Anaerobic treatment of tannery wastewater using ASBR for methane recovery and greenhouse gas emission mitigation

Journal of Water Process Engineering 19 (2017) 231–238

Contents lists available at ScienceDirect

Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe

Anaerobic treatment of tannery wastewater using ASBR for methane recovery and greenhouse gas emission mitigation

MARK

⁎

Andualem Mekonnena, , Seyoum Letaa, Karoli Nicholas Njaub a b

Center for Environmental Science, College of Natural Science, Addis Ababa University, P.O. Box 33348, Addis Ababa, Ethiopia The Nelson Mandela African Institute of Science and Technology, P.O. Box 447 Arusha, Tanzania

A R T I C L E I N F O

A B S T R A C T

Keywords: Anaerobic treatment ASBR GHG Methane yield and tannery wastewater

The objective of this study was to develop and optimize a pilot scale Anaerobic Sequencing Batch Reactor (ASBR) for the treatment tannery wastewater and reduction of greenhouse gas emission. The performance of the pilot scale ASBR was evaluated at the OLRs of 1.03, 1.23, 1.52 and 2.21 kg m−3 d−1 under mesophilic condition (31 °C). The removal efficiencies of COD and methane yield in the pilot scale ASBR were in the range of 69–85% and 0.17 ± 0.2–0.30 ± 0.02m3/kg COD removed, respectively. The optimum COD removal and methane yield were obtained at OLR of 1.03 kg m−3 d−1 (HRT of 4 days) in the stepwise feeding mode. The maximum amount of COD (83.3 ± 3.6%) converted to methane was also obtained in the same loading rate. At this OLR, the volumetric methane production would be 148,190 m3 per year when the digester will be operated at full scale level. The total amount of GHG emission reduction from factory is estimated in the range between 1500 and 3032 tons CO2-eq per year. Generally, the results of this study showed that ASBR is efficient on generating biogas and reducing green house gas emission while treating high strength wastewater such as tannery. Hence, a full scale ASBR should be developed and used to treat the wastewater generated in the tanning industries.

1. Introduction Leather tanning process consumes about 30–40 L of water/kg of skin or hides processed and generate 90% of the used water as effluent [1]. The amount of water consumption and effluent discharges, varies from tannery to tannery depends on the type of tanning processes, raw materials and type of products [2,3]. Tannery wastewater is characterized by high amount organic matter and suspended solids content with average total COD concentration of 6200 mg/l and a Suspended Solids (SS) concentration of 5300 mg/l. The wastewater contains very high salinity with an average TDS concentration of 37, 000 mg/l. Moreover, it contains significant amount of Total Kjeldah Nitrogen (TKN), N-NH3 and PO43− with average concentration of 273, 153 and 21 mg/l, respectively [4–9]. Discharged tannery effluents without proper treatment can pollute surface water, ground water and soil. As the results there is an increasing environmental concern against tanning industrial activity [10,11]. The tannery wastewater as a whole or wastewater from individual processes should be treated. Tannery wastewater can be treated using either physico-chemical or biological methods. There are a number of studies that focused on the treatability of tannery wastewater using chemical coagulation [12–15], electrochemical methods [5,16] and Advanced Oxidation Processes (AOPs)

⁎

[17,18]. The other physical or chemical methods used in the treatment of tannery wastewaters are filtration, air stripping, chemical precipitation, adsorption, ion exchange [19,17,20]. However, all the physical and chemical methods are either expensive (high operating and maintenance cost, and consumption of chemicals) and/or produce harmful byproducts. Biological treatment of tannery wastewater using a combination of aerobic and anaerobic methods has been reported by different researchers Lefebvre et al., 2006; [7]. Biological wastewater treatment methods with aerobic system recognized as a significant source of greenhouse gas emission and energy consumer [21]. The emission of greenhouse gases causes global climate change that affect all levels of the society from local, regional and global [22]. Greenhouse gases are emitted during course of wastewater treatment both from the onsite and off-site activities [23]. In aerobic treatment, carbon dioxide production attributed to the biological transformation of organic matter and electrical consumption. The total amount of greenhouse gases emitted are estimated to be 2.4 kg CO2-eq/kg COD removed for fully aerobic method. Electrical energy generation in aerobic system emit greenhouse gases up to 1.4 kg CO2-eq/kg COD removed [24]. The greenhouse gases emitted in anaerobic process is about 1.0 kg CO2-eq/ kg COD removed which less than half of the aerobic system [24,25]. Methane is the major emission from anaerobic treatment processes. It is

Corresponding author. E-mail addresses: [email protected] (A. Mekonnen), [email protected] (S. Leta), [email protected] (K.N. Njau).

http://dx.doi.org/10.1016/j.jwpe.2017.07.008 Received 7 March 2017; Received in revised form 8 July 2017; Accepted 12 July 2017 2214-7144/ © 2017 Elsevier Ltd. All rights reserved.

Journal of Water Process Engineering 19 (2017) 231–238

A. Mekonnen et al.

located in a closed box and included the electric system. Mixing of the digester system was performed with wastewater circulation between ASBR and mixing chamber. The heating of the digester was accompanied by re-circulating hot water in a closed loop from solar panel to the bottom of the digester through a stainless steel tube.

well confirmed that each molecule of methane causes about 25 times more global warming than one molecule of carbon dioxide [21,26]. However, anaerobic treatment with methane capturing has the potential to reduce greenhouse gases. Anaerobic treatment consumes little energy and produces renewable energy in the form of biogas and nutrient rich digestate [25]. It provides the opportunity for greenhouse gases emission reduction and global warming mitigation though substitution chemical fertilizer and fossil fuel for energy production [27]. Hence, anaerobic treatment of agricultural, domestic and industrial wastewater with methane recovery is considered as sustainable approach for the management of wastewater [28,29]. The ASBR systems has been successfully applied in laboratory and pilot scales for treatment of high strength wastewaters including landfill leachate, slaughterhouse wastewater, municipal sludge and dairy wastewaters, brewery wastewater [30–34]. The advantage of Anaerobic Sequencing Batch Reactor (ASBR) design compared to conventional reactors, ASBR can treat more volume of substrate per unit time compared to conventional reactors, thus reducing the required volume of the digester. In addition, the high food-tomicroorganism (F:M) ratio immediately after the feeding phase ensures high initial rates of substrate removal and more biogas production [35,36]. Solids are retained during the settling phase. Hydraulic Retention Time (HRT) can be controlled separately from Solids Retention Time (SRT). Hydraulic retention time can be set as short as required to change soluble organic matter to biogas, while longer SRT is maintained to avoid washout of methanogens [37]. A short HRT reduces the size and construction cost of the digester. SRT can be controlled by regulating the amount of Mixed Liquor Suspended Solids (MLSS) removed along with wasted sludge from the reactors [38]. The ASBR operational cycle-times can be as short as 6 h after the biomass granulation is achieved. ASBR systems are also popular largely due to possible elimination of equalization tanks and secondary clarifiers as well as relatively simple operations [35]. In Ethiopia, there are more than 30 tanning industries. The average amount wastewaters discharged from these industries are around 11,312 m3 per day and this disposed to the surrounding water bodies without proper treatments [39]. In this study pilot scale ASBR was developed at compound of Modjo share tannery in Ethiopia. This tannery has an annual processing capacity of 844, 000 sheep skins and 1,656,000 goat skins and discharges about 400 m3/day effluents into Modjo River. This study aimed to investigate the biogas production and greenhouse gas emission reduction potential of the pilot anaerobic sequencing batch reactor from tannery wastewater. The COD removal efficiency, biogas production and methane yield of the ASBR were investigated at different OLRS.

