Abstract
SummaryAir‐side stripping without a prior solid–liquid phase separation step is a feasible and promising process to control ammonia concentration in thermophilic digesters. During the process, part of the anaerobic biomass is exposed to high temperature, high pH and aerobic conditions. However, there are no studies assessing the effects of those harsh conditions on the microbial communities of thermophilic digesters. To fill this knowledge gap, the microbiomes of two thermophilic digesters (55°C), fed with a mixture of pig manure and nitrogen‐rich co‐substrates, were investigated under different organic loading rates (OLR: 1.1–5.2 g COD l−1 day−1), ammonia concentrations (0.2–1.5 g free ammonia nitrogen l−1) and stripping frequencies (3–5 times per week). The bacterial communities were dominated by Firmicutes and Bacteroidetes phyla, while the predominant methanogens were Methanosarcina sp archaea. Increasing co‐substrate fraction, OLR and free ammonia nitrogen (FAN) favoured the presence of genera Ruminiclostridium, Clostridium and Tepidimicrobium and of hydrogenotrophic methanogens, mainly Methanoculleus archaea. The data indicated that the use of air‐side stripping did not adversely affect thermophilic microbial communities, but indirectly modulated them by controlling FAN concentrations in the digester. These results demonstrate the viability at microbial community level of air side‐stream stripping process as an adequate technology for the ammonia control during anaerobic co‐digestion of nitrogen‐rich substrates.
Highlights
Anaerobic digestion (AD) is a microbial process based on the combined and syntrophic activities of a wide range of microorganisms from hydrolytic and fermentative bacteria to methanogenic archaea (Stams and Plugge, 2009)
Microbial communities are usually dominated by Firmicutes phylum and hydrogenotrophic methanogens, whereas Bacteroidetes and acetoclastic archaea are the prevalent groups under mesophilic conditions in reactors treating sewage sludge or co-digesting food waste and slaughterhouse residues (Sundberg et al, 2013)
During Period 4 of R2 (440–494 days), the stripping process was applied under the same conditions as in Period 3 of R1, but with a frequency of five times per week, decreasing free ammonia nitrogen (FAN) levels from 1.3 to 0.2 g N-FAN lÀ1, whereas organic loading rates (OLR) lowered slightly to 2.5–2.8 g chemical oxygen demand (COD) lÀ1 dayÀ1 (Fig. 1B) and methane production remained stable
Summary
Anaerobic digestion (AD) is a microbial process based on the combined and syntrophic activities of a wide range of microorganisms from hydrolytic and fermentative bacteria to methanogenic archaea (Stams and Plugge, 2009). Microbial communities are usually dominated by Firmicutes phylum and hydrogenotrophic methanogens, whereas Bacteroidetes and acetoclastic archaea are the prevalent groups under mesophilic conditions in reactors treating sewage sludge or co-digesting food waste and slaughterhouse residues (Sundberg et al, 2013).
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