Bioresource Technology 103 (2012) 415–424

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Bioresource Technology

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Low temperature thermo-chemical pretreatment of dairy waste activated sludgefor anaerobic digestion process

R. Uma Rani a, S. Adish Kumar a, S. Kaliappan b, Ick-Tae Yeom c, J. Rajesh Banu a,⇑a Department of Civil Engineering, Anna University of Technology, Tirunelveli 627007, Indiab Department of Civil Engineering, Anna University, Chennai 600025, Indiac Department of Civil and Environmental Engineering, Sungkyunkwan University, South Korea

a r t i c l e i n f o

Article history:Received 13 July 2011Received in revised form 28 September 2011Accepted 28 September 2011Available online 13 October 2011

Keywords:Waste activated sludgeThermo-chemical pretreatmentBiogasBiodegradabilityAnaerobic digestion

0960-8524/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.biortech.2011.09.124

⇑ Corresponding author. Tel.: +91 9444215544.E-mail address: rajeshces@gmail.com (J. Rajesh Ba

a b s t r a c t

An investigation into the influence of low temperature thermo-chemical pretreatment on sludge reduc-tion in a semi-continuous anaerobic reactor was performed. Firstly, effect of sludge pretreatment wasevaluated by COD solubilization, suspended solids reduction and biogas production. At optimized condi-tion (60 �C with pH 12), COD solubilization, suspended solids, reduction and biogas production was 23%,22% and 51% higher than the control, respectively. Secondly, semi-continuous process performance wasstudied in a lab-scale semi-continuous anaerobic reactor (5 L), with 4 L working volume. With three oper-ated SRTs, the SRT of 15 days was found to be most appropriate for economic operation of the reactor.Combining pretreatment with anaerobic digestion led to 80.5%, 117% and 90.4% of TS, SS and VS reductionrespectively, with an improvement of 103% in biogas production. Thus, low temperature thermo-chem-ical can play an important role in reducing sludge production.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Dairy industry is one of the prime sectors in India. Studiessay that Indian dairy industries have a growth at more than15% and are estimated to cross 150 million tons per annum.Water management in the dairy industry is well documented,but effluent production and disposal remain a problematic issuefor the dairy industry. Proper management of excess sludge is abig challenge to wastewater treatment operators because sludgehandling and disposal accounts for up to 60% of total treatmentplant operating costs (Neyens and Baeyens, 2003). Anaerobicdigestion is of particular interest in sludge treatment since itcan reduce the overall amount of sludge to be disposed, whileproducing an energy-rich biogas that can be valorized energeti-cally (Appels et al., 2010). Anaerobic digestion thus optimizeswastewater treatment costs, its environmental footprint and isconsidered a major and essential part of a modern wastewatertreatment plant. Anaerobic degradation can be achieved throughseveral stages: hydrolysis, acidogenesis, acetogenesis and metha-nogenesis. Anaerobic digestion of sludge is hampered due to therigid structure of the microbial cell walls, protecting the innercell products. Hence, hydrolysis of sludge requires longer reten-tion time and has been pointed as the rate limiting step. In order

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to improve the rate of hydrolysis and the anaerobic digestionperformance, sludge disintegration was developed as a pre-treat-ment process to accelerate the anaerobic digestion and to in-crease the degree of stabilization. Increasing the degree ofsludge stabilization using a disintegration process provides lesssludge production, more stable sludge and more biogasproduction compared with classical anaerobic digestion. Variousmethods like ultrasonic treatment (Pham et al., 2009), ozone oxi-dation (Ahn et al., 2002), alkaline treatment (Lin et al., 2007),thermal treatment (Barjenbruch and Kopplow, 2003), Fentonprocess (Kaynak and Filibeli, 2008) and biological hydrolysiswith enzymes (Ayol et al., 2008) were investigated for sludgedisintegration by several researchers in half-scale and lab-scaleplants to bypass the rate limiting stage of hydrolysis. Mechanicaltreatment employs several strategies for physically disintegratingthe cells and partly solubilizing their content. Although less re-sults are available for the other pretreatment methods, it is seenthat their efficiency of improving anaerobic digestion of sludge israther low, compared to the other methods. Ultrasonic pretreat-ment is the alternate method to disrupt sludge cells withoutdoubt. Although cell disintegrations of 100% can be obtained athigh-power levels, power consumption thus becomes a seriousproblem, and the ultrasound probes need replacement every1.5–2 years, which causes great concerns on its practicalapplication (Zhang et al., 2007). However its application as asludge pretreatment process in anaerobic digestion is scarcelymentioned.

