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Anaerobic ponds treatment of starch wastewater:case study in ThailandB.K. Rajbhandari, A.P. Annachhatre*Environmental Engineering and Management, Asian Institute of Technology, P.O. Box 4, Klong Luang, Pathumthani 12120, ThailandReceived in revised form 20 January 2004; accepted 26 January 2004Available online 12 March 2004AbstractAnaerobic ponds are particularly effective in treating high-strength wastewater containing biodegradable solids as they achievethe dual purpose of particulate settlement and organic removal. Performance of an anaerobic pond system for treatment of starchwastewater containing high organic carbon, biodegradable starch particulate matter and cyanide was assessed under tropical climateconditions. Approximately 5000 m3/d of wastewater from starch industry was treated in a series of anaerobic ponds with a total areaof 7.39 ha followed by facultative ponds with an area of 29.11 ha. Overall COD and TSS removal of over 90% and CN removal of51% was observed. Active biomass obtained from the anaerobic ponds sediments and bulk liquid layer exhibited specific methano-genic activity of 20.7 and 11.3 ml CH4/g VSSd, respectively. The cyanide degradability of sludge at initial cyanide concentration of10 and 20 mg/l were determined to be 0.43 and 0.84 mg CN?/g VSSd, respectively. A separate settling column experiment withstarch wastewater revealed that a settling time of approximately 120 min is sufficient to remove 9095% of the influent TSS.? 2004 Elsevier Ltd. All rights reserved.Keywords: Anaerobic pond; Cyanide degradability; Organic carbon; Settling characteristics; Specific methanogenic activity; Starch factorywastewater1. IntroductionAnaerobic ponds (APs) are popularly employed fortreatment of organic wastewater emanating from varietyof industries such as food, pulp and paper, sugar anddistillery. Anaerobic ponds are particularly effective intreating high-strength wastewaters containing biodegrad-able total suspended solids (TSS). In such cases the liquidlayer in anaerobic ponds act as a settling basin for thesuspended solids while the anaerobic biodegradationprimarily takes place in pond sediments (Toprak, 1994).Anaerobic reactions taking place in the sediment includesolubilization of biodegradable particulate matter fol-lowed by acidogenesis, acetogenesis and methanogenesis(Parker, 1979; Pescod, 1996). The reactions occurring inthe bulk liquid are often negligible as compared to thosein the pond sediments. Thus, anaerobic ponds achieve adual purpose of sedimentation of particulate matter aswell as anaerobic conversion of organics. However,anaerobic pond operation also has many intrinsic prob-lems such as high land requirements and emission ofobnoxiousandgreenhousegasessuchashydrogensulfide(H2S),carbondioxide(CO2)andmethane(CH4)(Parker,1979; Pescod, 1996; Toprak, 1997; Paing et al., 2003). Inspite of these problems, anaerobic ponds are popularparticularly wherever land is abundant (Arthur, 1983).Wastewater coming from starch factories is one suchtype of wastewater, which is treated extensively inanaerobic ponds. Starch is often produced in many partsof the world from tapioca. Tapioca roots contain 2025% starch. The starch extraction process essentiallyinvolves pre-processing of roots, followed by starchextraction, separation and drying. The process generates2060 m3/ton of wastewater with a low pH in the range3.85.2 (Economic and Social Commission for Asia andThe Pacific, 1982). The wastewater is highly organic innature with chemical oxygen demand (COD) up to25,000 mg/l (Bengtsson and Treit, 1994). The wastewaterconsists of high TSS comprising starch granules in therange 300015,000 mg/l, which are highly biodegradableby nature. Tapioca starch wastewater also has highcyanide content up to 1015 mg/l, which is highly toxic*Corresponding author. Tel./fax: +66-2-524-5644.E-mail address: ajitait.ac.th (A.P. Annachhatre).0960-8524/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2004.01.017Bioresource