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Recovery of biogas as a source of renewable energy from ice-cream production residues and wastewater a
a
a
a
b
Burak Demirel , Murat Örok , Elif Hot , Selin Erkişi , Metin Albükrek & Turgut T. Onay a a
Institute of Environmental Sciences , Bogazici University , Istanbul , Turkey
b
Unilever, Regional Environmental Manager AAR , Istanbul , Turkey Accepted author version posted online: 06 Feb 2013.Published online: 11 Mar 2013.
To cite this article: Burak Demirel , Murat Örok , Elif Hot , Selin Erkişi , Metin Albükrek & Turgut T. Onay (2013) Recovery of biogas as a source of renewable energy from ice-cream production residues and wastewater, Environmental Technology, 34:13-14, 2099-2104, DOI: 10.1080/09593330.2013.774055 To link to this article: http://dx.doi.org/10.1080/09593330.2013.774055
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, Nos. 13–14, 2099–2104, http://dx.doi.org/10.1080/09593330.2013.774055
Recovery of biogas as a source of renewable energy from ice-cream production residues and wastewater Burak Demirela∗ , Murat Öroka , Elif Hota , Selin Erki¸sia , Metin Albükrekb and Turgut T. Onaya a Institute
of Environmental Sciences, Bogazici University, Istanbul, Turkey; b Unilever, Regional Environmental Manager AAR, Istanbul, Turkey
Downloaded by [George Mason University] at 23:01 29 December 2014
(Received 22 October 2012; final version received 1 February 2013 ) Proper management of waste streams and residues from agro-industry is very important to prevent environmental pollution. In particular, the anaerobic co-digestion process can be used as an important tool for safe disposal and energy recovery from agro-industry waste streams and residues. The primary objective of this laboratory-scale study was to determine whether it was possible to recover energy (biogas) from ice-cream production residues and wastewater, through a mesophilic anaerobic co-digestion process. A high methane yield of 0.338 L CH4 /gCODremoved could be achieved from anaerobic digestion of ice-cream wastewater alone, with almost 70% of methane in biogas, while anaerobic digestion of ice-cream production residue alone did not seem feasible. When wastewater and ice-cream production residue were anaerobically co-digested at a ratio of 9:1 by weight, the highest methane yield of 0.131 L CH4 /gCODremoved was observed. Buffering capacity seemed to be imperative in energy recovery from these substrates in the anaerobic digestion process. Keywords: anaerobic digestion; biogas; ice-cream production residues; methane; renewable energy
1. Introduction An anaerobic digestion process can successfully be employed for energy recovery and waste disposal in agroindustry.[1] In particular, dairy plants produce strong waste streams, which can be characterized by their high biological oxygen demand (BOD) and chemical oxygen demand (COD) concentrations, representing their excess organic content.[2] These waste streams are concentrated in nature, and carbohydrates, proteins and fats from the milk contribute to this high organic load, making them an appropriate substrate for energy recovery through biological anaerobic digestion processes.[3,4] Ice-cream production activities generate wastewater and a residue that is generated as the production line begins to work. This residue is actually ice-cream, but since it has no commercial value it is regarded as waste and is disposed of. Ice-cream production residue can be characterized with a high total solids (TS) and organic content. Recovery of energy from dairy waste streams has been previously discussed in some studies.[5,6] However, no work was found in literature focusing on the potential of energy recovery through anaerobic digestion of ice-cream production residues. Therefore, the primary objective of this laboratory-scale experimental work was to determine the potential of biogas recovery as a source of renewable energy from ice-cream
∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
production residues and waste streams by a mesophilic anaerobic co-digestion process. 2. Materials and methods 2.1. The substrates The ice-cream production residue and process wastewater were obtained from an ice-cream production plant located close to Istanbul. Due to variable characteristics of both substrates, three different composite samples were obtained at different dates from the plant, in order to obtain reproducible results. The samples were kept at +4◦ C prior to use. The characteristics of the substrates are given in Table 1. The seed sludge (inoculum) was obtained from an anaerobic digester treating potato-processing wastewater. The seed sludge had an average pH value of 7.2, with TS and volatile solids (VS) values of 3.2 and 51%, respectively. The amount of inoculum added to the batch reactors in each trial is reported in Table 2. 2.2. The experimental set-up The experimental set-up used during the experiments is shown in Figure 1. Gas-tight glass reactors of 1 L capacity with a rubber plug were used for batch anaerobic digestion tests. The batch reactors were kept in a water bath
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Table 1. The characteristics of ice-cream wastewater and ice-cream production residue. Parameter pH TS VS VSS COD TKN TKN TP SO−2 4 Cl−1 Oil & grease
Unit
Wastewater (WW)
Residue (O)
% % % mg/L mg/L mg/kg mg/L mg/L mg/L mg/L
4.22–9.79 0.60–1.21 83–94 0.04–0.05 7312–10418 145–165 – 32–43 200–350 400–550 2424–2815
3.47–4.28 16–25 89–93 – 249960–303097 – 26627–144231 150–175 200–1600 – 7028–47365
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Figure 1.
and the temperature was automatically controlled to provide mesophilic conditions at 37◦ C. Each reactor was connected to a MilliGascounter for measurement of biogas production. Three different sets of experiments were conducted in order to compare the reproducibility of the experimental findings, since the characteristics of both substrates were quite different in each sampling period. Control reactors were also run for Set II and III (data not shown). The reactor loadings for each set of experiments are given in Table 2. The initial pH, TS, VS, and COD values of batch reactors are also given in Table 3. The batch reactors were loaded with wastewater, ice-cream production residue and seed sludge using pre-determined mixture ratios by weight (w/w). The pH of the medium was adjusted to a neutral pH range only at the beginning of the experiments, using a 6 N sodium hydroxide (NaOH) and a 6 N sulphuric acid (H2 SO4 ) solution. In order to provide adequate buffering capacity, potassium hydrogen bicarbonate (KHCO3 ) buffer solution was also used.[7] After loading, the reactors were Table 2.
The experimental set-up used for the experiment.
flushed with nitrogen (N2 ) gas for about 5 min to provide anaerobic conditions for the mixed culture. All of the chemicals were reagent grade obtained from commercial sources (Merck, Darmstadt, Germany). 2.3.
