Performance and microbial mechanism of eletrotrophic bio-cathode denitrification under low temperature

Insufficient amount of carbon in wastewater and low temperatures hinder the use of biological nitrogen removal for purification of wastewaters. Nitrogen removal using cold-tolerant electrotrophic cathodic microbes is a novel and unique autotrophic denitrification technique in which electrical current, not chemicals, is used as a source of electrons. In this study, integrated MFC (RW) and open-circuit MFC (RO) were cultured and acclimatized in stages at a low temperature (10 degrees C) to impart cold tolerance to electrotrophic cathodic microbes, investigate the effectiveness of simultaneous nitrification and denitrification (SND) process, and address the possible mechanism of microbial action. The results showed that (i) microbial communities in the RW system were successfully enriched with the cold-tolerant electrotrophic cathodic microbes after five stages, and (ii) the degree of NH4+-N removal and SND were 75.50% and 81.91%, respectively, but the respective values in the RO system were only 40.47% and 54.01%. The desirable SND efficiency was obtained in RW at a DO of-0.6 mg/L, a current of-20 mA, and pH-7.0. In RW, Thauera, Pesudomonas, and Hydrogenophaga were the main electrotrophic cathodic denitrifying bacteria with cold tolerance capable of degrading ammonia, nitrate, and nitrite through autotrophic denitrification and cathodic-driven bio-electrochemical denitrification. Besides, for RW, results from high throughput sequencing analysis revealed that the abundance of genes related to energy production and con-version, amino acid transport, and metabolism, signal transduction, environmental adaptation, and enzymatic activity (AMO, HAO, NAR, NIR, NOR, and NOS) were significantly higher than the corresponding parameters of the RO system. This may explain the reason behind RW having excellent ammonia and TN removal performance at low temperatures.