Mesophilic to thermophilic conditions in codigestion of sewage sludge and OFMSW: Evaluation of effluent stability using Dynamic Respirometric Index (DRI) and Biochemical Methane Potential (BMP)

The paper deals with the change from the mesophilic to thermophilic conditions in the codigestion process applied to secondary sludge from a biological nutrients removal (BNR) together with the organic fraction of municipal solid waste (OFMSW) or biowaste. The first part of the study was carried out at pilot scale in a 0.38 nr' pilot scale plant, in order to evaluate the process behaviour during short-time meso-thermo passage and the relative yield increase, in the second part, the same set of operational conditions was applied to a full scale 2000 m 3 digester: both pilot and full scale trials evidenced the same behaviour and results. The thermophilic range of temperature showed a general increase of performance of about 50% in terms of biogas yields and allowed for a better stabilisation of the effluent (digestate). In order to evaluate this aspect, both the aerobic and anaerobic rate of stability was evaluated: the effective rate of stabilisation of the digested material by means of aerobic (Dynamic Respirometric Index), and anaerobic batch tests (Biochemical Methane Potential) was determined. Preliminary tests on the effluents originated from three plants showed a DRI lower than 1000 mgO 2/kgVSh which is the minimum value required for a stable compost following the Italian regulation system, and an average SGP of 0,15-0,30 m 3/kgVS which demonstrated a partial conversion of the organic matter into biogas.