Analytical methods
Measurement of pH, solids, total Kjeldahl nitrogen, total phosphorus, COD, chloride (Cl−1 ), sulphate (SO−2 4 ) and oil and grease were performed according to Standard Methods.[8] Biogas production was continuously measured using MilliGascounter MGC-1 (Ritter, Bochum, Germany).[9] The biogas values were corrected for Standard Temperature and Pressure. The composition of biogas, namely methane (CH4 ) and carbon dioxide (CO2 ), was measured using a HP 6850 Gas Chromatograph equipped with a thermal conductivity detector. The volatile fatty acid (VFA) measurements were performed using a Perkin Elmer 600 Gas Chromatograph with a flame ionization detector.
The reactor loading conditions for each set of experiments.
Reactor
Mixing ratio by weight (WW:O)
Seed sludge (mL)
Wastewater (WW) (mL)
Residue (O) (mL)
Buffer addition (mL)
SET I R1 & R2 R3 & R4 R5 & R6
– – 1:1
250 250 250
500 – 250
– 500 250
24 24 24
SET II R2 & R3 R4 & R5 R6 & R7 R8 & R9 R10 & R11
– – 1:1 3:1 9:1
250 250 250 250 250
500 – 250 375 450
– 500 250 125 50
100 150 150 150 150
SET III R2 & R3 R4 & R5 R6 & R7 R8 & R9 R10 & R11
– – 3:1 6:1 9:1
250 250 250 250 250
500 – 375 430 450
– 500 125 70 50
100 150 150 150 150
Temperature (◦ C) Mesophilic
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Table 3.
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Initial pH, TS, VS and COD values in batch reactors for each set.
Reactor
pH
TS (%)
VS (%)
COD (mg/L)
Buffer addition (mL)
Temperature (◦ C)
SET I R1 & R2 R3 & R4 R5 & R6
7.6 6.8 6.9
1.19 17.66 9.75
73.1 92.8 88.6
4618 156030 113186
24 24 24
Mesophilic
SET II R2 & R3 R4 & R5 R6 & R7 R8 & R9 R10 & R11
7.5 7.0 7.3 7.5 7.6
2.18 11.03 6.55 4.59 2.80
80.2 76.6 88.3 77.2 82.2
17095 133697 69399 39723 22412
100 150 150 150 150
SET III R2 & R3 R4 & R5 R6 & R7 R8 & R9 R10 & R11
7.9 6.9 7.5 7.8 7.8
2.37 9.7 4.24 3.49 3.19
38.3 62.6 55.1 38.0 46.2
10501 136694 47436 38202 32662
100 150 150 150 150
3. Results and discussion The cumulative biogas and methane (CH4 ) production values, and the CH4 yields observed for each set are summarized in Table 4 and Figures 2– 4. The duration time of the experiments for Set I, II, and III were 17, 25 and 24 days, respectively. When ice-cream wastewater was anaerobically digested alone, without ice-cream production residue, the highest CH4 yield of 0.338 L CH4 /g CODremoved was obtained in Set III. The CH4 yields for wastewater for Set I and II were 0.194 and 0.160 L CH4 /g CODremoved , respectively. When ice-cream production residue was digested alone, without wastewater addition, the observed CH4 yields were almost negligible in both Set I and II, as indicated by very small amounts of cumulative CH4 production. Like wastewater, the highest CH4 yield for anaerobic
Figure 2. The cumulative biogas and methane values (mL) for ice-cream wastewater (WW), ice-cream production residue (O) and wastewater/residue mixture (WW:O) for batch reactors in Set I.
Table 4. Methane yields observed for batch anaerobic digestion tests.
Substrate WW O
WW+O
Set
Methane yield (L CH4 /g VSdestroyed )
Methane yield ( L CH4 /g CODremoved )
SET I SET II SET III SET I SET II SET III SET I (1:1) SET II (1:1) SET II (3:1) SET II (9:1) SET III (3:1) SET III (6:1) SET III (9:1)
0.091 0.147 0.222 0.000 0.001 0.079 0.000 0.001 0.006 0.049 0.121 0.175 0.369
0.194 0.160 0.338 0.000 0.001 0.032 0.000 0.002 0.015 0.083 0.016 0.050 0.131
Figure 3. The cumulative biogas and methane values (mL) for ice-cream wastewater (WW), ice-cream production residue (O) and wastewater/residue mixtures (WW:O) for batch reactors in Set II.
digestion of ice-cream production residue was obtained in Set III as 0.079 L CH4 /g VSdestroyed . However, this CH4 yield was quite poor in terms of biogas recovery from this production residue.
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Figure 4. The cumulative biogas and methane values (mL) for ice-cream wastewater (WW), ice-cream production residue (O) and wastewater/residue mixtures (WW:O) for batch reactors in Set III.
For batch anaerobic co-digestion of ice-cream wastewater and production residue, the highest CH4 yields were obtained in Set III as 0.369 L CH4 /g VSdestroyed and 0.131 L CH4 /g CODremoved , respectively, when wastewater and residue were mixed at a ratio of 9:1 by weight (w/w). In case of mixture ratios of 1:1, 3:1, and 6:1, poor CH4 yields were observed in Set I, II, and III. In addition, in Set II, although a mixture ratio of 9:1 was employed, the obtained CH4 yields were low again, as indicated by poor amount of cumulative CH4 production. In terms of biogas composition, the percentage of CH4 in biogas from anaerobic digestion of ice-cream wastewater ranged between 20–60% in Set I. Even though there was biogas production from anaerobic digestion of residue and the mixture of wastewater and residue, there was no CH4 detected in biogas. In both cases, the biogas was mainly composed of CO2 and N2 , indicating that the anaerobic digestion process could not be accomplished in these trials. In Set II, CH4 ranged from 40–80% in biogas from ice-cream wastewater alone, and on average it was about 60%. No CH4 production was observed from anaerobic digestion of residue and from the mixture of wastewater and residue at a ratio of 1:1. A low CH4 generation could be detected from the mixture of wastewater and residue at a ratio of 3:1. For mixtures of wastewater and residue, the highest CH4 composition in biogas was 60%, and this was obtained at a mixture ratio of 9:1. In terms of CH4 production, the best results were generally obtained in Set III, with Table 5.
