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© IWA Publishing 2014 Water Science & Technology
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2014
Experiences with anaerobic treatment of fat-containing food waste liquids: two full scale studies with a novel anaerobic flotation reactor C. T. M. J. Frijters, T. Jorna, G. Hesselink, J. Kruit, D. van Schaick and R. van der Arend
ABSTRACT Fat-containing food waste can be effectively treated in a new type of reactor, the so-called BIOPAQAnaerobic Flotation Reactor or BIOPAQ® anaerobic flotation reactor (AFR). In the reactor a flotation unit is integrated to retain the sludge. In this study results from two plants with a 430 and 511 m3AFR, respectively, are presented. In one reactor, which is fed with water originating from different food liquid streams, over 99% of fat and oils were removed. Over 90% of the chemical oxygen demand (COD) was removed. When the last solids were removed from the effluent with a tilted plate settler, 98% COD removal was attained. The effluent concentrations of extractable hydrolysed and non-hydrolysed fats were less than 40 mg/l. Apparently the variations in the liquid streams deriving from the tank cleaning activities did not disturb the system. Only extremely high concentrations of fats could disturb the system, but the inhibition was reversible. In the reactor treating water from an ice-cream factory, which contained fats up to approximately 50% of influent COD, a COD removal efficiency of 90% was achieved. At volumetric loading rates varying from 1 to 8 kg COD/m3/d, biogas was produced at an average specific gas production of 0.69 m3/kg COD–removed. Key words
C. T. M. J. Frijters (corresponding author) T. Jorna G. Hesselink J. Kruit PAQUES BV, T. de Boerstraat 24, 8561 EL, Balk, The Netherlands E-mail: [email protected] D. van Schaick ITC Holland Transport BV, Vorstengrafdonk 71, 5342 LW Oss, The Netherlands R. van der Arend Ben & Jerry’s, Reggeweg 15, 7447 AN Hellendoorn, The Netherlands
| anaerobic flotation reactor, fat, food waste water, ice-cream waste, long chain fatty acids (LCFA), oil
INTRODUCTION Waste water deriving from the food industry contains organic compounds that are very suitable for anaerobic conversion into biogas. The types of anaerobic reactors used for that purpose are high rate granular sludge reactors for more diluted streams (up to 30 g/l of chemical oxygen demand (COD)), or flocculent sludge continuous stirred reactors (CSTRs) for more concentrated wastewaters (from 70 g/l of COD). Food wastewaters that contain, besides carbohydrates and proteins, energy-valuable oils or fats like olive oil wastewater (5–25 g/l, Becker et al. ) and dairy wastewater (2–3 g/l, Kuang et al. ), are often difficult to treat in high rate granular sludge reactors. The granules can be trapped by the fats and oils and can float out of the reactor (Hwu et al. ), or toxicity of long chain fatty acids (LCFA) for the methanogens is found (Rinzema et al. ). Furthermore mass transfer limitations and performance reduction (Hwu et al. ) can be encountered in upflow anaerobic sludge blankets. Other systems doi: 10.2166/wst.2013.806
like biological filter systems with sludge retention, do not have very stable operation when treating dairy effluent. Scum formation and loss of solubilisation activity after a relatively short period of 1–2 weeks of non-feeding, were observed in a buoyant filter bioreactor (Haridas et al. ). Pretreatment is necessary to remove fat and other suspended solids in advance. A typical process configuration for this type of wastewater is a dissolved air flotation (DAF) unit followed by an EGSB (extended granular sludge blanket) system. However, the fat separation in the DAF unit is the bottleneck for the EGSB process. A breakthrough of a low concentration of fat (>50 mg/l) will result in process disturbance. In that case, treatment in a CSTR would be a better option. The food waste (water) streams often contain a low COD (7–50 g/l). Because CSTR technology is hydraulically designed, as no sludge is retained, large volumes of CSTR are required to treat these kinds of streams. The retention time needs to be high
