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Biological pretreatment of non-flocculated sludge augments the biogas production in the anaerobic digestion of the pretreated waste activated sludge a
a
b
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J. Merrylin , S. Adish Kumar , S. Kaliappan , Ick-Tae Yeom & J. Rajesh Banu
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Department of Civil Engineering , Regional Centre of Anna University , Tirunelveli , India
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Department of Civil Engineering , Ponjesley College of Engineering , Nagercoil , India
c
Department of Civil and Environmental Engineering , Sungkyunkwan University , Seoul , South Korea Accepted author version posted online: 03 Jun 2013.Published online: 27 Jun 2013.
To cite this article: J. Merrylin , S. Adish Kumar , S. Kaliappan , Ick-Tae Yeom & J. Rajesh Banu (2013) Biological pretreatment of non-flocculated sludge augments the biogas production in the anaerobic digestion of the pretreated waste activated sludge, Environmental Technology, 34:13-14, 2113-2123, DOI: 10.1080/09593330.2013.810294 To link to this article: http://dx.doi.org/10.1080/09593330.2013.810294
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Environmental Technology, 2013 Vol. 34, Nos. 13–14, 2113–2123, http://dx.doi.org/10.1080/09593330.2013.810294
Biological pretreatment of non-flocculated sludge augments the biogas production in the anaerobic digestion of the pretreated waste activated sludge J. Merrylina , S. Adish Kumara , S. Kaliappanb , Ick-Tae Yeomc and J. Rajesh Banua∗ of Civil Engineering, Regional Centre of Anna University, Tirunelveli, India; b Department of Civil Engineering, Ponjesley College of Engineering, Nagercoil, India; c Department of Civil and Environmental Engineering, Sungkyunkwan University, Seoul, South Korea
a Department
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(Received 26 February 2013; final version received 23 May 2013 ) High-efficiency resource recovery from municipal solid waste (MSW) has been a focus of attention. The objective of this research is to develop a bio-pretreatment process for application prior to the anaerobic digestion of MSW to improve methane productivity. Bacillus licheniformis was used for pretreating MSW (non-flocculated with 0.07% citric acid), followed by anaerobic digestion. Laboratory-scale experiments were carried out in semi-continuous bioreactors, with a total volume of 5 L and working volume of 3 L. Among the nine organic loading rates (OLRs) investigated, the OLR of 0.84 kg SS m−3 reactor day−1 was found to be the most appropriate for economic operation of the reactor. Pretreatment of MSW prior to anaerobic digestion led to 55% and 64% increase of suspended solids (SS) and volatile solids reduction, respectively, with an improvement of 57% in biogas production. The results indicate that the pretreatment of non-flocculated sludge with Bacillus licheniformis which consumes less energy compared to other pretreatment techniques could be a cost-effective and environmentally sound method for producing methane from MSW. Keywords: anaerobic digestion; methane production; municipal solid waste; biochemical methane potential; bacterial pretreatment
1. Introduction Anaerobic digestion is one of the more commonly used stabilization processes in sludge management, and provides effective pathogen destruction, reduction of volatile solids (VS) and odour potential, and an energy source in the form of biogas.[1] Much progress has been made in the fundamental understanding and control of the anaerobic digestion process, and the design and application of equipment. This process is one of the dominant processes for stabilizing sludge because of the energy conservation and recovery. Anaerobic digestion of municipal wastewater sludge can in many cases produce sufficient digester gas to meet most of the energy needs for plant operation.[2] Microorganisms and microbial–environment conditions are crucial in this process, as in other biological treatment processes. During the anaerobic digestion of biosolids, the first stage in the degradation of particulate organic matter is the solubilization and enhanced hydrolysis of complex polymeric organic carbon structures. The slow degradation rate of sludge in an anaerobic digester is due to the rate limiting step of sludge hydrolysis. This is caused by the low biodegradability of the cell walls and extracellular biopolymers in sludge. It is important to reduce the amount of sludge produced and to reduce its residual organic content. The methods of
∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
improvement of biodegradability of a particular substrate are mainly based on better accessibility of the substrate for microbes. Pretreatment can improve the subsequent anaerobic digestion. One advantage of sludge solubilization prior to anaerobic treatment is that increasing the amount of released soluble substrate significantly enhances volatile fatty acids (VFAs) generation for improved gas production during anaerobic digestion. Second, pretreatment decreases the viscosity of the sludge, permitting a greater solids concentration to be fed to an anaerobic digester. A higher solids concentration in the feed either enables increased digestion times in an existing digester, or allows for a small digester volume.[3] Various sludge pretreatment methods have achieved significant results in the lysis or disintegration of municipal sludge, and have thereby enhanced biogas production. Several methods have been studied to treat municipal sludge, including thermal,[4] chemical,[5] ultrasonic [6] and mechanical methods.[7] In addition, biological-microbial enzymes or bacterial methods,[8,9] and the combination of such processes, have also been used.[10] Bacteria are usually contained within a flocculated matrix of exopolymeric substances.[11,12] Extracellular polymeric substances (EPS) as a network detains the extracellular hydrolytic enzymes.[13] Researchers have
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also suggested that a higher concentration of EPS protein could bring more negatively charged amino groups,[14,15] and could thus strengthen the electrostatic interactions with cations. So the extraction of EPS from sludge could lead to a reduction in sludge volume and mass.[16] When enzymes are added to sludge, they are often adsorbed on to the sludge matrix, which can lead to inactivation. Treating the sludge with a cation binding agent prior to enzymatic pretreatment improves the organic matter in the sludge as a substrate for the enzymes through making components previously protected by the EPS structure more available to be degraded.[17] The use of microbial enzymes for the enhancement of degradation of waste activated sludge (WAS) is the basis for another process called the enzymatic hydrolysis process.[18,19] The primary benefit is the killing of pathogens, but a further benefit is the enhancement of biogas production in anaerobic digestion. A 10% improvement in biogas production was found in laboratory trials.[19] When a laboratoryscale mesophilic anaerobic reactor received this pretreated municipal WAS, a 1.5-fold increase in biogas production compared with a system receiving unconditioned sludge was found. In this study, a mesophilic anaerobic reactor is considered, which is fed with pretreated municipal WAS.
2. Materials and methods 2.1. Sludge sampling Municipal sewage sludge was collected from secondary clarifier in a local wastewater treatment plant in Trivandrum, Kerala in a sterilized container. The collected sample was aerated continuously at room temperature to prevent settling of the solids and to prevent constant changes in the biological sludge. The amount of biogas production from the sludge depends on the characteristics of the waste. Sludge characteristics such as protein, carbohydrates, total solids (TS), VS contents, and pH should be investigated before pretreatment. The pH of the raw sludge was 6.5, the CODS was 200 mg L−1 , the total COD was 12 g L−1 , the TS content was 13 g L−1 , the VS content was 8 g L−1 and the suspended solids (SS) content was 10 g L−1 .
2.2. Removal of EPS EPS is removed from sludge before pretreatment using the optimized citric acid concentration of 0.07%, according to the procedure by Merrylin et al. [20] The dosage of citric acid used is indicated as the gram equivalent, i.e 0.07 gm of citric acid is added to 100 mL of sludge. The mixture was kept at 4◦ C for 3 h with constant agitation to ensure proper mixing.[20] Through EPS removal, the sludge was dispersed and deflocculated. This deflocculation accelerates the bacterial activity. This deflocculated sludge is further subjected to bacterial pretreatment.
2.3. Bacterial pretreatment Bacillus licheniformis, a thermophilic protease producing bacterium, was used to pretreat the deflocculated sludge. This bacteria was mass cultivated in a nutrient broth at a temperature of 55◦ C in a 5 L fermenter, and the cells were harvested from the pure cell culture at the early stationary phase (20 h). These cells served as the inoculum. These cells were innoculated in a concentration of 77 × 106 bacterial cells mg−1 of SS. The inoculated sludge was pretreated for 24 h and this pretreated sludge was used for further studies.