respect to Set I and II, probably due to the characteristics of both wastewater and residue. CH4 production could be observed in all of the trials in Set III. The CH4 composition was measured to be around 70% for anaerobic digestion of ice-cream wastewater alone. The highest CH4 composition for mixtures of wastewater and residue was 60% at a mixture ratio of 9:1, while for the mixture ratio of 6:1, the CH4 varied from 20–40%. According to the experimental findings obtained, batch mesophilic anaerobic digestion of ice-cream wastewater alone (without co-substrate addition) could produce a high CH4 yield. The CH4 yield from mesophilic anaerobic treatment of a dairy industry wastewater by a pilot-scale upflow anaerobic filter was reported to vary between 0.32– 0.34 m3 CH4 /kg CODremoved with a CH4 composition ranging between 75–85% in biogas.[5] Typical CH4 generation values reported for anaerobic digestion of dairy industry waste streams range from 60–85%.[4,5,10] In this work, the CH4 content of biogas generated from anaerobic digestion of ice-cream wastewater could also reach more than 70%. Due to its high organic content, the ice-cream process wastewater seemed suitable for recovery of energy by anaerobic treatment. However, the CH4 yields seemed to vary greatly, depending strongly on the particular characteristics of the waste streams. Dairy wastewater flow rates and characteristics vary greatly and are quite difficult to predict, even when detailed information on processing operations is available.[11] A comparison of the characteristics of the ice-cream wastewater used in this work with similar waste streams is given in Table 5, indicating how the characteristics of dairy waste streams fluctuate.[5,12–16] In this work, the ice-cream wastewater pH was quite variable, and some batch tests performed without buffer solution indicated that the buffering capacity was quite low, resulting in rapid acidification of batch reactors (data not shown). The use of acid and alkaline cleaning agents in the dairy industry, in addition to various process operations, significantly influences the wastewater characteristics, particularly the pH.[11,17] The alkalinity of dairy wastewaters was reported to be less than 1000 mg CaCO3 /L in most cases,[13,14] not high enough to sustain an anaerobic treatment process successfully, showing that the buffering capacity is often lacking in these waste streams. Therefore, external use of
A comparison of the characteristics of dairy/ice-cream waste streams.
Wastewater type
pH
−1 (mg/L) Reference COD (mg/L) TKN (mg/L) TP (mg/L) SO−2 4 (mg/L) Oil & grease (mg/L) Cl
Ice-cream 4.22–9.79 7312–10418 Milk & cream 8–11 2000–6000 Ice-cream 5.2 5200 Milk processing 8.6 4420 Milk processing 5.8–11.4 1155–9185 7.78 13750 Mixed process 1410–1815
145–165 50–60
32–43
110 14–272 1027 66–84
30 8–68 8.2–10
200–350
2424–2815
400–550
35 7–62
525 233-993
This work [5] [12] [13] [14] [15] [16]
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Environmental Technology a buffering agent became an essential tool to accomplish anaerobic digestion process in this work. In addition to pH instability and low buffering capacity, the ice-cream wastewater also contained high concentrations of SO−2 4 , oil and grease and Cl−1 in each trial. Variable pH, inadequate buffering capacity along with high SO−2 4 and oil and grease concentrations are bottlenecks in an anaerobic digestion process.[18] The SO−2 4 and oil and grease concentrations in ice-cream wastewater ranged between 200–350 and 2424– 2815 mg/L, respectively, while Cl−1 concentration varied between 400–550 mg/L. It has been reported that the presence of high oil and grease content in dairy waste streams could cause problems due to their low biodegradability in the anaerobic digestion process.[19,20] High concentrations of oil and grease in ice-cream waste streams might have adversely affected the CH4 yields, while SO−2 4 and Cl−1 content seemed to be below the ranges considered to be inhibitory. Since oil and grease content of the batch reactor contents were not measured after the process has been accomplished, it is difficult to conclude more in detail about oil and grease degradation and its possible effects on the anaerobic digestion process in this work. Anaerobic digestion of ice-cream production residue alone (without ice-cream wastewater addition) did not seem to be feasible, since the observed CH4 yields were almost negligible in most trials. The ice-cream production residue also had differing characteristics in each trial, most probably due to production cycles employed, and it had a very low, acidic pH, with no buffering capacity. Even external supplementation of buffering solutions did not improve biological degradation and promote generation of CH4 . In addition, the ice-cream production residue contained very high concentrations of SO−2 4 , and oil and grease, both of which made the anaerobic digestion process quite unfavourable. Thus, in spite of its high organic content, anaerobic digestion of the ice-cream production residue without co-substrate (wastewater) addition for energy recovery purposes did not produce satisfactory results. Different mixture ratios by weight were employed to achieve high CH4 yields from anaerobic co-digestion of ice-cream wastewater and ice-cream production residues. The mixture of ice-cream wastewater and residue contained high concentrations of SO−2 4 and oil and grease, and it also had a quite low buffering capacity, mostly originating from the residue. As summarized above, all of these factors could easily result in acidification of an anaerobic digestion system. These factors eventually affected the CH4 yields adversely. The CH4 yields generally increased when the dilution ratio between the wastewater and residue increased. When the dilution ratios were low, such as 1:1 or 3:1, lower CH4 yields were achieved from anaerobic co-digestion. These results indicated that the ice-cream production residue had to be mixed with wastewater at high dilution ratios so that CH4 production from anaerobic co-digestion of ice-cream wastewater and residue could be realized. The dilution effect of wastewater seemed to
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Figure 5. The concentrations of VFA (as mg acetic acid equivalent/L) in batch reactors for each set.
