Anaerobic Digestion of Thickened Sludge from Fish Production in CSTR and Hybrid Pilot Scale Digesters

The present work is focused on the improvement of the sustainability of fish production by assessing the possible exploitation of the slurry derived from this type of farming. Tests were performed at the University of Udine, Italy, where experimental plants for the production of trout are installed. Wastewater from fish production was screened by drum sieve, and afterwards, subject to settling. The thickened sludge was characterized by a total and volatile solids content of 3969.1 – 9705.3 mg/l and 2916.4 – 7154.9 mg/l, respectively. Anaerobic digestion (AD) tests were performed on thickened sludge in a containerized unit equipped with a loading tank, two anaerobic digesters, biogas meters, and continuous monitoring of temperature, pH, and Redox. Biogas production and quality were monitored by Ritter gas meters and Infrared Analyzer. The reactors (D1 and D2) are completely stirred (CSTR) by means of recirculation of digestate and are characterized by a useful volume of 280l (D1) and 260l (D2). Reactor D2 is equipped with a plastic filling media characterized by relevant specific surface to promote the development of a fixed layer of anaerobic bacteria. The AD process was mesophilic (38°C) with varied loading rates intended to test various hydraulic retention times (HRT). HRT ranged from 28.9 to 20.3 days for digester D1 and from 18.3 days to 9.5 days for D2. Methane yields resulted high for the hybrid digester with the longest HRT (779.8 Nl CH4/kg VS, 18.3 days). The conventional digester presented its best performance, 648.8 Nl of CH4/kgVS, with HRT of 20.3 days.