Assessment of micro-scale anaerobic digestion for management of urban organic waste: A case study in London, UK.

This paper describes the analysis of an AD plant that is novel in that it is located in an urban environment, built on a micro-scale, fed on food and catering waste, and operates as a purposeful system. The plant was built in 2013 and continues to operate to date, processing urban food waste and generating biogas for use in a community café. The plant was monitored for a period of 319days during 2014, during which the operational parameters, biological stability and energy requirements of the plant were assessed. The plant processed 4574kg of food waste during this time, producing 1008m(3) of biogas at average 60.6% methane. The results showed that the plant was capable of stable operation despite large fluctuations in the rate and type of feed. Another innovative aspect of the plant was that it was equipped with a pre-digester tank and automated feeding, which reduced the effect of feedstock variations on the digestion process. Towards the end of the testing period, a rise in the concentration of volatile fatty acids and ammonia was detected in the digestate, indicating biological instability, and this was successfully remedied by adding trace elements. The energy balance and coefficient of performance (COP) of the system were calculated, which concluded that the system used 49% less heat energy by being housed in a greenhouse, achieved a net positive energy balance and potential COP of 3.16 and 5.55 based on electrical and heat energy, respectively. Greenhouse gas emissions analysis concluded that the most important contribution of the plant to the mitigation of greenhouse gases was the avoidance of on-site fossil fuel use, followed by the diversion of food waste from landfill and that the plant could result in carbon reduction of 2.95kg CO2eq kWh(-1) electricity production or 0.741kg CO2eq kg(-1) waste treated

Characterizing the variability of food waste quality: A need for efficient valorisation through anaerobic digestion

International audienceIn order to determine the variability of food waste (FW) characteristics and the influence of these variable values on the anaerobic digestion (AD) process, FW characteristics from 70 papers were compiled and analysed statistically. Results indicated that FW characteristics values are effectively very variable and that 24% of these variations may be explained by the geographical origin, the type of collection source and the season of the collection. Considering the whole range of values for physicochemical characteristics (especially volatile solids (VS), chemical oxygen demand (COD) and biomethane potential (BMP)), FW show good potential for AD treatment. However, the high carbohydrates contents (36.4%VS) and the low pH (5.1) might cause inhibitions by the rapid acidification of the digesters. As regards the variation of FW characteristics, FW categories were proposed. Moreover, the adequacy of FW characteristics with AD treatment was discussed. Four FW categories were identified with critical characteristics values for AD performance: (1) the high dry matter (DM) and total ammonia nitrogen (TAN) content of FW collected with green waste, (2) the high cellulose (CEL) content of FW from the organic fraction of municipal solid waste, (3) the low carbon-to-nitrogen (C/N) ratio of FW collected during summer, (4) the high value of TAN and Na of FW from Asia. For these cases, an aerobic pre-treatment or a corrective treatment seems to be advised to avoid instabilities along the digestion. Finally, the results of this review-paper provide a data basis of values for FW characteristics that could be used for AD process design and environmental assessment. © 2016 Elsevier Ltd

Performance of Coupling an Aerobic Pre-treatment Prior to a Solid-State Anaerobic Digestion of Food Waste

International audienceAn experimental design was proposed to test the influence of three parameters on the performance of food waste (FW) dry anaerobic digestion (AD) coupled with aerobic pre-treatment: (1) the type of aerobic pre-treatment (no pre-treatment/dynamic aeration/static storage); (2) the composition of FW (paper/no paper) and (3) the frequency of leachate recirculation in the batch leach bed reactor (LBR) used as dry AD process. Performance of AD was assessed by measuring pH, VFA, biogas production and by modelling the kinetics of methane production using modified Gompertz equation. A statistical analysis of variance showed that AD leachate recirculation condition was the most impacting parameter on methane production. A high production of VFA was noticed (~ 25 gDCO·L '1 ) that did not lead to inhibition due to high buffer capacity of inoculum (5.2 gN-NH 4 + ·L '1 ). 80% of the biomethane potential was reached in the batch test with a daily leachate recirculation and with FW without paper. According to simulations, an experimental duration of 31'32 days showed to be optimal to reach 90% of degradation potential under these LBR conditions. A lower frequency of leachate recirculation or the addition of paper decreased methane production. The introduction of a two-day aeration step stabilized the production of VFA along the digestion process and enhanced the methane production on FW with paper. Conversely a two-day storage step induced a rapid acidification and a subsequent significant decrease of methane production rate. © 2019, Springer Nature B.V