Technology 95 (2004) 135143to aquatic life at concentrations of cyanide as low as 0.3mg/l have been reported as cause for a massive fish kill(Bengtsson and Treit, 1994).Problems related to water pollution are reported tobe serious. The acidic nature of wastewater can harmaquaticorganismsandreducetheself-purificationcapacity of the receiving stream. Suspended solidspresent in the wastewater can settle on the streambedand spoil fish breeding areas in the stream. Since thesesolids are primarily organic in nature, they decomposeeasily and thus deoxygenate the water. Similarly, highbiochemical oxygen demand (BOD) of the wastewateralso can cause rapid depletion of oxygen content in thereceiving water body and promote the growth of nui-sance organisms. Water pollution caused by tapiocastarch production has been reported as a serious prob-lem in many Asian countries, particularly in Thailand(Kiravanich, 1977) and in India (Padmaja et al., 1990).Tapioca also contains bound cyanide as a natural de-fense mechanism. During the starch manufacturingprocess, bound cyanide in the form of linamarin andlotaustralin from tapioca roots is hydrolyzed by theenzyme linamarase with decomposition to hydrogencyanide (HCN), which finds its way into the wastewater.Cyanide containing starch wastewater can be effectivelydetoxified in anaerobic processes (Annachhatre andAmornkaew, 2000). Upflow anaerobic sludge blanket(UASB) processes are effective in treating starch waste-water (Annachhatre and Amatya, 2000), particularly, inremovingcyanide(AnnachhatreandAmornkaew,2001). Adaptation by methanogens to cyanide concen-trations of 530 mg/l has been reported in literature(Fedorak et al., 1986; Harper et al., 1983). Thus, intreating tapioca starch wastewater anaerobic pondsachieve a threefold objective namely: sedimentation ofparticulate matter, anaerobic conversion of organics anddetoxification of cyanide.Accordingly, the work presented here assesses theperformance of APs treating wastewater from tapiocastarch industry, particularly related to COD, TSS andcyanide removal. Since APs serve as a settling basin forstarch granules, the settling characteristics were alsoassessed by column experiments. Furthermore, the po-tential methane production rates of anaerobic biomass(sludge) obtained from the AP sediment as well as frombulk liquid layer were assessed from the specific meth-anogenic activity (SMA) test. The cyanide degradabilityof the anaerobic sludge obtained from the pond sedi-ment layer was also assessed.2. MethodsInvestigations on the existing wastewater anaerobicpond system were carried out in a tapioca starch andglucose factory situated in the Central province ofThailand with a capacity of 250 tons starch/day. Thefactory uses groundwater as a source for process water,and generates combined wastewater of approximately5000 m3/d. The operating ambient temperature duringthe period of investigation was in the range of 3035 ?C.2.1. Treatment pondsA schematic of waste stabilization pond system(WSPS) of the starch factory is presented in Fig. 1(Choi, 2001). The wastewater treatment system consistsof 21 APs and facultative ponds (FPs) connected inseries with total area of about 36.5 ha. Out of these, 6are AP with area of 7.39 ha and 15 are FP with 29.11 ha.The study concentrated on anaerobic ponds system.During the study period only four anaerobic ponds werein operation. The typical size of an AP is approximately250 m in length, 100 m in width and 45 m in depth.The pond parameters are presented in Table 1. Theanaerobic ponds treat wastewater from a starch as wellas from a glucose factory. The wastewater from thestarch factory was first introduced to Pond #2 andsubsequently flows to Pond #4 while the effluent fromthe glucose factory was introduced to Pond #3 and thenflows to Pond #5 where wastewater from the starch andglucose factory were combined. The combined waste-water then flows into a series of FPs and treated effluentwas finally discharged on to the groundwater rechargespreading basins.2.2. Sludge activity testsThe schematic of the SMA test set up is presented inFig. 