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

The Mexican pulp and paper industry and municipal authorities are facing increasing regulatory and cost-related 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 start-up 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 T_e (the time required to develop a full methanogenic regime) and the overall start-up time T_o (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 I₃ (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 I₄) 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.