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 operated by small and medium farmers. There is a gap of knowledge on the performance of these systems after several years of operation. The aim of this research is to provide a complete evaluation of a psychrophilic, low-cost, tubular digester operated for eight years. The thermal performance was monitored for 50 days, and parameters such as pH, total volatile fatty acid (tVFA), chemical oxygen demand (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 load rate (OLR) equal to 0.52 kg VS/m3digester *d and an estimated hydraulic retention time (HRT) of 25 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 influent and effluent showed that the digester reached a specific methane production of 0.40 Nm3CH4/kg VS and a 0.21 Nm3CH4/m3digester d with 63.1% CH4 in the biogas. These results, together with a microbiological analysis, show stabilized anaerobic digestion and a biogas production that was higher than expected for the psychrophilic range and the short HRT; this may have been due to the presence of an anaerobic digestion microorganism consortium which was extremely well-adapted to psychrophilic conditions over the eight-year study period.

Research trends and strategies for the improvement of anaerobic digestion of food waste in psychrophilic temperatures conditions

“…Jaimes-Estévez et al. [ 130 ] evaluated the AD of manure at 12 °C in a tubular digester that had been in operation for 8 years, microbiological analysis revealed a diverse population that had adapted to the conditions of AD; the process provided a selective environment that was favorable to the methanogenic archaea communities; by contrast, bacterial communities decreased over the course of AD. Park et al.…”

“… A fraction of the biogas is solubilized in the digestate; therefore, the digestate contains organic matter that is converted into NH3 and CH4, which translates into a loss of energy efficiency and greater environmental impacts. [ 24 , 29 , 30 , 42 , 48 , 103 , 107 , 111 , 115 , 117 , 126 , 130 , 140 , 141 , 142 , 143 , 144 , 145 , 146 ] Mesophilic Offers greater process stability and bacterial richness and requires lower energy costs compared with AD under psychrophilic and thermophilic conditions. Has low risk of inhibition by ammonium and LCFAs.…”

“… The choice of the appropriate mixing ratio is an important parameter in co-digestion that should be optimized. [ 33 , 92 , 130 , 156 ] …”

“…According to Feller [ 124 ], only microorganisms adapted to psychrophilic conditions can handle the limitations that occur at temperatures below 20 °C. These adapted microorganisms possess physiological conditions suitable for cold environments, due to the characteristics of their cell membranes in terms of proteins, lipids and enzymes, which makes the microorganisms remain active in psychrophilic environments and show excellent genetic responses to thermal changes [ 130 , 145 , 168 ]. In this sense, psychrophilic microorganisms render AD possible in cold regions and could overcome thermodynamic constraints at low temperatures [ 133 , 169 ].…”

High-Solid Anaerobic Digestion: Reviewing Strategies for Increasing Reactor Performance

“…When the performance of these systems is analyzed, better yields are obtained than those expected from control laboratory conditions. This is probably explained by the well-established consortium attained after an extended operation time in industrial operating reactors [147]. Zhao et al [148] studied digestion performance at 4 • C, indicating that the maximum treatment capacity was set at 4.33 g VS/Lr d of OLR.…”