2. To determine SMA, a known amount of sludgeobtained from the sediment layer of Pond #4 wastransferred into serum bottles (115 ml) after washingthree times with water to remove existing COD. While100 ml of bulk liquid from the same pond was kept in aserum bottle to determine SMA of sludge in suspensionin the bulk liquid layer. An appropriate amount ofstarch factory wastewater as substrate was added to theserum bottles so as to obtain an initial COD level inthe range of 20002500 mg/l. Nutrients were added tomaintain a carbon:nitrogen:phosphorus ratio of 300:5:1.pH was adjusted to between 7 and 7.8. Two grams perliter of NaHCO3were also added along with substrateto buffer the serum bottle contents to near neutral pHconditions during the test. Subsequently the bottles weresealed with a rubber septum and an aluminum cap afterpurging oxygen with nitrogen (N2) gas and attached tothe liquid displacement system. The liquid displacementbottle contained 3% NaOH solution. Methane gasproduction was measured at different time intervals upto 48 h. After every gas measurement, by swirlingmanually, the contents of the serum bottle were mixed.The tests were conducted in a 30 ?C temperature-136B.K. Rajbhandari, A.P. Annachhatre / Bioresource Technology 95 (2004) 135143controlled room. Likewise, cyanide degradation activityof anaerobic pond sediment was carried out also in theserum bottles. A known amount of sludge obtainedfrom Pond #4 sediment layer was kept in serum bottlesand filled with 70 ml of wastewater having substrate andnutrient similar to those mentioned for the SMA test. Astock cyanide solution was added into each serum bottleto achieve cyanide concentrations of 10 and 20 mg/lrespectively. The bottle was then purged with N2gasand immediately sealed with a rubber septum and analuminum cap. The bottle was kept in a 30 ?C temper-ature-controlledroom.SamplesweretakenwithHamilton syringe at every 8 h interval for 48 h andanalyzed for cyanide content.2.3. Suspended solid settling experimentBatch tests to investigate TSS settling characteristicsof starch factory wastewater under quiescent conditionFig. 1. Layout of WSPS of starch factory.Table 1Ponds parametersAPsSurface area (ha)Volume (m3104)Depth (m)Inflowb(m3/d)Detention timeb(d)1a1.074.414.521.345.574.5450173112.71.932.068.624.549882177.832.841.767.344.5450173116.72.550.491.784.549997853.60.56a0.672.464.5aPond not in operation at the time of this study.bResults are mean of nine valuesstandard deviation.Fig. 2. Sludge activity test setup.B.K. Rajbhandari, A.P. Annachhatre / Bioresource Technology 95 (2004) 135143137was carried out in a settling column of 10.0 cm diameterand 2.0 m height for different TSS concentrations.Wastewater of a desired TSS concentration for settlingexperiment was prepared by diluting the concentratedwastewater with tap water. The wastewater was pouredinto a settling column after stirring thoroughly. Samplesfrom top of column were collected at different timeintervals ranging from 2 to 60 min and analyzed for TSSconcentration.2.4. Analytical proceduresParameters including COD, BOD5, TSS, volatilesuspended solid (VSS) and dissolved solid (DS) wereanalyzed according to Standard Methods (APHA et al.,1998). The mass of the sludge used in the sludge activitytest was measured in terms of VSS. All the samples werefiltered through 0.45 lm glass fiber filters for the deter-mination of soluble COD and BOD5. Cyanide wasmeasuredspectrophotometrically(Spectroquant,E.Merck KGaA, Darmstadt, Germany) as per the proce-dure reported elsewhere (Annachhatre and Amornkaew,2000).2.5. Statistical analysisThe anaerobic ponds process performance data werepresented in terms of arithmetic averages of nine val-uesstandard deviation. The SMA tests were carriedout with two replicates. Common linear regression curvewas fitted to the data