C. T. M. J. Frijters et al.
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Two full scale studies with a novel anaerobic flotation reactor
enough to hydrolyse the particles. A new type of reactor, the so-called BIOPAQ®-Anaerobic Flotation Reactor or BIOPAQ® anaerobic flotation reactor (AFR) (PCT/NL02– 26-03-02, Vellinga & Mulder ), is especially designed to treat wastewater streams containing fats and oils with a COD content of 7–50 g/l. In the reactor, digestion is combined with a flotation unit placed inside the reactor (Figure 1). The decoupling of the hydraulic and solids retention time results in well adapted biomass and high hydrolysing capacities. Compared to a system with DAF followed by EGSB or CSTR, the system does not use polymers, needs less maintenance, has a smaller footprint and, compared to the first alternative, produces more biogas because all organic compounds can be anaerobically converted (Hülsen et al. ). In the AFR system the sludge is retained by the integrated flotation unit up to concentrations of 15–30 kg/m3-reactor. Because of these high concentrations, high volumetric loading rates (VLR) can be applied: up to 9 kg of COD/m3reactor/d, with an HRT (hydraulic sludge retention time) of 1.5 to 8 days. The solids retention time (SRT) can be longer than 50 days. The flocculent sludge is floated using white water, which releases small biogas bubbles when introduced in the reactor. The white water is produced by dissolving biogas in reactor effluent under pressure. Sludge on which fatty compounds are adsorbed, will be floated and will be retained in the system, as was found to be the case in a former study with this system (Frijters et al. ), unlike in granular systems where it would wash out.
Water Science & Technology
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In the cone of the flotation unit, the fluid with solids from the reactor is continuously pumped. The solids are floated by means of small biogas bubbles that are pumped in the flotation unit with the white water (Figure 1). The solids float over the edge of the unit, back into the anaerobic reactor. The reactor fluid is mixed with risers and downers driven by biogas (not shown in Figure 1). The effluent leaves the system at the bottom of the flotation unit beneath the flotation layer. In the system flocculent sludge develops, therefore no preacidification step, which is commonly used in granular sludge systems to provide enough acidification for a good granular sludge development, has to be applied. Other pretreatment steps, such as a fat removing pretreatment step, can be skipped. To avoid large fluctuations of COD or fat concentrations, a small buffer might be necessary in case of concentrated (side) streams. A pilot reactor treating wastewater derived from cleaning tanks used for transporting food liquids (ITC Holland Transport BV), showed promising results (Frijters et al. ). In the literature, problems with each of the degradation steps of fat and oil (roughly divided into hydrolysis, acidification (by β-oxidation) and methanogenesis) are described (Hanaki et al. ; Angeldaki & Ahring ; Hwu ; Sanders ). However in the pilot study these problems were not encountered. The first 430 m3-prototype reactor was built at the tank cleaning company in November 2009. In 2011 another full scale reactor (511 m3) was started, after a 4-month pilot plant study, at an ice-cream producing company (Ben & Jerry’s). The objective of these studies was to show that high removal efficiencies for COD and fat and oils can be attained in both industrial scale projects, at varying influent qualities (food liquid effluents) and loading rates (ice-cream effluents).
MATERIALS AND METHODS Set-up Prototype AFR at the tank cleaning company
Figure 1
|
Schematic overview of the AFR.
The prototype of BIOPAQ®AFR was started at the end of 2009 at ITC Holland Transport BV (Figure 2). The reactor had a maximum water level of 13.5 m and a diameter of 6.8 m, corresponding to a maximum water volume of 490 m3 (on average 430 m3). The water was buffered in a 40 m3-mixed buffer. A part of the non-soluble fats were skimmed and collected in a special mixed buffer. The fats could be fed to the reactor influent pipe by a controlled flow. In this way high peaks of fats and oils (high COD)
C. T. M. J. Frijters et al.
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Figure 2
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Two full scale studies with a novel anaerobic flotation reactor
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The BIOPAQ®AFR at ITC Holland Transport BV (left) and at Ben & Jerry’s (right; photographer: H. Roggen).
