Abstract
Wastewater treatment plants (WWTPs) are important for pollutant removal from wastewater, elimination of point discharges of nutrients into the environment and water resource protection. The anaerobic/anoxic/oxic (A2/O) process is widely used in WWTPs for nitrogen removal, but the requirement for additional organics to ensure a suitable nitrogen removal efficiency makes this process costly and energy consuming. In this study, we report mixotrophic denitrification at a low COD (chemical oxygen demand)/TN (total nitrogen) ratio in a full-scale A2/O WWTP with relatively high sulfate in the inlet. Nitrogen and sulfur species analysis in different units of this A2/O WWTP showed that the internal sulfur cycle of sulfate reduction and reoxidation occurred and that the reduced sulfur species might contribute to denitrification. Microbial community analysis revealed that Thiobacillus, an autotrophic sulfur-oxidizing denitrifier, dominated the activated sludge bacterial community. Metagenomics data also supported the potential of sulfur-based denitrification when high levels of denitrification occurred, and sulfur oxidation and sulfate reduction genes coexisted in the activated sludge. Although most of the denitrification genes were affiliated with heterotrophic denitrifiers with high abundance, the narG and napA genes were mainly associated with autotrophic sulfur-oxidizing denitrifiers. The functional genes related to nitrogen removal were actively expressed even in the unit containing relatively highly reduced sulfur species, indicating that the mixotrophic denitrification process in A2/O could overcome not only a shortage of carbon sources but also the inhibition by reduced sulfur of nitrification and denitrification. Our results indicate that a mixotrophic denitrification process could be developed in full-scale WWTPs and reduce the requirement for additional carbon sources, which could endow WWTPs with more flexible and adaptable nitrogen removal.
Highlights
With the increasing realization of the impacts of excess nitrogen (N) discharge on the environment and human health, N effluent regulations have become increasingly stringent worldwide
The effluent quality could meet the first class A criteria of effluent discharge, and the average removal efficiency of COD, TN and NH4+-N could reach 83%, 72.4%, and 98.6%, respectively [19]. Both COD/TN and ΔCOD/ ΔTN showed no significant relationship with TN removal efficiency (p > 0.05, Fig 1), which indicated that the ratio of COD/TN did not significantly influence the N removal processes in the YXM WWTP
In the PRAN unit, NO3--N decreased from 7.41 ± 0.44 mg L-1 in the influent to 1.03 ± 0.25 mg L-1 in the effluent, and the concentration of NO3--N continued to decrease to 0.41 ± 0.08 mg L-1 in the ANA unit effluent (Table 1)
Summary
With the increasing realization of the impacts of excess nitrogen (N) discharge on the environment and human health, N effluent regulations have become increasingly stringent worldwide. Sulfur-based autotrophic denitrification, such as the sulfate reduction autotrophic denitrification nitrification integrated (SANI1) process and sulfur-limestone autotrophic denitrification (SLAD), has been comprehensively studied [3,5,6,7,8]. In these processes, chemolithotrophic sulfide-oxidizing denitrifying bacteria (SONB) (Thiobacillus sp., Sulfurimonas denitrificans, Beggiatoa sp., and Thiothrix sp.) and heterotrophic sulfide-oxidizing denitrifiers (Thauera-like taxa, Azoarcus, Pseudomonas, and Dechloromonas) cooperate in the processes of N removal [3,5,9,10,11,12,13,14]. Sulfate reducing bacteria (SRB), such as Desulfobacteraceae, Desulfonema, and Thermotogaceae, in activated sludge (AS), which convert sulfate to sulfur, sulfide, and poly-S, could cooperate with SONB and enhance N removal by providing electron donors [3,8,15]
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