obtained from the two replicatestest and a relation between volumes of methane pro-duction with respect to time was established. The SMAwas calculated based on the slope of methane volumeversus time curve and mass of sludge taken for the SMAtest. Likewise a linear relation was established betweenthe cumulative cyanide degradation with respect to time.Data from settling experiments was used to establish anon-linear relationship between the half-removal timeand influent total suspended solid concentrations. Allstatistical analyses (arithmetic average, standard devia-tion, linear and non-linear regression and correlationcoefficient) were performed using Microsoft Excel 2000.3. Results and discussion3.1. Analysis of existing wastewater processCharacteristics of raw wastewater: The pond systemtreats approximately 4500 m3/d of wastewater fromstarch and approximately 500 m3/d of wastewater fromglucose factory. A scheme of anaerobic ponds andsampling points is shown in Fig. 3. The characterizationof raw, influent and effluent wastewater of the pondsystems is shown in Table 2. The wastewater charac-teristics at sampling point, a and d, in Table 2 cor-responds, respectively to raw wastewater from starchand glucose factory. Raw wastewater from starch fac-tory was highly acidic in nature while from glucosefactory was low acidic to neutral. As can be seen inTable 2, the major pollution load was due to wastewaterfrom starch factory having BOD5of 12,776499 mg/las compared to BOD5of 1046153 mg/l from glucosefactory. The starch factory wastewater also had TSS of91303067 mg/l mainly as starch granules, which werehighly biodegradable by nature. A cyanide concentra-tion of 17.51.5 mg/l was found in starch factorywastewater while no cyanide was detected in wastewaterfrom glucose factory.Performance of anaerobic ponds: The details of thepond area and residence time are presented in Table 1.The overall residence time works out to be 335 days forstarch and 18133 days for glucose factory wastewater.The average pollution load for the total wastewater flowsof 4999785 m3/d calculated to be 63,25810,198 kgCOD/d with 62,73210,152 kg COD/d from starch and658138 kg COD/d from glucose factory. The averageoverall volumetric loading in the anaerobic ponds was49782 kg BOD5/m3d (51482 kg COD/m3d).abcdefAnaerobic pondsSampling point: a.Starch wastewater/influent to pond 2b. effluent from pond 2/ influent to pond 4c.effluent from pond 4/ influent to pond 5d.Glucose wastewater/ influent to pond 3e.effluent from pond 3/ influent to pond 5f.effluent from pond 5/ influent to pond 61Starch wastewaterGlucose wastewater23456Fig. 3. Scheme of anaerobic ponds and sampling points.138B.K. Rajbhandari, A.P. Annachhatre / Bioresource Technology 95 (2004) 135143Out of six anaerobic ponds, Ponds #1 and #6 werenot in operation during the study period. Pond #1 wasfilled up due to accumulation of starch granules fromstarch wastewater so wastewater from the starch factorywas introduced into Pond #2. The average COD, BOD5and TSS removal in Pond #2 is very small, about10.56.8%, 8.66.2% and 18.010.9%, respectively(Table 3). It was observed that Pond #2 was also par-tially filled up by starch granules and a channel wasformed where wastewater flowed to Pond #4. Thisindicates that Ponds #1 and #2 operate mainly as set-tling basins for the suspended solids, and hence, theyneed to be desludged regularly. Accumulation of starchgranules in the pond also reduces the residence time inthe pond significantly.The pH in Pond #2 was acidic, in the range 4.14.3.Under this condition, methanogenesis cannot occur, asthis condition is highly unfavorable for the growth ofmethanogenic bacteria (Duarte and Anderson, 1982).This is further brought out by the fact that BOD5re-moval in Pond #2 was less than 10%. However, in Ponds#4 and #5, the pH was between 6 and 8 as these pondswere active anaerobically. In fact, intense biologicalactivity was observed in these two ponds as evidenced byformation of excessive