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The heat treatment of waste activated sludge was shown to bean effective pretreatment method for anaerobic digestion. Theoptimum treatment conditions and digestion improvement are lar-gely depending on the nature of the sludge. Various temperatures,ranging from 60 to 270 �C have been studied in literature. How-ever, temperatures higher than 180 �C lead to the production of re-calcitrant soluble organics or toxic/inhibitory intermediates, hencereducing the biodegradability (Wilson and Novak, 2009). The onlyalternative to overcome this drawback is the application of lowtemperature treatment, and it has been pointed out as an effectivetreatment for increasing biogas production (Climent et al., 2007).In thermo-chemical methods, an acid or base is added to solubilizethe sludge. The addition of acid or base avoids the necessity of hightemperature and these methods are carried out at ambient or mod-erate temperatures. These methods are shown to be effective andcumbersome for sludge solubilization. The objective of the currentwork is to study the solubilization of organic and inorganic com-pounds during thermo-chemical treatment and the effects onanaerobic biodegradability. Finally, this thermo-chemical pretreat-ment was combined with semi continuous anaerobic digesters inorder to evaluate their effects in terms of solids reduction and ofbiogas production.

2. Methods

2.1. Sludge sampling and characterization

The waste activated sludge was obtained from a dairy plant atTirunelveli in Tamil Nadu (India). Samples were collected andstored at 4 �C. The characteristics of the raw sludge were asfollows: pH was 6.96, CODS (SCOD) was 420 mg/L, total COD(TCOD) was 22,000 mg/L, total solids (TS) content was 8513 mg/L, volatile solids (VS) content was 5160 mg/L, suspended solids(SS) content was 4700 mg/L, Proteins was 780 mg/L and carbohy-drates were 320 mg/L.

2.2. Thermo-chemical pretreatment

In this study, the effect of low temperature thermo-chemicalpretreatment was investigated. The low temperaturethermo-chemical pretreatment was carried out at 50, 60, 70 and80 �C in order to enhance solubilization of particulate material,as well as enzymatic hydrolysis. The effect of thermo-chemicaltreatment depends on both temperature and time. In this work,the effect of pretreatment time was evaluated by taking samplesat different pretreatment times (6, 9, 12, 24, 36 and 48 h) in orderto study the combined effect.

Batch reactors containing 1 L of sludge were submersed in athermostatic bath at various temperatures (50, 60, 70 and80 �C) during 6, 9, 12, 24, 36 and 48 h. The reactors were coveredwith an aluminium foil, to avoid water evaporation. The sludge inthe reactor was kept in suspension by a slow-speed stirrer (Digi-tal Overhead IKA RW 20), to ensure temperature homogeneity.For the waste activated sludge studied, different concentrationsof NaOH were added to reach different pH values of 10, 11 and12. Even though, sodium decrease the dewaterability of thesludge, the choice of this alkaline agent was made from differentstudies indicated that, for anaerobic digestion, pretreatment withNaOH was more efficient than using other alkaline agents (Kimet al., 2003, Lin et al., 2007). The negative influence of sodiumover sludge dewaterability is reduced when NaOH is combinedwith other treatment methods such as microwave and thermal(Jin et al., 2008; Ilgin and Dilek., 2009). Dewaterability of the so-dium pretreated sludge can be improved by the subsequentsludge management using lime. Banu et al. (2012) have used lime

to decrease the capillary suction time of sodium pretreatedsludge.

