Effects of NaCl salinity on wastewater pollutants removal and microorganism in A2/O technology process

Abstract: A large amount of wastewater containing NaCl was produced in the rapid developed industries such as food processing, leather manufacturing and seawater recycling. The salty wastewater entered the urban sewage treatment plants and contacted with the activated sludge, and would affect the properties of activated sludge and influence the pollutant treatment efficiency. For example, the change of microbial community structure in the sewage treatment plants is known to be closely related to the pollutants' removal efficiency of the biological wastewater treatment process. The A2/O technology process is widely applied in the wastewater treatment due to its simple processing flow and ideal denitrification performance. The growth environment for microorganisms is different in anaerobic zone, anoxic zone and oxic zone, which leads to different microbial community structure in different zones of the system. However, few studies have been performed with regard to the effects of salinity on microbial community structure in different zones of A2/O technology process, and the distribution of main populations affecting biological denitrification remains unclear. In order to provide concrete information for improving the pollutant removal efficiency, the effects of NaCl salinity variations on the microbial community structure in different zones of A2/O technology process were investigated through high-throughput sequencing by analyzing the microbial community structures in anaerobic, anoxic and oxic zones. The results showed that: 1) With the increase of NaCl salinity, COD removal efficiency decreased. For example, when the salinity increased from 0 to 40 g/L, the COD removal efficiency decreased from 52%, 80% and 56% to 30%, 50% and 40% in anaerobic, anoxic and oxic zones, respectively, while the removal efficiency of NH4+-N decreased from 33% and 61% to 11% and 39% in anaerobic zone and oxic zone, respectively, and the removal efficiency of NO3?-N in anoxic zones decreased form 63% to 47%. 2) Compared with wastewater without NaCl, the addition of NaCl resulted in a decrease of microbial diversity; The microbial communities detected under higher salinity (40 g/L) and lower salinity (0 and 10 g/L) showed big difference; The decrease of the relative abundance of Thauera and Paracoccus in anoxic area, Clostridium and Nitrospira in oxic area, attributed to the decrease of nitrogen removal efficiency of the A2/O technology process; The relative abundance of some strains in Firmicutes (such as: Lactobacillus, Streptococcus, Tepidibacterium, Veillonella, Lachnoclostridium and Zoogloea) with stronger salinity tolerance increased as the increasing of salinity; The abundance of microorganisms related to denitrification (such as: Proteobacteria, Bacteroidetes and Firmicutes) always maintained at relatively high levels and they were dominant with the increasing salinity all the time in different zones of A2/O technology process, which guaranteed the system always having a certain denitrification efficiency.