W
could be avoided. The temperature in the reactor was 38 C, regulated by a heat-exchanger cooling the wastewater. The pH in the reactor was operated at a minimum of 6.3, if necessary caustic was dosed. Urea and phosphoric acid were dosed in the influent pipe line to supply macronutrients. The produced biogas was used in a boiler to heat water for tank cleaning purposes. The full scale AFR at the ice-cream producing company
The wastewater The wastewater from the tank cleaning company originated from the cleaning of tanks which had been used for transporting liquids from a variety of food producing companies. The wastewater composition varied (Table 1) according to the variation of the tank contents. The fat-COD (ratio of fat:fat-COD was assumed to be 1: 2.5), could make up over 30% of the T-COD. If necessary the water (in the buffer up to 55 C) was cooled in a heatexchanger to a temperature of 38 C. The flow during 6 days of the week was on average 81 m3/d, on Sunday it was 50% of the T-COD, data not shown) in the sludge, the system was disturbed: the VFA in the reactor increased and fat accumulated. Similar results were described in the literature for LCFA-fed digesters (Kuang et al. ). However, the inhibition was reversible, this was also found in another study (Pereira et al. ). The effect of inhibition of LCFA has recently been modelled and the high sensitivity of the acetoclastic population to LCFA was evidenced (Zonta et al. ). In the current study it is shown that the inhibition of fat degradation could be prevented by operating the AFR at low VFA concentrations in the reactor. The separation of the skimmed insoluble fats and oils in the small buffer tank, prevented high ratios of fat-COD:T-COD and provided controlled dosing of fats and oily compounds. No fat layers were formed in the reactor: the mixing with biogas lifts appeared to be very effective. It was observed that the presence of fat had a positive influence on the compactness of the floc. In the literature the syntrophic populations that are needed for good LCFA conversion were described in which acetogens, acetoclastic and hydrotrophic methanogens play an important role (Sousa et al. ). A compact biomass structure is needed for this purpose. Furthermore it is described that adsorbed LCFA or fat might facilitate the transport of the poor water-soluble hydrogen (Alves et al. ). In that case the compact flocs with adsorbed fatty compounds could be the optimum biomass structure to perform the conversions. The floc structure might facilitate the hydrolysing processes that precede the β-oxidation and methanogenesis, as well. This was supported by the results of a previous pilot study with the AFR system (Frijters et al. ): at high loading rates of easily biodegradable organics (with only a low concentration of fat and oils) the process was disturbed. The flocs became very fluffy. This resulted in minor flotation and less effective fat degradation. Maybe the microbiological aspects such as a limited transport of acetate and hydrogen in these fluffy flocs (with a relative high distance between bacteria) had a negative influence on the conversion rates as well. These results indicate that the compactness of the flocs is very important for both physical and biological processes. These aspects together with the (shift in) composition of the bacteria and archaea in the flocs need to be elucidated in the future.
CONCLUSIONS The anaerobic flotation reactor was successfully applied to convert organic compounds, of which fats could make up 50% of the COD, into biogas. Fat and oils were removed
Water Science & Technology
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2014
to concentrations of less than 40 mg/l. Fat peaks could be buffered in the system, being converted later into biogas. Extreme high peaks of fats could disturb the system, but the inhibition was reversible. However, peaks of COD and fat should be avoided to obtain a more constant quality of sludge. In both industrial scale reactors, COD removal efficiencies of 90% were obtained. The effluent could be post-treated by means of a tilted plate settler, resulting in COD removal efficiencies of over 98%.
REFERENCES Alves, M. M., Pereira, M. A., Sousa, D. Z., Cavaleiro, A. J., Picavet, M., Smidt, H. & Stams, A. J. M. Waste lipids to energy: how to optimize methane production from long-chain fatty acids (LCFA). Microbial Biotechnology 2 (5), 538–550. Angeldaki, I. & Ahring, B. K. Effects of free long chain fatty acids on thermophilic anaerobic digestion. Applied Microbiology and Biotechnology 37 (6), 808–812. Becker, P., Koster, D., Popov, M. N., Markossian, S., Antranikian, G. & Markl, H. The biodegradation of olive oil and the treatment of lipid-rich wool scouring wastewater under aerobic thermophilic conditions. Water Research 33 (3), 653–660. Field, J., Lettinga, G. & Geurts, M. The methanogenic toxicity and anaerobic degradability of potato starch wastewater phenolic amino acids. Biological Wastes 21, 37–45. Frijters, C. T. M. J., Havinga, S., de Jong, K., van der Meer, W., Wessels, C., van Schaick, D., de Wit, L., van Eekert, M., Zeeman, G. & Vellinga, S. A new type of anaerobic reactor for treatment of concentrated waste(water streams): an anaerobic reactor with integrated flotation unit. In: Proceedings of IWA Conference on Industrial Water Treatment Systems, September 30–October 03 2008 [CD], Amsterdam. Hanaki, K., Matsuo, T. & Nagase, M. Mechanisms of inhibition caused by long chain fatty acids in anaerobic digesting process. Biotechnology and Bioengineering 23 (7), 1591–1610. Haridas, A., Suresh, S., Chitra, K. R. & Manilal, V. B. The Buoyant Filter Bioreactor: a high-rate anaerobic reactor for complex wastewater-process dynamics with dairy effluent. Water Research 39 (6), 993–1004. Hülsen, T., van Zessen, E., Kruit, J., van Bosch, P. L. F. & Frijters, C. T. M. J. Production of valuable biogas out of fat and protein containing wastewaters using compact BIOPAQ AFR and the THIOPAQ-technology. In: Proceedings of IWA Conference. November 2010, Water and Energy, Amsterdam. Hwu, C.-S. Enhancing Anaerobic Treatment of Wastewaters Containing Oleic Acid. PhD Thesis. Wageningen Agricultural University, Wageningen, The Netherlands.
