Effects of NaCl concentration on simultaneous nitrification and denitrification process and N2O emission in SBBR

The sequencing batch biofilm reactor (SBBR) is widely applied in the wastewater treatment due to its strong adaptability to the unstable influent substrate concentrations. The growth environment for microorganisms is different in the outer and inner space of the biofilm, which leads to different microbial community structure in different zones of the system. Salinity is one of the key factors that affect biological nitrogen removal (BNR) performance for domestic wastewater treatment. Higher salinity could also promote the nitrite accumulation. In particular, nitrite accumulation was considered to be a major parameter for affecting the emission of N2O in both nitrification and denitrification stages, and therefore mitigate the environmental benefits of nitrogen removal process. In present study, the feasibility of simultaneous nitrification and denitrification process achievement in a SBBR was evaluated treating domestic wastewater with NaCl addition (0, 5, 10, 15 and 20 g/L) salinity addition. For more detailed insights, the changes of polyhydroxyalkanoate (PHA) and glycogen (Gly) were also analyzed to evaluate the salinity effect on nitrite accumulation and N2O emission. The results showed that with the increase of NaCl concentration, the nitrogen removal efficiency decreased, while the N2O emission ratio increased. The NH4+ removal efficiency was more than 95% as the NaCl concentration was no more than 10 g/L. When the NaCl concentration increased to 20 g/L, the average NH4+ decreased to 70.6%. As the NaCl increased from 0 to 20 g/L, the increment of PHA and Gly decreased from 43.6 mg/g and 34.5 mg/g to 28.2 mg/g and 22.7 mg/g, respectively, while the NO2- accumulation and the N2O emission ratio increased from 1.12 mg/L and 4.08 % to 18.87 mg/L and 13.60%. The more NaCl was added, the higher the ratio of NO2- to NOx- accomplished. The accumulated NO2- contributed to the occurrence of nitrifier denitrification (ND) by AOB. Most nitrous oxide emission was via ND process with NH4+ as electron donor and NO2- as electron acceptor. The higher amount of N2O, formed in the transition zone, could be consumed in deeper regions of the biofilm when the COD was sufficient. In the absence of external carbon source, both PHA and glycogen Gly were used as internal carbon source for the endogenous denitrification. The higher NaCl concentration inhibited the PHA and Gly production, which decreased the internal electron donors for denitrification. The competition for electron between Nir and Nos during the endogenous denitrification process in the deeper region, as well as the nitrifier denitrification of AOB in the transition region, contributed to the high N2O emission, especially in the high NaCl concentration of 15 and 20 g/L. Furthermore, higher NaCl concentration reduced the density of the biofilm, which made it possible for more DO diffusing into the biofilm. It can not be ignored that DO possessed high inhibition on Nos, which attributed to the high N2O emission under high NaCl concentration. The "feast" time increased at a high salinity, revealing the inhibition of microbial activity. High salinity hindered the denitrification rate, and the inhibition degree was dependent on the influent COD/N and terminal electron acceptors. Compared with the nitrate reduction rate (DNAR) and the nitrite reduction rate (DNIR), the nitrous oxide reduction rate (DN2OR) was much more reduced by high salinity. In saline wastewater BNR process, the higher NO2- accumulation, the competion between Nir and Nos, as well as the higher DO concentration in the inner region of the biofilm, led to the increase in N2O yield.