Advancement in Anaerobic Digestion for Scaling‐Up Biogas Production Through Food Waste Valorization

Managing food waste is key to tackling climate change

Energy from food waste

Day by day as the population increases, food waste keeps on growing. This waste needs to be managed in order to reduce the number of landfills and to use food waste efficiently. Among the various processes available, Anaerobic Digestion (AD) of food waste is one of the alternatives for processing food waste. The two biggest obstacles to anaerobic digestion of food waste are high biodegradability and high C/N ratio. The C/N ratio determines the ratio between substrate and nutrients; the latter is essential for microbial synthesis and for providing alkalinity through ammonia metabolism. Biogas, a product of the anaerobic digestion process, is a clean and renewable form of energy that can replace conventional energy sources that cause ecological-environmental problems and at the same time are depleted more quickly. The aim of this work was to increase the nitrogen content to enhance the production of biogas from canteen waste. During the process, two digesters of the same capacity were operated. Anaerobic digestion of canteen waste along with addition of ammonium chloride was carried out in Digester 1 while AD of only canteen waste was carried out in Digester 2. The amount of biogas produced in Digester 1 was in the range of 0.04 m3/kg–0.075 m3/kg, while in Digester 2 the volume range was 0.02 m3/kg–0.04 m3/kg. The average biogas produced in digester 1 consisting of canteen waste and nitrogen source was 0.053 m3/kg while biogas production in digester 2 with only canteen waste was 0.030 m3/kg. So, biogas produced by addition of nitrogen source was 77 % higher than that of only canteen waste. From this study we obtained a higher amount of biogas by addition of ammonium chloride as an external nitrogen source. Nitrogen demand of methanogens was fulfilled by additional supply of nitrogen resulting in increased quantity of biogas. Therefore, in anaerobic digestion addition of ammonium chloride was beneficial for food waste digestion. Hence, nitrogen content in canteen waste turned out to be the main parameter affecting anaerobic digestion of canteen waste which is justified in this research.

This study evaluates the effects of the varying substrate to inoculum ratios (S:I) of 0.5, 1, 2, 3, 4, 5, and 6 (volatile solids/VS basis) on the kinetics of biogas production during batch mesophilic (35 ± 1 °C) anaerobic digestion (AD) of simulated food waste (FW), using anaerobic digestate as the inoculum. Kinetic parameters during biogas production (scrubbed with NaOH solution) are predicted by the first-order and the modified Gompertz model. The observed average specific biogas yields are in descending order corresponding to the S:I ratios 1, 2, 4, 6, 3, 5, and 0.5, respectively, and the significant effect of the S:I ratio was observed. The tests with the S:I of 1 have the maximum average biogas production rates of 88.56 NmL/gVS.d, whereas tests with the S:I of 6 exhibited the lowest production rates (24.61 NmL/gVS.d). The maximum biogas yields, predicted by the first order and the modified Gompertz model, are 668.65 NmL/gVS (experimental 674.40 ± 29.10 NmL/gVS) and 653.17 NmL/gVS, respectively. The modified Gompertz model has been proven to be suitable in predicting biogas production from FW. VS removal efficiency is greater in higher S:I ratios, with a maximum of 78.80 % at the S:I ratio of 6, supported by the longer incubation time. Moreover, a significant effect of the S:I ratio is seen on kinetics and energy recovery from the AD of FW.

Due to increase in the amount of food waste, the proper method to dispose food waste has become a concern. Anaerobic digestion has received increasing attention because of its advantages such as reducing waste pollution and producing clean energy. Hydrolysis, the first step in anaerobic digestion, has been identified as rate-limiting step in this process. Enzyme addition during hydrolysis of a substrate has been reported as a promising alternative to stimulate waste degradation, thus improving the efficiency of the anaerobic digestion system. In this work, Candida Antarctica Lipase B (CALB) was used to facilitate the anaerobic digestion of food waste for 40 days. The effects of lipase addition on the total organic carbon (TOC), biogas production, pH, electrical conductivity and moisture content were studied. The finding showed that the food waste digestion with lipase achieved higher reduction in pH at a shorter time, indicating a higher degradation rate when lipase was added into the system. However, lipase addition had no effect on the final values of both pH and conductivity after 40 days of digestion. It was also found that the usage of lipase in this study did not help in increasing the biogas production due to acidification of food waste during digestion that inhibited the biogas production. On the other hand, it was found that the final moisture content of 88% achieved by food waste with lipase was higher as compared to the control sample. The finding also showed that higher reduction in TOC value was achieved when lipase was added into the digestion system, showing higher degradation rate by microorganisms. The complex molecule in food waste was hydrolysed to a simple molecule assisted by the enzyme added so that it can be easily utilised by the microorganisms.