2.4. Biochemical methane potential A biochemical methane potential (BMP) test was used to evaluate biogas recovery from sludge after bacterial pretreatment. In this test, biogas production of raw and pretreated sludge samples (both flocculated and deflocculated) was initially determined by batch tests at 35◦ C in three reactors. In reactor 1, 50 mL of deflocculated sludge (EPS removed and bacterially pretreated) was inoculated with 150 mL of cow rumen and fed into the 300 mL reactor. In reactor 2, 50 mL of flocculated sludge (bacterially pretreated) was inoculated with 150 mL of cow rumen and was fed into a 300 mL reactor. Reactor 3 was run as a control in which 50 mL and 150 mL of inoculum (cow rumen) was used to determine endogenous amount of biogas production. Rumen bacteria from the digestive tract of a cow were used as an innoculum. The rumen is an exclusive organ of ruminant animals in which the digestion of cellulose and other polysaccharide molecules occur through the activity of specific microbial populations. The capacity of cellulose digestion that these animals possess is related to the presence of anaerobic microorganisms in its rumen, which decompose up the glucose polymer chains to acetate. Methanogenic microorganisms that convert acetate into methane and carbon dioxide also exist in rumen.[21] The better performance of the inoculated reactors may be related to the potential increase in the number of indigenous anaerobic microorganisms of rumen, which contributed substantially to the degradation of the organic material in the reactor. After adding the substrates and the inoculum, the head space above the liquid in the assay reactors is purged with 30% CO2 and 70% N gas to enhance the anaerobic conditions in the reactor. The gas mixture was introduced into the reactor at the rate of 1 L min−1 for 5 min. Then, the reactor was sealed with a rubber septum and aluminium to make it air tight, and then placed on a shaker (150–200 rpm). The biogas was measured by inserting a needle into the septum. The gas pressure in the reactor was allowed to displace the syringe plunger, and the displaced volume is recorded. The cumulative gas production was measured using a water displacement method. The reactors were continuously shaken to allow sufficient blending. The methane content in the biogas was measured using a gas chromatograph. The modified Gompertz equation was to study the cumulative biogas
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Figure 1.
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Semi-continuous anaerobic reactor set-up.
generation and the kinetics of biogas production. Rb Bt = B∗ exp − exp , B∗ exp(λ − t) + 1
(1)
where Bt is the cumulative biogas produced (mL) at any time (t), B is the biogas production potential (mL), Rb is the maximum biogas production rate (mL day−1 ), and λ is the lag phase (days), which is the minimum time taken to produce biogas, or the time taken for bacteria to acclimatize to the environment, in days. The constants B, Rb and λ were determined using the non-linear regression approach with the aid of Polymath software. In many biological fields, the basic knowledge of phenomena is insufficient to build a mechanistic model. In this study, the effect of EPS removal in sludge to produce methane in the anaerobic digester was analysed using a Gompertz model,[22] as shown in Equation (1). This equation was utilized by researchers to study the cumulative methane production in biogas production.[23]
times (SRT) ranging from 30 to 10 days were sequentially applied to investigate the performance of the anaerobic digestion of the pretreated sludge. Moreover, in order to preserve the nitrification capability of the activated sludge, high SRT were applied to the biomass in the activated sludge process (≥10 days). The first three loadings were carried out with different mixed liquor SS (MLSS) of 5000, 7500 and 10,000 at the constant SRT of 30 days. The rest of the loadings were carried out with different SRT of 25, 20, 17, 15, 12 and 10 days with a constant MLSS of 10,000. The CR was also run under the same conditions as that of the ER. Feeding and withdrawals were carried out each day using peristaltic pumps in a semi-continuous mode. Biogas production was measured by the liquid displacement method, in which the difference in the water level in a cylinder linked to the reactors is observed. The displaced liquid is therefore considered to have the same volume as the produced biogas. 2.6.
2.5. Anaerobic semi-continuous reactor Two identical semi-continuous reactors (control and experimental) were run. The control reactor (CR) was fed with the raw sludge and the experimental reactor (ER) was fed with pretreated sludge. Their total volume was 5 L, and their working volume was 3 L, the reactor set-up is shown in Figure 1. The reactors were inoculated with the activated slurry collected from a local biogas plant in India. Organic loading rate (OLR) of 0.16, 0.26, 0.34, 0.4, 0.5, 0.58, 0.66, 0.83 and 1 kg SS m−3 reactor d−1 with the solids retention
Analytical parameters
The SS, VS and alkalinity parameters were measured according to the standard methods [24] to determine the sludge reduction efficiency for each reactor. The pH was measured using a digital pH meter. VFAs were analysed by distillation-titration method, and the result was expressed in acetic acid. The extent of MLSS reduction during the experiment was calculated by the equation mentioned below: Initial − Final MLSS reduction (%) = × 100. (2) Initial
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The methane content in the biogas was analysed using a Baroda gas chromatograph equipped with a thermal conductivity detector and porapack Q column with hydrogen as the carrier gas at a flow rate of 40 mL min−1 .
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3.
Results and discussion
3.1. Bacterial pretreatment Sludge deflocculated with 0.07% citric acid was pretreated with a protease secreting bacteria, Bacillus licheniformis.[25] Sludge disintegration in biological systems incorporates the mechanism of cell lysis transforming the cell content into the medium, the breakdown of EPS fractions in the flocculated matrix, and the biodegradation of the end products by microbial metabolism.[26] The efficiency of pretreatment was measured in terms of COD solubilization and was found to be in the range of 10–12% throughout the study (data not shown). The anaerobic degradability of the pretreated sludge was assayed using the BMP test.
3.2. BMP test The biogas production from non-flocculated, flocculated and raw sludge was examined in three reactors. The estimated kinetic constants using non-linear regression and other characteristics of the reactors 1–3 are shown in Table 1. A graphical representation of cumulative biogas production is shown in Figure 2. Biogas production is slow at the beginning and at the end of observation; this indicates that the biogas produced under batch condition corresponds to specific growth rate of methanogenic bacteria.[27] To analytically quantify the parameters of the batch growth curve, a modified Gompertz equation was fitted to the cumulative biogas production data. Values of parameters obtained are listed in Table 1. The best fit to the Gompertz equation is compared with the experimental data in Figure 2. During the first 5 days, biogas production is low due to the lag phase of microbial growth. After 7 days, the biogas production increased significantly due to exponential growth of microorganisms. After 20 days, the biogas production stabilized due to the stationary phase of microbial growth. At the end of 21 days, reactor 1 produced the highest cumulative biogas production potential (B) of 195 mL at a maximum biogas production rate (Rb) of 127.37 mL h−1 ,
Table 1.
with a lag phase (λ) of 1.8 days. Reactor 2 had an estimated biogas production potential of 114 mL at a maximum biogas production rate of 86 mL h−1 , with a lag phase of 4 days. In reactor 3, which contained the raw sludge, the biogas production potential was 21 mL at a maximum biogas production rate of 17 mL h−1 . The modified Gompertz equation was observed to adequately describe biogas production with R2 values of 0.9917, 0.9909 and 0.9962 for reactors 1, 2 and 3, respectively.
3.3. Semi-continuous anaerobic digesters 3.3.1. Start-up of the reactor During the acclimatization phase the biologically pretreated sludge was fed at an OLR of 0.16 kg SS m−3 reactor d−1 and the set-up was continued until a stable period was achieved. Stable periods were defined by the process showing a fairly constant performance in terms of biogas production, VFA concentration and pH in the reactor, without showing symptoms of any process unbalance or failure (i.e. cease in biogas production, VFA accumulation or pH drop for at least one loading rate). With this assessment after 50 days, the fluctuations in these parameters were less than 10% and it was believed that steady state was achieved. After normal operation had been established, fresh solids were added to the digester by mixing them with the digesting sludge, which greatly improves the rate of digestion.
3.3.2. OLR variation The OLR is a measure of the biological conversion capacity of the anaerobic digestion system. Feeding the system above its sustainable OLR, results in low biogas yield due to biomass concentration and the mass transfer rate of substrates to bacteria.[28] In such a case, the feeding rate to the system must be reduced. OLR is a particularly important control parameter in continuous systems. Both the biogas yield and methane production have a close relation with OLR, so special emphasis should placed on reactor performance in steady-state conditions in the OLR that do not result in reactor failure.[29] In this study, during a total period of 300 days, the performance of the semi-continuous anaerobic reactors was evaluated at different OLRs. The feeding was initialized with a low OLR of 0.16 kg SS m−3 reactor d−1 and the corresponding
Kinetic parameters calculated from the theoretical model for various samples. Modified Gompertz parameters (model)
Digester Control (1) Flocculated (2) Non-Flocculated (3)
B, L/(gVS)
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0.395313 2.02478 3.571262
17.5008 106.9286 127.37811
4.216581 3.594685 1.845676
0.996254 0.990903 0.99175
0.000132 0.001179 0.001882
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Figure 2.
