Solid substrate anaerobic co-digestion of paper mill sludge, biosolids, and municipal solid waste

Abstract

The Mexican pulp and paper industry and municipal authorities are facing increasing regulatory and costrelated pressures regarding the handling, treatment and disposal of waste sludge and solid wastes. The dry anaerobic digestion (DASS) is a promising alternative for the co-stabilization of waste sludge, municipal and industrial solid wastes. However, appropriate and fast process start-up is a bottleneck for the dissemination of DASS technology in developing countries. This work aimed at determining a reliable and fast DASS startup procedure from non-anaerobic inocula for the digestion of a mixture of paper sludge, waste sludge and municipal solid waste. Three types of inoculum were used: cattle manure (CM), soil (S) and waste activated sludge (WAS). Results were analyzed in terms of the stabilization time Te (the time required to develop a full methanogenic regime) and the overall start-up time To (time required to reach at least 25% TS inside the reactor since the inoculation). A factorial experiment was implemented; factors were the inoculum type (five combinations of CM, S and WAS), temperature (35 and 55°C) and loading rate (4.5 and 8.2 g VS/kg.d). Results showed that the fastest start-up was obtained with reactors using inoculum 13 (33% CM, 33% S and 33% WAS) at 55°C and 8.2 g VS/kg.day loading rate. Interestingly, thermophilic regime favoured shorter stabilization times, in spite of the fact that the inocula used were meso- or psychrophilic. Results from DASS reactors that reached steady state after start-up (I3 and 14) showed typical performance responses of 60% removal efficiency (% total VS basis), biogas productivity between 2.7–3.5 NL/kg wrm.day, a in the range 0.3–1.3 and pH around 8. This suggested that the DASS process is a feasible alternative for co-digesting paper-mill sludge, MSW and biosolids.