2.2. Operation of the ASBR The performance of the ASBR was evaluated using four different OLRs. The performance was monitored by measuring COD removal, biogas production and quality. In the first phase, the reactor was operated at the organic loading rate (OLR) of 1.03 kg m−3 d−1 with 4 days HRT. In the second phase, the OLR was increased from 1.03 to 1.23 kg m−3 d−1 and HRT was 3.5 days. In the third phase, the reactor was operated at OLR of 1.52 kg m−3 d−1 and HRT of 3 days. Finally, it was operated at OLR of 2.21 kg m−3 d−1 and 2 days of HRT. 2.3. Laboratory analysis The influent and effluents samples were collected for analysis at the pumping chamber and the outlet pipe, respectively. The samples were analyzed for COD, Total Nitrogen (TN), ammonium-nitrogen (NH4+-N), nitrate-nitrogen (NO3−-N), sulphate (SO42−), sulphides (S2−), orthophosphate (PO43−) and total phosphorous (TP) colorimetrically using spectrophotometer (DR/2010 HACH, Loveland, USA) according to HACH instructions. Total Solid (TS) and Volatile Solid (VS) were also measured according to the methods described in standard methods of water and wastewater [40]. Total Dissolved Solid (TDS), Electrical Conductivity (EC) and Salinity were measured using TDS/EC/Salinity meter (CON2700). pH of was measured using a pH meter (JENWAY Model:3510). Calcium, chromium, copper, iron, lead, magnesium, potassium and sodium were determined using an atomic absorption spectrometer (novAA, 400P). The biogas production was measured using wet gas flow meter fitted on the ASBR and the biogas composition was determined using biogas meter (Biogas meter Geotechnical instruments, UK, England). 2.4. Data analysis The data was analyzed using Microsoft EXCE L spread sheets and OriginPro 8.0 software was used to draw graphs. The analysis of variance and comparison of means were performed on the data using SPSS package version 22. The comparison between mean was performed at 5% level of significance. 3. Result and discussion

2. Materials and methods 3.1. Characteristics of tannery wastewater used in the study 2.1. Experimental setup Table 1 shows the average characteristics of the tannery wastewaters used. The wastewater was alkaline with pH in the range between 9.09 ± 0.49 and 9.64 ± 0.46. The level of total dissolved solids and salinity were in the range from 6.81 ± 0.42 to 7.49 ± 0.36 g/l and 8.77 ± 0.72 to 9.26 ± 0.51 g/l, respectively. These were due to the different chemicals used in the soaking and beam house operation. The wastewater contained high amount of organic matter (4221 ± 359 to 4586 ± 292 mg/l) and nitrogenous compounds (451 ± 47.5 to 517 ± 112 mg/l). Sulfate values ranging 390 ± 76.9 to 520 ± 99.13 mg/l; likewise, phosphate and sulfide concentrations were ranging from 18 ± 4.5 to 23.5 ± 6.5 mg/l and 93 ± 22.27 to 126 ± 38.9 mg/l, respectively. Seyoum et al. [9] reported higher concentration of COD, TN and ammonium in tannery wastewater. The levels of COD/SO4−2 ratio were in the range of 7.7–12.5. The highest ratio was observed in the influent used in the last phase of the study. On the other hand, the lowest ratio was recorded in the first phase. There would be vigorous competition between the Methane

The pilot scale ASBR and the accessories used in this study are shown Fig. 1. It was constructed in a cylindrical shape using concrete materials at the compounds of Modjo share Tannery in Ethiopia. The total volume of the digester was 100m3 with dimension of 4 m height and 4 m diameter. The working volume was 80 m3. The internal part of the digester was insulated using foam and covered with geo-membrane. The top of the digester has two holes. One of the holes was fitted with PVC pipe for biogas outlet to gas flow meter and to the gas storage bag. The other one was fitted with stainless steel tubing extended to the bottom of the digester for hot water circulation. Stainless steel tubes were installed 30 cm above the bottom of the digester surface for the circulation of hot water. Two peristaltic pumps were also installed one of which was used for feeding and the other for agitating. Moreover, there are four holes on right side of the digester. One for feeding, two for agitation and the other one remaining for excess sludge out let. The control panel was 232

Journal of Water Process Engineering 19 (2017) 231–238

A. Mekonnen et al.

Fig. 1. Schematic diagram of the pilot scale ASBR and installed accessories. ASBR(1); Control panel (2); Mixing chamber (3); Feeding pipe (4); Gas pipe (5);Gas flow meter (6); Moisture trap unit (7); Biogas storage bag (8): Gas valve (9); Gas blower (10); Sulfur scrubber (11); Generator (12) and gas line to the kitchen (13).

Excessive amount of light metals retard growth of micro-organism and even higher concentrations might cause severe inhibition or toxicity. The level of calcium (327 ± 55 to 352 ± 34 mg/l) in the tannery wastewater was within the optimum range for anaerobic process. Magnesium (41.3 ± 7.8 to 53 ± 6.2 mg/l) and potassium (83 ± 7 to 99 ± 8 mg/l) was below the optimum concentration for anaerobic digestions. On the other hand, sodium concentration (2486 ± 419 to 2991 ± 22 mg/l) was above the optimum level for anaerobic process and lower than the maximum tolerable levels. Feijoo et al. [49] showed that sodium concentration at 3000 mg/l might cause inhibition of methanogenic bacteria. However, anaerobic processes can be operated with sodium concentrations as high as 16,000 mg/l. The concentration of chromium (4.9 ± 0.59 to 7.4 ± 1.1 mg/l) and iron (5.6 ± 1.0 to 9.6 ± 1.7 mg/l) in the tannery wastewater were higher compared to lead (0.14 ± 0.01 to 0.19 ± 0.03 mg/l) and copper (0.73 ± 0.27 to 0.98 ± 0.13 mg/l). The trend of heavy metal variability in mean concentrations of the wastewater samples were Fe > Cr > Cu > Pb. However, heavy metals concentration varies depending on type of tanning the process of tanning adopted in the industries [46]. For example, Deepali [50] has also reported similar chromium level (7.21 mg/l) and Jahan et al. [47] reported higher chromium level (10.348 mg/l) than this study. On the other hand, Tariq et al. [51] reported extremely higher chromium level (391 mg/l) while Koizhaiganova et al. [52] reported lowest level of chromium (3.769 mg/l). The highest level of chromium was due to the chromium salts used. In comparison with the results of this study, Jahan et al. [47] found higher concentration of copper (0.4112 mg/l), iron (14.675 mg/ l) and lead (0.1818 mg/l). Similarly, Sugasini and Rajagopal [46] and Tariq et al. [51] found higher level of iron and lead, respectively. Like light metals, trace amount of heavy metals stimulate the growth of anaerobic microorganisms and it is toxic at higher concentrations. All the metals measured in this study were found within the maximum tolerable limit for anaerobic digestion. Thus, the wastewater can be treated with anaerobic digestion process.