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Hwu, C. S., Tseng, S. K., Yuan, C. Y., Kulik, Z. & Lettinga, G. Biosorption of long-chain fatty acids in UASB treatment process. Water Research 32 (5), 1571–1579. Kuang, Y., Lepesteur, M., Pullammanappallil, P. & Ho, G. E. Influence of co-substrates on structure of microbial aggregates in long-chain fatty acid-fed anaerobic digesters. Letters in Applied Microbiology 35 (3), 190–194. Palatsi, J., Laureni, M., Andrés, M. V., Flotats, X., Nielsen, H. B. & Angelidaki, I. Strategies for recovering inhibition caused by long chain fatty acids on anaerobic thermophilic biogas reactors. Bioresource Technology 100 (20), 4588–4596. Pereira, M. A., Pires, O. C., Mota, M. & Alves, M. M. Anaerobic biodegradation of oleic and palmitic acids: evidence of mass transfer limitations caused by long chain fatty acid accumulation onto the anaerobic sludge. Biotechnology and Bioengineering 92 (1), 15–23. Perle, M., Shelef, G. & Kimchie, S. Some biochemical aspects of the anaerobic degradation of dairy wastewater. Water Research 29 (6), 1549–1554.
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Rinzema, A., Boone, M., van Knippenberg, K. & Lettinga, G. Bactericidal effect of long chain fatty acids in anaerobic digestion. Water Environmental Research 66, 40–49. Sanders, W. T. M. Anaerobic Hydrolysis During Digestion of Complex Substrates. PhD Thesis, Wageningen Agricultural University. Sousa, D. Z., Balk, M., Alves, M. M., Schink, B., McInerney, M. J., Smidt, H., Plugge, C. M. & Stams, A. J. M. Degradation of long-chain fatty acids by sulfate-reducing and methanogenic communities. In: Handbook of Hydrocarbon and Lipid Microbiology. (K. N. Timmis, ed.). Springer-Verlag, Berlin, Heidelberg. Vellinga, S. H. J. & Mulder, R. (Patent) Method and device for the anaerobic purification of a slurry which contains organic constituents (WO02076893). Zonta, Z., Alvez, M. M., Flotats, X. & Palatsi, J. Modelling inhibitory effects of long chain fatty acids in the anaerobic digestion process. Water Research 47 (3), 1369–1380.