2.3. Biochemical methane potential assay

Biogas production of raw and pretreated sludge samples at var-ious temperatures (50–80 �C) for 6, 9, 12, 24 h was initially deter-mined by batch tests at mesophilic conditions. After pretreatmentapplications and before the reactor setup, pH of all pretreated sam-ples were neutralized to 7. The biodegradability assays were con-ducted in batch reactors with 300 mL serum bottles. The rumenmicro-organisms of cattle dung were used as an inoculum. More-over in case of anaerobic biodegradability, the use of a highly activeanimal inoculum waste will reduce the experimental time signifi-cantly, or reduced the amount of inoculum required in full-scalebatch digesters, and consequently, the corresponding digester vol-ume (Borja et al., 2003). Each serum bottle was filled with 100 g ofinoculum and 50 g of substrate. A blank treatment with only 150 gof inoculum was used to determine biogas production due toendogenous respiration. After adding the substrates and inoculum,the reactors were closed with a rubber septum and an aluminiumseal to make them air tight and was subsequently purged withnitrogen gas at the rate of 10 mL/s for 20 min into the reactors tomaintain anaerobic conditions. Bottles were maintained at 35 �Cunder shaking (220 rpm). Batch reactors were operated with a res-idence time of 50 days. Samples were analysed in duplicate.Enhancement of biodegradability was evaluated by comparisonof biogas volumes produced by treated and untreated samples.Theanaerobic biodegradability test’s performance was evaluated byfitting the cumulative biogas production data to the modifiedGompertz equation. The Gompertz equation describes cumulativebiogas production assuming that, biogas production is a functionof bacterial growth (Redzwan and Banks, 2004). The modifiedGompertz equation is presented below:

B ¼ Pf1� exp ½�Rmðt � kÞ=P�g

where B is the cumulative biogas production (mL), Rm is themaximum biogas production rate, P is the biogas yield potential(mL), and k is the duration of lag phase, days. Using Matlab 7.0,the unknown parameters were calculated in order to measure thedifference between the experimental measurement and the corre-sponding stimulated value.

2.4. Anaerobic digestion reactors

Two identical laboratory scale semi continuous reactors with aworking volume of 4 L were used as anaerobic digesters atmesophilic temperature (35 �C). Among the two, one is designatedas Control Semi Continuous Anaerobic Reactor (CSCAR) which actsas control and another is designated as Experimental SemiContinuous Anaerobic Reactor (ESCAR), where sludge reductionwas carried out. Sludge retention times (SRT) of 20, 15 and 12 dayswere sequentially tried to investigate the performance of theanaerobic digestion of the pretreated sludge. A control reactorfed with untreated sludge was operated at same SRTs. Feedingand withdrawals were carried out each day by peristaltic pumpsin a semi continuous mode. Biogas production was measured bywater moving in graduated test tubes linked to the reactors.

2.5. Analytical parameters

The following parameters were analysed before and after ther-mo-chemical treatment: total solids (TS), suspended solids (SS),chemical oxygen demand (COD), carbohydrate concentration,protein concentration and pH as per Standard Methods for theExamination of Water and Wastewater (American Public Health

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Association, 2005). The analyses were performed on both thesludge and the supernatant to identify the total and soluble frac-tions of the specific component. TS, VS and SS were measured inthe whole sludge (Total TS, VS and SS) and in the supernatant aftercentrifugation at 30,130�g, for 15 min and subsequent filtrationthrough 0.45 lm microfiber filter paper. VFA was analysed bydistillation–titration method, and the result was expressed in ace-tic acid. The methane content in the biogas was analysed using aBaroda gas chromatograph equipped with a thermal conductivitydetector and porapack Q column with hydrogen as carrier gas ata flow rate of 40 mL/min.

Protein concentration was determined on total sludge and onthe supernatant using the Lowry method (Takahashi et al., 2009).After reactions with salts and Folin’s phenol reagent, absorbanceof samples was determined at 620 nm, using a spectrophotometer.Folin’s phenol reagent is a mixture of phosphomolybdate andphosphotungstate used for the colorimetric assay of phenolic andpolyphenolic antioxidants. It works by measuring the amount ofthe substance being tested needed to inhibit the oxidation of thereagent. Carbohydrate concentration was determined on totalsludge and on the supernatant using the anthrone method (Tapiaet al., 2009). After reactions with anthrone and sulphuric acid,absorbance of samples was determined at 625 nm, using aspectrophotometer. The increase in SCOD was calculated as givenbelow:

SCOD ð%Þ ¼ ðSCODafter pretreatment � SCODbefore pretreatmentÞSCODbefore pretreatment

� 100

3. Results and discussion

3.1. Thermo-chemical pretreatment

In this study, thermo-chemical pretreatment of waste activatedsludge was performed to improve the treatment efficiency. Thecontrol was performed using non-pretreated WAS. The treatmentwas performed for different temperature and treatment time.The expected effect of the thermo-chemical treatment of sludgewas an increase in soluble materials, with interest focused onSCOD solubilization, suspended solids reduction and biogas pro-duction, thus enhancing hydrolysis.