Acetate production from food waste or sewage sludge was evaluated in four semi-continuous anaerobic digestion processes. To examine the importance of inoculum and substrate for acid production, two different inoculum sources (a wastewater treatment plant (WWTP) and a co-digestion plant treating food and industry waste) and two common substrates (sewage sludge and food waste) were used in process operations. The processes were evaluated with regard to the efficiency of hydrolysis, acidogenesis, acetogenesis, and methanogenesis and the microbial community structure was determined. Feeding sewage sludge led to mixed acid fermentation and low total acid yield, whereas feeding food waste resulted in the production of high acetate and lactate yields. Inoculum from WWTP with sewage sludge substrate resulted in maintained methane production, despite a low hydraulic retention time. For food waste, the process using inoculum from WWTP produced high levels of lactate (30 g/L) and acetate (10 g/L), while the process initiated with inoculum from the co-digestion plant had higher acetate (25 g/L) and lower lactate (15 g/L) levels. The microbial communities developed during acid production consisted of the major genera Lactobacillus (92–100%) with food waste substrate, and Roseburia (44–45%) and Fastidiosipila (16–36%) with sewage sludge substrate. Use of the outgoing material (hydrolysates) in a biogas production system resulted in a non-significant increase in bio-methane production (+5–20%) compared with direct biogas production from food waste and sewage sludge.

Enhanced energy recovery via separate hydrogen and methane production from two-stage anaerobic digestion of food waste with nanobubble water supplementation

Cutting edge technologies to end food waste

In a world facing increasing environmental and energy challenges, anaerobic digestion of agrifood by-products and food waste could contribute to the production of green energy while reducing greenhouse gas emissions into the atmosphere. Anaerobic digestion is a biological process capable of breaking down and stabilising organic matter in the absence of oxygen and converting it into a renewable source of energy, known as biogas. Biomethane production also enables the generation of electricity and produces digestate, a by-product of the digestion process that can be used as a soil conditioner or fertiliser. This review aims to highlight how substrate pretreatment, together with the optimisation of operating parameters, application of additives, recirculation of digestate and frequent feeding, can increase biogas production. An overview of the basics of the anaerobic digestion of agrifood by-products and food waste is provided, including feedstock characteristics (nutrient content, particle size and inhibitory compounds) and process parameters (process configuration, pH, temperature, total and volatile solids, total Kjeldahl nitrogen, ammonium, chemical oxygen demand, carbon/nitrogen ratio, retention time, organic loading rate, etc.). In addition, recent studies in the field of processes, equipment and pretreatments that can significantly improve the anaerobic digestion process of agricultural and food wastes were classified and discussed. Finally, the challenges and future perspectives of biogas production from the agrifood sector are addressed.

Several treatment alternatives for food waste can result in both energy and nutrient recovery, and thereby potential environmental benefits. However, according to the European Union waste management hierarchy, waste prevention should be the prioritized strategy to decrease the environmental burdens from all solid waste management. The aim of the present study was therefore to investigate the potential for food waste minimization among Swedish households through an investigation of the amount of avoidable food waste currently disposed of. A further aim was to investigate the effect on the national biogas production potential through anaerobic digestion of food waste, considering minimization potentials. A method for waste composition analyses of household food waste, where a differentiation between avoidable and unavoidable food waste is made, was used in a total of 24 waste composition analyses of household waste from Swedish residential areas. The total household food waste generation reached 3.4 kg (household and week)(-1), on average, of which 34% is avoidable. The theoretical methane (CH4) potential in unavoidable food waste reached 442 Ndm(3) (kg VS)(-1) or 128 Nm(3) tonne(-1) wet waste, while the measured (mesophilic CH4 batch tests) CH4 production reached 399 Ndm(3) (kg VS)(-1), which is lower than several previous assessments of CH4 production from household food waste. According to this study the combination of a decrease in food waste generation-in case of successful minimization-and decreased CH4 production from unavoidable food waste will thus result in lower total potential energy recovery from household food waste through anaerobic digestion CH4 potential than previously stated.

Anaerobic digestion (AD) of food waste (FW) has become of great interest in recent years due to the high organic removal rates and positive net energy balance. However, two key issues should be highlighted. On the one hand, AD effluent still needs to be purified in order to meet ecologically acceptable requirements for direct disposal. On the other hand, AD plants have been mainly based on mesophilic and thermophilic temperatures, which may represent an important economic barrier for extending AD to small- and medium-sized plants. Hence, the aim of this paper is to assess the AD of FW at low temperature and the post-treatment of anaerobic digestate by using microalgae cultivation at laboratory scale. This study explores an economical alternative for small- and medium-size treatment plants loaded with FW. Inoculum and FW were physical and chemically characterized, and 5-L glass batch reactors in triplicate were used for determining the biochemical methane potential. The chemical oxygen demand (COD), biogas production and its composition were measured until the end of batch tests. The post-digestate was assessed as cultivation media for the microalgae Scenedesmus sp. by varying the dilution rate with fresh water. After 15 days, microalgae was harvested and the liquid fraction was assessed in accordance with the Chilean legal requirements. Although AD of FW at psychrophilic temperature leads to a lower biogas yield, the COD was reduced up to 97.5%, microalgae was successfully cultivated in all post-digestate dilutions, and some of them allowed the use of effluent for irrigation.

Biogas production from brown grease using a pilot-scale high-rate anaerobic digester

Food waste minimization from a life-cycle perspective

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