Distribution of micro- and macro-elements in anaerobic co-digestion of sewage sludge and food processing wastes

Abstract

Recently, anaerobic co-digestion has become an indispensable strategy for improving the effectiveness of municipal sewage sludge (MSS) mono-digestion. This technology is recognised as a disposal method for organic wastes with renewable energy generation and production of nutrient-rich digestate. The food processing wastes are among the most recommended substrates. Its introduction to MSS might provide valuable micro- and macro- elements which can result in increased methane production and improved quality of digestate. In this study, micro- and macro-elements distribution in anaerobic co-digestion of MSS and food processing wastes represented by ice-cream processing residues (IC), apple pomace (AP) and orange wastes (OW) has been evaluated. The batch anaerobic digestion experiments at temperature range of 37°C were conducted differing feedstock composition. In the feedstock and digestate the following metals were analysed: Na, Mg, P, K, Ca, S, Fe, Cr, Mn, Ni, Cu, Zn, As, Cd, Hg, Pb. In almost all cases, the introduction of co-substrates to MSS resulted in the improvement of macro-elements contents, simultaneously the reduction of the heavy metals (HMs) content was observed. The most favourable results were observed in three-component mixture in the presence of MSS, IC and OW, therein enhanced methane production of 401.6 Ndm³∙g⁻¹ VS (volatile solids) accompanied by improved organic compounds removal was noticed. The opposite trend, i.e. decreased methane production, was obtained in the reactor supplied by AP, therein also the highest bioaccumulation of almost all HMs occurred indicating the deterioration on process efficiency. The composition of all digestates meets the requirements for MSS intended for agricultural use.