decrease the concentrations of inhibitory components in the ice-cream production residue, thereby promoting CH4 production along with adequate buffering capacity. The SO−2 4 and oil and grease concentrations in residue ranged between 200–1600 and 7028–47365 mg/L, respectively, much higher than those of ice-cream wastewater, probably causing perturbation. Among the wastewater and residue mixtures, the highest dilution of residue with wastewater provided the highest CH4 yield. The concentrations of VFAs as acetic acid equivalent for each set is displayed in Figure 5. In Set I, the highest VFA concentration of about 750 mg/L as acetic acid equivalent was measured in reactors that only contained ice-cream production residue. In reactors containing wastewater and residue mixture (R5 & R6), a lower VFA concentration was measured than those of reactors containing only wastewater (R1 & R2). However, a negligible methane yield was calculated for R5 & R6 (Table 4). In Set II, the highest VFA concentration was determined in reactors containing only wastewater. The VFA concentration was around 1000 mg/L as acetic acid equivalent. In Set III, the amount of VFA measured in reactors with wastewater (R2 & R3) was almost negligible, while all the other reactors containing residue and residue/wastewater mixtures contained higher levels of VFA. In Set III, the highest VFA concentration was determined to be around 2250 mg/L as acetic acid equivalent in reactors containing wastewater and residue mixture with a ratio of WW:O = 3:1. The VFA measurements indicated that, particularly in Set III, the VFAs were effectively consumed and converted to CH4 in reactors containing ice-cream wastewater only, providing the highest CH4 yields. VFA concentrations above 2000 mg/L have been reported to cause inhibition in batch anaerobic digestion tests.[21] Higher VFA contents in all other reactors in Set III showed that the methanogenesis could not have been accomplished successfully, most probably due to presence of high concentrations of inhibitory compounds such as SO−2 4 and oil and grease, both in residue and wastewater/residue mixtures. Milk fat was found to be a major cause for reduction of the methanogenic activity for anaerobic degradation of dairy streams.[3] In particular, the oil and grease content of the ice-cream production residue, which was much above the inhibitory concentrations reported, might have adversely affected the
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process. In Set I and II, even though higher VFA content in reactors containing only wastewater was observed, these reactors could produce higher CH4 yields than those of the reactors with only residue and wastewater/residue mixtures. It seemed that in Set I and II, acidogenesis and acetogenesis phases could not have been properly accomplished in reactors containing residue and wastewater/residue mixtures. Therefore, the particular characteristics of the ice-cream production residue, in addition to ice-cream wastewater, were most probably quite influential on the process stages in Set I and II. In addition, the VFAs could not be consumed both in Set I and II in reactors only with ice-cream wastewater, resulting in lower CH4 yields than those of reactors in Set III. In this case, it can be speculated that the methanogenesis was the rate-limiting step in the anaerobic digestion process for reactors containing wastewater. 4.
Conclusions
Depending on the production cycles used in the ice-cream production plant, both ice-cream wastewater and ice-cream production residues had quite variable characteristics and these particular characteristics had adverse impacts on the methane yields in terms of energy recovery through the anaerobic digestion process. Recovery of energy from icecream wastewater alone seemed favourable as long as adequate buffering capacity was provided by external supplementation of buffering chemicals. On the other hand, energy recovery from ice-cream production residue alone did not seem feasible, particularly due to high oil and grease, and SO−2 4 content, which could significantly decrease the methane yields or could cause cessation of methane production. When wastewater and residue were mixed at an appropriate ratio, and adequate buffering capacity was provided, anaerobic co-digestion of ice-cream wastewater and residue could produce methane as a source of renewable energy. However, the methane yields and the eventual energy recovery would strongly depend on the characteristics of both substrates. Acknowledgements The authors thank Bogazici University Research Fund for supporting this work (Project numbers 12Y00P1 and 11Y00P3).
References [1] Weiland P. Biomass digestion in agriculture: a successful pathway for the energy production and waste treatment in Germany. Eng Life Sci. 2006;6:302–309. [2] Orhon D, Gorgun E, Germirli F, Artan N. Biological treatability of dairy wastewaters. Water Res. 2003;27:625–633.
[3] Perle M, Kimchie S, Shelef G. Some biochemical aspects of the anaerobic digestion of dairy wastewater. Water Res. 1995;29:1549–1554. [4] Demirel B, Onay TT, Yenigun O. Anaerobic treatment of dairy wastewaters: a review, Process Biochem. 2005;40:2583–2595. [5] Ince O. Potential energy production from anaerobic digestion of dairy wastewater. J Environ Sci Heal A. 1998;33: 1219–1228. [6] Ramasamy EV, Abbasi SA. Energy recovery from dairy wastewaters. Energ Appl. 2000;65:91–98. [7] Demirel B, Scherer P. Bio-methanization of energy crops through mono-digestion form continuous production of renewable biogas. Renew Energ. 2009;34:2940–2945. [8] APHA. Standard methods for the examination of water and wastewater. 20th ed. Washington (DC): American Public Health Association; 1999. [9] Scherer P, Neumann L, Demirel B, Schmidt O, Unbehauen M. Long term fermentation studies about the nutritional requirements for biogasification of fodder beet silage as mono-substrate. Biomass Bioenerg. 2009;33:873–881. [10] Rao PV, Baral SS, Mutnuri S. Biogas generation potential by anaerobic digestion for sustainable energy development in India. Renew Sust Energ Rev. 2008;14:2086–2094. [11] Danalewich JR, Papagiannis TG, Belyea RL, Tumbleson ME, Raskin L. Characterization of dairy waste streams, current treatment practices, and potential for biological nutrient removal. Water Res. 1998;32:3555–3568. [12] Borja R, Banks C. Kinetics of an upflow anaerobic sludge blanket reactor treating ice-cream wastewater. Environ Technol. 1994;15:219–232. [13] Demirel B, Yenigun O. Anaerobic acidogenesis of dairy wastewater: the effects of variations in hydraulic retention time with no pH control. J Chem Technol Biot. 2004;79:755– 760. [14] Demirel B, Yenigun O. Changes in microbial ecology in an anerobic reactor. Bioresour Technol. 2006;97:1201–1208. [15] Ndegwa PM, Wang L, Vaddella VK. Stabilisation of dairy wastewater using limited-aeration treatments in batch reactors. Biosystems Eng. 2007;97:379–385. [16] Cokgor EU, Sozen S, Insel G, Orhon D. Respirometric evaluation of biodegradation characteristics of dairy wastewater for organic carbon removal. Environ Technol. 2009;30:1169–1176. [17] Kasapgil B, Anderson GK, Ince O. An investigation into pretreatment of dairy wastewater prior to aerobic treatment. Water Sci Technol. 1994;29:205–212. [18] Chen Y, Cheng JJ, Creamer KS. Inhibition of anaerobic digestion process: a review. Bioresour Technol. 2008;99:4044–4064. [19] Vidal G, Carvalho A, Mendez R, Lima JM. Influence on the content of fats and proteins on the anaerobic biodegradability of dairy wastewaters. Bioresour Technol. 2000;74: 231–239. [20] Cammarota MC, Freire DMG. A review on hydrolytic enzymes in the treatment of wastewater with high oil and grease content. Bioresour Technol. 2006;97:2195–2210. [21] Siegert I, Banks C. The effect of volatile fatty acid additions on the anaerobic digestion of cellulose and glucose in batch reactors. Process Biochem. 2005;40:3412–3418.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Recovery of biogas as a source of renewable energy from ice-cream production residues and wastewater a
a
a
a
b
Burak Demirel , Murat Örok , Elif Hot , Selin Erkişi , Metin Albükrek & Turgut T. Onay a a
Institute of Environmental Sciences , Bogazici University , Istanbul , Turkey
b
Unilever, Regional Environmental Manager AAR , Istanbul , Turkey Accepted author version posted online: 06 Feb 2013.Published online: 11 Mar 2013.
