Appl Biochem Biotechnol DOI 10.1007/s12010-015-1559-5
Biogas by Semi-Continuous Anaerobic Digestion of Food Waste Cunsheng Zhang & Haijia Su & Zhenbin Wang & Tianwei Tan & Peiyong Qin
Received: 5 November 2014 / Accepted: 4 March 2015 # Springer Science+Business Media New York 2015
Abstract The semi-continuous anaerobic digestion of food waste was investigated in 1-L and 20-L continuously stirred tank reactors (CSTRs), to identify the optimum operation condition and the methane production of the semi-continuous anaerobic process. Results from a 1-L digester indicated that the optimum organic loading rate (OLR) for semi-continuous digestion is 8 g VS/L/day. The corresponding methane yield and chemical oxygen demand (COD) reduction were 385 mL/g VS and 80.2 %, respectively. Anaerobic digestion was inhibited at high OLRs (12 and 16 g VS/L/day), due to volatile fatty acid (VFA) accumulation. Results from a 20-L digester indicated that a higher methane yield of 423 mL/g VS was obtained at this larger scale. The analysis showed that the methane production at the optimum OLR fitted well with the determined kinetics equation. An obvious decrease on the methane content was observed at the initial of digestion. The increased metabolization of microbes and the activity decrease of methanogen caused by VFA accumulation explained the lower methane content at the initial of digestion. Keywords Semi-continuous . Anaerobic digestion . Food waste . Methane . Biogas . Kinetic equation
Introduction The generation of municipal solid waste (MSW) continues to increase worldwide [1]. Food waste (FW), which comprises the food residuals, e.g., rice meat and vegetables, is a main organic fraction of MSW, accounting for 37–55 % of the total MSW [2]. Due to the high organic substrate content, it is considered to be a source of energy instead of waste. Currently, the most attractive approach for FW disposal is anaerobic digestion [3, 4]. Due to its C. Zhang (*) : Z. Wang School of Food and Biological Engineering, Jiangsu University, Zhenjiang 212013, People’s Republic of China e-mail: [email protected] H. Su : T. Tan : P. Qin Beijing Key Laboratory of Bioprocess, Beijing University of Chemical Technology, 100029 Beijing, People’s Republic of China
Appl Biochem Biotechnol
high potential of generating biogas, this technology has significant benefits toward economics [5, 6]. Organic substrates are degraded via enzymatic and bacterial activity during anaerobic digestion. It is accepted that the surface area and particle size of feedstock are important parameters which influence the initial degradation rate and methane yield. To compare the results obtained from anaerobic digestion, the particle size should be comparable. As stated by Raposo et al. [7], the particle size should be below 10 mm. The biodegradation process includes hydrolysis, acidogenesis, acetogenesis, and methanogenesis [6]. Anaerobic digestion of organic substrates is considered to be a balance between the rates of hydrolysis and methanogenesis, because methanogenic bacteria are more sensitive than hydrolytic and acidogenic bacteria to higher volatile fatty acid (VFA) concentration and the corresponding pH drop. The rates of hydrolysis and acidogenesis exceed the rate of methanogenesis if the organic load is excessive, resulting in VFA accumulation and even digestion failure [8, 9]. Considerable work focusing on batch tests has been done in the treatment of FW. Zhang et al. found that the FW is a highly desirable substrate for anaerobic digesters with regard to its high biodegradability and methane yield [10]. Liu et al. pointed out that the biogas production rate was significantly affected by feed to inoculum (F/I) ratios [4]. While identifying the parameters influencing anaerobic efficiency, various kinetic equations have been used in batch anaerobic digestion as tools for describing and predicting the performance of anaerobic digestion systems [11]. Nevertheless, the drawbacks of batch anaerobic digestion were obvious: longer digestion time and lower organic loads, thus limiting the effective processing capacity of a given digester within its set geometry. By contrast, the treatment capability of a digester in semi-continuous mode was obviously improved. Wang et al. developed a hybrid anaerobic solid–liquid (HASL) system treating FW with high efficiencies [12]. They pointed out that semi-continuous process was more effective than batch process. A higher chemical oxygen demand (COD) reduction of 85 % could be achieved in a 3-L lab-scale HASL system. Due to its high capability, semicontinuous anaerobic digestion has attracted a lot of interest in treating FW. Most of the previous studies mainly focused on the methane production improvement [2, 13] and were conducted on the scale of 0.5-L volumes or even smaller [14]. However, they failed in both defining the optimum operation condition for semi-continuous mode and in developing an appropriate kinetic equation for the methane production. Moreover, more data are required by the biogas plant decision makers for designing full-scale reactors. In order to provide more data on methane production for particular biogas plant, a 20-L digester operated in semi-continuous mode was employed. The objectives of this study were as follows: (i) to determine the optimum operation condition for anaerobic digestion treating FW while avoiding acidification in the AD and (ii) to identify the methane production in a semi-continuous anaerobic process.