Modified Gompertz model fit to the experimental data.
SRT was 30 days. When the OLR was increased to 0.26 kg SS m−3 reactor d−1 , the SRT was maintained at 30 days but the MLSS in the feed sludge was increased from 5000 to 7500 mg L−1 . Similarly for the consecutive OLR (0.34 kg SS m−3 reactor d−1 ), the MLSS was increased from 7500 to 10,000 mg L−1 . The OLR was shifted to the next higher value, once the reduction efficiencies of SS and VS were found to be consistent with a particular OLR. An increase in the OLR increases the extent of the reactions, and vice versa. Each time sludge is withdrawn, a fraction of the bacterial population is removed, thus implying that the cell growth must at least compensate for the cell removal to ensure steady-state operation and to avoid process failure.[30] 3.3.3. SS reduction SS reduction is an indication of sludge stability, and it is used for assessing the effectiveness of a process in stabilizing the sludge.[31] It is observed in Figure 3(a) that the overall performance efficiency in terms of reduction in the SS increases as the SRT was reduced. SRT has a significant effect on the digester performance. At individual SRTs, steady-state operations were achieved, and the results mentioned are the average of five consecutive consistent readings. The SRT is determined by the average time it takes for organic material to be digested completely, as measured by the chemical and biological oxygen demand of the exiting effluent. Speeding up the process will make the process more efficient. Microorganisms that consume organic material control the rate of digestion that determines the time for which the substrate must remain in the digestion chamber. Figure 3(a) shows the variation of SS in the digested sludge. There was an SS reduction of about 26% in the ER, and 11%
SS reduction in the CR. SS analysis shows that there was an increase of 55% in the amount of SS reduction in the ER compared to the CR. This indicates that the biological pretreatment with Bacillus licheniformis has a great advantage over the non-pretreated sludge (control). It is also observed in the figure that the maximum SS reduction was achieved when the loading rate was 0.84 kg SS m−3 reactor d−1 , and the reduction % declined with further increases in OLR. 3.3.4. VS reduction VS reduction was examined as well to evaluate the reactor performance and stability of the digestate. During the digestion process, volatile solids are degraded to a certain extent and converted into biogas. The sludge volume is thereby reduced. The degree of stabilization is often expressed as the per-cent reduction in volatile solids.[2] It was observed from Figure 3(b) that with the increase in OLR, there is an increase in VS reduction. A maximum VS degradation value of 30% was achieved when operating with loading rate of 0.84 kg SS m−3 reactor d−1 . On the other hand, when the loading rate was increased further, there was a decrease in VS reductions, and only 28.6% was obtained as illustrated in Figure 3(b). Comparably, these VS reduction were lower than the results by Castillo et al. who also reported that VS removal efficiency is decreased with decreases in the retention time.[32] 3.3.5. Biogas production Figure 4 shows the total biogas production and methane production at different OLRs and SRTs. Initially, the biogas production was 24 mL g−1 with addition of VS, and then it followed a zig-zag path for quite some time
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(a) Removal profile of SS of digested sludge at different OLRs. (b) Removal profile of VS of digested sludge at different OLRs. 200.00
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Figure 4.
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Environmental Technology during the acclimatization period. During each phase of OLR, the total biogas production in the digester showed appreciable increases until a stage when methanogenesis could not work quickly enough to convert acetic acid into methane. The results showed that compared with the control, the methane amount and the biogas production were increased by 57% and 53%, respectively, due to the biological pretreatment. In contrast to the present study, in works with physical and chemical pretreatment, significantly higher levels of biogas production in the range of 84–88% were obtained.[33] However these pretreatment techniques had intensive energy demands and high operation costs.[23] Biogas production was higher with 0.84 kg SS m−3 reactor d−1 , but with an increase in OLR to 1 kg SS m−3 reactor d−1 , it was decreased, which may have been due to either a lack of a critical requirement of inoculum to take the load of additional OLR, or that a lag period proportional to the growth inhibitorrich recalcitrant chemical composition was required.[34] The cumulative biogas produced during the entire reactor operation period of the ER was 51 L, and the cumulative biogas produced during the entire reactor operation period of the CR was 22 L. The decreasing methane content can also be attributed to the higher OLR, which favours a higher growth rate of the acid-forming bacteria over the methanogenic bacteria. Thus, the methane conversion process was adversely affected by reducing the methane content, which led to the formation of carbon dioxide at a higher rate.[35] Therefore, the methane yield was reduced at higher OLR due to the lower methane content. Thus, at OLR greater than 0.84 kg SS m−3 reactor d−1 , the methane production was decreased. The methane gas production rate increased appreciably and ranged from 20 to 412 mL day−1 , as the methane content in the biogas was varied from 65% to 70%.[2] The higher gas production in the bacterially pretreated non-flocculated sludge evidently indicated that it hydrolysed more organic material solution, which was immediately used by anaerobic bacteria, and eventually facilitated the digestion processes. With a decrease in SRT from 12 to 10 days, there was a decrease in biogas production. A shorter SRT results in decreasing biogas production.[23] Therefore, considering both biogas and methane production, an SRT of 12 days was found to be a suitable retention time for effective sludge degradation. Therefore, the digester efficiency (kg of VS fed and its conversion to methane) and the digestion efficiency as a function of OLR were optimal at 0.84 kg SS m−3 reactor d−1 . 3.3.6. pH During the start-up of the digester, the pH was in the range of 8.2, which gradually decreased with continuous feeding. The decrease in pH can be explained by the formation of acidic compounds through the depolymerization of the organic matter by enzymes produced by microorganisms
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present in mainly the secondary sludge.[36] After the acclimatization period, the pH of the effluent from the semi-continuous digester remained steady in the range of 7.3–8.2 but dropped when the OLR increased from 0.84 to 1 kg SS m−3 reactor d−1 , as shown in Figure 5. It was also observed that during every shift to the next lower SRT, the pH dropped. The pH of a digester may drop to below 6.6 if there is an excessive accumulation of VFAs, due to excessive organic loading or the presence of toxic materials in the digester. Low pH has inhibitory effects on the methanogen.[37] The operational range of pH should be between 6.6 and 7.6, with the optimum range being 7.0– 7.2.[38] It is evident from the graph that with an increase in OLR, there is decrease in pH. The results also reveal that there were fluctuations, but the pH remained in the methanogenic range, which proves that the digester could maintain its pH to some extent, even though higher organic loading was supplied. 3.3.7. Alkalinity pH cannot be an effective measure of the stability of an anaerobic process when there is a high buffering capacity.[39] Under this condition, alkalinity levels reveal a potential anaerobic process performance directly. Calcium, magnesium, and ammonium bicarbonates are examples of buffering substances found in a digester. The digestion process produces ammonium bicarbonate from breakdown of protein in the pretreated sludge feed. The concentration of alkalinity in a digester is, to a great extent, proportional to the solids feed concentration. A well–established digester has a total alkalinity of 2000–5000 mg L−1 .[2] The variation of alkalinity and pH with digestion time in the present study is displayed in Figure 5. The total alkalinity in this digester was in the range of 3000–5000 mg L−1 . The alkalinity of feed sludge and digested sludge varied significantly as pretreatments were used. An increase in alkalinity is normally due to the activity of methanogenic bacteria, which can produce alkalinity in the form of carbon dioxide, ammonia, and bicarbonate.[40] Carbon dioxide is produced in the fermentation and methanogenesis phases of the digestion process. Due to the partial pressure of gas in a digester, the carbon dioxide solubilizes and forms carbonic acid. The carbon dioxide concentration in the digester gas is therefore reflective of the alkalinity requirements. 3.3.8. Volatile fatty acid VFA is an important intermediate produced during the anaerobic digestion process. VFA reduces the pH of the system, which has an inhibitory effect on the growth of the methanogenic bacteria. VFAs should be less than 250 mg L−1 for satisfactory performance of the anaerobic process. In the present study, VFA in the initial stage was above 250 mg L−1 , which was in turn neutralized by increases in alkalinity. Higher levels of VFA in the
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Variation of pH and alkalinity in the digested sludge at various OLRs. 400
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Figure 6.
