Anaerobic digestion as attractive multistep bio-refinery: research and application with your bio-waste

Biowaste as food waste and sewage sludge are traditionally seen as a costly problem in economic and environmental terms. The challenge is to reverse this equation by designing more effective recovery and processing systems to turn these biodegradable waste into a source of value and contribute to restoring natural capital. Biorefineries could be a central technology in this perspective. Operating in a similar way to petrochemical refineries, they employ a range of techniques - such as pre-treatments, biological processes and enzymatic conversions - to transform organic feedstock into valuable chemicals, products and energy. Anaerobic digestion is a very attractive multi-step bioprocess that can be applied to a wide range of organic materials to generate biogas, leaving a nutrient-rich substance called digestate. Among the most typical forms of waste used for biogas production by Anaerobic Digestion (AD) there are sewage sludge and food waste. Their chemical-physical characteristics largely influence the AD performances leading to different results in biogas production and composition. An accurate characterization is an essential step also to determine whether a pre-treatment is needed and to help understanding which one is the most appropriate to modify the structural and compositional properties to make the substrate more accessible. Pre-treatment methods, as thermal hydrolysis, capable of significantly increasing the fraction of fermentable organic carbon can furthermore change the status of the feedstock to become more suitable for production of higher added value products as volatile fatty acids (VFAs). One of the step of the AD process, namely the dark fermentation, involves in fact the transformation of sugars, proteins and lipids into a mixture of products, e.g. acetic acid, butyric acid, propionic acid, with high market prices (800-2500 USD/tonn). At Water Research Institute, feedstocks pretreatments as AD processes are deeply investigated by means of a) Biomethane potential tests to understand the biomass performance in substrate degradation, to identify potential inhibition phenomena, and to evaluate the impact of pre-treatments; 2) in continuous operation with semi-pilot scale reactors to understand the AD process under dynamic loading conditions and to define limits of acceptable operative clauses, as correlations among microbial consortia. Long term AD of canteen food waste (FW), carried out at moderate organic load in semi-continuous mode was unstable, due to dramatic VFA accumulation because of high fermentative bacterial activity with respect to methanogenesis. By adding a small amount of sludge to the feedstock, no process failure was observed despite higher loads, allowing steady state methane yields (up to 0.31 Nm3CH4 kg-1VSfed, corresponding to 55 Liters CH4 per kg FW). The preliminary results from a novel platform approach to produce short chain fatty acids by acidogenic fermentation (35°C, controlled pH 6, HRT 4d) of the released organics (mainly sugars) after food waste thermal pretreatment combined with parallel bio-methane recovery from the residue, are very promising (up to 16 g VFA L-1, composed by 37% of acetic and 37% of butyric acid).