Table 1 Characteristics of tannery wastewater used at four different OLR. Parameters

Phase I

Phase II

Phase III

Phase IV

pH E.C. (mS) TDS (g/l) Salinity (g/l) COD (mg/l) TN (mg/l) NH4+-N (mg/l) Total phosphorus, (mg/l) Sulfide (mg/l) Sulfate (mg/l)

9.64 ± 0.46 8.76 ± 0.40 7.49 ± 0.36 9.26 ± 0.57 4221 ± 359 451 ± 47.5 231 ± 45 22.2 ± 6.8

9.20 ± 0.33 8.46 ± 0.39 7.24 ± 0.44 9.19 ± 0.38 4265 ± 215 517 ± 112 270 ± 66 18 ± 4.5

9.28 ± 0.311 8.08 ± 0.38 6.81 ± 0.42 8.91 ± 0.33 4586 ± 292 492.5 ± 89.9 255 ± 58 19.3 ± 4.16

9.09 ± 0.49 8.43 ± 0.72 7.26 ± 0.68 9.07 ± 0.71 4458 ± 396 458 ± 58.6 248 ± 44.46 23.5 ± 6.5

93 ± 22.27 470 ± 75.

126 ± 38.9 390 ± 76.9

123.5 ± 33.8 520 ± 99.13

117.5 ± 29.4 469 ± 69

Producing Bacteria (MPB) and Sulfate Reducing Bacteria (SRB) if the COD/sulfate ratios were 1.7–2.7 and sulfate reducing bacteria (SRB) would be more competitive at lower COD/SO4−2 ratios below 1.5 [41]. However, the ratios of COD/SO4−2 were greater than six in all the feeding used in the four different phases of the study. These levels of ratio support effective performance of anaerobic reactor in both COD removal and increase biogas yield [42]. Methane gas generation is directly proportional to the removal organic matter (COD) through the action of anaerobic methanogenic bacteria. Low concentration of sulfate would support the growth of methanogenic bacteria than sulfate reducing bacteria [43]. The concentration of sulfide was 93 ± 22.27 to 126 ± 38.9 mg/l in the influent. These levels of sulfide does not considered as problem for anaerobic digestion of tannery wastewater. The toxicity depends primarily on the levels of free hydrogen sulfide. Different studies indicated that sulphide concentration ranging from 50 to 100 mg/l with little or no acclimation can be endured in anaerobic treatment. Sulfide concentration up to 200 mg/l with a little acclimation and continuous operation would not cause significant inhibition effect on anaerobic processes [44]. Moreover, Vijayaraghavan and Murthy [44] showed that tannery wastewater containing sulfide up to 180 mg/l did not affect the anaerobic contact filter reactor performance.

3.3. COD removal efficiency COD was used as a major parameter to compare the performance of the reactors and to monitor the effect of OLR throughout the study. Table 3 summarizes the performance of the ASBR in terms of COD removal (in kg and percent) and biogas production in the four different phases of the experiment. During the first phase of the operation, the average COD removal efficiency and mass removal rate in the single feeding mode were 81 ± 2% and 69.9 ± 4.4kgCOD per day, respectively. It was 85 ± 1.5% for COD removal efficiency and 74.1 ± 5.1 for mass removal rate when stepwise feeding was employed (Table 3). The average concentration of COD also decreased from 791 ± 149.5 to

3.2. Metals and heavy metals concentration in tannery wastewater The concentration of light metals and heavy metals in the tannery wastewater were characterized to determine the suitable of tannery wastewater for biogas production (Table 2). Light metal ions (calcium, magnesium, sodium and potassium) and heavy metals (cadmium, chromium, copper, iron, lead and zinc) are commonly present in tannery wastewater [46–48]. Moderate concentrations of these light metals are important to stimulate microbial growth. 233

Journal of Water Process Engineering 19 (2017) 231–238

A. Mekonnen et al.

Table 2 Metals and heavy metals levels in tannery wastewater and comparison with standards. Metals (mg/l)

Phase I

Phase II

Phase III

Phase IV

Maximum tolerable

Calcium Magnesium Sodium Potassium Copper Chromium Iron Lead

339 ± 45 46.7 ± 6 2991 ± 221 99 ± 8 0.73 ± 0.27 4.9 ± 0.59 9.6 ± 1.7 0.14 ± 0.01

352 ± 34 53 ± 6.2 2685 ± 398 83 ± 7 0.83 ± 0.33 7.4 ± 1.1 6.5 ± 1.9 0.18 ± 0.04

327 ± 55 51 ± 3 2486 ± 419 85 ± 10 0.98 ± 0.13 6.4 ± 0.7 7 ± 1.4 0.16 ± 0.02

332 ± 56 41.3 ± 7.8 2578 ± 199 90 ± 7.9 0.77 ± 0.16 5.5 ± 0.9 5.6 ± 1.0 0.19 ± 0.03

2500–4500a 1000–1500a 3500–5500 2500–4500 100a 130 a 20–100a 2.0a

a

Minale and Worku [45].

