Improving the biomethane yield and biogas quality of food waste during anaerobic digestion by sequential process optimisation and biomethanation
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In spite of global efforts to reduce the generation of food waste, overwhelming quantities are still generated annually. In the United Kingdom for example, a third of the food crops produced annually for consumption end up in the bins. Anaerobic digestion (AD) is currently the most suitable technology for treating food waste, providing energy in the form of methane. However, the highly organic nature of food waste enriches the release of nutrients up to levels, which can be toxic or inhibitory to the acting microorganisms. As a result, the biomethane yields are much lower than the theoretical potential. This study investigates the possibility of improving the stability of AD and enhancing biomethane yield from mono-digestion of food waste, by a sequential optimisation of the biomethane production process.
The first level of optimisation was to identify suitable combinations of food waste particle size and microbial availability (inoculum-to-substrate ratio – ISR), to improve the process stability and biomethane yield. This investigation revealed that PS reduction (≤ 3 mm) resulted in a rapid digestion of food waste, and while this is expected to result in higher rates of acidification within the system, the variation in ISR helped to reduce such effects. Hence, an optimum condition of 1 mm PS and 3:1 ISR was determined; resulting in 38% increase in methane, and was used henceforth.
The second level of optimisation explored the potential for incorporating biomethanation into food waste AD. To optimise the conversion of the injected hydrogen to biomethane, three hydrogen injection points were investigated. As a result, 12.1%, 4% and 9.6% increases in biomethane yield were achieved, when hydrogen was added before hydrolysis, at the peak of acidification and during active methanogenesis respectively.
The third level of optimisation adopted the principle of acclimation to further improve the biomethane yield and explore the possibility of using formic acid (FA) as an alternative source of H2. The H2-acclimated systems performed better than the FA-acclimated systems, and yielded up to 81% biomethane against 65% without acclimation. Based on the results obtained in this study, it is possible to obtain up to 98% biomethane content, with continuous hydrogen acclimation. This reveals that the energy and revenue potential of food waste AD can be improved, by opening up multiple end uses beyond combined heat and power, such as gas-to-grid injection and vehicle fuel.Keywords:
Food Waste
Biogas
Biodegradable waste
Green waste
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Biogas
Bioconversion
Energy crop
Anaerobic respiration
Digestion
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Biogas
Food Waste
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Digestate
Food Waste
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Green waste
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The increasing demand of energy supply requires the development of systems of energy production based on the exploitation of renewable energy sources as an alternative to fossil fuels in common use. Through the process of anaerobic digestion it is possible to convert into biogas agricultural biomass, zootechnical waste, sewage sludge and organic fraction of municipal solid waste. After that it is possible to generate energy from biogas through the process of cogeneration. More recent concerns about global warming have stimulated further anaerobic digestion application and the improvement the processes in order to maximize biogas production, which is a renewable and versatile energy source that can be used for heat and electricity production, and as transportation fuel. It is in the interest of operators of anaerobic digestion plants to maximize methane production whilst concomitantly reducing the chemical oxygen demand of the digested material. The pre-treatment of solid waste is regarded as a prerequisite of the anaerobic digestion process to reduce volume and increase methane yield. The aim of the mechanical treatment is the reduction of the size of the biomass and its degree of crystallization, in order to increase the surface area available to enzymatic hydrolysis. This generates an increase on biogas production and a decrease in the time required for the digestion. In this work the link between mechanical pretreatment and the increase of methane yield of some samples of a dedicated crop (triticale) was discussed.
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The role of anaerobic digestion in the present bioeconomy concept exceeds the boundaries of on-site electricity and heat production, as it can serve as a process for renewable energy recovery and the production of bio-based products. To apply this concept, three agricultural feedstocks were evaluated for potential biogas production: cocoa waste, pumpkin and animal manure.
First, the optimization of cocoa waste anaerobic digestion was evaluated employing batch and fed-batch reactors in high (dry AD) and low (wet AD) total solids content of the feedstock. Dry AD performed significantly better than wet AD. A case study included a theoretical energy potential calculation for a full-scale plant with data obtained experimentally. AD from cocoa waste could supply up to 82% of the electricity demand of a rural region of Ecuador where cocoa is grown (20,000 inhabitants).
