Effective and stable nitrogen removal from wastewater requires abundant and active denitrifying populations. In this study, a one-year investigation of the population dynamics of phylogenetic groups known to harbor nitrate reducers was conducted in three municipal wastewater treatment plants (WWTPs). The bacterial community composition was determined by amplicon sequencing of the 16S rRNA gene, and putative nitrate reducers were identified by sequencing narG and napA genes. Fluorescence in situ hybridization with oligonucleotide probes targeting known nitrate reducers in wastewater revealed that certain bacteria predominated in the WWTPs: Curvibacter-related bacteria, Comamonadaceae, Azoarcus, Thauera, Dechloromonas, and Candidatus Accumulibacter within Rhodocyclaceae. The data showed high diversity in the nitrate-reducing community and a large degree of redundancy, with a relatively stable core group of bacteria in each plant that ensured small yearly variation in nitrate reduction rates.

The influence of ultrasonic pretreatment with specific energy input ranging from 25 to 550 kJ/kg volatile solids on the mixture of Sida hermaphrodita (L.) Rusby mixed with cattle manure disintegration and subsequent anaerobic digestion was assessed. The pretreatment process led to significant increase in the biomass solubility by 21.9% as chemical oxygen demand and enhanced biogas yield by 157% (567.1 L biogas/kg volatile solids) when the specific energy input was from 200 to 550 kJ/kg. However, only pretreatments where ultrasound was applied at 25–50 kJ/kg led to positive net energy gain, indicating that the biomass processing with this method does not always compensate the energy consumption for irradiation.