Assessing Volatile Fatty Acids production from food waste at MBT plants: focusing on temperature influence

Abstract

The implementation of the circular economy paradigm requires a change in economic dynamics towards more sustainable and renewable strategies. Under this scenario, waste is not considered a residue but instead a source of add-value products. The Organic Fraction of Municipal Solid Waste (OFMSW) represents around the 30% of the municipal solid waste and its treatment is a big challenge to deal with since the European union restricted its landfilling. Thermal treatment such as incineration and pyrolysis have high energy requirements and greenhouse gas emissions (GHG) while biological treatments, such as composting and anaerobic digestion (AD), operation cost is lower but generate products (compost and biogas) with low market value. Mechanical-Biological Treatment plants (MBT) incorporate biological stages that reduce and stabilize the biodegradable matter present in the OFMSW. The AD process can be engineered to promote the accumulation of fermentation products such as Volatile Fatty Acids (VFA) including acetic, propionic and butyric acid, among others. VFA have a higher market price than biogas and have a wide range of utilities. However, there is still some controversy regarding the impact of operational conditions on VFA yield and VFA profile. Therefore, this research investigates the influence of temperature as operational parameter to maximize VFA production from OFMSW and minimize methanogenic activity at different temperatures. Experimental results have shown that the configuration of MBT plant using wet AD with supernatant from AD recirculation is ideal for VFA production since the waste liquid stream under treatment has high alkalinity, a pH near 6 and readily organic matter available to be fermented into VFA. Fermentation under mesophilic conditions (35 ⁰C) have been increased VFA concentration from 6 g VFA/L to 20 g VFA/L which is in the top-tier of values in the literature. Experiments at different temperatures (20, 45, 55 and 70 ⁰C) have shown that both the ratio CODVFA/sCOD and specific production (gCODVFA/gVS) decreased as temperature increased even that the differences at 45 and 55 ⁰C from 35 ⁰C were slight. Nevertheless, 20 ⁰C resulted to be an unfeasible temperature for VFAs production. This investigation could generate an alternative for biorefinery innovations and valorization of OFMSW