Abstract
The present study focuses on a two-step process for treatment and stabilisation of primary sludge. The process consists of a hyper-thermophilic hydrolysis step operated at 70 °C and a hydraulic retention time (HRT) of 2 days followed by a thermophilic (55 °C) anaerobic digestion step at a HRT of 13 days. A one-step anaerobic digester operated at 55 °C and 15 days HRT was used as a reference process. The two-step process was characterized by a 12% higher organic suspended solids removal efficiency and better pathogen reduction effect than the conventional one-step digestion. The microbial community of the digester fed with pre-treated sludge was characterised by a higher activity compared to that of the digester treating raw sludge. Moreover, the pre-treatment of the primary sludge resulted up to 48% increase of the methane potential (20.09 and 13.56 mmol CH 4 g − VS −1 with and without pre-treatment, respectively) and up to 115% increase of the methane production rate. Finally it was shown that the extra energy requirements for the operation of a pre-treatment step would be covered by the energy produced from the extra methane production and in addition there would be a significant energy surplus of 2.17 kJ d −1 for the system tested.
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