Argo Kuusik, Karin Pachel, Aare Kuusik, Enn Loigu

Anaerobic co-digestion of sewage sludge with fish farming waste

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Abstract

Costly management of wastes from Estonia fish processing plants usually sends their sludge from the sludge filter press process to wastewater treatment plant or composting. To reduce the cost, the potential use of this waste for the production of biogas through the anaerobic process was investigated. Anaerobic digestion has long proven to be an efficient way for the production of a renewable fuel, biogas, which can be used as a source of energy to produce electricity or heat. This renewable energy resource can be used to reduce processing costs of plants. As a result, fish waste becomes a valuable resource instead of a waste which has tipping fee. In this study, both batch and continuous flow anaerobic digestion experiments were performed at mesophilic (38 ± 1°C) condition. The inoculum used was from an anaerobic mesophilic digester from municipal wastewater treatment plant (WWTP). Primary substrate was sewage sludge from WWTP and secondary substrate was sludge from Saaremaa salmon breeding farm. The mesophilic anaerobic treatment of concentrated sludge from an Saaremaa salmon breeding farm pools with total solids (TS) from 3.2 to 7.0% was investigated in a one-stage periodically stirred tank reactor at 38 °C and 20–25 days hydraulic retention time (HRT). Organic loading rate (OLR) ranged from 1.08 up to 1.22 kg volatile solids (VS)/(m3*d). Methane yields between 223.13–370.19 m3 CH4/ton VS and 4.34–8.65 m3 CH4/ton were achieved. The pH- value was hold at 6.92–7.45 during the whole operation. The fertilizing value of the treated sludge was estimated to be 0.87–1.12 kg N and 0.42–0.99 kg P per ton. The biogas analysis shows that CH4 content varied from 63.3 to 74.6% and CO2 content ranged from 11.2 to 29.0%.

Keywords: Anaerobic treatment; fish farming sludge; energy production; biogas; sludge treatment.