Nitrogen And Phosphorus Removal Performances In Integrated Fixed-film And Activated Sludge Process With Suspended Carriers

The activated sludge (AS) anoxic/oxic (A/0) and anaerobic/anoxic/oxic (A2/O) processes have been widely used in the municipal wastewater treatment plants (WWTPs) for nitrogen and phosphorus removal. However, the method of increasing the nitrate liquor recirculation ratio is usually employed to improve the nitrogen removal efficiencies of these processes, which will increase the energy consumption on recirculation. In addition, the nutrients removal performances of these processes will be limited by several problems, such as the shortage of carbon source in the influent and the poor nitrification performance under low temperature. Due to the increasingly stringent discharge standard for nitrogen and phosphorus, a number of existing WWTPs are facing the challenges of improving the nutrients removal efficiencies. The integrated fixed-film and activated sludge (IFAS) process was easy to be established by only adding suspended carriers into the aerobic reactor of AS process, which could save the construction cost for building new wastewater treatment facilities. Therefore, the nutrients removal performances of the IFAS process under low temperature and low carbon source were investigated in this study. In addition, it was also explored whether the preferable nitrogen removal performance in the IFAS process can be achieved under lower nitrate liquor recirculation ratio. Furthermore, the molecular biology was applied to analyze the correlation between the nutrients removal performances and the microbial community structure. The purposes of this study are to provide some theoretical basises and technical supports for the modification of the existing WWTPs, increasing the nutrients removal efficiencies and reducing the operational cost. The main contents and* results are as follows:(1) The COD, NH4+-N and TN removal performances of the IFAS aerobic reactor, the moving-bed biofilm aerobic reactor and the activated sludge (AS) aerobic reactor were investigated contrastively under aerobic condition. The results showed that when the influent COD, NH+-N and TN averaged 130,38.0 and 42.0 mg/L, the COD removal performances of each reactor were roughly the same, whereas the nitrification efficiency of the IFAS aerobic reactor was higher than those of the other reactors. In addition, the IFAS aerobic reactor exhibited a stronger resistance impact load capability for NH4+-N. When the hydraulic retention time (HRT), the dissolve oxygen (DO) and the carriers packing ratios (PR) were controlled at 5-6 h,2-3 mg/L and 30%, the effluent COD and NH4+-N of IFAS aerobic reactor could meet the Chinese discharge standard. However, the lower TN removal efficiency had been achieved in IFAS aerobic reactor. Hence, it needs to increase TN removal efficiency with an anoxic reactor.(2) In order to increase the nitrogen removal efficiency of the IFAS aerobic reactor, an A/O-IFAS process was built by combining the IFAS aerobic reactor with an anoxic reactor. The nitrogen removal performances of the A/O-IFAS and A/O activated sludge process was investigatedunder different nitrate liquor recirculation ratios (R=100,200 and 300%). The results showed that the simultaneous nitrification and denitrification (SND) performance was enhanced in the A/O-IFAS process, which resulted in the effluent COD, NH4+-N and TN of A/O-IFAS process at R of 200% meeting the Chinese discharge standard. However, the effluent COD, NH4+-N and TN of A/O process at R of 200% and 300% could not meet the discharge standard. It indicated that the A/O-IFAS process could achieve the excellent nitrogen removal efficiency under lower R. The polymerase chain reaction and denaturing gradient gel electrophoresis (PCR-DGGE) results suggested that nitrifiers and denitrifiers co-existed in one microbial community. Due to the cooperation of these microorganisms, the SND occurred in aerobic reactor of the A/O-IFAS process. In addition, the nitrogen removal performances of the A/O-IFAS process and A/O activated sludge process under low wastewater temperature (8-12 ?) were investigated, and the FISH technique was used to analyze the relative abundance of nitrifiers in these processes at 8-12 ? and 19-23 ?, respectively. The results showed that the relative abundance of nitrifiers in the suspended sludge of each process decreased significantly at 8-12 ? compared with that at 19-23 ? However, the relative abundance of nitrifiers on the biofilm of the A/O-IFAS process exhibited no obvious variation, which ensured the superior nitrification performance of the A/O-IFAS process under low temperature. When the influent NH4+-N and TN averaged 53.7 and 70.7 mg/L, the effluent NH4+-N and TN of the A/O process averaged 12.9 and 21.8 mg/L, whereas the average NH4+-N and TN in the effluent of the A/O-IFAS process were 1.2 and 12.9 mg/L, meeting the winter discharge standard of China (NH4+-N<8 mg/L, TN<15 mg/L).(3) The nutrients removal efficiencies could be decreased due to the shortage of the carbon source in the influent. Hence, the A2O-IFAS process was introduced to treat the low-carbon source (C/N:3.0-3.4) wastewater, aiming to investigate the nutrients removal efficiencies of this process. The results showed that the phenomenon of denitrifying phosphorus removal (DPR) appeared in anoxic reactor of the A2O-IFAS process. The bypass flow method was employed to bring a part of influent into anoxic reactor, which could provide more carbon source for denitrifying phosphorus accumulating organisms (DPAOs) to improve DPR performance. After the modification, the DPR efficiency was increased by 23.3%, and the TN and TP removal efficiencies were increased by 25.8% and 41.2%, respectively. When the sludge retention time (SRT) was controlled at 8 days, the effluent NH4+-N, TN and TP could meet the Chinese discharge standard.(4) The A2O-IFAS was introduced to treat the municipal wastewater in a full-scale WWTP, and operated with the existing AS process of the WWTP to investigate the nutrients removal performances. In addition, the PCR-DGGE and FISH techniques were introduced to analyze the distribution and relative abundance of the microbial communities in the two processes, aiming to discuss the relevance between nutrients removal and the microbial community structure. The PCR-DGGE and the FISH results showed that the species and the relative abundances of nitrifiers, denitrifiers and PAOs in the A O-IFAS process were higher than those in the AS process, demonstrating the superiority on nutrients removal of the A2O-IFAS process. When the influent COD, NH+-N, TN and TP were in the range of 109-302 mg/L,25.0-42.1 mg/L,28.0-49.4 mg/L and 2.2-4.5 mg/L, respectively. The effluent COD, NH/-N, TN and TP of the A2O-IFAS process were less than 50,5,15 and 1 mg/L, meeting the Chinese discharge standard (for the WWTP built before Dec 21th,2005:COD< 50 mg/L, NH4+-N< 5 mg/L, TN< 15 mg/L and TP< 1 mg/L). However, the effluent NH4+-N, TN and TP of the AS process could not meet the discharge standard.