Anaerobic Mesophilic and Thermophilic Municipal Sludge Digestion

M. de la Rubia, M. Pérez, L. I. Romero and D. Sales

Abstract
In this study the performance of mesophilic and thermophilic anaerobic digestion of sludge at pilot-scale, have been presented. The thermophilic reactor was operated at solids retention time between 75–20 days, while the mesophilic process was operated at 27 days. Evaluation of performance was in terms of a number of quantities including: OLR removal (as kg m–3 d–1 VS and kg m–3 d–1 COD), methane generation (as m3 kg–1 CH4 in the VSr), individual volatile fatty acids (VFAs) generation, and total acidity. Experimentally it was confirmed that the maximum loading rate achieved was 1.87 kg m–3 d–1 VS and 3.12 kg m–3 d–1 COD (at SRT: 20 days) for the themophilic process. Under these conditions, the volatile solids removal efficiency and organic removal efficiency was 38.0% VSr and 24.7 % CODr respectively, and the methane generation was 0.24 m3 kg–1 CH4 in the VSr. At SRT 27 days and mesophilic conditions, the organic loading rate removal was 0.69 kg m–3 d–1 VSr and 1.37 kg m–3 d–1 CODr and the methane generation was 0.40 m3 kg–1 CH4 in the VSr. At the same SRT, the thermophilic process operated with 0.80 kg m–3 d–1 VSr and 0.96 kg m–3 d–1 CODr organic loading rate removal, and the methane generation was 0.24 m3 kg–1 CH4 in the VSr. The experimental results showed, that with 27 days retention time there was little difference in the VS removal efficiency by two types of processes at organic loading rates (OLR) up to 1.3 kg m–3 d–1 VS. However, at low SRT, the thermophilic reactor produced more gas than the mesophilic at OLR up to 2.19 kg m–3 d–1 COD.

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Keywords
anaerobic digestion, mesophilic, thermophilic, sludge, solids retention time, VFA