Anaerobic co‐composting of municipal solid waste and waste sludge at hlgh total solids levels

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Abstract
The feasibility of anaerobic co‐composting of waste activated sludge (WSS) and simulated organic fraction of typical North‐American municipal solid waste (NA‐MSW), rich in lignocellulosics, at high total solids (TS) content, was examined. Sixteen semicontinuous reactors were run, at 4 levels of TS (25, 30, 35 and 40%), 2 levels of retention time (15 and 21 days), 2 temperature regimes (mesophilic at 39°C and thermophilic at 53°C), and 2 ratios of MSW‐to‐WSS (1: 1 and 2: 1 on total weight basis). Reactors operated at 35% TS and 21 days gave the best organic matter removal efficiencies (up to 44% and 53% on volatile solids basis, for mesophilic and thermophilic regimes respectively) and highest biogas productivities (up to 6 L kg‐1 reactor content day‐1). The methane concentration in biogas was between 55 and 60%. Successful thermophilic reactors generated 15 to 30% more biogas than the corresponding mesophilic digesters. Instability and failure of some thermophilic reactors at 15 days retention time were observed. Addition of WSS (ratio 1: 1, richest in sludge) had a beneficial effect on the process.