The superiority of citrate as denitrification carbon source in an activated sludge system: Substrate metabolism, electron transfer and microbial characteristics

Citrate, a low-molecular-weight organic substance, was rich in citric acid production wastewater with the potential for reuse. Although citrate has been extensively used as a denitrification carbon source, previous studies mainly focus on its application by pure cultures of denitrifying bacteria. The information about denitrification performance and mechanisms using citrates as carbon source in an activated sludge system is still very limited. This study investigated the denitrification efficiency and mechanisms using citrate as carbon source through a long-term operation of continuous reactor. Under the C/N ratio of 5, the total nitrogen (TN) removal efficiency using citrate as carbon source was 93.21 ± 4.24 %, which was comparable to that of acetate (99.84 ± 0.44 %), and much higher than glucose (65.64 ± 3.86 %). Higher denitrification rate was achieved using citrate as carbon source, which was slightly higher than acetate and >4 times of that using glucose. Due to the high activities of denitrifying enzymes including nitrite reductase (NIR), nitric oxide reductase (NOR) and nitrous oxide reductase (N2OR), there was almost no NO2‐-N and N2O accumulation in the reactor fed with citrate, which was superior to the reactors fed with acetate or glucose. Azoarcus (48.2 %) with the ability of complete denitrification was the most dominant bacteria in the reactor fed with citrate. Considering denitrification kinetics, activities of key enzymes and microbial community characteristics, citrate had a superiority as an effective denitrification carbon source.