Performance and behaviour of the microbial community of an anaerobic biogas digester using sugar beet silage as mono-substrate

Abstract

The performance and the behaviour of the microbial community of an anaerobic mesophilic biogas digester fed with the sugar beet silage (SBS) were investigated in this laboratory-scale work. The biogas digester was operated using a fuzzy logic control (FLC) technique, which was developed at the Hamburg University of Applied Sciences. The SBS, which had extremely low pH of 3.3–3.4, was used as mono-input, without addition of manure. The main objective of this study was to achieve and maintain a stable and safe anaerobic conversion process, along with satisfactory process efficiency. With use of FLC, the biogas digester could be operated with hydraulic retention times (HRTs) in the range 8–25 days, and up to an organic loading rate (OLR) of 7.41 g volatile solids [VS] l−1 d−1. The average levels of the specific gas production (GP) rate (SGPR) and the volumetric GP rate (VGPR) were 0.55 l g [VS]−1 d−1 and 3 l l−1 d−1, respectively, with 66% of methane (CH4) content in digester biogas. Despite the SBS having an extremely low pH, low amounts of nutrients and buffering capacity, instability problems were not encountered during the experimental period of 156 days, as indicated by the neutral range of the digester pH and the high OLR applied. A daily addition of 1 M KHCO3 to the digester also provided adequate buffering capacity. The application of the FLC also exhibited a positive effect on the methanogenic population. After starting the FLC, the methanogenic population doubled from 4.80 × 108 to 1.19 × 109 cells ml−1 at the end of the investigation period, whereas the number of the total bacteria remained constant around 2.27–2.30 × 1010 cells ml−1. The FLC application appeared to have a positive effect on the number of the fluorescent methanogens. However, no significant effect on the morphology was found.