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Variation of VFA and VFA/alkalinity in the digested sludge at various OLRs.
reactor during the first phase of operation indicate the prevalence of acid fermentation. Furthermore, with the decrease in SRT there was decrease in VFA and it was within 250 mg L−1 resulting in the better performance of
the anaerobic digestion (Figure 6). The ratio of VFA to total alkalinity was in the range of 0.02–0.07 (Figure 6). The range of 0.02–0.08 for the ratio of VFAs to total alkalinity ratio is a good working range of an anaerobic
7.5 7.0 2980 11.4 10.3 189 84 360 7.5 7.2 3350 11.3 10.2 143 72 375 6.9 6.9 3845 24.9 28.7 393 175 239 6.9 7.3 3800 25.8 29.5 412 184 240 6.9 7.8 4500 25.7 24.8 325 162 245 6.9 7.6 4200 21.1 24.1 216 123 243 6.9 7.8 4400 17 20.1 161 96 247 Feed pH Outlet pH Alkalinity (mg L−1 ) SS removal (%) VS removal (%) Methane production (mL) Total biogas (mL gVS−1 added) VFA production (mL day−1 )
SRT (days) Experimental run (days) OLR (kg SS m−3 reactor d−1 )
Parameters
30 121 (a) 0.16 (b) 0.26 (c) 0.33 6.9 7.9 4500 10.1 12.4 67 60 246
6.9 7.4 4000 13.9 17.2 106 78 244
25 24 0.4
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12 24 0.84
10 17 1
30 121 (a) 0.16 (b) 0.26 (c) 0.33 7.5 7.4 3700 5.1 3.9 32 29 364
7.5 7.3 3650 6.9 5.7 52 38 366
12 24 0.84 15 24 0.66
7.5 6.9 2970 11.7 10.6 185 83 350 7.5 7.3 3500 10.4 9.2 106 60 383 7.5 7.4 3600 8.3 7.2 83 50 377
4.
10 17 1 18 24 0.58
process.[41] VFAs are the main precursors of methane formation. Studies have shown that 70% of the methane in an anaerobic system is produced from VFAs in the form of acetic acid. During digestion, VFAs are formed and consumed in the hydrolysis or acidogenic phases. It is therefore difficult to determine the exact VFA concentration during digestion, since the formation rate is expected to be equal to the further disappearance during methanogenesis.[30] During digestion, organic substances degraded by microorganisms are the only carbon source, and the basic source of easily assimilated organic carbon comprises short chain carboxylic acids.
25 24 0.4
CR ER Digester’s performance after stabilization. Table 2.
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Selection of the significant OLR and SRT
The results obtained after reactor stabilization are reported in Table 2. The optimal loading rates and retention time for methanogenic digestion will depend on the quality of the feedstock and on the desired efficiency of the overall process. For the most cost-effective operation, it is important to operate a reactor at the highest loading rate possible. A high organic loading will normally result in excessive VFA production in the digester with a consequent decrease in pH, and will adversely affect the methanogenic bacteria. A low organic loading will not provide a sufficient quantity of biogas for other uses, but will make the digester unnecessarily large. An SRT that is too short will not allow sufficient time for anaerobic bacteria to metabolize the wastes, especially for methane-forming bacteria. Excessive SRT could result in the excessive accumulation of digested materials in the digester, and the construction of a digester that is too large.[42] An optimum range of SRT for suspended growth digesters falls within the range 10–60 days, while attached growth and high-rate digesters can be operated at much shorter SRTs.[43] It was identified from the results that an OLR of 0.84 kg SS m−3 reactor d−1 operated with 12 days SRT is the most appropriate OLR for efficient and economical operation of the digester. The OLR and retention times reported in various studies on municipal solid waste digestion have ranged from 0.07 to 0.35 lb VS ft−3 day and from 10 to 30 days, respectively. Most of these studies have included co-digestion with raw sewage sludge. Uma et al. [23] have reported OLRs of 3.55–5.9 g VS L−1 day with retention times of 20, 15 and 12 days, and it was indicated that good performance of the reactor was achieved with an SRT of 15 days while using dairy sludge. 5. Conclusions The influence of bacterial pretreatment (after EPS removal) prior to anaerobic digestion was studied. BMP assay results of pretreated sludge confirmed that the reactors fed with bacterially pretreated sludge indicated better performance in terms of organic matter stabilization and gas production compared with the CR. BMP results also indicated that there is more gas production in the reactor with
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non-flocculated pretreated sludge compared with the flocculated pretreated sludge. Thus, non-flocculated pretreated sludge was chosen for semi-continuous digesters, and by combining pretreatment with anaerobic digestion, it led to 56% and 58% of SS and VS reduction, respectively, with a 57% improvement in biogas production when operated at an efficient OLR of 0.84 kg SS m−3 reactor d−1 .
Acknowledgements
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The authors are thankful to the Department of Biotechnology, India, for financial assistance to this project (BT/PR13124/GBD/ 27/192/2009) under their scheme Rapid Grant for Young Investigator.
References [1] Novak JT, Sadler ME, Murthy SN. Mechanisms of floc destruction during anaerobic and aerobic digestion and the effect on conditioning and dewatering of biosolids. Water Res. 2003;37:3136–3144. [2] Metcalf & Eddy. Wastewater engineering treatment and reuse. 4th ed. New York: McGraw Hill; 2003. [3] Elliott A, Mahmood T. Pretreatment technologies for advancing anaerobic digestion of pulp and paper biotreatment residues. Water Res. 2007;41:4273–4286. [4] Jolis D, Jones B, Marneri M, Kan H, Jones S, Panter K. Thermal hydrolysis pretreatment for high solids anaerobic digestion. Proceedings of the Anaerobic Digestion Conference, Montreal (QC); 2004. [5] Heo N, Park S, Kang H. Solubilization of waste activated sludge by alkaline pretreatment and biochemical methane potential (BMP) test for anaerobic co-digestion of municipal organic waste. Water Sci Technol. 2003;48: 211–219. [6] Muller C, Abu-Orf M, Blumenschein C, Novak J. The impact of ultrasound energy on mesophilic anaerobic digestion. WEF Proceedings of the Residuals and Biosolids Management Conference, Covington (KY); 2005. [7] Springer A, Higgins J. Mechanical cell lysis and recycle to reduce waste activated sludge. TAPPI Proceedings of the International Environmental Conference, Nashville (TN); 1999. [8] Hasegawa S, Shiota N, Katsura K, Akashi A. Solubilization of organic sludge by thermophilic aerobic bacteria as a pretreatment for anaerobic digestion. Water Sci Technol. 2000;41:163–169. [9] Miah M, Tada C, Yang Y, Sawayama S. Aerobic thermophilic bacteria enhance biogas production. Mater Cycles Waste Manage. 2005;7:48–54. [10] Valo A, Carrère H, Delgenè J. Thermal, chemical, and thermo-chemical pre-treatment of waste activated sludge for anaerobic digestion. J Chem Technol Biotechnol. 2004;79:1197–1203. [11] Ayol A. Enzymatic treatment effects on dewaterability of anaerobically digested biosolids-I: performance evaluations. Process Biochem. 2005;40:2427–2434. [12] Ayol A, Dentel SK. Enzymatic treatment effects on dewaterability of anaerobically digested biosolids-II: laboratory characterizations of drainability and filterability. Process Biochem. 2005;40:2435–2442. [13] Vavilin VA, Rytov SV, Lokshina LY. A description of hydrolysis kinetics in anaerobic degradation of particulate organic matter. Biosour Technol. 1996;56:229–237.
