Strategies to optimise anaerobic co-digestion of complex organic wastes

Resumo

Anaerobic co-digestion is established as an appropriate alternative for managing organic wastes surpassing both technical and economic barriers that mono-digestion technologies offer. Accordingly, digesters treating residues like livestock manure or sewage sludge are usually oversized and thus their treatment together with other organic wastes is appeared as an opportunity to increase the economic profitability of biogas plants. A wide range of organic wastes susceptible to be treated by anaerobic co-digestion is encountered. Those residues with high readily biodegradability such as biodiesel waste or grease coming from food industry are preferred. However, these residues often have a limited availability or they are seasonal. Other organic complex wastes can be also suitable for co-digestion purpose although their use is sometimes dependent on the application of certain strategies to endorse the feasibility of the anaerobic co-digestion process. The aim of this thesis is focused on the assessment of strategies for anaerobic co-digestion to foster the treatment of complex organic wastes from different industrial sectors: waste streams from machining processes, agricultural residues (livestock manure), sewage sludge and agrowastes (slaughterhouse residues, fish waste, etc), among others. Many of these complex organic wastes present sevel problems like the presence of toxic compounds (heavy metals, complex organic compounds, etc.), high protein and/or lipids, high values of dry matter or the presence of recalcitrant material or hardly biodegradable. A major conclusion from this Thesis is that anaerobic co-digestion of complex organic wastes is a feasible treatment technology when particular operational strategies are applied. Thus, it was demonstrated the i) treatment of residues containing highly toxic compounds such as spent metalworking fluids from machining processes was proven by means of a pulse feeding strategy; ii) poultry manure, which high levels of proteins and hardly biodegradable organic matter, can be used as co-substrate through the application of thermochemical pretreatments coupled to ammonia stripping processes; iii) and slaughterhouse residues together with potato waste resulted to be suitable for the controlled transition from mono- to co-digestion. Besides assessing the feasibility from the technical, environmental and economic perspectives was also considered.