It was confirmed that the use of synthetic nutrients and cow manure improved biogas and methane yields from cocoa waste, significantly. Interestingly, the treatment of synthetic nutrient addition and co-digestion with sterile cow manure had no significant differences. However, the best treatment was co-digestion with raw cow manure in terms of stability and methane yields in the long-term operation. A possible explanation behind the long-term stabilization of co-digestion with raw cow manure might be the high presence of Metanosaetaceae in the cow manure feedstock, which remained stable throughout the experiment.
After AD optimization, a cyclic valorization of agricultural residues with different processes was performed. AD was integrated with slow pyrolysis to evaluate the energy and mass balances towards new products. The integrated process of co-digestion (cocoa waste + cow manure), followed by the slow pyrolysis of the digestate at 500 °C recovers most of the intrinsic energy of the waste into the gas phase in the form of biogas and non-condensable gases (up to 48%). In synthesis, these integrated processes recover more energy than each of processes separated, being equivalent to 61% mass conversion from the feedstock to fuel (liquid and gas).
Finally, we demonstrated that electrochemical biogas upgrading is a successful method to separate CO2 from biogas. The process achieved almost perfect CO2 removal efficiency. Electrochemical biogas separation produces ‘customized’ gas mixtures. The final cathode and anode off-gas blend varied with the current applied, indicating that it is possible to have specific gas blends according to our necessities by changing operational parameters of the electrochemical upgrading unit. Animal feed, in the form of microbial protein, were generated from real stream biogas, cathode and anode off-gases. Cathode off-gas achieved the highest biomass concentration and the highest protein content in the microbial biomass.
In conclusion, co-digestion proved to be a powerful technology in dealing with agricultural residues and brings stability and buffer capacity in the overall anaerobic digestion process in the long term. Slow pyrolysis has a strong potential for dealing with agricultural waste. Digestate, biogas, biochar, bio-oil and even syngas can supply agricultural areas that have a high demand for fuel and soil amendments. The best-case scenario is the integration of co-digestion and slow pyrolysis. This alternative has the highest energy efficiency in terms of gross energy recovery and in terms of net energy return. Finally, new products can be obtained from agricultural waste, such as microbial protein using anaerobic digestion as central technology.
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Anaerobic digestion (AD) is widely considered a more sustainable food waste management method than conventional technologies, such as landfilling and incineration. To improve economic performance while maintaining AD system stability at commercial scale, food waste is often co-digested with animal manure, but there is increasing interest in food waste-only digestion. We investigated the stability of anaerobic digestion with mixed cafeteria food waste (CFW) as the main substrate, combined in a semi-continuous mode with acid whey, waste bread, waste energy drinks, and soiled paper napkins as co-substrates. During digestion of CFW without any co-substrates, the maximum specific methane yield (SMY) was 363 mL gVS−1d−1 at organic loading rate (OLR) of 2.8 gVSL−1d−1, and reactor failure occurred at OLR of 3.5 gVSL−1d−1. Co-substrates of acid whey, waste energy drinks, and waste bread resulted in maximum SMY of 455, 453, and 479 mL gVS−1d−1, respectively, and it was possible to achieve stable digestion at OLR as high as 4.4 gVSL−1d−1. These results offer a potential approach to high organic loading rate digestion of food waste without using animal manure. Process optimization for the use of unconventional co-substrates may help enable deployment of anaerobic digesters for food waste management in urban and institutional applications and enable increased diversion of food waste from landfills in heavily populated regions.
Food Waste
Biogas
Biodegradable waste
Digestion
Green waste
Energy Recovery
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Biogas is a fuel obtained from organic waste fermentation and can be an interesting solution for producing electric energy, heat and fuel. Recently, many European countries have incentivized the production of biomethane to be injected into natural gas grids or compressed and used as biofuel in vehicles. The introduction of an upgrading unit into an existing anaerobic digestion plant to convert biogas to biomethane may have a strong impact on the overall energy balance of the systems. The amount of biomethane produced may be optimized from several points of view (i.e., energy, environmental and economic). In this paper, the mass and energy fluxes of an anaerobic digestion plant were analyzed as a function of the biogas percentage sent to the upgrading system and the amount of biomethane produced. A numerical model of an anaerobic digestion plant was developed by considering an existing case study. The mass and energy balance of the digesters, cogeneration unit, upgrading system and auxiliary boiler were estimated when the amount of produced biomethane was varied. An internal combustion engine was adopted as the cogeneration unit and a CO2 absorption system was assumed for biogas upgrading. Results demonstrated that the energy balance of the plant is strictly dependent on the biomethane production and that an excess of biomethane production makes the plant totally dependent on external energy sources. As for the environmental impact, an optimal level of biomethane production exists that minimizes the emissions of equivalent CO2. However, high biomethane subsides can encourage plant managers to increase biomethane production and thus reduce CO2 savings.