651.6 ± 106 mg/l when feeding mode changed from single feeding to stepwise feeding. The COD removal efficiency in step feeding was significantly higher than single time feeding (p < 0.05). The major cause for this is that simple organic compounds produced in the hydrolysis are further converted in to volatile fatty acids by acidogenic bacteria. These intermediary compounds are changed to acetic acid, carbon dioxide and hydrogen in the acetogenic stage and the methanogenic bacteria use this molecule to produce methane and carbon dioxide as end product [53]. Increasing feeding time would cause a decrease in volatile acids (VA). This could reduce the substrate availability to microorganism and cause low accumulation of these acids in the reactor. This in turn increases the COD removal efficiency [54]. In the second phase, the COD removal efficiency was varied in the range of 76 − 83% with average removal efficiency of 79 ± 2.3%. The organic mass removal rate was also varied in the range of 74–82.1 kg COD per day while the average concentration of COD was 898.9 ± 122 mg/l in the final effluent. In the third phase, the average COD removal efficiency was decreased to 74–79% while the organic mass removal rate was varied between 83.4 and 98.2 kg COD per day. In the final phase, the COD removal efficiency was varied between 67 and 72% and mass removal rate was in the range between 101 and 143 kg COD/d. The average removal COD removal efficiency and effluent concentration were 69 ± 1.7% with 1358.3 ± 170 mg/l, respectively. The anaerobic digester showed significant variation in both COD removal efficiency and mass removal rate with variation of organic loading rate (ANOVA, P < 0.05). The results of this study indicate that COD removal efficiency was highest in the first phase of operation specially in the stepwise feeding and lowest in the final phase of operation. The final phase showed residual COD 31% from the influent. The organic matter mass removal rate was highest in the final phase and lowest in the first phase of operation. The COD removal efficiency significantly negatively correlated with mass removal rate with

Fig. 2. COD removal efficiency and mass removal rate with OLR.

increasing of organic loading rate (p < 0.05; Pearson Correlation = −0.8). The organic matter mass removal rate was significantly strongly associated with increasing of organic loading rate (p < 0.05; Pearson Correlation = 0.99). Moreover, Fig. 2 also shows the linear relationship between mass removal rates and organic loading rate. The slope of the line is 49.5 with 0.99 regression coefficient. Hence, mass COD removal rate highly depends on organic loading rate if the biomasses in the reactor have sufficient time to utilize the waste [55]. The COD removal efficiency of the first phase with one time feeding was not significantly different from phase II (p = 0.223) and significantly higher than the other two phases (P < 0.05). The removal efficiency in the second phase was also significantly higher than phase three and four and removal efficiency in phase four was significantly lower than all the three phases of the operation (p < 0.05). Hence, the COD removal efficiency decreased with increasing of organic loading

Table 3 Summary of the performance of the pilot scale ASBR for tannery wastewater. Parameters

OLR, kg.m−3 day−1 HRT, day COD removal, % COD removed, kg day−1 COD in, mg/l Biogas production, m3 day−1 Methane, % Methane yield BPR, m3 m−3 kg−1 SBR, m3kg−1 COD in converted to CH4, % CODrem. converted to CH4, %

Phase I

Phase II

Phase III

Phase IV

One time feeding

Step feeding

One time feeding

One time feeding

One time feeding

1.03 ± 0.09 4 81 ± 2.1 66.9 ± 4.4 791 ± 149.5 26.2 ± 1.6 70 ± 1.6 0.28 ± 0.02 0.33 ± 0.02 0.32 ± 0.03 64.2 ± 5.4 79.2 ± 5.4

1.03 ± 0.09 4 85 ± 1.5 74.1 ± 5.1 651.6 ± 106 29.8 ± 2.1 70 ± 2.7 0.30 ± 0.02 0.38 ± 0.02 0.35 ± 0.03 70.0 ± 4.5 83.3 ± 3.6

1.23 ± 0.06 3.5 79 ± 2.3 77.4 ± 3.6 898.9 ± 122 28.1 ± 1.8 68 ± 1.7 0.25 ± 0.01 0.35 ± 0.02 0.29 ± 0.02 55.7 ± 2.9 70.6 ± 3.5

1.52 ± 0.1 3 76 ± 1.6 92.3 ± 5.2 1101.4 ± 123 31.8 ± 2.7 64 ± 3.0 0.22 ± 0.01 0.4 ± 0.03 0.26 ± 0.01 47.9 ± 3.2 63.1 ± 4.1

2.21 ± 0.23 2 69 ± 1.7 122.3 ± 12.6 1358.3 ± 170 36.7 ± 2.8 55 ± 1.9 0.17 ± 0.02 0.46 ± 0.4 0.21 ± 0.03 32.9 ± 3.4 47.5 ± 4.3

Methane yield = CH4 m3/kgCOD removed, SBR = Specific biogas productivity, BPR = Biogas production rate, COD in = kg of COD added to the digester per day, CODrem = kg of COD removed per day.

234

Journal of Water Process Engineering 19 (2017) 231–238

A. Mekonnen et al.

rate (Fig. 2). This might be attributed to the accumulation of Volatile Fatty Acids (VFA) due to the reduction of Hydraulic Retention Time (HRT) in the anaerobic reactor. The COD removal efficiencies obtained in this study were higher than the results recorded by Song et al. [55] from the treatment of tannery wastewater (COD removal 60–75%) using upflow anaerobic fixed biofilm reactor at varying organic loading rate of 0.16–3.14 kg m−3d−1 and HRT of 16 days to 1 day. The results obtained in phase II (79 ± 2.3%) are comparable with results reported by Lefebvre et al. [56] for the treatment of tannery soak liquor (COD removal 78%) at a HRT of 5 days and an OLR of 0.5 kg m−3d−1 using up flow anaerobic sludge blanket bed reactor. Other comparable mean COD removal (78.2%) was reported El-Sheikh et al. [57] in the treatment of tannery wastewater using two stage UASB reactors. On the other hand, Banu and Kaliappan [58] reported slightly higher COD removal efficiency (86% at OLR of 2.74 kg m−3d−1 and HRT of 60 h; 88% at OLR of 3.22 kg m−3d−1 for 70 h) in the treatment of tannery wastewater using hybrid upflow anaerobic sludge blanket reactor. In addition, various researchers were reported high COD removal efficiencies using Laboratory and pilot scale Anaerobic Sequencing Batch Reactor (ASBR). Xiangwen et al. [30] recorded higher COD removal (90% at OLR of 5 kg m−3d−1 and HRT of 1 day) than the present study from the anaerobic treatment of brewery wastewater. On the other hand, Gregor et al. [31] reported comparable COD removal (81.6% at OLR of 6.27 kg m−3d−1 and HRT of 16 days) from the treatment of brewery wastewater. Timur and Oèzturk [34] obtained COD removal in the range of 64–85% at OLR of 0.4–9.4 kg m−3d−1 and HRT of 1.5–10 days in the treatment of strong landfill leachate. Kennedy and Lentz [33] were also reported 71–92% COD removal efficiency in the treatment of landfill leachate at OLR of 0.6–18.4 kg m−3d−1 and HRT of 0.5–1 day.