gas bubbles and the existence offloating sludge on the pond surface. According toZehnder et al. (1982), the optimum pH range for allmethanogenic bacteria is between 6.0 and 8.0, but theoptimum value for the group as a whole is close to 7.0.Van Haandel and Lettinga (1994) reported the sameobservation.Based on data in Table 3, it is apparent that theperformance of Ponds #4 and #5 is satisfactory. Pond#4 was the most efficient one and provided averageCOD, BOD5and TSS removal of 88.60.6%, 90.50.6% and 87.62.8%, respectively. The average volu-metric loading of 1031165 g BOD5/m3d in Pond #2was very high and 62 g BOD5/m3d in Pond #3 wasvery low, while 716128 and 30047 g BOD5/m3d inPonds #4 and #5, respectively, were within the rangefound in much of the literature (Ellis, 1980; Arthur,1983; Gomes de Sousa, 1987; Mara and Pearson, 1998).The starch wastewater also contained 17.51.5 mg/l ofcyanide. Since the ponds have been in operation for over20 years, it was anticipated that the sludge would be wellacclimatized to cyanide present in the wastewater.Average cyanide removal of 2.82.5%, 38.42.6% and9.25.0% was observed in anaerobic Ponds #2, #4 and#5, respectively.The overall removal rate for COD, BOD5and TSSwere 96.20.6%, 98.20.4% and 94.71.3% respec-tively (Table 3), whereas the removal efficiencies for DSand CN were 71.41.0% and 51.21.1%, respectively.However, the quality of treated effluent from the seriesof anaerobic ponds (Table 3, corresponding to samplingpoint f) still did not meet the effluent standard,therefore, further treatment of treated wastewater fromthe anaerobic pond system is required. The CODremoval efficiency is in agreement with results reportedelsewhere (Annachhatre and Amatya, 2000) for UASBreactor, treating the wastewater from the same starchfactory. Pena et al. (2000) studied the performance of anTable 3Average loading and removal rate of anaerobic pondsPondVolumetric loading rate(g BOD5/m3d)Removal rateCOD (%)BOD (%)TSS (%)DS (%)CN (%)1Not in operation210311656.918.010.92.536226.97.825.07.257.27.020.87.3471612888.70.890.50.687.62.849.63.838.42.652994760.76.610.628.06Not in operationOverall4978296.10.898.20.594.71.371.41.051.21.1Results are mean of nine valuesstandard deviation.Table 2Characterization of the influent and effluent of the pond systemParametersSampling pointsabcdefCOD (mg/l)13,94135912,46893014143513141279523653894BOD5(mg/l)12,77649911,700124911024710461537754223060TSS (mg/l)91303067774032109003149702854007345083DS (mg/l)12,400133990011504950235590076346251903540125pHDO (mg/l)2.02.71.5CN (mg/l)17.51.517.01.510.51.3NilNil8.50.7Results are mean of nine valuesstandard deviation.B.K. Rajbhandari, A.P. Annachhatre / Bioresource Technology 95 (2004) 135143139AP and a UASB reactor treating the same domesticsewage under the same environmental conditions andreported similar performance of these two systems.3.2. Sludge activityThe SMA test results (Fig. 4) of sludge taken from thepond sediment layer exhibited a negligible level ofmethane production during the first 13 h and increasedafterwards. This revealed that approximately 13 h wasrequired for conversion of organic matter from starchfactory wastewater to produce a sufficient amount oforganic acid (substrate for methane producing bacteria)that is required for substantial methanogenic activity.However instantaneous methane production was ob-served in the case of sludge taken from pond bulk liquidlayer because of the presence of residual organic acids inthe liquid. The SMA test result obtained from this studyis given in Table 4 along with the other values reported inliterature (Valcke and Verstraete, 1983; James et al.,1990; Ince et al., 1995). As can be seen from Table 4, thepotential methane production rate of 20.7 and 11.3 mlCH4/g VSSdfor the sludge from pond sediment and bulkliquid layer, respectively, obtained from this study waslower than those reported values. This could explain therelatively long retention time required for AP comparedto UASB when