Biochemical and microbial changes reveal how aerobic pre-treatment impacts anaerobic biodegradability of food waste

National audienceAerobic pre-treatment of food waste (FW) was performed at different oxygen concentrations (0%, 5%, 10% and 21%O2) and different durations (1, 2, 3 and 4 days) to investigate its impact on biochemical and microbial community characteristics of the waste and its ability to improve anaerobic biodegradability. Whatever the duration, the highest effect of pre-treatment was observed at full aerobic pre-treatment (21%O2) while 5%O2 and 10%O2 showed lower transformation performances. Biochemical variations at 21%O2 were mainly a decrease of simple carbohydrates, volatile fatty acids (VFA) and low molecular weight water soluble compounds and an increase of high weight water soluble compounds. Microbial community analysis showed a clear modification of populations after 21%O2 aerobic pre-treatment, changing from an initial dominance of lactic acid bacteria to a final dominance of VFA consumers (like Acetobacter) and a higher presence of Fungi. Enzymatic tests showed an increase of exoenzymes content and a higher presence of protein and carbohydrates degrading enzymes. Finally, the aerobic pre-treatment did not negatively impact methane potential of FW (496 NLCH4·kgVS'1) which remained unchanged after two days of pre-treatment at 21%O2. These latter optimal pre-treatment conditions are proposed to be tested in future investigation of anaerobic digestion (AD) process with low inoculum to substrate ratio in order to assess their ability to avoid acidification risk during AD of FW. © 2018 Elsevier Lt

Influence de la température sur la méthanisation de boues primaires d’épuration

The influence of temperature and organic load on the anaerobic digestion of primary sludge was studied in the range of 28-60°C with organic loads ranging from 2 to 8.5gCOD.L-1.j-1. The work was carried out from a mesophilic inoculum with prior adaptation. The incubation was conducted in 10-L perfectly mixed thermo-regulated digesters which were fed fed daily. Results showed that the daily production of methane was more influenced by the load than by temperature. The performances obtained at 28°, 37° and 49°C were found similar but slightly reduced at 55°C. The digester operated at 60°C with the highest load quickly led to acid inhibition resulting in a drastic drop of methane production. The rise in temperature lead to an accumulation of ammonium NH4+ in the digesters as well as soluble COD (measured after filtration to 0.45 microns). These variations suggested a change in the balance of biodegradation pathway. The effect of temperature on the acceleration of hydrolysis / acidogenesis phases was more acute than on the acetogenic and methanogenic phases, thereby leading to the accumulation of intermediate metabolites.L’influence de la température et de la charge organique sur la méthanisation des boues primaires de station d’épuration a été étudiée dans la plage de 28 à 60°C pour une charge organique allant de 2 à 8.5gDCO.L-1.j-1. Les travaux ont été réalisés à partir d’un inoculum mésophile, préalablement adapté. L’incubation des boues a été conduite dans des digesteurs thermostatés parfaitement agités de 10L alimentés quotidiennement. Les résultats révèlent que la production journalière de méthane est davantage influencée par la charge que par la température. Les performances épuratoires obtenues à 28°, 37° et 49°C s’avèrent similaires. Elles sont légèrement moins bonnes à 55°C, et le digesteur opéré à 60°C avec la charge la plus élevée a rapidement conduit à une inhibition acide avec arrêt de production de méthane. L’élévation de température conduit à une accumulation d’ammonium NH4+ dans les digesteurs, ainsi que de la DCO soluble (après filtration à 0,45µm). Ces variations suggèrent une modification des équilibres de biodégradation. L’effet de la température sur d’accélération des phases hydrolyse / acidogenèse s’avère plus accentué que sur les phases d’acétogenèse et de méthanisation, conduisant ainsi à l’accumulation de métabolites intermédiaires