Psychrophilic Full Scale Tubular Digester Operating over Eight Years: Complete Performance Evaluation and Microbiological Population

Abstract

Most biogas plants in the world run under psychrophilic conditions and are operatedby small and medium farmers. There is a gap of knowledge on the performance of these systemsafter several years of operation. The aim of this research is to provide a complete evaluation of apsychrophilic, low-cost, tubular digester operated for eight years. The thermal performance wasmonitored for 50 days, and parameters such as pH, total volatile fatty acid (tVFA), chemical oxygendemand (COD) and volatile solids (VS) were measured every week for the influent and effluent.The digester operated at a stabilized slurry temperature of around 17.7◦C, with a mean organic loadrate (OLR) equal to 0.52 kg VS/m3digester*d and an estimated hydraulic retention time (HRT) of25 days. The VS reduction in the digester was around 77.58% and the COD reduction was 67±3%,with a mean value for the effluent of 3.31±1.20 g COD/Lt, while the tVFA decreased by 83.6±15.5%and the presence of coliforms decreased 10.5%. A BioMethane potential test (BMP) for the influentand effluent showed that the digester reached a specific methane production of 0.40 Nm3CH4/kgVS and a 0.21 Nm3CH4/m3digesterd with 63.1% CH4in the biogas. These results, together witha microbiological analysis, show stabilized anaerobic digestion and a biogas production that washigher than expected for the psychrophilic range and the short HRT; this may have been due to thepresence of an anaerobic digestion microorganism consortium which was extremely well-adapted topsychrophilic conditions over the eight-year study period.