3.1.1. COD solubilization and SCOD releaseThe pretreatment was done to improve the bioavailability of

sludge particulate material. SCOD calculations were consideredthe main parameter for evaluation of sludge particulate material,and it enables an evaluation of the maximum level of sludge solu-bilization. Increased SCOD is determined as the substance that canbe readily used to produce methane during anaerobic digestion(Wang et al., 2005). The SCOD of pretreated sludge increased withincreasing temperature. Fig. 1a shows the optimization of time andpH for COD solubilization during low temperature thermo-chemical pretreatment. From the figure, it is evident that, as thetreatment time was increased from 6 to 24 h, an increase inCOD solubilization was observed. This may be due to the disrup-tion of chemical bonds in cell walls and membranes bythermo-chemical treatment. Therefore intracellular organic mate-rial is released to the liquid phase and increases the SCOD (Appelset al., 2010; Banu et al., 2009). As reaction time increases from 24to 48 h SCOD was found to be decreased. The sludge is subjected tolow temperature thermo-chemical treatment for a significantlyprolonged time (24 h). Thermal treatments are prone to enhancethe formation of refractory substances. The fall in SCOD after24 h might be due to the formation of refractory substances. Thus,for the waste activated sludge sample, a treatment time of 24 h

was found to be the optimum. Similarly, a pH 10 seems to be toolow for an effective COD solubilization, since even after a treatmenttime for 24 h only a very limited amount was set free. Thus, in or-der to achieve better solubilization, pH 12 was considered to be anoptimum condition. Likewise, at 50 �C for pH 12, the COD solubili-zation was found only 17%. However, for 60, 70 and 80 �C irrespec-tive of pH, the temperature plays a major role in enhancing CODsolubilization, and it was found to be 23%, 24% and 25%, respec-tively. From the above, it is clear that temperature above 60 �Cdoes not help in solubilizing the excess sludge. Thus, comparingcost economics and considering energy generation, low tempera-ture thermo-chemical treatment (60 �C) was considered to be anoptimum condition for rupturing cell membranes. The study iscarried out at low temperature range of 50–80 �C which are less in-tense when compared to high temperatures. Thus, the solubiliza-tion does not show a rapid rise, but the overall solubilization forthe treatment is satisfactory.

3.1.2. SS reductionSS reduction is an indication of sludge stability, and it is used for

assessing the effectiveness of a process in stabilizing sludge (Gho-lamreza et al., 2008). Fig. 1b shows the optimization of time and pHfor SS reduction during low temperature thermo-chemical pre-treatment. From the figure, it is evident that, as the treatment timewas increased from 6 to 24 h, an increase in SS reduction was ob-served. The main reason for mass reduction of sludge during thethermo-chemical pretreatment might be to rupture the cell walland to release of extracellular and intracellular matter. Thus, it isevident that at 60 �C with pH 12, COD solubilization and SS reduc-tion was found to be 23% and 22%, respectively, which might be re-garded as a threshold for the pre-digestion step. In contrast, pHwas found to be decreased during thermo-chemical pretreatment,and it may be explained by acidic compound’s formation due tofloc disintegration. Lipids were hydrolysed to volatile fatty acids,and these compounds led to be decreased pH (Bougrier et al.,2005).

3.1.3. Soluble carbohydrate and protein releaseCell lysis releases protein content into the medium is the first

stage of floc disintegration. Proteins are the principal constituentsof organisms, and they contain carbon, which is a common organicsubstance as well as hydrogen, oxygen and nitrogen (Nah et al.,2000). For this reason, it was considered that as the level of solubleprotein increased, the efficiency of anaerobic digestion would beimproved. Due to thermo-chemical treatment, solids were solubi-lized, especially organic solids. Fig. 2 presents the solubilizationrate calculated for proteins and carbohydrates for the samples.As given in figure, a higher pH value of 12 led to more carbohydrateand protein releases than pH 10. Increase in the carbohydrate andprotein releases with the increase in pH was stated by Chen et al.(2007). Furthermore, it was found that increase in temperaturedoesn’t play a major role, i.e., the solubilization rate increases lin-early with temperature only till 60 �C, after which the rise is nearlyflat. Thus, 60 �C with pH 12 is significant for protein and carbohy-drates release. According to Liu and Fang (2002), during the sludgetreatment with NaOH, protein is released more compared tocarbohydrate and this result is similar to that obtained in thepresent study. The protein releases presented in the figure arethe sums of protein released from EPS as well as the cell lysis.Hence protein concentration is more than carbohydrate.