Fig. 3. Biogas production with variation of OLR.

utilization substrate in the anaerobic reactor [62]. Biogas production was significantly strongly associated with increasing of organic loading rate (p < 0.05; Pearson Correlation = 0.97). Fig. 3 also shows the linear relationship between biogas production rate per day and organic loading rate with regression coefficient of R2 = 0.80. This showed that 20% of the total variations were not explained in the regression model. Hence, this revealed that the microbial biomass in digester was continuously acclimatizing the changing environmental conditions due to increasing in OLR [63]. This might be also the possible reason for the reduction of methane yield and methane content in the biogas with increasing of loading rate. The methane yield obtained in this study were lower than the results recorded by Song et al. [55] from the treatment of tannery wastewater (0.36m3CH4/kg COD removed) using upflow anaerobic fixed biofilm reactor at varying organic loading rate of 0.16–3.14 kg m−3d−1 and HRT of 16 days to 1 day. The methane yield obtained in phase I (0.28 ± 0.02m3CH4/kg COD removed in one time feeding and 0.30 ± 0.02m3CH4/kg COD removed in step feeding) are comparable with results reported by Timur and Oèzturk [34] and Kennedy and Lentz [33] from the treatment of strong landfill leachate using ASBR. Timur and Oèzturk [34] obtained 0.29 m3CH4/kg COD removed at OLR of 0.4–9.4 kg m−3d−1 and HRT of 1.5–10 days. Kennedy and Lentz [33] were also reported 0.29–0.34 m3CH4/kg COD removed at OLR of 0.6–18.4 kg m−3d−1 and HRT of 0.5–1 day. Xiangwen et al. [30] obtained higher methane yield (0.48 m3CH4/ kg COD removed) from the anaerobic treatment of brewery wastewater at OLR of 5 kg m−3d−1 and HRT of 1 day. Gregor et al. [31] also obtained higher methane yield (0.34 m3CH4/kg COD removed) at OLR of 6.27 kg m−3d−1 and HRT of 16 days from the treatment of the same type of wastewater (brewery wastewater). The variation of amount COD converted to methane at different organic loading rates is shown in Fig. 4. At the OLR of 1.03 kg m−3d−1 (phase I), the amount of added and removed COD converted to methane in one time feeding mode were 64.2 ± 5.4 and 79.2 ± 5.4%, respectively (Table 3). The amount of added COD and removed COD converted to methane in the step time feeding were 70.0 ± 4.5% and 83.3 ± 3.6%, respectively. The amount converted to methane in the step feeding mode were significantly higher than one time feeding methods (p < 0.05). Around 16.7% of the removed COD was not converted to methane. Some part of this removed COD might be changed to carbon dioxide and the other part might be remained in the biomass. When the OLR increased to 1.23 kg m−3d−1 (phase II), 70.6 ± 3.5% of the removed COD and 55.7 ± 2.9% added COD were converted to methane. The average value of amount of added and removed COD converted to methane at the OLR of 1.52 kg m−3d−1 were 47.9 ± 3.2% and

3.4. Biogas production, gas composition and methane yield The variations of volumetric biogas generation and methane yield during all the experimental phases are illustrated in Table 3. At the OLR of 1.03 kg m−3d−1 (phase I), significantly higher biogas generation was observed in the step feeding than the onetime feeding (p < 0.05). The average methane and carbon dioxide content were 70 ± 1.6% and 24 ± 2.1% in one time feeding and 70 ± 2.7% and 22.8 ± 2.2% in the step feeding methods, respectively (Table 3). Like biogas production, higher methane yield was recorded in the step feeding (0.30 ± 0.02 m3 CH4 per kg COD per day) method than one time feeding method (0.28 ± 0.02 m3 CH4 per kg COD per day). When the OLR increased to 1.23 kg m−3d−1 (phase II), the biogas production showed slight increment with an average methane and carbon dioxide content of 68 ± 1.7% and 24.2 ± 2%, respectively. On the other hand, the methane yield was significantly reduced from 0.30 ± 0.02 to 0.25 ± 0.01 m3 CH4 per kg COD per day. The biogas production was significantly increased when OLR rise from 1.23 to 1.52 kg m−3d−1 (p < 0.05) while the methane yield was significantly reduced from 0.25 ± 0.01 to 0.22 ± 0.01 m3 CH4 per kg COD per day (p < 0.0) and average methane content dropped to 64 ± 3.0%. When the OLR was increased from 1.52 to 2.21 kg m−3d−1, the biogas production was significantly higher than all the other loading rates (p < 0.05). On the other hand, it resulted in a significantly level of reduction in methane yield. Similarly, the methane content was decreased from 64 ± 3.0% to 55 ± 1.9% while the carbon dioxide content of the biogas was increased from to 37 ± 1.9%, respectively. This might be due to the inadequate HRT period at the corresponding OLR values [59,60]. Hence, the high carbon dioxide percentages could be a result of improper balance of substrate supply and digestion time [59,61]. The results of this study showed that the biogas generation was low at the beginning of the experiment due to low OLR and then increased with increasing organic loading rate. This was due to the maximum 235

Journal of Water Process Engineering 19 (2017) 231–238

A. Mekonnen et al.

Table 4 The estimated methane production, energy content and diesel replaced in a full scale ASBR for Modjo tannery. Phases

OLR

Biogas (m3/y)

Methane (m3/y)

Energy (MJ)

Diesel oil replaced (l/y)

Electricity (Mwh)

I(1) I(2) II III IV

1.03 1.03 1.23 1.52 2.21

191260 211700 178120 175200 133955

133,882 148,190 121,122 112,128 73,675

4,779,587 5,290,383 4,324,041 4,002,969 2,630,206

128738.7 142490.4 116463 107815 70841

527.8 584.2 477.5 442.0 290.5

OLR = kg.m−3 d−1, (1) eOne time feeding and (2) = step feeding.

3.6. Estimated renewable energy production potential of ASBR The biogas generated in the anaerobic process can be used to replace diesel oil or produce electricity. The amount of renewable energy in the form of heat, electricity and biodiesel oil that could be produced through anaerobic treatment of tannery wastewater was determined by using the methane yield obtained in the pilot scale ASBR experiments and this extrapolated to the full scale system when the designed parameters would be scaled-up for the actual daily wastewater treatment of the tanning industry. Table 4 illustrates the results obtained for the full scale system. At the OLR of 1.03 kg m−3 d−1, the estimated methane production is 133, 882 m3 per year in the onetime feeding and 148,190m3 per year in the step feeding mode. This is equivalent to 4,779,587 and 5,290,383 MJ or 128739 and 142290 L diesel, respectively. The estimated methane production will be 121,122; 112,128 and 73,675 m3 per year when the digester will be operated at OLR of 1.23, 1.52 and 2.21 kg m−3 d−1, respectively. The estimated energy value 4,324,041; 4,002,969 and 2,630,206 MJ and this will replace 43, 40 and 26% of the diesel oil, respectively. The biogas produced in the ASBR might also be used to generate 290.5–584.2Mwh of electricity per year depending on the mode of feeding and OLR used to operate the system. Chotwattanasak and Puetpaiboon [65] have also reported the generation of 2.2 million kwh electricity from full scale reactor treating palm oil mill wastewater.