treating the same wastewater. Since APsare a low rate system they require retention times be-tween 1 and 2 days at temperature around 25 ?C toachieve 7080% of BOD5removal efficiency, dependingon the wastewater strength (Mara et al., 1992). UASBreactors also achieve the same level of treatment but atshorter retention times of around 68 h (Van Haandeland Lettinga, 1994).The cyanide degradability test of anaerobic sludgeobtained from the Pond #4 sediment layer was con-ducted for initial cyanide concentration of 10 and 20mg/l. The slope of the line for an initial cyanide con-centration of 10 mg/l (Fig. 5) provides an average cya-nide degradation rate of 4.02 mg CN?/ld, correspondingto an average cyanide degradability of 0.43 mg CN?/gVSSd. Likewise, an average cyanide degradation rate of7.83 mg CN?/ld was obtained for an initial cyanideconcentration of 20 mg/l, yielding an average cyanidedegradability of 0.84 mg CN?/g VSSd.3.3. Suspended solid settling in pondsRemoval of organic matter in anaerobic ponds isbrought about both by sedimentation and anaerobicdigestion (Oswald, 1968; McGarry and Pescod, 1970).Anaerobic wastewater stabilization ponds are consid-ered to be an important first-step treatment as theyy = 29.393x - 16.248R2 = 0.977601020304050600.00.51.01.52.02.5Cum. methane (ml)VSS = 1.42 g(a)y = 1.8153x + 1.6847R2 = 0.8188024680.00.51.01.52.02.5Time (day)Cum. methane (ml)VSS = 0.16 g(b)Replica 1Replica 2Fig. 4. SMA on sludge from (a) anaerobic pond sediment layer and (b)anaerobic pond bulk liquid layer.Table 4Comparison of SMA resultsBiomass fromFeedTemperature (?C)SMA (ml CH4/g VSSd)ReferencePond sediment layerStarch wastewater3020.7This studyPond bulk liquid layerStarch wastewater3011.3This studyUASBBrewery wastewater36150Ince et al. (1995)UASBAcetate3071103Valcke and Verstraete (1983)UASBAcetate35200400James et al. (1990)y = 7.828xR2 = 0.9581y = 4.0225xR2 = 0.9728051015200.00.51.01.52.02.5Time (day)Cum. CN- degration (mg /l)VSS = 0.65 gInitial CN=10 mg/lInitial CN=20 mg/lFig. 5. Cumulative cyanide degradation against time.140B.K. Rajbhandari, A.P. Annachhatre / Bioresource Technology 95 (2004) 135143allow for settleable materials in the raw wastewater toseparate out and fall to the bottom sludge zone (Saqqarand Pescod, 1995). Considering the importance of set-tling of suspended particles, the settling experiment wascarried out to study the settling characteristics of sus-pended solid from starch wastewater. The relationshipbetween settling time and suspended solids removalunder various influent suspended solid concentrationsfound from experiment is shown in Fig. 6. The figureshows that approximately 9095% of influent TSS re-moval occurs within 120 min due to sedimentation atand above 1600 mg/l influent TSS concentration.However nearly 70% and 60% removal occurs within120 min in the case of 630 and 490 mg/l of influent TSS,respectively. The settling time of 120 min was verysmall compared to the retention time of the ponds,which revealed that the suspended solid settlement andaccumulation mainly occurs near the inlet zone of theponds.However the actual suspended solids removal inanaerobic ponds was observed to be less than the valueexpected from settling experiment. This was because ofshort-circuiting in case of Pond #2 due to the accumu-lation of starch granules and reducing the actual volumeof ponds. In the case of APs #3 to 5, the re-suspensionof settled solids occurred due to bubbling up of biogasas well as scouring of settled materials near the pondoutlet zone along the outflow. The re-suspended solidscarried out with pond effluent were the cause of reducedsuspended solid removal efficiency of the ponds.Tay (1982) proposed a settling model given in Eq. (1)based on the settling characteristics of suspension andhydraulic characteristics of the tank for settling perfor-mance. The model considered detention time and half-settling time as hydraulic characteristics and settlingcharacteristics, respectively. Half-removal time