3.1.4. Biochemical methane potential assayBiodegradability batch assay was carried out to assess the

feasibility of using thermo-chemical pre-treatment to improvethe anaerobic biological degradation of dairy sludge. Biodegrad-ability assays, in which cumulative biogas production was

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monitored, were assessed to both raw and pre-treated substratesunder mesophilic conditions. Cumulative biogas production in ser-um bottles was maintained for 50 days, and the biogas productionwas monitored every day. Daily biogas was measured by insertingthe needle attached to a syringe (10 and 25 mL). The gas pressurein the bottle displaces the syringe plunger, and the displacedvolume was recorded. The results showed that thermo-chemicalpretreatment of dairy sludge improved anaerobic degradation.Comparative analysis of biogas generation is depicted in Fig. 3a.It was observed that, initial biogas production up to day 5 was sim-ilar in all cases, and this may be due to the acclimatization. The in-crease in biogas production was observed for 60, 70 and 80 �Cpretreated sludge and there is no significant difference betweenbiogas productions. The accumulated biogas production at theend of 50 days of the digestion period, was nearly 415 mL forraw sample, and at 60 �C with pH 12 it was around 845 mL, nearly51% higher biogas production was obtained than the raw sludge.These results suggest that a low temperature thermo-chemicalpretreatment enhances the biogas production. After 50 days ofthe digestion period, the biogas production ceased indicating thatthe digestate did not undergo any further degradation.

Fig. 3b shows the model fit with the experimental data (solidline), from each assay (circles) for the samples. Table 1 presentsthe kinetic parameters obtained in the optimization process. Fromthe experiment conducted, it can be noted that biogas productionis enhanced by an increase in temperature from 50 �C (535 mg/L)to 60 �C (845 mg/L) by 57%. When further increase up to 80 �C(890 mg/L) the increase in biogas production is only 5.3%. Thetrend of biogas production (Rm and P values from Table 1) showsthat the solubilization of sludge at 60 �C with pH 12 attributes tothe significant biogas production. Increasing the temperature hasnot influenced the process to a larger extent other than power con-sumption. At 60 �C with pH 12 the lag time for biogas production isdecreased to 0.99 from 1.4 at 50 �C. The values were calculated forall experimental conditions and are found to be fit (which are notshown).

3.2. Semi continuous anaerobic digestion reactors

During this part of the study, the dairy sludge was thermo-chemically pretreated at 60 �C with pH 12 was used as a feed. Ina semi continuous anaerobic reactors, steady state is assessed by

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the stability of daily gas production, TS, SS and VS concentrationsdigested sludge from each reactor. With this assessment after50 days, the fluctuations in these parameters were less than 10%and it was believed that steady state was achieved. Results ob-tained after reactor stabilization is reported in Table 2.

3.2.1. SRT variationDuring the total period of 200 days, the performance evaluation

of semi continuous anaerobic reactors was carried out at three dif-ferent SRTs such as 20, 15 and 12 days. The particular SRT wasmaintained constant by feeding the sludge at a constant rate bythe peristaltic pump. The feeding was initialized with low organicloading rate of 3.55 g VS/Ld for maintaining a long SRT of 20 days.The SRT was shifted to the next lower value and so on, once thereduction efficiencies of TS, SS, and VS were found to be consistentwith a particular SRT. At individual SRTs, steady state of operationswas retrieved and the results mentioned were average of five con-secutive consistent readings. It was observed that the overall per-formance efficiency in terms of reduction in TS, SS and VS waslowering down as the SRT was lowered.

3.2.2. pHThe digester performance was influenced by pH. The value and

stability of the pH in an anaerobic reactor are extremely importantbecause methanogenesis only proceeds at a high rate, when the pHis maintained in the neutral range. pH values in these experimentalremained in the range from 7.9 to 7.1 during anaerobic digestion. Itwas observed that during every shift to the next lower SRT, the pHdropped rapidly. The dropping down of pH at shorter SRT is due tothe high organic loading rate, which results in high VFA concentra-tion during the acidogenesis phase. The results reveal that, digestercould maintain their pH themselves though higher organic loadingwas supplied.