First received 18 July 2013; accepted in revised form 11 December 2013. Available online 28 December 2013
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© IWA Publishing 2014 Water Science & Technology
|
69.7
|
2014
Experiences with anaerobic treatment of fat-containing food waste liquids: two full scale studies with a novel anaerobic flotation reactor C. T. M. J. Frijters, T. Jorna, G. Hesselink, J. Kruit, D. van Schaick and R. van der Arend
ABSTRACT Fat-containing food waste can be effectively treated in a new type of reactor, the so-called BIOPAQAnaerobic Flotation Reactor or BIOPAQ® anaerobic flotation reactor (AFR). In the reactor a flotation unit is integrated to retain the sludge. In this study results from two plants with a 430 and 511 m3AFR, respectively, are presented. In one reactor, which is fed with water originating from different food liquid streams, over 99% of fat and oils were removed. Over 90% of the chemical oxygen demand (COD) was removed. When the last solids were removed from the effluent with a tilted plate settler, 98% COD removal was attained. The effluent concentrations of extractable hydrolysed and non-hydrolysed fats were less than 40 mg/l. Apparently the variations in the liquid streams deriving from the tank cleaning activities did not disturb the system. Only extremely high concentrations of fats could disturb the system, but the inhibition was reversible. In the reactor treating water from an ice-cream factory, which contained fats up to approximately 50% of influent COD, a COD removal efficiency of 90% was achieved. At volumetric loading rates varying from 1 to 8 kg COD/m3/d, biogas was produced at an average specific gas production of 0.69 m3/kg COD–removed. Key words
C. T. M. J. Frijters (corresponding author) T. Jorna G. Hesselink J. Kruit PAQUES BV, T. de Boerstraat 24, 8561 EL, Balk, The Netherlands E-mail: [email protected] D. van Schaick ITC Holland Transport BV, Vorstengrafdonk 71, 5342 LW Oss, The Netherlands R. van der Arend Ben & Jerry’s, Reggeweg 15, 7447 AN Hellendoorn, The Netherlands
| anaerobic flotation reactor, fat, food waste water, ice-cream waste, long chain fatty acids (LCFA), oil
INTRODUCTION Waste water deriving from the food industry contains organic compounds that are very suitable for anaerobic conversion into biogas. The types of anaerobic reactors used for that purpose are high rate granular sludge reactors for more diluted streams (up to 30 g/l of chemical oxygen demand (COD)), or flocculent sludge continuous stirred reactors (CSTRs) for more concentrated wastewaters (from 70 g/l of COD). Food wastewaters that contain, besides carbohydrates and proteins, energy-valuable oils or fats like olive oil wastewater (5–25 g/l, Becker et al. ) and dairy wastewater (2–3 g/l, Kuang et al. ), are often difficult to treat in high rate granular sludge reactors. The granules can be trapped by the fats and oils and can float out of the reactor (Hwu et al. ), or toxicity of long chain fatty acids (LCFA) for the methanogens is found (Rinzema et al. ). Furthermore mass transfer limitations and performance reduction (Hwu et al. ) can be encountered in upflow anaerobic sludge blankets. Other systems doi: 10.2166/wst.2013.806
like biological filter systems with sludge retention, do not have very stable operation when treating dairy effluent. Scum formation and loss of solubilisation activity after a relatively short period of 1–2 weeks of non-feeding, were observed in a buoyant filter bioreactor (Haridas et al. ). Pretreatment is necessary to remove fat and other suspended solids in advance. A typical process configuration for this type of wastewater is a dissolved air flotation (DAF) unit followed by an EGSB (extended granular sludge blanket) system. However, the fat separation in the DAF unit is the bottleneck for the EGSB process. A breakthrough of a low concentration of fat (>50 mg/l) will result in process disturbance. In that case, treatment in a CSTR would be a better option. The food waste (water) streams often contain a low COD (7–50 g/l). Because CSTR technology is hydraulically designed, as no sludge is retained, large volumes of CSTR are required to treat these kinds of streams. The retention time needs to be high
C. T. M. J. Frijters et al.
1387
|
Two full scale studies with a novel anaerobic flotation reactor
enough to hydrolyse the particles. A new type of reactor, the so-called BIOPAQ®-Anaerobic Flotation Reactor or BIOPAQ® anaerobic flotation reactor (AFR) (PCT/NL02– 26-03-02, Vellinga & Mulder ), is especially designed to treat wastewater streams containing fats and oils with a COD content of 7–50 g/l. In the reactor, digestion is combined with a flotation unit placed inside the reactor (Figure 1). The decoupling of the hydraulic and solids retention time results in well adapted biomass and high hydrolysing capacities. Compared to a system with DAF followed by EGSB or CSTR, the system does not use polymers, needs less maintenance, has a smaller footprint and, compared to the first alternative, produces more biogas because all organic compounds can be anaerobically converted (Hülsen et al. ). In the AFR system the sludge is retained by the integrated flotation unit up to concentrations of 15–30 kg/m3-reactor. Because of these high concentrations, high volumetric loading rates (VLR) can be applied: up to 9 kg of COD/m3reactor/d, with an HRT (hydraulic sludge retention time) of 1.5 to 8 days. The solids retention time (SRT) can be longer than 50 days. The flocculent sludge is floated using white water, which releases small biogas bubbles when introduced in the reactor. The white water is produced by dissolving biogas in reactor effluent under pressure. Sludge on which fatty compounds are adsorbed, will be floated and will be retained in the system, as was found to be the case in a former study with this system (Frijters et al. ), unlike in granular systems where it would wash out.