[14] Brown MJ, Lester JN. Comparison of bacterial extracellular polymer extraction methods. Appl Environ Microbiol. 1980;92:107–185. [15] Barber JB, Veenstra JN. Evaluation of biological sludge properties influencing volume reduction. J Water Pollut Control Fed. 1986;58:149–156. [16] Tian Y. Behavior of bacterial extracellular polymeric substances from activated sludge: a review. Int J Environ Pollut. 2008;32:78–89. [17] Wawrzynczyk J, Szewczyk E, Norrlöw O, Dey SE. Application of enzymes, sodium tripolyhospate and cation exchange resin for the release of extracellular polymeric substance from sewage sludge characterization of the extracted polysaccharide/glycoconjugates by panel of lection. J Biotechnol. 2007;130:274–281. [18] Mayhew M, Le M, Brade C, Harrison D. The united utilities enzymic hydrolysis process validation of phased digestion at full-scale to enhance pathogen removal. WEF Proceedings of the Residuals and Biosolids Conference, Baltimore (MD); 2003. [19] Mayhew M, Le M, Ratcliff R. A novel approach to pathogen reduction in biosolids: the enzymic hydrolyser. Water Sci Technol. 2002;46:427–434. [20] Merrylin J, Adish Kumar S, Kaliappan S, Yeom IT, Rajesh BJ. Effect of extracellular polymeric substances on sludge reduction potential of Bacillus licheniformis. Int J Environ Sci Technol. 2013;10:85–92. [21] Brock TD. Biology of microorganisms. 10th ed. Upper Saddle River, NJ: Prentice-Hall; 2002. p. 1104. [22] Elaiyaraju P, Partha N. Biogas production from co-digestion of orange peel waste and jatropha de-oiled cake in an anaerobic batch reactor. Afr J Biotechnol. 2012;11: 3339–3345. [23] Uma R, Adish KS, Kaliappan S, Ick TY, Rajesh BJ. Low temperature thermo-chemical pretreatment of dairy waste activated sludge for anaerobic digestion process. Bioresour Technol. 2011;103:415–424. [24] APHA. Standard methods for the examination of water and wastewater. 21st ed. Washington (DC): American Public Health Association; 2005. [25] Merrylin J, Adish Kumar S, Kaliappan S, Yeom IT, Rajesh BJ. Effect of EPS removal on the sludge reduction potential of Bacillus licheniformis on its optimized pH conditions. Water Environ J. 2012 (in press, doi/10.1111/wej1204). [26] Ayol A, Filibeli A, Sir D, Kuzyaka E. Aerobic and anaerobic bioprocessing of activated sludge: floc disintegration by enzymes. J Environ Sci Health A Tox Hazard Subst Environ Eng. 2008;43:1528–1535. [27] Luengo PL, Alvarez JM. Influence of temperature, buffer, composition and straw particle length on the anaerobic digestion of wheat straw-pig manure mixtures. Resour Conserv Recycl. 1988;1:27–37. [28] Vandevivere P, De Baere L, Verstraete W. Biomethanization of the organic fraction of municipal solid wastes. London: IWA; 2003. p. 111–140. [29] Linke B. Kinetic study of thermophilic anaerobic digestion of solid wastes from potato processing. Biomass Bioenerg. 2006;30:892–896. [30] Appels L, Dewil R, Baeyens J. Ultrasonically enhanced anaerobic digestion of municipal sewage. Int J Sust Eng. 2008;1:94–104. [31] Gholamreza M, Hassan A, Akram J. Effect of ozonation on reduction of volume and mass of waste activated sludge. J Appl Sci Res. 2008;4:122–127. [32] Castillo MEF, Cristancho DE, Arellano VA. Study the operational condition for anaerobic digestion of urban solid waste. Waste Manage. 2006;26:546–556.
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[33] Zhang HJ. Sludge treatment to increase biogas production. Trita-LWR Degree Project 10–20; 2010. [34] Kirtane RD, Suryawanshi PC, Patil MR, Chaudhari AB, Kothari RM. Optimization of organic loading rate for different fruit wastes during biomethanization. J Sci Ind Res. 2009;68:252–255. [35] Abdullah AGL, Idris A, Ahmadun FR, Baharin BS, Emby F, Megat MN, Nour AH. A kinetic study of a membrane anaerobic reactor (MAR) for treatment of sewage sludge. Desalination. 2005;185:439–445. [36] Kaseng K, Ibrahim K, Pancerselvam SV, Hassan RS. Extracellular enzymes and acidogen profiles of a laboratory-scale two-phase anaerobic digestion system. Process Biochem. 1992;27:43–47. [37] Chuthathip S, Supusanee D. Continuous co-digestion of agro-industrial residues [MS Thesis]. Boras: University of Boras; 2012.
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Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Biological pretreatment of non-flocculated sludge augments the biogas production in the anaerobic digestion of the pretreated waste activated sludge a
a
b
c
J. Merrylin , S. Adish Kumar , S. Kaliappan , Ick-Tae Yeom & J. Rajesh Banu
a
a
Department of Civil Engineering , Regional Centre of Anna University , Tirunelveli , India
b
Department of Civil Engineering , Ponjesley College of Engineering , Nagercoil , India
c
Department of Civil and Environmental Engineering , Sungkyunkwan University , Seoul , South Korea Accepted author version posted online: 03 Jun 2013.Published online: 27 Jun 2013.
To cite this article: J. Merrylin , S. Adish Kumar , S. Kaliappan , Ick-Tae Yeom & J. Rajesh Banu (2013) Biological pretreatment of non-flocculated sludge augments the biogas production in the anaerobic digestion of the pretreated waste activated sludge, Environmental Technology, 34:13-14, 2113-2123, DOI: 10.1080/09593330.2013.810294 To link to this article: http://dx.doi.org/10.1080/09593330.2013.810294
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Environmental Technology, 2013 Vol. 34, Nos. 13–14, 2113–2123, http://dx.doi.org/10.1080/09593330.2013.810294
Biological pretreatment of non-flocculated sludge augments the biogas production in the anaerobic digestion of the pretreated waste activated sludge J. Merrylina , S. Adish Kumara , S. Kaliappanb , Ick-Tae Yeomc and J. Rajesh Banua∗ of Civil Engineering, Regional Centre of Anna University, Tirunelveli, India; b Department of Civil Engineering, Ponjesley College of Engineering, Nagercoil, India; c Department of Civil and Environmental Engineering, Sungkyunkwan University, Seoul, South Korea
a Department
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(Received 26 February 2013; final version received 23 May 2013 ) High-efficiency resource recovery from municipal solid waste (MSW) has been a focus of attention. The objective of this research is to develop a bio-pretreatment process for application prior to the anaerobic digestion of MSW to improve methane productivity. Bacillus licheniformis was used for pretreating MSW (non-flocculated with 0.07% citric acid), followed by anaerobic digestion. Laboratory-scale experiments were carried out in semi-continuous bioreactors, with a total volume of 5 L and working volume of 3 L. Among the nine organic loading rates (OLRs) investigated, the OLR of 0.84 kg SS m−3 reactor day−1 was found to be the most appropriate for economic operation of the reactor. Pretreatment of MSW prior to anaerobic digestion led to 55% and 64% increase of suspended solids (SS) and volatile solids reduction, respectively, with an improvement of 57% in biogas production. The results indicate that the pretreatment of non-flocculated sludge with Bacillus licheniformis which consumes less energy compared to other pretreatment techniques could be a cost-effective and environmentally sound method for producing methane from MSW. Keywords: anaerobic digestion; methane production; municipal solid waste; biochemical methane potential; bacterial pretreatment
1. Introduction Anaerobic digestion is one of the more commonly used stabilization processes in sludge management, and provides effective pathogen destruction, reduction of volatile solids (VS) and odour potential, and an energy source in the form of biogas.[1] Much progress has been made in the fundamental understanding and control of the anaerobic digestion process, and the design and application of equipment. This process is one of the dominant processes for stabilizing sludge because of the energy conservation and recovery. Anaerobic digestion of municipal wastewater sludge can in many cases produce sufficient digester gas to meet most of the energy needs for plant operation.[2] Microorganisms and microbial–environment conditions are crucial in this process, as in other biological treatment processes. During the anaerobic digestion of biosolids, the first stage in the degradation of particulate organic matter is the solubilization and enhanced hydrolysis of complex polymeric organic carbon structures. The slow degradation rate of sludge in an anaerobic digester is due to the rate limiting step of sludge hydrolysis. This is caused by the low biodegradability of the cell walls and extracellular biopolymers in sludge. It is important to reduce the amount of sludge produced and to reduce its residual organic content. The methods of
∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
improvement of biodegradability of a particular substrate are mainly based on better accessibility of the substrate for microbes. Pretreatment can improve the subsequent anaerobic digestion. One advantage of sludge solubilization prior to anaerobic treatment is that increasing the amount of released soluble substrate significantly enhances volatile fatty acids (VFAs) generation for improved gas production during anaerobic digestion. Second, pretreatment decreases the viscosity of the sludge, permitting a greater solids concentration to be fed to an anaerobic digester. A higher solids concentration in the feed either enables increased digestion times in an existing digester, or allows for a small digester volume.[3] Various sludge pretreatment methods have achieved significant results in the lysis or disintegration of municipal sludge, and have thereby enhanced biogas production. Several methods have been studied to treat municipal sludge, including thermal,[4] chemical,[5] ultrasonic [6] and mechanical methods.[7] In addition, biological-microbial enzymes or bacterial methods,[8,9] and the combination of such processes, have also been used.[10] Bacteria are usually contained within a flocculated matrix of exopolymeric substances.[11,12] Extracellular polymeric substances (EPS) as a network detains the extracellular hydrolytic enzymes.[13] Researchers have
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also suggested that a higher concentration of EPS protein could bring more negatively charged amino groups,[14,15] and could thus strengthen the electrostatic interactions with cations. So the extraction of EPS from sludge could lead to a reduction in sludge volume and mass.[16] When enzymes are added to sludge, they are often adsorbed on to the sludge matrix, which can lead to inactivation. Treating the sludge with a cation binding agent prior to enzymatic pretreatment improves the organic matter in the sludge as a substrate for the enzymes through making components previously protected by the EPS structure more available to be degraded.[17] The use of microbial enzymes for the enhancement of degradation of waste activated sludge (WAS) is the basis for another process called the enzymatic hydrolysis process.[18,19] The primary benefit is the killing of pathogens, but a further benefit is the enhancement of biogas production in anaerobic digestion. A 10% improvement in biogas production was found in laboratory trials.[19] When a laboratoryscale mesophilic anaerobic reactor received this pretreated municipal WAS, a 1.5-fold increase in biogas production compared with a system receiving unconditioned sludge was found. In this study, a mesophilic anaerobic reactor is considered, which is fed with pretreated municipal WAS.