is de-fined as the time at which 50% of influent suspendedsolid is removed.S0? SS0TT th1where S0influent TSS, mg/l; S effluent TSS, mg/l;T detention time, min; thhalf-removal time, min.The half-removal time with respect to influent sus-pended solid concentration obtained from the settlingexperiment with wastewater from starch factory isshown in Fig. 7. It appears that the half-removal timevalue at 1600 mg/l of influent TSS is on character data(Fig. 7). By ignoring this data a correlation line is drawnwhich follows a relationship of the type shown in Eq. (2)(R2 0:80).th 705:61S0?0:46072For influent TSS of 500 and 12,500 mg/l, the half-re-moval time is calculated to be 9 and 40 min, respectively,from Eq. (2). The relation of TSS removal and settlingtime at half-removal time of 9 and 40 min obtained frommodel Eq. (1) is shown in Fig. 6. Most of the pointsobtained from settling experiment above influent TSS of630 mg/l falls within the range of model values for 9 and40 min half-removal times. It implies that the model isapplicable to define the settling characteristics of starchwastewater.4. ConclusionsThis study essentially focused on evaluating the effi-ciency of a series of anaerobic ponds treating high or-ganic carbon and cyanide containing wastewater from atapioca starch factory. SMA of sludge obtained frompond sediments as well from pond bulk liquid layer wasassessed in order to determine the relative conversionrates in bulk liquid and sediment layer. Cyanide degrad-ability of sludge taken from the pond sediment layer wasalso assessed. Furthermore, investigation on TSS set-tling characteristics of starch wastewater was also car-ried out.The existing anaerobic pond system effectively re-moved organic carbon and suspended solids. However,0.00.81.0050 100 150 200 250 300Fraction of TSS removal (S0-S)/S0Settling time (min)Model(th=40 min)Model(th=9 min)S0=490 mg/lS0=630 mg/lS0=1632 mg/lS0=3302 mg/lS0=3508 mg/lS0=4588 mg/lS0=5112 mg/lS0=7780 mg/lS0=9126 mg/lS0=12528 mg/lFig. 6. Settling curve for various influent suspended solid concentra-tions.y = 705.61x-0.4607R2 = 0.803602040608003000600090001200015000Influent TSS concentration (mg/l)Half removal time (min)Fig. 7. Half-removal time against influent TSS.B.K. Rajbhandari, A.P. Annachhatre / Bioresource Technology 95 (2004) 135143141the treated effluent required further treatment to meetthe effluent standard before final discharge into anysurface water. Overall COD and TSS removal of over90% was achieved as influent COD of 13,941359 mg/lwas reduced to less than 700 mg/l and influent TSSof about 91303067 mg/l was reduced to less than 600mg/l. Overall CN removal of 51% was observed from thesystem by reducing the concentration from 17.51.5mg/l in starch factory wastewater to 8.50.7 mg/l in thefinal effluent of series of anaerobic pond system.Active biomass from the anaerobic pond sedimentsand bulk liquid had SMA of 20.7 and 11.3 ml CH4/gVSSd, respectively. Likewise cyanide degradability ofsludge obtained were 0.430.84 mg CN?/g VSSd atinitial cyanide concentration of 1020 mg/l, respectively.Settling column test on wastewater from starch fac-tory revealed that settling time of approximately 120min were sufficient to remove 9095% of the influentTSS.AcknowledgementsThis research was carried out under ModelingTools for Environment and Resource Management(MTERM) project funded by Danish InternationalDevelopment Assistance (Danida) and Waste WaterTreatment and Management project under AsianRegional Research Program on Environmental Tech-nology (ARRPET) funded by Swedish InternationalDevelopment Agency (SIDA). The authors are thank-ful to Prof. Jean-Luc VASEL, Foundation UniversitaireLuxembourgeoise, Arlon, Belgium for his critical sug-gestions throughout this research.ReferencesAnnachhatre, A.P., Amatya, P.L., 2000. UASB treatment of tapiocastarch wastewater. J. Environ. Eng. 126 (12), 11491152.Annachhatre, A.P., Amornkaew, A., 2000. 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