Water Science & Technology
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2014
In the cone of the flotation unit, the fluid with solids from the reactor is continuously pumped. The solids are floated by means of small biogas bubbles that are pumped in the flotation unit with the white water (Figure 1). The solids float over the edge of the unit, back into the anaerobic reactor. The reactor fluid is mixed with risers and downers driven by biogas (not shown in Figure 1). The effluent leaves the system at the bottom of the flotation unit beneath the flotation layer. In the system flocculent sludge develops, therefore no preacidification step, which is commonly used in granular sludge systems to provide enough acidification for a good granular sludge development, has to be applied. Other pretreatment steps, such as a fat removing pretreatment step, can be skipped. To avoid large fluctuations of COD or fat concentrations, a small buffer might be necessary in case of concentrated (side) streams. A pilot reactor treating wastewater derived from cleaning tanks used for transporting food liquids (ITC Holland Transport BV), showed promising results (Frijters et al. ). In the literature, problems with each of the degradation steps of fat and oil (roughly divided into hydrolysis, acidification (by β-oxidation) and methanogenesis) are described (Hanaki et al. ; Angeldaki & Ahring ; Hwu ; Sanders ). However in the pilot study these problems were not encountered. The first 430 m3-prototype reactor was built at the tank cleaning company in November 2009. In 2011 another full scale reactor (511 m3) was started, after a 4-month pilot plant study, at an ice-cream producing company (Ben & Jerry’s). The objective of these studies was to show that high removal efficiencies for COD and fat and oils can be attained in both industrial scale projects, at varying influent qualities (food liquid effluents) and loading rates (ice-cream effluents).
MATERIALS AND METHODS Set-up Prototype AFR at the tank cleaning company
Figure 1
|
Schematic overview of the AFR.
The prototype of BIOPAQ®AFR was started at the end of 2009 at ITC Holland Transport BV (Figure 2). The reactor had a maximum water level of 13.5 m and a diameter of 6.8 m, corresponding to a maximum water volume of 490 m3 (on average 430 m3). The water was buffered in a 40 m3-mixed buffer. A part of the non-soluble fats were skimmed and collected in a special mixed buffer. The fats could be fed to the reactor influent pipe by a controlled flow. In this way high peaks of fats and oils (high COD)
C. T. M. J. Frijters et al.
1388
Figure 2
|
|
Two full scale studies with a novel anaerobic flotation reactor
Water Science & Technology
|
69.7
|
2014
The BIOPAQ®AFR at ITC Holland Transport BV (left) and at Ben & Jerry’s (right; photographer: H. Roggen).
W
could be avoided. The temperature in the reactor was 38 C, regulated by a heat-exchanger cooling the wastewater. The pH in the reactor was operated at a minimum of 6.3, if necessary caustic was dosed. Urea and phosphoric acid were dosed in the influent pipe line to supply macronutrients. The produced biogas was used in a boiler to heat water for tank cleaning purposes. The full scale AFR at the ice-cream producing company
The wastewater The wastewater from the tank cleaning company originated from the cleaning of tanks which had been used for transporting liquids from a variety of food producing companies. The wastewater composition varied (Table 1) according to the variation of the tank contents. The fat-COD (ratio of fat:fat-COD was assumed to be 1: 2.5), could make up over 30% of the T-COD. If necessary the water (in the buffer up to 55 C) was cooled in a heatexchanger to a temperature of 38 C. The flow during 6 days of the week was on average 81 m3/d, on Sunday it was 50% of the T-COD, data not shown) in the sludge, the system was disturbed: the VFA in the reactor increased and fat accumulated. Similar results were described in the literature for LCFA-fed digesters (Kuang et al. ). However, the inhibition was reversible, this was also found in another study (Pereira et al. ). The effect of inhibition of LCFA has recently been modelled and the high sensitivity of the acetoclastic population to LCFA was evidenced (Zonta et al. ). In the current study it is shown that the inhibition of fat degradation could be prevented by operating the AFR at low VFA concentrations in the reactor. The separation of the skimmed insoluble fats and oils in the small buffer tank, prevented high ratios of fat-COD:T-COD and provided controlled dosing of fats and oily compounds. No fat layers were formed in the reactor: the mixing with biogas lifts appeared to be very effective. It was observed that the presence of fat had a positive influence on the compactness of the floc. In the literature the syntrophic populations that are needed for good LCFA conversion were described in which acetogens, acetoclastic and hydrotrophic methanogens play an important role (Sousa et al. ). A compact biomass structure is needed for this purpose. Furthermore it is described that adsorbed LCFA or fat might facilitate the transport of the poor water-soluble hydrogen (Alves et al. ). In that case the compact flocs with adsorbed fatty compounds could be the optimum biomass structure to perform the conversions. The floc structure might facilitate the hydrolysing processes that precede the β-oxidation and methanogenesis, as well. This was supported by the results of a previous pilot study with the AFR system (Frijters et al. ): at high loading rates of easily biodegradable organics (with only a low concentration of fat and oils) the process was disturbed. The flocs became very fluffy. This resulted in minor flotation and less effective fat degradation. Maybe the microbiological aspects such as a limited transport of acetate and hydrogen in these fluffy flocs (with a relative high distance between bacteria) had a negative influence on the conversion rates as well. These results indicate that the compactness of the flocs is very important for both physical and biological processes. These aspects together with the (shift in) composition of the bacteria and archaea in the flocs need to be elucidated in the future.
