Effect of plant-based carbon source supplements on denitrification of synthetic wastewater: focus on the microbiology

The effects of plant-based carbon source addition on wastewater NO₃^--N removal and the involved microorganisms, especially denitrifying bacteria, were investigated. A synthetic wastewater (NO₃^--N, 15 mg/L) was treated through the batch experiment, which included three inoculation cycles (7 days/cycle), and was conducted at 25 °C. Four natural plant substrates, namely, rice straw (RS), wheat straw (WS), ryegrass (RG), and reed (RD), were used as carbon sources and supplemented at the rate of 1% (w/v). The results showed that both RS and WS performed well in promoting NO₃^--N removal (79.55-97.07%). While RG removed only 22.08% of NO₃^--N in the first cycle, the removal efficiency increased afterward (86.09-95.82%). Conversely, the NO₃^--N removal rate of RD decreased from 95.10 to 24.77% as a result of its low ability to supply carbon. With respect to the microorganisms, the RS treatment resulted in more bacteria and denitrifying genes such as narG, nirK, nirS, and norB than other treatments, while the highest number of nosZ gene copies was recorded in the WS treatment. Sequencing results revealed that Firmicutes (18.19-56.96%), Proteobacteria (38.82-74.80%), and Bacteroidetes (3.15-4.15%) were three dominant bacterial phyla for RS, WS, and RD treatments. Furthermore, the genera Enterobacter, Massilia, and Bacillus were the main denitrifying bacteria participating in the NO₃^--N removal. Furthermore, correlation analysis indicated that the denitrifying genus Sphingobacterium played an important role in enhancing nitrogen removal. This study suggested that RS is the superior plant-based carbon source for denitrifying bioreactors used in agricultural runoff treatment.