To cite this article: Burak Demirel , Murat Örok , Elif Hot , Selin Erkişi , Metin Albükrek & Turgut T. Onay (2013) Recovery of biogas as a source of renewable energy from ice-cream production residues and wastewater, Environmental Technology, 34:13-14, 2099-2104, DOI: 10.1080/09593330.2013.774055 To link to this article: http://dx.doi.org/10.1080/09593330.2013.774055
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, Nos. 13–14, 2099–2104, http://dx.doi.org/10.1080/09593330.2013.774055
Recovery of biogas as a source of renewable energy from ice-cream production residues and wastewater Burak Demirela∗ , Murat Öroka , Elif Hota , Selin Erki¸sia , Metin Albükrekb and Turgut T. Onaya a Institute
of Environmental Sciences, Bogazici University, Istanbul, Turkey; b Unilever, Regional Environmental Manager AAR, Istanbul, Turkey
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(Received 22 October 2012; final version received 1 February 2013 ) Proper management of waste streams and residues from agro-industry is very important to prevent environmental pollution. In particular, the anaerobic co-digestion process can be used as an important tool for safe disposal and energy recovery from agro-industry waste streams and residues. The primary objective of this laboratory-scale study was to determine whether it was possible to recover energy (biogas) from ice-cream production residues and wastewater, through a mesophilic anaerobic co-digestion process. A high methane yield of 0.338 L CH4 /gCODremoved could be achieved from anaerobic digestion of ice-cream wastewater alone, with almost 70% of methane in biogas, while anaerobic digestion of ice-cream production residue alone did not seem feasible. When wastewater and ice-cream production residue were anaerobically co-digested at a ratio of 9:1 by weight, the highest methane yield of 0.131 L CH4 /gCODremoved was observed. Buffering capacity seemed to be imperative in energy recovery from these substrates in the anaerobic digestion process. Keywords: anaerobic digestion; biogas; ice-cream production residues; methane; renewable energy
1. Introduction An anaerobic digestion process can successfully be employed for energy recovery and waste disposal in agroindustry.[1] In particular, dairy plants produce strong waste streams, which can be characterized by their high biological oxygen demand (BOD) and chemical oxygen demand (COD) concentrations, representing their excess organic content.[2] These waste streams are concentrated in nature, and carbohydrates, proteins and fats from the milk contribute to this high organic load, making them an appropriate substrate for energy recovery through biological anaerobic digestion processes.[3,4] Ice-cream production activities generate wastewater and a residue that is generated as the production line begins to work. This residue is actually ice-cream, but since it has no commercial value it is regarded as waste and is disposed of. Ice-cream production residue can be characterized with a high total solids (TS) and organic content. Recovery of energy from dairy waste streams has been previously discussed in some studies.[5,6] However, no work was found in literature focusing on the potential of energy recovery through anaerobic digestion of ice-cream production residues. Therefore, the primary objective of this laboratory-scale experimental work was to determine the potential of biogas recovery as a source of renewable energy from ice-cream
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author. Email: [email protected]
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production residues and waste streams by a mesophilic anaerobic co-digestion process. 2. Materials and methods 2.1. The substrates The ice-cream production residue and process wastewater were obtained from an ice-cream production plant located close to Istanbul. Due to variable characteristics of both substrates, three different composite samples were obtained at different dates from the plant, in order to obtain reproducible results. The samples were kept at +4◦ C prior to use. The characteristics of the substrates are given in Table 1. The seed sludge (inoculum) was obtained from an anaerobic digester treating potato-processing wastewater. The seed sludge had an average pH value of 7.2, with TS and volatile solids (VS) values of 3.2 and 51%, respectively. The amount of inoculum added to the batch reactors in each trial is reported in Table 2. 2.2. The experimental set-up The experimental set-up used during the experiments is shown in Figure 1. Gas-tight glass reactors of 1 L capacity with a rubber plug were used for batch anaerobic digestion tests. The batch reactors were kept in a water bath
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Table 1. The characteristics of ice-cream wastewater and ice-cream production residue. Parameter pH TS VS VSS COD TKN TKN TP SO−2 4 Cl−1 Oil & grease
Unit
Wastewater (WW)
Residue (O)
% % % mg/L mg/L mg/kg mg/L mg/L mg/L mg/L
4.22–9.79 0.60–1.21 83–94 0.04–0.05 7312–10418 145–165 – 32–43 200–350 400–550 2424–2815
3.47–4.28 16–25 89–93 – 249960–303097 – 26627–144231 150–175 200–1600 – 7028–47365
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Figure 1.
and the temperature was automatically controlled to provide mesophilic conditions at 37◦ C. Each reactor was connected to a MilliGascounter for measurement of biogas production. Three different sets of experiments were conducted in order to compare the reproducibility of the experimental findings, since the characteristics of both substrates were quite different in each sampling period. Control reactors were also run for Set II and III (data not shown). The reactor loadings for each set of experiments are given in Table 2. The initial pH, TS, VS, and COD values of batch reactors are also given in Table 3. The batch reactors were loaded with wastewater, ice-cream production residue and seed sludge using pre-determined mixture ratios by weight (w/w). The pH of the medium was adjusted to a neutral pH range only at the beginning of the experiments, using a 6 N sodium hydroxide (NaOH) and a 6 N sulphuric acid (H2 SO4 ) solution. In order to provide adequate buffering capacity, potassium hydrogen bicarbonate (KHCO3 ) buffer solution was also used.[7] After loading, the reactors were Table 2.
The experimental set-up used for the experiment.
flushed with nitrogen (N2 ) gas for about 5 min to provide anaerobic conditions for the mixed culture. All of the chemicals were reagent grade obtained from commercial sources (Merck, Darmstadt, Germany). 2.3.