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Anaerobic Digestion is a biological process that takes place naturally when microorganisms break down organic matter in the absence of oxygen. In an enclosed chamber, controlled anaerobic digestion of organic matter produces biogas which is predominantly methane. The produced methane then can be directly used for rural cooking; or after certain conditioning, can be used in onsite power generation, heating homes or as vehicular fuel. Besides, food waste is increasingly becoming a major problem in every society imposing serious economic and environmental concerns. For this reason, many contemporary researches are emphasizing in finding sustainable solutions to recycle and produce energy from such waste. In this context, this paper aims to study and optimize the production of biogas from food waste (rice). For the experiment, an existing wet digestion biogas plant installed in Islamic University of Technology was used. The food waste (rice) for the research was collected from the cafeteria of Islamic University of Technology. Furthermore, a process simulation was performed by PROII software to estimate the methane production rate. Eventually, the simulated and experimental results were compared. The duration of the study period was 120 days. The experimental results showed that an average specific gas production of 14.4 kg-mol/hr can be obtained for 0.05 kg-mol/hr of starch loading rate. In case of the simulated results, the gas production was found to be 19.82 kg-mol/hr for the same loading rate of starch. The percentage of methane and CO2 obtained in the biogas plant was 69% and 29% respectively.
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Food Waste
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Digestate
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Anaerobic digestion provides an array of positive environmental benefits such as reducing greenhouse gas emissions, replacing mineral fertilizers, producing renewable energy and treating waste. However, pitfalls in anaerobic digestion such as poor methane yields, process instability, process failure and regional shortages of feedstock have limited the full exploitation of the anaerobic digestion process.
The research presented in this thesis deals with the assessment of the possible negative or positive impacts of feedstock characteristics on the efficiency of anaerobic digestion. In addition, it investigates ways of enhancing the methane yield of the feedstock by improving the feedstock characteristics. The feedstocks investigated were various energy crops, food industrial waste and sewage sludge. The improvement methods investigated were ensiling, nutrient supplementation, co-digestion and anaerobic pretreatment.
It was found that ensiling crops results in insignificant losses in energy, total solid and wet weight. In addition, no significant difference was found in methane yields between the ensiled and fresh crop samples. The importance of correcting for losses of volatiles in total solids determination was pointed out and it was shown that failing to do so could be the main reason why many previous publications report increased total solid based methane yields after ensiling. Increased methane yield in silages may therefore be an effect of an analytical error rather than an effect of using ensiling as a pretreatment prior to anaerobic digestion.
Anaerobic digestion of crop biomass is known to be particularly limited by nutrient availability. Direct nutrient supplementation in crop mono-digestion in this research demonstrated an efficient biogas process at the shorter hydraulic retention times commonly applied in co-digestion of crop biomass and manure. The high degradation efficiency was evidenced by high methane yields, comparable to maximum expected yields generated under controlled conditions, and low volatile fatty acids accumulation. As a result of nutrient addition, the digestate could comply with certification standards for bio-fertilizer. Also, viscosity problems commonly reported for crop mono-digestion were not observed in this study, which could be another effect of nutrient addition.
Co-digesting of waste biomass and crop biomass led to significant improvement in methane yield per ton of feedstock and carbon to nitrogen ratio as compared to digestion of only the waste biomass. Biogas production from crops in combination with waste biomass also eliminated the need for addition of micronutrients normally required in crop mono-digestion. Co-digestion was also presented as a means of feedstock supplementation to curb feedstock shortages in waste-based anaerobic digestion processes. In addition, inhibitors in anaerobic digestion such as free ammonia and light metal ions were diluted, a condition which can lead to an overall viable biogas process
Anaerobic pre-treatment led to the solubilisation of particulate organic matter in sewage sludge. This solubilisation could have led to the improved methane yield, methane production rate and reduction in volatile solids.
Applying different feedstock improvement solutions to the various feedstocks investigated, i.e. nutrient addition, co-digestion and pretreatment, were demonstrated as effective means of enhancing the methane yield of the feedstock thereby improving the overall anaerobic digestion process.
Biogas
Digestate
Energy crop
Food Waste
Digestion
Biodegradable waste
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