2. Materials and methods 2.1. Sludge sampling Municipal sewage sludge was collected from secondary clarifier in a local wastewater treatment plant in Trivandrum, Kerala in a sterilized container. The collected sample was aerated continuously at room temperature to prevent settling of the solids and to prevent constant changes in the biological sludge. The amount of biogas production from the sludge depends on the characteristics of the waste. Sludge characteristics such as protein, carbohydrates, total solids (TS), VS contents, and pH should be investigated before pretreatment. The pH of the raw sludge was 6.5, the CODS was 200 mg L−1 , the total COD was 12 g L−1 , the TS content was 13 g L−1 , the VS content was 8 g L−1 and the suspended solids (SS) content was 10 g L−1 .
2.2. Removal of EPS EPS is removed from sludge before pretreatment using the optimized citric acid concentration of 0.07%, according to the procedure by Merrylin et al. [20] The dosage of citric acid used is indicated as the gram equivalent, i.e 0.07 gm of citric acid is added to 100 mL of sludge. The mixture was kept at 4◦ C for 3 h with constant agitation to ensure proper mixing.[20] Through EPS removal, the sludge was dispersed and deflocculated. This deflocculation accelerates the bacterial activity. This deflocculated sludge is further subjected to bacterial pretreatment.
2.3. Bacterial pretreatment Bacillus licheniformis, a thermophilic protease producing bacterium, was used to pretreat the deflocculated sludge. This bacteria was mass cultivated in a nutrient broth at a temperature of 55◦ C in a 5 L fermenter, and the cells were harvested from the pure cell culture at the early stationary phase (20 h). These cells served as the inoculum. These cells were innoculated in a concentration of 77 × 106 bacterial cells mg−1 of SS. The inoculated sludge was pretreated for 24 h and this pretreated sludge was used for further studies.
2.4. Biochemical methane potential A biochemical methane potential (BMP) test was used to evaluate biogas recovery from sludge after bacterial pretreatment. In this test, biogas production of raw and pretreated sludge samples (both flocculated and deflocculated) was initially determined by batch tests at 35◦ C in three reactors. In reactor 1, 50 mL of deflocculated sludge (EPS removed and bacterially pretreated) was inoculated with 150 mL of cow rumen and fed into the 300 mL reactor. In reactor 2, 50 mL of flocculated sludge (bacterially pretreated) was inoculated with 150 mL of cow rumen and was fed into a 300 mL reactor. Reactor 3 was run as a control in which 50 mL and 150 mL of inoculum (cow rumen) was used to determine endogenous amount of biogas production. Rumen bacteria from the digestive tract of a cow were used as an innoculum. The rumen is an exclusive organ of ruminant animals in which the digestion of cellulose and other polysaccharide molecules occur through the activity of specific microbial populations. The capacity of cellulose digestion that these animals possess is related to the presence of anaerobic microorganisms in its rumen, which decompose up the glucose polymer chains to acetate. Methanogenic microorganisms that convert acetate into methane and carbon dioxide also exist in rumen.[21] The better performance of the inoculated reactors may be related to the potential increase in the number of indigenous anaerobic microorganisms of rumen, which contributed substantially to the degradation of the organic material in the reactor. After adding the substrates and the inoculum, the head space above the liquid in the assay reactors is purged with 30% CO2 and 70% N gas to enhance the anaerobic conditions in the reactor. The gas mixture was introduced into the reactor at the rate of 1 L min−1 for 5 min. Then, the reactor was sealed with a rubber septum and aluminium to make it air tight, and then placed on a shaker (150–200 rpm). The biogas was measured by inserting a needle into the septum. The gas pressure in the reactor was allowed to displace the syringe plunger, and the displaced volume is recorded. The cumulative gas production was measured using a water displacement method. The reactors were continuously shaken to allow sufficient blending. The methane content in the biogas was measured using a gas chromatograph. The modified Gompertz equation was to study the cumulative biogas
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Figure 1.
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Semi-continuous anaerobic reactor set-up.
generation and the kinetics of biogas production. Rb Bt = B∗ exp − exp , B∗ exp(λ − t) + 1
(1)
where Bt is the cumulative biogas produced (mL) at any time (t), B is the biogas production potential (mL), Rb is the maximum biogas production rate (mL day−1 ), and λ is the lag phase (days), which is the minimum time taken to produce biogas, or the time taken for bacteria to acclimatize to the environment, in days. The constants B, Rb and λ were determined using the non-linear regression approach with the aid of Polymath software. In many biological fields, the basic knowledge of phenomena is insufficient to build a mechanistic model. In this study, the effect of EPS removal in sludge to produce methane in the anaerobic digester was analysed using a Gompertz model,[22] as shown in Equation (1). This equation was utilized by researchers to study the cumulative methane production in biogas production.[23]
times (SRT) ranging from 30 to 10 days were sequentially applied to investigate the performance of the anaerobic digestion of the pretreated sludge. Moreover, in order to preserve the nitrification capability of the activated sludge, high SRT were applied to the biomass in the activated sludge process (≥10 days). The first three loadings were carried out with different mixed liquor SS (MLSS) of 5000, 7500 and 10,000 at the constant SRT of 30 days. The rest of the loadings were carried out with different SRT of 25, 20, 17, 15, 12 and 10 days with a constant MLSS of 10,000. The CR was also run under the same conditions as that of the ER. Feeding and withdrawals were carried out each day using peristaltic pumps in a semi-continuous mode. Biogas production was measured by the liquid displacement method, in which the difference in the water level in a cylinder linked to the reactors is observed. The displaced liquid is therefore considered to have the same volume as the produced biogas. 2.6.
2.5. Anaerobic semi-continuous reactor Two identical semi-continuous reactors (control and experimental) were run. The control reactor (CR) was fed with the raw sludge and the experimental reactor (ER) was fed with pretreated sludge. Their total volume was 5 L, and their working volume was 3 L, the reactor set-up is shown in Figure 1. The reactors were inoculated with the activated slurry collected from a local biogas plant in India. Organic loading rate (OLR) of 0.16, 0.26, 0.34, 0.4, 0.5, 0.58, 0.66, 0.83 and 1 kg SS m−3 reactor d−1 with the solids retention
Analytical parameters
The SS, VS and alkalinity parameters were measured according to the standard methods [24] to determine the sludge reduction efficiency for each reactor. The pH was measured using a digital pH meter. VFAs were analysed by distillation-titration method, and the result was expressed in acetic acid. The extent of MLSS reduction during the experiment was calculated by the equation mentioned below: Initial − Final MLSS reduction (%) = × 100. (2) Initial
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The methane content in the biogas was analysed using a Baroda gas chromatograph equipped with a thermal conductivity detector and porapack Q column with hydrogen as the carrier gas at a flow rate of 40 mL min−1 .
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3.
Results and discussion
3.1. Bacterial pretreatment Sludge deflocculated with 0.07% citric acid was pretreated with a protease secreting bacteria, Bacillus licheniformis.[25] Sludge disintegration in biological systems incorporates the mechanism of cell lysis transforming the cell content into the medium, the breakdown of EPS fractions in the flocculated matrix, and the biodegradation of the end products by microbial metabolism.[26] The efficiency of pretreatment was measured in terms of COD solubilization and was found to be in the range of 10–12% throughout the study (data not shown). The anaerobic degradability of the pretreated sludge was assayed using the BMP test.