Analytical methods
Measurement of pH, solids, total Kjeldahl nitrogen, total phosphorus, COD, chloride (Cl−1 ), sulphate (SO−2 4 ) and oil and grease were performed according to Standard Methods.[8] Biogas production was continuously measured using MilliGascounter MGC-1 (Ritter, Bochum, Germany).[9] The biogas values were corrected for Standard Temperature and Pressure. The composition of biogas, namely methane (CH4 ) and carbon dioxide (CO2 ), was measured using a HP 6850 Gas Chromatograph equipped with a thermal conductivity detector. The volatile fatty acid (VFA) measurements were performed using a Perkin Elmer 600 Gas Chromatograph with a flame ionization detector.
The reactor loading conditions for each set of experiments.
Reactor
Mixing ratio by weight (WW:O)
Seed sludge (mL)
Wastewater (WW) (mL)
Residue (O) (mL)
Buffer addition (mL)
SET I R1 & R2 R3 & R4 R5 & R6
– – 1:1
250 250 250
500 – 250
– 500 250
24 24 24
SET II R2 & R3 R4 & R5 R6 & R7 R8 & R9 R10 & R11
– – 1:1 3:1 9:1
250 250 250 250 250
500 – 250 375 450
– 500 250 125 50
100 150 150 150 150
SET III R2 & R3 R4 & R5 R6 & R7 R8 & R9 R10 & R11
– – 3:1 6:1 9:1
250 250 250 250 250
500 – 375 430 450
– 500 125 70 50
100 150 150 150 150
Temperature (◦ C) Mesophilic
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Table 3.
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Initial pH, TS, VS and COD values in batch reactors for each set.
Reactor
pH
TS (%)
VS (%)
COD (mg/L)
Buffer addition (mL)
Temperature (◦ C)
SET I R1 & R2 R3 & R4 R5 & R6
7.6 6.8 6.9
1.19 17.66 9.75
73.1 92.8 88.6
4618 156030 113186
24 24 24
Mesophilic
SET II R2 & R3 R4 & R5 R6 & R7 R8 & R9 R10 & R11
7.5 7.0 7.3 7.5 7.6
2.18 11.03 6.55 4.59 2.80
80.2 76.6 88.3 77.2 82.2
17095 133697 69399 39723 22412
100 150 150 150 150
SET III R2 & R3 R4 & R5 R6 & R7 R8 & R9 R10 & R11
7.9 6.9 7.5 7.8 7.8
2.37 9.7 4.24 3.49 3.19
38.3 62.6 55.1 38.0 46.2
10501 136694 47436 38202 32662
100 150 150 150 150
3. Results and discussion The cumulative biogas and methane (CH4 ) production values, and the CH4 yields observed for each set are summarized in Table 4 and Figures 2– 4. The duration time of the experiments for Set I, II, and III were 17, 25 and 24 days, respectively. When ice-cream wastewater was anaerobically digested alone, without ice-cream production residue, the highest CH4 yield of 0.338 L CH4 /g CODremoved was obtained in Set III. The CH4 yields for wastewater for Set I and II were 0.194 and 0.160 L CH4 /g CODremoved , respectively. When ice-cream production residue was digested alone, without wastewater addition, the observed CH4 yields were almost negligible in both Set I and II, as indicated by very small amounts of cumulative CH4 production. Like wastewater, the highest CH4 yield for anaerobic
Figure 2. The cumulative biogas and methane values (mL) for ice-cream wastewater (WW), ice-cream production residue (O) and wastewater/residue mixture (WW:O) for batch reactors in Set I.
Table 4. Methane yields observed for batch anaerobic digestion tests.
Substrate WW O
WW+O
Set
Methane yield (L CH4 /g VSdestroyed )
Methane yield ( L CH4 /g CODremoved )
SET I SET II SET III SET I SET II SET III SET I (1:1) SET II (1:1) SET II (3:1) SET II (9:1) SET III (3:1) SET III (6:1) SET III (9:1)
0.091 0.147 0.222 0.000 0.001 0.079 0.000 0.001 0.006 0.049 0.121 0.175 0.369
0.194 0.160 0.338 0.000 0.001 0.032 0.000 0.002 0.015 0.083 0.016 0.050 0.131
Figure 3. The cumulative biogas and methane values (mL) for ice-cream wastewater (WW), ice-cream production residue (O) and wastewater/residue mixtures (WW:O) for batch reactors in Set II.
digestion of ice-cream production residue was obtained in Set III as 0.079 L CH4 /g VSdestroyed . However, this CH4 yield was quite poor in terms of biogas recovery from this production residue.
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Figure 4. The cumulative biogas and methane values (mL) for ice-cream wastewater (WW), ice-cream production residue (O) and wastewater/residue mixtures (WW:O) for batch reactors in Set III.
For batch anaerobic co-digestion of ice-cream wastewater and production residue, the highest CH4 yields were obtained in Set III as 0.369 L CH4 /g VSdestroyed and 0.131 L CH4 /g CODremoved , respectively, when wastewater and residue were mixed at a ratio of 9:1 by weight (w/w). In case of mixture ratios of 1:1, 3:1, and 6:1, poor CH4 yields were observed in Set I, II, and III. In addition, in Set II, although a mixture ratio of 9:1 was employed, the obtained CH4 yields were low again, as indicated by poor amount of cumulative CH4 production. In terms of biogas composition, the percentage of CH4 in biogas from anaerobic digestion of ice-cream wastewater ranged between 20–60% in Set I. Even though there was biogas production from anaerobic digestion of residue and the mixture of wastewater and residue, there was no CH4 detected in biogas. In both cases, the biogas was mainly composed of CO2 and N2 , indicating that the anaerobic digestion process could not be accomplished in these trials. In Set II, CH4 ranged from 40–80% in biogas from ice-cream wastewater alone, and on average it was about 60%. No CH4 production was observed from anaerobic digestion of residue and from the mixture of wastewater and residue at a ratio of 1:1. A low CH4 generation could be detected from the mixture of wastewater and residue at a ratio of 3:1. For mixtures of wastewater and residue, the highest CH4 composition in biogas was 60%, and this was obtained at a mixture ratio of 9:1. In terms of CH4 production, the best results were generally obtained in Set III, with Table 5.