Fig. 4. percent of added and removed COD converted to methane with OLR.

63.1 ± 4.1%, respectively. When the OLR increased from 1.52 to 2.21 kg m−3d−1 (phase IV), 32.9 ± 3.4% of the added COD and 47.5 ± 4.3% the removed COD were converted to methane (Table 3). The COD converted to methane (both the added and removed) was significantly highest at the OLR of 1.03 kg m−3d−1 (p < 0.05) and significantly lowest at the OLR of 2.21 kg m−3d−1 (p < 0.05). 3.5. Relationship between biogas production rates and COD removal The amount of biogas production in the anaerobic reactor is the result of the degradation of organic matter by the action of consortium of anaerobic bacteria. The average amount of biogas production rate were 0.33 ± 0.02, 0.35 ± 0.02, 0.4 ± 0.03 and 0.46 ± 0.4m3/m3.d at the organic loading rate of 1.03 ± 0.09, 1.23 ± 0.06, 1.52 ± 0.1 and 2.21 ± 0.23 kg m−3 d−1, respectively (Table 3). The biogas production rates were increased with increasing of organic loading rate. The biogas production rate was significantly highest at the highest OLR of 2.21 ± 0.23 kg m−3 d−1 (p < 0.05). At the lowest OLR (1.03 ± 0.09 kg m−3 d−1), the biogas production rate was significantly lower than the other loading rate (p < 0.05). Other researchers were also reported that the biogas production rate is highly dependent on the organic loading rate [64]. Fig. 5 illustrates the relation of biogas production rate (m3/m3.day) with organic matter mass removal rate. As it is shown, the total amount of biogas production rate increased linearly (R2 = 0.95) with increasing COD mass removal rate (p < 0.05). In the regression model only around 5% of the total variations were not explained. The remaining 95% of the total variations were explained in the regression model. This shows that biogas production rate is direct function organic matter destruction through anaerobic processes. Hence, biogas production rate increased with increasing of organic matter removal.

3.7. Greenhouse gas emission mitigation potential of ASBR The capturing and utilization of biogas from the anaerobic digestion of tannery wastewater would contribute to the mitigation of greenhouse gas (GHG) emission. This has on-site and off-site emission reduction potential. Table 5 shows the variation of estimated greenhouse gas reduction due to both biogas recovery and diesel oil replacement. The amount of CO2 emission reduced due to diesel oil burning displacement will be in the ranges from 186219 kg per year (at OLR of 2.21 kg m−3 d−1) to 374559 kg per year (at OLR of 1.03 kg m−3 d−1 with step feeding). Moreover, there is reduction in CH4 emission (in the range 7.89–15.87 kg per year) and N2O emission (in the range of Table 5 Estimated greenhouse gas emission reduction. OLR

kg CO2 yr−1

1.03 (1) 1.03(2) 1.23 1.52 2.21

338395 374559 306142 283410 186219

a

kg CH4 yr−1 14.34 15.87 12.97 12.01 7.89

(1) = one time feeding. (2) = step feeding and OLR = kg.m−3 d−1. a = reduction from diesel replacement. b = reduction from CH4 recovery.

Fig. 5. Biogas production rate with organic mass matter removal rate.

236

a

kg N2O yr−1 2.87 3.18 2.59 2.40 1.58

a

kg CH4 yr−1 95592 105808 86481 80060 52604

b

Journal of Water Process Engineering 19 (2017) 231–238

A. Mekonnen et al.

Innovations Network for Eastern Africa Development (Bio-Innovate) program. References [1] IFC, Environmental, Health, and Safety Guidelines, Tanning and Leather Finishing, Washington, D.C, 2007. [2] W. Wiegant, T.J. Kaker, V. Sontakke, R. Zwaag, Full scale experience with tannery water management: an integrated approach, Water Sci. Technol. 39 (5) (1999) 169–176. [3] UNIDO, Pollutants in Tannery Effluent. Regional Program for Pollution Control in the Tanning Industry in South-East Asia, UNIDO, 2000. [4] S. Kongjao, S. Damronglerd, M. Hunsom, Simultaneous removal of organic and inorganic pollutants in tannery wastewater using electrocoagulation technique, Korean J. Chem. Eng. 25 (2008) 703–709. [5] O. Apaydin, U. Kurt, M.T. Gonullu, An investigation on the tannery wastewater by electrocoagulation, Global NEST J. 11 (2009) 546–555. [6] G. Farabegoli, A. Carucci, M. Majone, E. Rolle, Biological treatment of tannery wastewater in the presence of chromium, J. Environ. Manage. 71 (2004) 345–349. [7] R. Ganesh, G. Balaji, R.A. Ramanujam, Biodegradation of tannery wastewater using sequencing batch reactor-respirometric assessment, Bioresour. Technol. 97 (2006) 1815–1821. [8] D. Orhon, E. Ates, S. Sozen, Experimental evaluation of the nitrification kinetics for tannery wastewaters, Water SA 26 (2000) 43–52. [9] Leta Seyoum, L. Gumelius, Assefa Fassil, G. Dalhammar, Biological nitrogen and organic matter removal from tannery wastewater in pilot operations in Ethiopia, Appl. Micrbiol. Biotechnol. 66 (3) (2004) 333–339. [10] P. Saravanabhavan, J. Thanikaivelan, R. Raghave, U.N. Balachandran, A one bath chrome tanning together with wet finishing process for reduced water usage and discharge, Clean Technol. Environ. Policy 7 (2006) 168–178. [11] D.A. Bernal, R.A. Contreras, T. Trujillo, P.V. Olalde, L. Dendooven, Effects of tanneries wastewater on chemical and biological soil characteristics, Appl. Soil Ecol. 33 (2006) 269–277. [12] S. Haydar, J.A. Aziz, Coagulation-flocculation studies of tannery wastewater using combination of alum with cationic and anionic polymers, J. Hazard. Mater. 168 (2009) 1035–1040. [13] Z. Song, C.J. Williams, R.G.J. Edyvean, Coagulation and anaerobic digestion of tannery wastewater, Trans I ChemE 79 (Part B) (2001) 2001. [14] E. Ates, D. Orhon, O. Tunay, Characterization of tannery wastewaters for pretreatmentselected case studies, Water Sci. Technol. 36 (1997) 217–223. [15] J.I. Garrote, M. Bao, P. Castro, M.J. Bao, Treatment of tannery effluents by a twostep coagulation/flocculation process, Water Res. 29 (1995) 2605. [16] G. Varank, H. Erkan, S. Yazýcý, A. Demir, G. Engin, Electrocoagulation of tannery wastewater using monopolar electrodes: process optimization by response B surface methodology, Int. J. Environ. Res. 8 (1) (2014) 165–180. [17] G. Lofrano, S. Meriç, G.E. Zengin, D. Orhon, Chemical and biological treatment technologies for leather tannery chemicals and wastewaters: a review, Sci. Total Environ. (2013) 265–281. [18] D. Rameshraja, S. Suresh, Treatment of tannery wastewater by various oxidation and combined processes, Int. J. Environ. Res. 5 (2) (2011) 349–360. [19] B. Ramesh, N.S. Bhadrinarayana, S.B. Meera, N. Anantharaman, Treatment of tannery wastewater by electrocoagulation, Chem. Technol. Metall. 42 (2) (2007) 201–206. [20] A.L. Linda, K.J. Peter, Physico-chemical treatment methods for the removal of microcystins (cyanobacterial hepatotoxins) from potable waters, Chem. Soc. Rev. 28 (1999) 217–224. [21] T. Abbasi, S.M. Tauseef, S.A. Abbasi, Anaerobic digestion for global warming control and energy generation-An overview, Renew. Sustain. Energy Rev. 16 (2012) 3228–3242. [22] R. Kothari, V.V. Tyagi, A. Pathak, Waste-to-energy: a way from renewable energy sources to sustainable development, Renew. Sustain. Energy Rev. 14 (2010) 3164–3170. [23] C. Chai, D. Zhang, Y. Yu, Y. Feng, M.S. Wong, Carbon footprint analyses of mainstream wastewater treatment technologies under different sludge treatment scenarios in china, Water 7 (2015) 918–938. [24] H. Insam, B. Wett, Control of greenhouse gas emission at microbial community level, Waste Manage. 28 (2008) 699–706. [25] P. Kaparaju, J. Rintala, Mitigation of greenhouse gas emission by adopting anaerobic digestion technology on dairy, sow and pig farms in Finland, Renew. Energy 36 (2011) 31–41. [26] IPCC, Climate Change Work Group III; Mitigation of Climate Change, IPPC, Paris, 2007. [27] H. Pathak, N. Jain, A. Bhatia, S. Mohanty, N. Gupta, Global warming mitigation potential of biogas plants in India, Environ. Monit. Assess. 157 (2009) 407–418. [28] L. Seghezzo, G. Zeeman, B.V. Jules, H.V.M. Hamelers, G. Lettinga, A Review: the anaerobic treatment of sewage in UASB and EGSB reactors, Bioresour. Technol. 65 (1998) 175–190. [29] P.B. William, C.S. David, The use of the anaerobic baffled reactor (ABR) for wastewater treatment; a review, Water Res. 33 (7) (1999) 1559–1578. [30] S. Xiangwen, P. Dangcong, T. Zhaohua, J. Xinghua, Treatment of brewery wastewater using anaerobic sequencing batch reactor (ASBR), Bioresour. Technol. 99 (2008) 3182–3186. [31] D.Z. Gregor, M. Straqinbar, M. Ron, Treatment of brewery slurry in thermophilic anaerobic sequencing batch reactor, Bioresour. Technol. 98 (2007) 2714–2722.