3.2. BMP test The biogas production from non-flocculated, flocculated and raw sludge was examined in three reactors. The estimated kinetic constants using non-linear regression and other characteristics of the reactors 1–3 are shown in Table 1. A graphical representation of cumulative biogas production is shown in Figure 2. Biogas production is slow at the beginning and at the end of observation; this indicates that the biogas produced under batch condition corresponds to specific growth rate of methanogenic bacteria.[27] To analytically quantify the parameters of the batch growth curve, a modified Gompertz equation was fitted to the cumulative biogas production data. Values of parameters obtained are listed in Table 1. The best fit to the Gompertz equation is compared with the experimental data in Figure 2. During the first 5 days, biogas production is low due to the lag phase of microbial growth. After 7 days, the biogas production increased significantly due to exponential growth of microorganisms. After 20 days, the biogas production stabilized due to the stationary phase of microbial growth. At the end of 21 days, reactor 1 produced the highest cumulative biogas production potential (B) of 195 mL at a maximum biogas production rate (Rb) of 127.37 mL h−1 ,
Table 1.
with a lag phase (λ) of 1.8 days. Reactor 2 had an estimated biogas production potential of 114 mL at a maximum biogas production rate of 86 mL h−1 , with a lag phase of 4 days. In reactor 3, which contained the raw sludge, the biogas production potential was 21 mL at a maximum biogas production rate of 17 mL h−1 . The modified Gompertz equation was observed to adequately describe biogas production with R2 values of 0.9917, 0.9909 and 0.9962 for reactors 1, 2 and 3, respectively.
3.3. Semi-continuous anaerobic digesters 3.3.1. Start-up of the reactor During the acclimatization phase the biologically pretreated sludge was fed at an OLR of 0.16 kg SS m−3 reactor d−1 and the set-up was continued until a stable period was achieved. Stable periods were defined by the process showing a fairly constant performance in terms of biogas production, VFA concentration and pH in the reactor, without showing symptoms of any process unbalance or failure (i.e. cease in biogas production, VFA accumulation or pH drop for at least one loading rate). With this assessment after 50 days, the fluctuations in these parameters were less than 10% and it was believed that steady state was achieved. After normal operation had been established, fresh solids were added to the digester by mixing them with the digesting sludge, which greatly improves the rate of digestion.
3.3.2. OLR variation The OLR is a measure of the biological conversion capacity of the anaerobic digestion system. Feeding the system above its sustainable OLR, results in low biogas yield due to biomass concentration and the mass transfer rate of substrates to bacteria.[28] In such a case, the feeding rate to the system must be reduced. OLR is a particularly important control parameter in continuous systems. Both the biogas yield and methane production have a close relation with OLR, so special emphasis should placed on reactor performance in steady-state conditions in the OLR that do not result in reactor failure.[29] In this study, during a total period of 300 days, the performance of the semi-continuous anaerobic reactors was evaluated at different OLRs. The feeding was initialized with a low OLR of 0.16 kg SS m−3 reactor d−1 and the corresponding
Kinetic parameters calculated from the theoretical model for various samples. Modified Gompertz parameters (model)
Digester Control (1) Flocculated (2) Non-Flocculated (3)
B, L/(gVS)
Rb, L/(gVS d)
λ, d
R2
RMSD
0.395313 2.02478 3.571262
17.5008 106.9286 127.37811
4.216581 3.594685 1.845676
0.996254 0.990903 0.99175
0.000132 0.001179 0.001882
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0.30 Non-flocculated-exp
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Specific gas production (L/gVS)
0.25
Non-flocculated fit Flocculated-exp
0.20 Flocculated-fit Control-exp
0.15
Control-fit 0.10
0.05
0.00 0
5
10
15
20
25
Days
Figure 2.
Modified Gompertz model fit to the experimental data.
SRT was 30 days. When the OLR was increased to 0.26 kg SS m−3 reactor d−1 , the SRT was maintained at 30 days but the MLSS in the feed sludge was increased from 5000 to 7500 mg L−1 . Similarly for the consecutive OLR (0.34 kg SS m−3 reactor d−1 ), the MLSS was increased from 7500 to 10,000 mg L−1 . The OLR was shifted to the next higher value, once the reduction efficiencies of SS and VS were found to be consistent with a particular OLR. An increase in the OLR increases the extent of the reactions, and vice versa. Each time sludge is withdrawn, a fraction of the bacterial population is removed, thus implying that the cell growth must at least compensate for the cell removal to ensure steady-state operation and to avoid process failure.[30] 3.3.3. SS reduction SS reduction is an indication of sludge stability, and it is used for assessing the effectiveness of a process in stabilizing the sludge.[31] It is observed in Figure 3(a) that the overall performance efficiency in terms of reduction in the SS increases as the SRT was reduced. SRT has a significant effect on the digester performance. At individual SRTs, steady-state operations were achieved, and the results mentioned are the average of five consecutive consistent readings. The SRT is determined by the average time it takes for organic material to be digested completely, as measured by the chemical and biological oxygen demand of the exiting effluent. Speeding up the process will make the process more efficient. Microorganisms that consume organic material control the rate of digestion that determines the time for which the substrate must remain in the digestion chamber. Figure 3(a) shows the variation of SS in the digested sludge. There was an SS reduction of about 26% in the ER, and 11%
SS reduction in the CR. SS analysis shows that there was an increase of 55% in the amount of SS reduction in the ER compared to the CR. This indicates that the biological pretreatment with Bacillus licheniformis has a great advantage over the non-pretreated sludge (control). It is also observed in the figure that the maximum SS reduction was achieved when the loading rate was 0.84 kg SS m−3 reactor d−1 , and the reduction % declined with further increases in OLR. 3.3.4. VS reduction VS reduction was examined as well to evaluate the reactor performance and stability of the digestate. During the digestion process, volatile solids are degraded to a certain extent and converted into biogas. The sludge volume is thereby reduced. The degree of stabilization is often expressed as the per-cent reduction in volatile solids.[2] It was observed from Figure 3(b) that with the increase in OLR, there is an increase in VS reduction. A maximum VS degradation value of 30% was achieved when operating with loading rate of 0.84 kg SS m−3 reactor d−1 . On the other hand, when the loading rate was increased further, there was a decrease in VS reductions, and only 28.6% was obtained as illustrated in Figure 3(b). Comparably, these VS reduction were lower than the results by Castillo et al. who also reported that VS removal efficiency is decreased with decreases in the retention time.[32] 3.3.5. Biogas production Figure 4 shows the total biogas production and methane production at different OLRs and SRTs. Initially, the biogas production was 24 mL g−1 with addition of VS, and then it followed a zig-zag path for quite some time
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Figure 3.
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(a) Removal profile of SS of digested sludge at different OLRs. (b) Removal profile of VS of digested sludge at different OLRs. 200.00
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Figure 4.