respect to Set I and II, probably due to the characteristics of both wastewater and residue. CH4 production could be observed in all of the trials in Set III. The CH4 composition was measured to be around 70% for anaerobic digestion of ice-cream wastewater alone. The highest CH4 composition for mixtures of wastewater and residue was 60% at a mixture ratio of 9:1, while for the mixture ratio of 6:1, the CH4 varied from 20–40%. According to the experimental findings obtained, batch mesophilic anaerobic digestion of ice-cream wastewater alone (without co-substrate addition) could produce a high CH4 yield. The CH4 yield from mesophilic anaerobic treatment of a dairy industry wastewater by a pilot-scale upflow anaerobic filter was reported to vary between 0.32– 0.34 m3 CH4 /kg CODremoved with a CH4 composition ranging between 75–85% in biogas.[5] Typical CH4 generation values reported for anaerobic digestion of dairy industry waste streams range from 60–85%.[4,5,10] In this work, the CH4 content of biogas generated from anaerobic digestion of ice-cream wastewater could also reach more than 70%. Due to its high organic content, the ice-cream process wastewater seemed suitable for recovery of energy by anaerobic treatment. However, the CH4 yields seemed to vary greatly, depending strongly on the particular characteristics of the waste streams. Dairy wastewater flow rates and characteristics vary greatly and are quite difficult to predict, even when detailed information on processing operations is available.[11] A comparison of the characteristics of the ice-cream wastewater used in this work with similar waste streams is given in Table 5, indicating how the characteristics of dairy waste streams fluctuate.[5,12–16] In this work, the ice-cream wastewater pH was quite variable, and some batch tests performed without buffer solution indicated that the buffering capacity was quite low, resulting in rapid acidification of batch reactors (data not shown). The use of acid and alkaline cleaning agents in the dairy industry, in addition to various process operations, significantly influences the wastewater characteristics, particularly the pH.[11,17] The alkalinity of dairy wastewaters was reported to be less than 1000 mg CaCO3 /L in most cases,[13,14] not high enough to sustain an anaerobic treatment process successfully, showing that the buffering capacity is often lacking in these waste streams. Therefore, external use of
A comparison of the characteristics of dairy/ice-cream waste streams.
Wastewater type
pH
−1 (mg/L) Reference COD (mg/L) TKN (mg/L) TP (mg/L) SO−2 4 (mg/L) Oil & grease (mg/L) Cl
Ice-cream 4.22–9.79 7312–10418 Milk & cream 8–11 2000–6000 Ice-cream 5.2 5200 Milk processing 8.6 4420 Milk processing 5.8–11.4 1155–9185 7.78 13750 Mixed process 1410–1815
145–165 50–60
32–43
110 14–272 1027 66–84
30 8–68 8.2–10
200–350
2424–2815
400–550
35 7–62
525 233-993
This work [5] [12] [13] [14] [15] [16]
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Environmental Technology a buffering agent became an essential tool to accomplish anaerobic digestion process in this work. In addition to pH instability and low buffering capacity, the ice-cream wastewater also contained high concentrations of SO−2 4 , oil and grease and Cl−1 in each trial. Variable pH, inadequate buffering capacity along with high SO−2 4 and oil and grease concentrations are bottlenecks in an anaerobic digestion process.[18] The SO−2 4 and oil and grease concentrations in ice-cream wastewater ranged between 200–350 and 2424– 2815 mg/L, respectively, while Cl−1 concentration varied between 400–550 mg/L. It has been reported that the presence of high oil and grease content in dairy waste streams could cause problems due to their low biodegradability in the anaerobic digestion process.[19,20] High concentrations of oil and grease in ice-cream waste streams might have adversely affected the CH4 yields, while SO−2 4 and Cl−1 content seemed to be below the ranges considered to be inhibitory. Since oil and grease content of the batch reactor contents were not measured after the process has been accomplished, it is difficult to conclude more in detail about oil and grease degradation and its possible effects on the anaerobic digestion process in this work. Anaerobic digestion of ice-cream production residue alone (without ice-cream wastewater addition) did not seem to be feasible, since the observed CH4 yields were almost negligible in most trials. The ice-cream production residue also had differing characteristics in each trial, most probably due to production cycles employed, and it had a very low, acidic pH, with no buffering capacity. Even external supplementation of buffering solutions did not improve biological degradation and promote generation of CH4 . In addition, the ice-cream production residue contained very high concentrations of SO−2 4 , and oil and grease, both of which made the anaerobic digestion process quite unfavourable. Thus, in spite of its high organic content, anaerobic digestion of the ice-cream production residue without co-substrate (wastewater) addition for energy recovery purposes did not produce satisfactory results. Different mixture ratios by weight were employed to achieve high CH4 yields from anaerobic co-digestion of ice-cream wastewater and ice-cream production residues. The mixture of ice-cream wastewater and residue contained high concentrations of SO−2 4 and oil and grease, and it also had a quite low buffering capacity, mostly originating from the residue. As summarized above, all of these factors could easily result in acidification of an anaerobic digestion system. These factors eventually affected the CH4 yields adversely. The CH4 yields generally increased when the dilution ratio between the wastewater and residue increased. When the dilution ratios were low, such as 1:1 or 3:1, lower CH4 yields were achieved from anaerobic co-digestion. These results indicated that the ice-cream production residue had to be mixed with wastewater at high dilution ratios so that CH4 production from anaerobic co-digestion of ice-cream wastewater and residue could be realized. The dilution effect of wastewater seemed to
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Figure 5. The concentrations of VFA (as mg acetic acid equivalent/L) in batch reactors for each set.
