reserved*Corresponding author. Desalination 183 (2005) 439–445  Membrane separation techniques haveproven to be an effective method in separat-ing biomass solids from digester suspensionsand recycle them to the digester [1]. Several studies have been conducted on membraneanaerobic processes for the treatment of var-ious wastewaters [2 – 5]. These studies have showed that the membrane anaerobic pro-cesses had retained all particulate mattersand consequently liquefied and decomposedthem due to long solids retention time.For the correct design of a bioreactor, it isvery important to have values of kinetic para-meters involved in the bioreaction. Theseparameters depend on type of substrate,microorganisms and temperature. Domesticsewage, on the other hand, is a complex sub-strate and kinetic data on its biodegradationin anaerobic systems are limited. Threewidely used kinetic models used in this studyare shown in Table 1. Therefore, the aims of this study are to provide some data on thekinetics and performance of membrane anae-robic system. 2. Materials and methods A schematic diagram of the experimentalsystem is shown in Fig. 1. It consists of ananaerobic reactor coupled with two tubularcross flow ultrafiltration membrane modules(PCI model FP200). This polysulphone mem-brane has a tube diameter of 1.25 cm and anaverage pore size of 0.1  m m with molecularcut-off weight of 200,000. The length of eachtube is 30 cm. The total effective areas of thetwo membranes is 0.024 m 2 . The maximumoperating pressure on the membrane is 55 barat 70  C and it can be used in the pH rangefrom 2 to 12. The anaerobic reactor is madeof clear PVC which has a dimension of 30 cmdiameter and a total height of 100 cm. Thetotal effective liquid volume of the reactor is50 l.The volumetric COD loading rate wasincreased stepwise throughout the experimen-tal period. The pH of the reactor was in therange between 7.0 and 7.7. The membraneunit was operated at the pressure of 1.5–2.0bars. A 20-l water displacement bottle wasused in this study to monitor the biogas pro-duction during the steady state. Since almost95 %  of biogas produced during anaerobicdigestion is CH 4  and CO 2  [6], the composi-tion of gases can be determined by absorbingCO 2  into sodium hydroxide solution inside asimple J-tube gas analyzer. Therefore, in thisstudy, the system was said to achieve steadystate when the operating and control para-meters were within    ten percent of the aver-age value. There were six steady stateconditions performed in this study. 3. Results and discussion Table 2 shows the results obtained fromthis study. The kinetic coefficients of theselected model were derived from Eq. (2) inTable 1 by using a linear relationship. Thecoefficients are summarized in Table 3. It Table 1Mathematical expressions of specifics substrate utilization rates for known kinetic modelsKinetic Model Equation 1 Equation 2Monod  U   ¼  kS K  s þ S  1 U   ¼ K  s K  1 S    þ 1 k Contois  U   ¼  U  max S Y BX  þ S  ð Þ 1 U   ¼  aX   max S  þ Y   1 þ a ð Þ  max Chen & Hashimoto  U   ¼   max S YKS  o þ  1  K  ð Þ SY  1 U   ¼  YKS  o  max S  þ Y   1  K  ð Þ  max 440  A.G.L. Abdullah et al. / Desalination 183 (2005) 439–445  Membrane UFModulePumpSludge Wastage Feeder Tank AnaerobicReactorPermeateRetentateTo gas collectorValvePressure Gauge Fig. 1. Layout of the experimental setup.Table 2Summary of resultsSteady state (SS) 1 2 3 4 5 6Feed, l/d 0.053 0.58 1.875 4.0 4.52 6.41COD feed, mg/l 94000 86500 80000 80000 77500 78000COD permeate, mg/l 1090 1480 1900 2200 2500 3800Total Gas Yield, L/g.COD/d 0.28 0.34 0.60 0.74 0.70 0.81 % methane 69 76.3 74.4 71.8 69.1 66.3CH4 yield 0.19 0.26 0.45 0.53 0.48 0.54MLSS, mg/l 12760 13870 15400 16300 18200 21800MLVSS, mg/l 9540 10400 11200 12400 14000 17000HRT, d 943.4 86.2 26.7 12.5 11.06 7.80SRT, d 1250 625 200 55.6 21.7 16.1OLR, kg COD/m 3 /d 0.1 1 3 6 7 10BLR, kg COD/kg VSS/d 0.01 0.10 0.27 0.48 0.50 0.59SSUR, kg COD/kg VSS/d 0.183 0.201 0.218 0.222 0.226 0.241SUR, kg COD/m 3 /d 0.0249 0.9174 2.9008 5.8759 6.8676 9.8427 A.G.L. Abdullah et al. / Desalination 183 (2005) 439–445  441  was found that the three models produced agood relationship with  R 2 >  99 %  for themembrane anaerobic reactor treating sewagesludge as shown in Figs. 2,3 and 4.The better performance of both Contoisand Chen & Hashimoto implied that organicloadings should be taken into considerationfor digester performance. In fact these twomodels suggested that the predicted permeateCOD concentration ( S  ) is a function of influ-ent COD concentration ( S  o ). However, inMonod model,  S   is independent of   S  o . The Table 3Results of the application of three known substrate utilization modelsModel Equation  R 2 ( % )Monod  U   1 ¼ 2025 S   1 þ 3 : 61  99.4 K  s  ¼ 498 K   ¼ 0 : 350  max  ¼ 0 : 259 Contois  U   1 ¼ 0 : 306 S   1 þ 2 : 78  99.7 B ¼ 0 : 111 U  max  ¼ 0 : 344 a ¼ 0 : 115 K   ¼ 0 : 519  max  ¼ 0 : 384 Chen & Hashimoto  U   1 ¼ 0 : 0190 S  o S   1 þ 3 : 77  99.5 k ¼ 0 : 374 a ¼ 0 : 006 K   ¼ 0 : 006  max  ¼ 0 : 277 1/U   =   2.0221(1/S)   +   3.5933R 2   =   0.993944·24·44·64·855·25·45·60 0·1 0·2 0·3 0·4 0·5 0·6 0·7 0·8 0·9 1 1/S (m3/kg COD)    1   /   U   (  g   V   S   S .   d   /  g   C   O   D   ) Fig. 2. Monod model.442  A.G.L. Abdullah et al. / Desalination 183 (2005) 439–445  excellent fitting of these three models( R 2 >  99 % ) in this study suggests that themembrane anaerobic reactor process is ableto sustain loadings between 0.1 and 10.0 kgCOD/m 3 /d. 4. Gas production and composition Fig. 5 shows the gas production rate andthe methane content of the biogas producedusing the membrane anaerobic reactor. Thebiogas production steadily increased with the 1/U   =   0.3145X/S   +   2.7246R 2   =   0.99454 5·5 6·5 7·5 8·54·5 5 6 7 8 9 X/S    1   /   U   (  g   V   S   S .   d   /  g   C   O   D   ) 5·65·45·254·84·64·44·24 Fig. 3. Contois model. 1/U   =   0.0198So/S   +   3.7765R 2   =   0.99540 10 20 30 40 50 60 70 80 90 100 So/S    1   /   U   (  g   V   S   S .   d   /  g   C   O   D   ) 5·65·45·24·84·64·44·245 Fig. 4. Chen and Hashimoto model. A.G.L. Abdullah et al. / Desalination 183 (2005) 439–445  443
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