Dynamics of Biological Processes during Composting of Anaerobically Digested Wastewater Sludge

Microbiological processes occurring in the course of controlled thermophilic composting of dehydrated wastewater sludge anaerobically digested in a reactor at the Lyubertsy waste treatment plant (Moscow, Russia) were studied. Dynamics of the concentrations of nitrogen species in the presence of microorganisms of the nitrogen cycle (ammonifying, nitrifying, denitrifying, and nitrogen-fixing) was studied at the changed temperature modes at sequential stages of long-term composting (98 days). During the active stage of composting, considerable emission of ammonia (553–861 mg m–3) and nitrogen oxide (67–86 mg m–3) were observed on days 6–10; these values significantly exceeded the accepted allowances for atmospheric emissions. Abundance of culturable heterotrophic microorganisms, CFU numbers of nitrogen-fixers, and the presence and activity of cultured nitrifying and denitrifying microorganisms were determined using microbiological techniques. The data of real-time PCR and NGS profiling by the 16S rRNA genes were used to analyze the taxonomic diversity of the microorganisms involved in composting (bacteria, archaea, and fungi); the patterns of succession within the microbial community during this process were established. Laboratory experiments revealed the potential for further decomposition of organic matter of digested sludge, which was probably unavailable to microorganisms under anoxic conditions, but could be degraded by the aerobic microbiota, resulting in a product stimulating plant growth and containing 2.3% of total nitrogen and containing ammonium and nitrate nitrogen (890 and 3750 mg kg–1, respectively).