3.2.3. AlkalinitypH cannot be an effective measure of the stability of an

anaerobic process when there is a high buffering capacity (Bprns-son et al., 2000). Under this condition, alkalinity levels reveal a po-tential anaerobic process performance directly. The alkalinity of asteady system is between 1000 and 5000 mg CaCO3 L�1 (Ren and

Wang, 2004). Lower values of effluent alkalinity warn about theimpending reactor failure. The variation of pH and alkalinity withdigestion time is presented in Fig. 4a. The alkalinity of feed sludgeand digested sludge varied significantly as pretreatments wereused. The ESCAR digester had higher alkalinity compared to CSCARdue to the addition of NaOH. At 20 and 15 days SRTs, the effluentalkalinity levels were 19% and 34% more than the influent alkalin-ity. Manariotis and Grigoropoulos (2002) observed the increasedlevels between 19% and 21%, while Bodkhe (2009) observed therange of 25–33%. Similar to pH, the alkalinity reduced at shorterSRT is since a more organic matter was supplied, it eventually in-creased with acid generation and subsequently neutralizes thealkalinity capacity.

3.2.4. Volatile fatty acidsVolatile fatty acids are considered as a central parameter that

governs the healthiness of the anaerobic treatment (Pind et al.,2002). These are the most important intermediates, where theyare degraded by proton-reducing acetogens in association withhydrogen consuming methanogenic bacteria (Mechichi and Sayadi,2005). Thus, VFA determination in conjunction with pH measure-ment is an essential prerequisite for maintenance of desired envi-ronmental conditions in the anaerobic reactor. Fig. 4b presentedthe VFA concentration of anaerobic digester which followed thetrend of increasing, firstly, and then decreasing. The VFA concen-tration varied in a range of 227–790 mg/L and fell well withinthe recommended value for healthy anaerobic digestion. As a re-sult of increased concentrations of VFA, the acids accumulate andthe pH decreases to such a low value that the hydrolysis andacetogenesis can be inhibited (Siegert and Banks, 2005). The aver-age VFA concentration of ESCAR (363 mg/L) was found to be lowerthan CSCAR (590 mg/L) indicating greater utilization of VFA bymethanogens and subsequent biogas production in ESCAR. Accord-ing to Dearman and Bentham (2006), when VFA concentrationstarted to decrease, biogas production rate increase.

3.2.5. TS removalFig. 5a shows the variation of TS in digested sludge. For TS re-

moval, CSCAR was in the range of 6.1–9.6% and digester ESCAR13–25.1%. TS analysis shows that there was an increase of 161%

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Table 1Kinetic parameters calculated from the theoretical model for various samples.

Samples P (mL) Rm (d) R2

Control 694 23.09 1.4026 0.967750 at pH 10 696 23.41 1.2249 0.984750 at pH 11 816 22.46 0.9554 0.987750 at pH 12 986 22.52 0.3818 0.997660 at pH 10 904 28.77 1.2125 0.998460 at pH 11 993 28.95 1.4096 0.986560 at pH 12 1071 39.36 0.9944 0.998770 at pH 10 1072 39.57 0.8938 0.985470 at pH 11 1075 39.95 0.7440 0.976770 at pH 12 1090 39.91 0.6773 0.989880 at pH 10 1091 40.40 0.6242 0.994280 at pH 11 1103 41.14 0.5966 0.967580 at pH 12 1107 42.78 0.6529 0.9695

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in TS reduction when compared with CSCAR, since TS reductionwas 25.1% in ESCAR at 15 days SRT. The results revealed that ther-mo-chemical is an effective technique for breaking down the

microbial cells or difficult hydrolysed compounds to easy biode-gradable compounds.

3.2.6. SS and VS removalThe SS and VS reductions can be regarded as the reductions that

are expected to be achieved in total mass of sludge. The variation ofSS and VS removal is graphically presented in Fig. 5b. For SS re-moval, CSCAR was in the range of 17–19.5% and digester ESCARwas in the range of 33–39.2%. The highest SS removal achievingfrom ESCAR digester at 15 days SRT was 39.2% with an increaseof 101% over CSCAR. Thus, a pretreatment with thermo-chemicalgave a great advantage in SS removal improvement compared tonon-pretreatment.

For VS removal, CSCAR was in the range of 14–17% and digesterESCAR 25–33%. The VS removal of ESCAR at 20 and 15 days SRTwas almost constant and slightly reduced at 12 days SRT. However,it reduced for CSCAR from long to short SRT. This higher perfor-mance is due to the combination effects of thermal and chemical,which help to break down the microbial cells for faster subsequent

Table 2Digester’s performance after stabilization.