Fig. 6. Greenhouse gas emission reduction due to CH4 recovery and Fossil fuel displacement.

1.58–3.18 kg per year). The highest gas reduction will be at OLR of 1.03 kg m−3 d−1 and the lowest will be at OLR of 2.21 kg m−3 d−1. This directly associated with amount of diesel oil replaced by the biogas. As it is shown in Table 3, the estimated annual methane production decrease with increasing organic loading rate and this in turn reduce the amount of diesel oil to be replaced. The reduction of diesel oil displacement with increasing organic loading rate will result in reduction in the amount of green house gas emission mitigation. Moreover, the estimated amount of methane produced in the anaerobic digester would also reduce from 105,808 kg per year at the OLR of 1.03 kg m−3 d−1 with step feeding to 52,604 kg per year at OLR of 2.21 kg m−3 d−1. The variation of GHG reduction resulted from displacement of diesel burning and methane capturing is shown in Fig. 6. The GHG reduction due to diesel replacement would range between 187 and 340 ton CO2eq per year and it would be between 1315 and 2646 ton CO2-eq per year due to methane recovery. The total amount of GHG reduction can be in the range between 1500 and 3032 ton CO2-eq per year. The highest estimated reduction will be at OLR of 1.03 kg m−3 d−1 when step feeding mode employed and the lowest will be at OLR of 2.21 kg m−3 d−1.This is mainly due to the reduction of the volume of methane produced with increasing of OLR. Chotwattanasak and Puetpaiboon [65] obtained 14,836 ton CO2-eq per year from the treatment of palm oil mill wastewater using at full scale anaerobic digester. Moreover, there will be also GHG emissions reductions when the nutrient rich digestate/slurry substitutes the production of mineral fertilizer [66]. The emission factors for the production of mineral fertilizer are considered to be 6.41 kg CO2-eq per kg of nitrogen (N), 1.18 kg CO2-eq per kg of phosphorous (P2O5) and 0.663 kg CO2-eq per kg of potassium oxide [67]. 4. Conclusion The results of this study showed that COD removal, methane yield and GHG reduction were decreased with increasing OLR or decreasing HRT. The COD removal and GHG reduction was estimated at the OLR of 1.03 kg m−3 d−1 in the stepwise feeding mode and the lowest was obtained at OLR of 2.21 kg m−3 d−1. As the results of this study shows anaerobic treatment of tannery wastewater with methane recovery reduce emission of greenhouse gases. Acknowledgment This research work was supported by the Swedish International Development Cooperation Agency (SIDA) through Bioresources 237