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Environmental Technology during the acclimatization period. During each phase of OLR, the total biogas production in the digester showed appreciable increases until a stage when methanogenesis could not work quickly enough to convert acetic acid into methane. The results showed that compared with the control, the methane amount and the biogas production were increased by 57% and 53%, respectively, due to the biological pretreatment. In contrast to the present study, in works with physical and chemical pretreatment, significantly higher levels of biogas production in the range of 84–88% were obtained.[33] However these pretreatment techniques had intensive energy demands and high operation costs.[23] Biogas production was higher with 0.84 kg SS m−3 reactor d−1 , but with an increase in OLR to 1 kg SS m−3 reactor d−1 , it was decreased, which may have been due to either a lack of a critical requirement of inoculum to take the load of additional OLR, or that a lag period proportional to the growth inhibitorrich recalcitrant chemical composition was required.[34] The cumulative biogas produced during the entire reactor operation period of the ER was 51 L, and the cumulative biogas produced during the entire reactor operation period of the CR was 22 L. The decreasing methane content can also be attributed to the higher OLR, which favours a higher growth rate of the acid-forming bacteria over the methanogenic bacteria. Thus, the methane conversion process was adversely affected by reducing the methane content, which led to the formation of carbon dioxide at a higher rate.[35] Therefore, the methane yield was reduced at higher OLR due to the lower methane content. Thus, at OLR greater than 0.84 kg SS m−3 reactor d−1 , the methane production was decreased. The methane gas production rate increased appreciably and ranged from 20 to 412 mL day−1 , as the methane content in the biogas was varied from 65% to 70%.[2] The higher gas production in the bacterially pretreated non-flocculated sludge evidently indicated that it hydrolysed more organic material solution, which was immediately used by anaerobic bacteria, and eventually facilitated the digestion processes. With a decrease in SRT from 12 to 10 days, there was a decrease in biogas production. A shorter SRT results in decreasing biogas production.[23] Therefore, considering both biogas and methane production, an SRT of 12 days was found to be a suitable retention time for effective sludge degradation. Therefore, the digester efficiency (kg of VS fed and its conversion to methane) and the digestion efficiency as a function of OLR were optimal at 0.84 kg SS m−3 reactor d−1 . 3.3.6. pH During the start-up of the digester, the pH was in the range of 8.2, which gradually decreased with continuous feeding. The decrease in pH can be explained by the formation of acidic compounds through the depolymerization of the organic matter by enzymes produced by microorganisms
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present in mainly the secondary sludge.[36] After the acclimatization period, the pH of the effluent from the semi-continuous digester remained steady in the range of 7.3–8.2 but dropped when the OLR increased from 0.84 to 1 kg SS m−3 reactor d−1 , as shown in Figure 5. It was also observed that during every shift to the next lower SRT, the pH dropped. The pH of a digester may drop to below 6.6 if there is an excessive accumulation of VFAs, due to excessive organic loading or the presence of toxic materials in the digester. Low pH has inhibitory effects on the methanogen.[37] The operational range of pH should be between 6.6 and 7.6, with the optimum range being 7.0– 7.2.[38] It is evident from the graph that with an increase in OLR, there is decrease in pH. The results also reveal that there were fluctuations, but the pH remained in the methanogenic range, which proves that the digester could maintain its pH to some extent, even though higher organic loading was supplied. 3.3.7. Alkalinity pH cannot be an effective measure of the stability of an anaerobic process when there is a high buffering capacity.[39] Under this condition, alkalinity levels reveal a potential anaerobic process performance directly. Calcium, magnesium, and ammonium bicarbonates are examples of buffering substances found in a digester. The digestion process produces ammonium bicarbonate from breakdown of protein in the pretreated sludge feed. The concentration of alkalinity in a digester is, to a great extent, proportional to the solids feed concentration. A well–established digester has a total alkalinity of 2000–5000 mg L−1 .[2] The variation of alkalinity and pH with digestion time in the present study is displayed in Figure 5. The total alkalinity in this digester was in the range of 3000–5000 mg L−1 . The alkalinity of feed sludge and digested sludge varied significantly as pretreatments were used. An increase in alkalinity is normally due to the activity of methanogenic bacteria, which can produce alkalinity in the form of carbon dioxide, ammonia, and bicarbonate.[40] Carbon dioxide is produced in the fermentation and methanogenesis phases of the digestion process. Due to the partial pressure of gas in a digester, the carbon dioxide solubilizes and forms carbonic acid. The carbon dioxide concentration in the digester gas is therefore reflective of the alkalinity requirements. 3.3.8. Volatile fatty acid VFA is an important intermediate produced during the anaerobic digestion process. VFA reduces the pH of the system, which has an inhibitory effect on the growth of the methanogenic bacteria. VFAs should be less than 250 mg L−1 for satisfactory performance of the anaerobic process. In the present study, VFA in the initial stage was above 250 mg L−1 , which was in turn neutralized by increases in alkalinity. Higher levels of VFA in the
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Figure 6.
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Variation of VFA and VFA/alkalinity in the digested sludge at various OLRs.
reactor during the first phase of operation indicate the prevalence of acid fermentation. Furthermore, with the decrease in SRT there was decrease in VFA and it was within 250 mg L−1 resulting in the better performance of
the anaerobic digestion (Figure 6). The ratio of VFA to total alkalinity was in the range of 0.02–0.07 (Figure 6). The range of 0.02–0.08 for the ratio of VFAs to total alkalinity ratio is a good working range of an anaerobic
7.5 7.0 2980 11.4 10.3 189 84 360 7.5 7.2 3350 11.3 10.2 143 72 375 6.9 6.9 3845 24.9 28.7 393 175 239 6.9 7.3 3800 25.8 29.5 412 184 240 6.9 7.8 4500 25.7 24.8 325 162 245 6.9 7.6 4200 21.1 24.1 216 123 243 6.9 7.8 4400 17 20.1 161 96 247 Feed pH Outlet pH Alkalinity (mg L−1 ) SS removal (%) VS removal (%) Methane production (mL) Total biogas (mL gVS−1 added) VFA production (mL day−1 )
SRT (days) Experimental run (days) OLR (kg SS m−3 reactor d−1 )
Parameters
30 121 (a) 0.16 (b) 0.26 (c) 0.33 6.9 7.9 4500 10.1 12.4 67 60 246
6.9 7.4 4000 13.9 17.2 106 78 244
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30 121 (a) 0.16 (b) 0.26 (c) 0.33 7.5 7.4 3700 5.1 3.9 32 29 364
7.5 7.3 3650 6.9 5.7 52 38 366
12 24 0.84 15 24 0.66
7.5 6.9 2970 11.7 10.6 185 83 350 7.5 7.3 3500 10.4 9.2 106 60 383 7.5 7.4 3600 8.3 7.2 83 50 377
4.
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process.[41] VFAs are the main precursors of methane formation. Studies have shown that 70% of the methane in an anaerobic system is produced from VFAs in the form of acetic acid. During digestion, VFAs are formed and consumed in the hydrolysis or acidogenic phases. It is therefore difficult to determine the exact VFA concentration during digestion, since the formation rate is expected to be equal to the further disappearance during methanogenesis.[30] During digestion, organic substances degraded by microorganisms are the only carbon source, and the basic source of easily assimilated organic carbon comprises short chain carboxylic acids.
25 24 0.4
CR ER Digester’s performance after stabilization. Table 2.
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Selection of the significant OLR and SRT
The results obtained after reactor stabilization are reported in Table 2. The optimal loading rates and retention time for methanogenic digestion will depend on the quality of the feedstock and on the desired efficiency of the overall process. For the most cost-effective operation, it is important to operate a reactor at the highest loading rate possible. A high organic loading will normally result in excessive VFA production in the digester with a consequent decrease in pH, and will adversely affect the methanogenic bacteria. A low organic loading will not provide a sufficient quantity of biogas for other uses, but will make the digester unnecessarily large. An SRT that is too short will not allow sufficient time for anaerobic bacteria to metabolize the wastes, especially for methane-forming bacteria. Excessive SRT could result in the excessive accumulation of digested materials in the digester, and the construction of a digester that is too large.[42] An optimum range of SRT for suspended growth digesters falls within the range 10–60 days, while attached growth and high-rate digesters can be operated at much shorter SRTs.[43] It was identified from the results that an OLR of 0.84 kg SS m−3 reactor d−1 operated with 12 days SRT is the most appropriate OLR for efficient and economical operation of the digester. The OLR and retention times reported in various studies on municipal solid waste digestion have ranged from 0.07 to 0.35 lb VS ft−3 day and from 10 to 30 days, respectively. Most of these studies have included co-digestion with raw sewage sludge. Uma et al. [23] have reported OLRs of 3.55–5.9 g VS L−1 day with retention times of 20, 15 and 12 days, and it was indicated that good performance of the reactor was achieved with an SRT of 15 days while using dairy sludge. 5. Conclusions The influence of bacterial pretreatment (after EPS removal) prior to anaerobic digestion was studied. BMP assay results of pretreated sludge confirmed that the reactors fed with bacterially pretreated sludge indicated better performance in terms of organic matter stabilization and gas production compared with the CR. BMP results also indicated that there is more gas production in the reactor with
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non-flocculated pretreated sludge compared with the flocculated pretreated sludge. Thus, non-flocculated pretreated sludge was chosen for semi-continuous digesters, and by combining pretreatment with anaerobic digestion, it led to 56% and 58% of SS and VS reduction, respectively, with a 57% improvement in biogas production when operated at an efficient OLR of 0.84 kg SS m−3 reactor d−1 .
Acknowledgements
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The authors are thankful to the Department of Biotechnology, India, for financial assistance to this project (BT/PR13124/GBD/ 27/192/2009) under their scheme Rapid Grant for Young Investigator.
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