Parameters CSCAR ESCAR

SRT (days) 20 15 12 20 15 12Experimental run (days) 65 49 36 65 49 36OLR (g VS/Ld) 3.5 4.7 5.9 3.5 4.7 5.9Feed pH 6.9 6.9 6.9 7.5 7.5 7.5Outlet pH 6.9 6.9 6.8 7.4 7.3 7.1Alkalinity (mg/L) 2973 2504 2235 3538 3354 2982TS removal (%) 6.2 9.6 9 13 25.1 22Increasea (%) 113 161 144SS removal (%) 17 19.5 19 33 39.2 32.3Increasea (%) 94 101 70VS removal (%) 14 17 14.9 27 33 25Increasea (%) 92 94 67Biogas production (mL/g VS added) 196 332 415 378 674 862Increasea (%) 93 103 107Methane production (mL/g VS added) 129 222 270 246 465 569Methane content (%) 65 67 65 65 69 66VFA production (mL/day) 786 570 302 580 260 201

a Increase is expressed as percentage over CSCAR.

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0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

TS ESCAR TS CSCAR

Anaerobic Digestion Period (days)

TS

Red

ucti

on (

%)

50

40

30

20

10

0

Fig. 5. Removal trend of TS, SS and VS of digested sludges at different SRTs: (a) TS reduction; (b) SS and VS reduction.

422 R. Uma Rani et al. / Bioresource Technology 103 (2012) 415–424

degradation. It eventually facilitates the decomposition reactionwhich leads to biodegrade more compounds in the digester.

3.2.7. Biogas productionFig. 6 shows the comparison of biogas production of the digest-

ers at different SRTs. The first SRT (20 days) was started at diges-tion run time of 55 days until 115 days, then second SRT(15 days) until 165 days, and finally, the third SRT (12 days) until200 days. By comparing to control, the methane production ofthermo-chemically pretreated sludge increased by 90%, 109% and111% at 20, 15 and 12 days SRT, respectively. It clearly showed thatthe methane production rate appreciably increased from 20 to15 days SRT, while minor improvement was observed from 15 to12 days SRT. The methane content in the biogas varied from 65to 70%. The higher gas production in thermo-chemically pretreatedsludge evidently indicated that it hydrolysed more organic mate-rial solution, which is immediately used by anaerobic bacteriaand eventually facilitates the digestion processes. Furthermore,

the biogas production rate was high at short SRT and low at longSRT. Once the SRT was reduced, the organic loading rate automat-ically increases, which eventually come to over loading phenome-non. It was clearly shown by dropping pH at 12 days SRT.Therefore, by considering both biogas and methane production,SRT of 15 days was found to be the suitable retention time foreffective sludge degradation.

3.2.8. Selection of pertinent SRTThe minimum SRT at which the reactor yields the designed

treatment efficiency highly influences the reactor volume, its com-pactness and the economy of operation. The economic viability ofsludge management is not feasible when the reactor volume is lar-ger. Therefore, the selection of the most suitable minimum possi-ble SRT is very important to ensure the cost-effectiveness of thereactor operation. The results obtained at various SRTs identified,the SRT of 15 days to be the most appropriate SRT for economicoperation of the reactor.

0

100

200

300

400

500

600

700

800

900

1000

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Biogas-ESCARBiogas-CSCAR

Bio

gas

Pro

duct

ion

(mL

/gV

S ad

ded)

Anaerobic Digestion Period (days)

Fig. 6. Biogas production rate for the digesters with digestion period.

R. Uma Rani et al. / Bioresource Technology 103 (2012) 415–424 423

4. Conclusions

The influence of low temperature thermo-chemical pretreat-ment of dairy sludge was studied. At 60 �C with pH 12, COD solu-bilization and suspended solids reduction was 23% and 22% higherthan that of control. BMP assay results of pretreated sludge con-firmed that the observed solubilization led to an increase in sludgebiodegradability, nearly 51% higher biogas production than control.Thus, 60 �C with pH 12 was chosen for semi-continuous digestersand by combining pretreatment with anaerobic digestion led to80.5%, 117% and 90.4% of TS, SS and VS reduction, respectively,with an improvement of 103% in biogas production.

Acknowledgement

Authors are thankful to Department of Biotechnology, India forpartial financial assistant to this project (BT/PR13124/GBD/27/192/2009) under their scheme Rapid Grant for Young Investigator(RGYI).

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