Materials and Methods Characteristics of Food Waste and Seed Sludge (Inoculums) FW was provided by the cafeteria of the Beijing University of Chemical Technology and mainly composed of rice, meat, vegetables, oil, and some inorganic substance such as plastics to a minor extent. These plastics and big bones were manually removed, and the residual FW was ground into small particles (
Biogas by Semi-Continuous Anaerobic Digestion of Food Waste Cunsheng Zhang & Haijia Su & Zhenbin Wang & Tianwei Tan & Peiyong Qin
Received: 5 November 2014 / Accepted: 4 March 2015 # Springer Science+Business Media New York 2015
Abstract The semi-continuous anaerobic digestion of food waste was investigated in 1-L and 20-L continuously stirred tank reactors (CSTRs), to identify the optimum operation condition and the methane production of the semi-continuous anaerobic process. Results from a 1-L digester indicated that the optimum organic loading rate (OLR) for semi-continuous digestion is 8 g VS/L/day. The corresponding methane yield and chemical oxygen demand (COD) reduction were 385 mL/g VS and 80.2 %, respectively. Anaerobic digestion was inhibited at high OLRs (12 and 16 g VS/L/day), due to volatile fatty acid (VFA) accumulation. Results from a 20-L digester indicated that a higher methane yield of 423 mL/g VS was obtained at this larger scale. The analysis showed that the methane production at the optimum OLR fitted well with the determined kinetics equation. An obvious decrease on the methane content was observed at the initial of digestion. The increased metabolization of microbes and the activity decrease of methanogen caused by VFA accumulation explained the lower methane content at the initial of digestion. Keywords Semi-continuous . Anaerobic digestion . Food waste . Methane . Biogas . Kinetic equation
Introduction The generation of municipal solid waste (MSW) continues to increase worldwide [1]. Food waste (FW), which comprises the food residuals, e.g., rice meat and vegetables, is a main organic fraction of MSW, accounting for 37–55 % of the total MSW [2]. Due to the high organic substrate content, it is considered to be a source of energy instead of waste. Currently, the most attractive approach for FW disposal is anaerobic digestion [3, 4]. Due to its C. Zhang (*) : Z. Wang School of Food and Biological Engineering, Jiangsu University, Zhenjiang 212013, People’s Republic of China e-mail: [email protected] H. Su : T. Tan : P. Qin Beijing Key Laboratory of Bioprocess, Beijing University of Chemical Technology, 100029 Beijing, People’s Republic of China
Appl Biochem Biotechnol
high potential of generating biogas, this technology has significant benefits toward economics [5, 6]. Organic substrates are degraded via enzymatic and bacterial activity during anaerobic digestion. It is accepted that the surface area and particle size of feedstock are important parameters which influence the initial degradation rate and methane yield. To compare the results obtained from anaerobic digestion, the particle size should be comparable. As stated by Raposo et al. [7], the particle size should be below 10 mm. The biodegradation process includes hydrolysis, acidogenesis, acetogenesis, and methanogenesis [6]. Anaerobic digestion of organic substrates is considered to be a balance between the rates of hydrolysis and methanogenesis, because methanogenic bacteria are more sensitive than hydrolytic and acidogenic bacteria to higher volatile fatty acid (VFA) concentration and the corresponding pH drop. The rates of hydrolysis and acidogenesis exceed the rate of methanogenesis if the organic load is excessive, resulting in VFA accumulation and even digestion failure [8, 9]. Considerable work focusing on batch tests has been done in the treatment of FW. Zhang et al. found that the FW is a highly desirable substrate for anaerobic digesters with regard to its high biodegradability and methane yield [10]. Liu et al. pointed out that the biogas production rate was significantly affected by feed to inoculum (F/I) ratios [4]. While identifying the parameters influencing anaerobic efficiency, various kinetic equations have been used in batch anaerobic digestion as tools for describing and predicting the performance of anaerobic digestion systems [11]. Nevertheless, the drawbacks of batch anaerobic digestion were obvious: longer digestion time and lower organic loads, thus limiting the effective processing capacity of a given digester within its set geometry. By contrast, the treatment capability of a digester in semi-continuous mode was obviously improved. Wang et al. developed a hybrid anaerobic solid–liquid (HASL) system treating FW with high efficiencies [12]. They pointed out that semi-continuous process was more effective than batch process. A higher chemical oxygen demand (COD) reduction of 85 % could be achieved in a 3-L lab-scale HASL system. Due to its high capability, semicontinuous anaerobic digestion has attracted a lot of interest in treating FW. Most of the previous studies mainly focused on the methane production improvement [2, 13] and were conducted on the scale of 0.5-L volumes or even smaller [14]. However, they failed in both defining the optimum operation condition for semi-continuous mode and in developing an appropriate kinetic equation for the methane production. Moreover, more data are required by the biogas plant decision makers for designing full-scale reactors. In order to provide more data on methane production for particular biogas plant, a 20-L digester operated in semi-continuous mode was employed. The objectives of this study were as follows: (i) to determine the optimum operation condition for anaerobic digestion treating FW while avoiding acidification in the AD and (ii) to identify the methane production in a semi-continuous anaerobic process.
Materials and Methods Characteristics of Food Waste and Seed Sludge (Inoculums) FW was provided by the cafeteria of the Beijing University of Chemical Technology and mainly composed of rice, meat, vegetables, oil, and some inorganic substance such as plastics to a minor extent. These plastics and big bones were manually removed, and the residual FW was ground into small particles (