decrease the concentrations of inhibitory components in the ice-cream production residue, thereby promoting CH4 production along with adequate buffering capacity. The SO−2 4 and oil and grease concentrations in residue ranged between 200–1600 and 7028–47365 mg/L, respectively, much higher than those of ice-cream wastewater, probably causing perturbation. Among the wastewater and residue mixtures, the highest dilution of residue with wastewater provided the highest CH4 yield. The concentrations of VFAs as acetic acid equivalent for each set is displayed in Figure 5. In Set I, the highest VFA concentration of about 750 mg/L as acetic acid equivalent was measured in reactors that only contained ice-cream production residue. In reactors containing wastewater and residue mixture (R5 & R6), a lower VFA concentration was measured than those of reactors containing only wastewater (R1 & R2). However, a negligible methane yield was calculated for R5 & R6 (Table 4). In Set II, the highest VFA concentration was determined in reactors containing only wastewater. The VFA concentration was around 1000 mg/L as acetic acid equivalent. In Set III, the amount of VFA measured in reactors with wastewater (R2 & R3) was almost negligible, while all the other reactors containing residue and residue/wastewater mixtures contained higher levels of VFA. In Set III, the highest VFA concentration was determined to be around 2250 mg/L as acetic acid equivalent in reactors containing wastewater and residue mixture with a ratio of WW:O = 3:1. The VFA measurements indicated that, particularly in Set III, the VFAs were effectively consumed and converted to CH4 in reactors containing ice-cream wastewater only, providing the highest CH4 yields. VFA concentrations above 2000 mg/L have been reported to cause inhibition in batch anaerobic digestion tests.[21] Higher VFA contents in all other reactors in Set III showed that the methanogenesis could not have been accomplished successfully, most probably due to presence of high concentrations of inhibitory compounds such as SO−2 4 and oil and grease, both in residue and wastewater/residue mixtures. Milk fat was found to be a major cause for reduction of the methanogenic activity for anaerobic degradation of dairy streams.[3] In particular, the oil and grease content of the ice-cream production residue, which was much above the inhibitory concentrations reported, might have adversely affected the
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process. In Set I and II, even though higher VFA content in reactors containing only wastewater was observed, these reactors could produce higher CH4 yields than those of the reactors with only residue and wastewater/residue mixtures. It seemed that in Set I and II, acidogenesis and acetogenesis phases could not have been properly accomplished in reactors containing residue and wastewater/residue mixtures. Therefore, the particular characteristics of the ice-cream production residue, in addition to ice-cream wastewater, were most probably quite influential on the process stages in Set I and II. In addition, the VFAs could not be consumed both in Set I and II in reactors only with ice-cream wastewater, resulting in lower CH4 yields than those of reactors in Set III. In this case, it can be speculated that the methanogenesis was the rate-limiting step in the anaerobic digestion process for reactors containing wastewater. 4.
Conclusions
Depending on the production cycles used in the ice-cream production plant, both ice-cream wastewater and ice-cream production residues had quite variable characteristics and these particular characteristics had adverse impacts on the methane yields in terms of energy recovery through the anaerobic digestion process. Recovery of energy from icecream wastewater alone seemed favourable as long as adequate buffering capacity was provided by external supplementation of buffering chemicals. On the other hand, energy recovery from ice-cream production residue alone did not seem feasible, particularly due to high oil and grease, and SO−2 4 content, which could significantly decrease the methane yields or could cause cessation of methane production. When wastewater and residue were mixed at an appropriate ratio, and adequate buffering capacity was provided, anaerobic co-digestion of ice-cream wastewater and residue could produce methane as a source of renewable energy. However, the methane yields and the eventual energy recovery would strongly depend on the characteristics of both substrates. Acknowledgements The authors thank Bogazici University Research Fund for supporting this work (Project numbers 12Y00P1 and 11Y00P3).
References [1] Weiland P. Biomass digestion in agriculture: a successful pathway for the energy production and waste treatment in Germany. Eng Life Sci. 2006;6:302–309. [2] Orhon D, Gorgun E, Germirli F, Artan N. Biological treatability of dairy wastewaters. Water Res. 2003;27:625–633.
[3] Perle M, Kimchie S, Shelef G. Some biochemical aspects of the anaerobic digestion of dairy wastewater. Water Res. 1995;29:1549–1554. [4] Demirel B, Onay TT, Yenigun O. Anaerobic treatment of dairy wastewaters: a review, Process Biochem. 2005;40:2583–2595. [5] Ince O. Potential energy production from anaerobic digestion of dairy wastewater. J Environ Sci Heal A. 1998;33: 1219–1228. [6] Ramasamy EV, Abbasi SA. Energy recovery from dairy wastewaters. Energ Appl. 2000;65:91–98. [7] Demirel B, Scherer P. Bio-methanization of energy crops through mono-digestion form continuous production of renewable biogas. Renew Energ. 2009;34:2940–2945. [8] APHA. Standard methods for the examination of water and wastewater. 20th ed. Washington (DC): American Public Health Association; 1999. [9] Scherer P, Neumann L, Demirel B, Schmidt O, Unbehauen M. Long term fermentation studies about the nutritional requirements for biogasification of fodder beet silage as mono-substrate. Biomass Bioenerg. 2009;33:873–881. [10] Rao PV, Baral SS, Mutnuri S. Biogas generation potential by anaerobic digestion for sustainable energy development in India. Renew Sust Energ Rev. 2008;14:2086–2094. [11] Danalewich JR, Papagiannis TG, Belyea RL, Tumbleson ME, Raskin L. Characterization of dairy waste streams, current treatment practices, and potential for biological nutrient removal. Water Res. 1998;32:3555–3568. [12] Borja R, Banks C. Kinetics of an upflow anaerobic sludge blanket reactor treating ice-cream wastewater. Environ Technol. 1994;15:219–232. [13] Demirel B, Yenigun O. Anaerobic acidogenesis of dairy wastewater: the effects of variations in hydraulic retention time with no pH control. J Chem Technol Biot. 2004;79:755– 760. [14] Demirel B, Yenigun O. Changes in microbial ecology in an anerobic reactor. Bioresour Technol. 2006;97:1201–1208. [15] Ndegwa PM, Wang L, Vaddella VK. Stabilisation of dairy wastewater using limited-aeration treatments in batch reactors. Biosystems Eng. 2007;97:379–385. [16] Cokgor EU, Sozen S, Insel G, Orhon D. Respirometric evaluation of biodegradation characteristics of dairy wastewater for organic carbon removal. Environ Technol. 2009;30:1169–1176. [17] Kasapgil B, Anderson GK, Ince O. An investigation into pretreatment of dairy wastewater prior to aerobic treatment. Water Sci Technol. 1994;29:205–212. [18] Chen Y, Cheng JJ, Creamer KS. Inhibition of anaerobic digestion process: a review. Bioresour Technol. 2008;99:4044–4064. [19] Vidal G, Carvalho A, Mendez R, Lima JM. Influence on the content of fats and proteins on the anaerobic biodegradability of dairy wastewaters. Bioresour Technol. 2000;74: 231–239. [20] Cammarota MC, Freire DMG. A review on hydrolytic enzymes in the treatment of wastewater with high oil and grease content. Bioresour Technol. 2006;97:2195–2210. [21] Siegert I, Banks C. The effect of volatile fatty acid additions on the anaerobic digestion of cellulose and glucose in batch reactors. Process Biochem. 2005;40:3412–3418.