Journal of Water Process Engineering 19 (2017) 231–238

A. Mekonnen et al. [32] A. Sarti, B.S. Fernandes, M. Zaiat, E. Foresti, Anaerobic sequencing batch reactors in pilot-scale for domestic sewage treatment, Desalination 216 (2007) 174–182. [33] K.J. Kennedy, E.M. Lentz, Treatment of landfill leachate using sequencing batch and continuous flow upflow anaerobic sludge blanket (UASB) reactors, Water Res. 34 (14) (2000) 3640–3656. [34] H. Timur, I. Oèzturk, Anaerobic sequencing batch reactor treatment of landfill leachate, Water Res. 33 (15) (1999) 3225–3230. [35] G.D. Zupancic, A. Jemec, Anaerobic digestion of tannery waste: semi-continuous and anaerobic sequencing batch reactor processes, Bioresour. Technol. 101 (2010) 26–33. [36] Z. Wang, W. Wang, X. Zhang, G. Zhang, Digestion of thermally hydrolyzed sewage sludge by anaerobic sequencing batch reactor, J. Hazard. Mater. 162 (2002) 799–803. [37] R.H. Zhang, Y. Yin, S. Sung, R.R. Dague, Anaerobic treatment of swine waste by the anaerobic sequencing batch reactor, Trans. ASAE 40 (3) (1997) 761–767. [38] D.W. Hamilton, M.T. Steele, Operation and performance of a farm-scale anaerobic sequencing batch reactor treating dilute swine manure, Trans. ASABE 57 (5) (2014) 1473–1482. [39] LIDI, Leather industry development institute and united nation development program (UNDP), July, 2010: Proceeding of National Workshop on Enhancing the Ethiopian Leather Industry and Its Market Competitiveness, Adama, Ethiopia, 2010. [40] APHA, Standard Methods for the Examination of Water and Wastewater, 19th edn, American public Health Association, Washington DC, 1998. [41] T.Y. Jeong, G. Cha, Y.C. Seo, C. Jeon, S.S. Choi, Effect of COD/sulfate ratios on batch anaerobic digestion using waste activated sludge, J. Ind. Eng. Chem. 14 (2008) 693–697. [42] S. Venkata Mohan, N.C. Rao, K.K. Prasad, P.N. Sarma, Bioaugmentation of an anaerobic sequencing batch biofilm reactor (AnSBBR) with immobilized sulphate reducing bacteria (SRB) for the treatment of sulphate bearing chemical wastewater, Process Biochem. 40 (2005) 2849–2857. [43] D. Erdirencelebi, I. Ozturk, E.U. Cokgor, G.U. Tonuk, Degree of sulfate-reducing activities on COD removal in various reactor configurations in anaerobic glucose and acetate-fed reactors, Clean 35 (2) (2007) 178–182. [44] K. Vijayaraghavan, D.V.S. Murthy, Effect of toxic substances in anaerobic treatment of tannery wastewaters, Bioprocess. Eng. 16 (1997) 151–155. [45] M. Minale, T. Worku, Anaerobic co-digestion of sanitary wastewater and kitchen solid waste for biogas and fertilizer production under ambient temperature: waste generated from condominium house, Int. J. Environ. Sci. Technol. 11 (2014) 509–516. [46] A. Sugasini, K. Rajagopal, Characterization of physicochemical parameters and heavy metal analysis of tannery effluent, Int. J. Curr. Microbiol. Appl. Sci. 4 (9) (2015) 349–359. [47] M.A.A. Jahan, N. Akhtar, N.M.S. Khan, C.K. Roy, R. Islam, Nurunnabi, Characterization of tannery wastewater and its treatment by aquatic macrophytes and algae Bangladesh, J. Sci. Ind. Res. 49 (4) (2014) 233–242. [48] M.K. Bhatnagar, S. Raviraj, G. Sanjay, B. Prachi, Study of tannery effluents and its effects on sediments of river ganga in special reference to heavy metals at Jajmau, Kanpur, India, J. Environ. Res. Dev. 8 (1) (2013). [49] G. Feijoo, M. Soto, R. Méndez, J.M. Lema, Sodium inhibition in the anae-robic

[50] [51]

[52] [53] [54]

[55] [56]

[57]

[58] [59]

[60] [61]

[62]

[63]

[64] [65] [66]

[67]

238

digestion process: antagonism and adaptation phenomena, Enzyme Microb. Technol. 17 (2) (1995) 180–188. K.K.G. Deepali, Metals concentration in textile and tannery effluents, associated soils and ground water, N. Y. Sci. J. 3 (4) (2010) 82–89. S.R. Tariq, M.H. Shah, N. Shaheen, A. Khalique, S. Manzoor, M. Jaffar, Multivariate analysis of selected metals in tannery effluents and related soil, J. Hazard. Mater. A122 (2005) 17–22. M. Koizhaiganova, N.O. Üzüm, İ. Cireli, B.K. Assenova, Heavy metal contents in tannery wastewater, J. Selçuk Univ. Nat. Appl. Sci. ICOEST (Part I) (2014). M.H. Gerardi, The Microbiology of Anaerobic Digesters. Wastewater Microbiology Series, John Wiley and Sons Inc, New Jersey, USA, 2003. D.M. Bagley, T.S. Brodkorb, Modeling microbial kinetics in an anaerobic sequencing batch reactor-model development and experimental validation, Water Environ. Res. 71 (1999) 1320. Z. Song, C.J. Williams, R.G.J. Edyvean, Tannery wastewater treatment using an upflow anaerobic fixed biofilm reactor (UAFBR), Environ. Eng. Sci. 20 (6) (2003). O. Lefebvre, N. Vasudevan, M. Torrijos, K. Thanasekaran, R. Moletta, Anaerobic digestions of tannery soak liquor with an aerobic post-treatment, Water Res. 40 (2006) 1492–1500. M.A. El-Sheikh, H.I. Saleh, J.R. Flora, M.R. AbdEl-Ghany, Biological tannery wastewater treatment using two stage UASB reactors, Desalination 276 (2011) 253–259. J.R. Banu, S. Kaliappan, Treatment of tannery wastewater using hybrid upflow anaerobic sludge blanket reactor, J. Environ. Eng. Sci. 6 (2007) 415–421. E. Senturk, M. Ince, G.O. Engin, Treatment efficiency and VFA composition of a thermophilic anaerobic contact reactor treating food industry wastewater, J. Hazard. Mater. 176 (2010) 843–848. P.E. Poh, M.F. Chong, Development of anaerobic digestion methods for palm oil mill effluent (POME) treatment, Bioresour. Technol. 00 (2009) 1–9. H.W. Zhao, T. Viraraghavan, Analysis of the performance of an anaerobic digestion system at the Regina wastewater treatment plant, Bioresour. Technol. 95 (2004) 301–307. K. Kavitha, A.G. Murugesan, Efficiency of Upflow anaerobic granulated sludge blanket reactor in treating fish processing effluent, J. Ind. Pollut. Control 23 (1) (2007) 77–92. R. Borja, C.J. Bank, Z. Wang, A. Mancha, Anaerobic digestion of slaughterhouse wastewater using combination sludge blanket and filter arrangement in a single reactor, Bioresour. Technol. 65 (1998) 125–133. R.T. Romano, R. Zhang, Co-digestion of onion juice and wastewater sludge using an anaerobic mixed biofilm reactor, Bioresour. Technol. 99 (2008) 631–637. J. Chotwattanasak, U. Puetpaiboon, Full scale anaerobic digester for treating palm oil mill wastewater, J. Sustain. Energy Environ. 2 (136) (2011). S. Bonetta, S. Bonetta, E. Ferretti, G. Fezia, G. Gilli, E. Carraro, Agricultural reuse of the digestate from anaerobic co-digestion of organic waste: microbiological contamination, metal hazards and fertilizing performance, Water Air Soil Pollut. 225 (2014) 2046. J. Daniel-Gromke, J. Liebetrau, V. Denysenko, C. Krebs, Digestion of bio-waste-GHG emissions and mitigation potential, Energy Sustain. Soc. 5 (3) (2015).