CONCLUSIONS The anaerobic flotation reactor was successfully applied to convert organic compounds, of which fats could make up 50% of the COD, into biogas. Fat and oils were removed
Water Science & Technology
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2014
to concentrations of less than 40 mg/l. Fat peaks could be buffered in the system, being converted later into biogas. Extreme high peaks of fats could disturb the system, but the inhibition was reversible. However, peaks of COD and fat should be avoided to obtain a more constant quality of sludge. In both industrial scale reactors, COD removal efficiencies of 90% were obtained. The effluent could be post-treated by means of a tilted plate settler, resulting in COD removal efficiencies of over 98%.
REFERENCES Alves, M. M., Pereira, M. A., Sousa, D. Z., Cavaleiro, A. J., Picavet, M., Smidt, H. & Stams, A. J. M. Waste lipids to energy: how to optimize methane production from long-chain fatty acids (LCFA). Microbial Biotechnology 2 (5), 538–550. Angeldaki, I. & Ahring, B. K. Effects of free long chain fatty acids on thermophilic anaerobic digestion. Applied Microbiology and Biotechnology 37 (6), 808–812. Becker, P., Koster, D., Popov, M. N., Markossian, S., Antranikian, G. & Markl, H. The biodegradation of olive oil and the treatment of lipid-rich wool scouring wastewater under aerobic thermophilic conditions. Water Research 33 (3), 653–660. Field, J., Lettinga, G. & Geurts, M. The methanogenic toxicity and anaerobic degradability of potato starch wastewater phenolic amino acids. Biological Wastes 21, 37–45. Frijters, C. T. M. J., Havinga, S., de Jong, K., van der Meer, W., Wessels, C., van Schaick, D., de Wit, L., van Eekert, M., Zeeman, G. & Vellinga, S. A new type of anaerobic reactor for treatment of concentrated waste(water streams): an anaerobic reactor with integrated flotation unit. In: Proceedings of IWA Conference on Industrial Water Treatment Systems, September 30–October 03 2008 [CD], Amsterdam. Hanaki, K., Matsuo, T. & Nagase, M. Mechanisms of inhibition caused by long chain fatty acids in anaerobic digesting process. Biotechnology and Bioengineering 23 (7), 1591–1610. Haridas, A., Suresh, S., Chitra, K. R. & Manilal, V. B. The Buoyant Filter Bioreactor: a high-rate anaerobic reactor for complex wastewater-process dynamics with dairy effluent. Water Research 39 (6), 993–1004. Hülsen, T., van Zessen, E., Kruit, J., van Bosch, P. L. F. & Frijters, C. T. M. J. Production of valuable biogas out of fat and protein containing wastewaters using compact BIOPAQ AFR and the THIOPAQ-technology. In: Proceedings of IWA Conference. November 2010, Water and Energy, Amsterdam. Hwu, C.-S. Enhancing Anaerobic Treatment of Wastewaters Containing Oleic Acid. PhD Thesis. Wageningen Agricultural University, Wageningen, The Netherlands.
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Two full scale studies with a novel anaerobic flotation reactor
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First received 18 July 2013; accepted in revised form 11 December 2013. Available online 28 December 2013
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