| When Wastewater Treatment Plant (WWTP) effluent is reused as second water source, its high TN concentration and more than 80% NO3--N ratio is the most serious limiting factor, and NO3--N removal is becoming the key problem to be resolved for nitrogen removal. On the basis of long-term WWTP effluent quality investigation, the moving bed biofilm reactors (MBBRs) were used for the residual NO3--N removal, and the influence of carriers, filling ratio(FR), C/N ratio, carbon source and Hydraulic Retention Time (HRT) on denitrification were studied. Using Terminal Restriction Fragment Length Polymorphism (TRFLP) and Real-time Quantitative PCR (qPCR) molecular biology methods, the MBBR microbiological characteristics were also analyzed under different different operating conditions.Popular Polyethylene (PE), polypropylene (PP), polyurethane foam (PUF) and Haydite on the market were compared as the carriers for MBBRs on the conditions of 25±1℃ temperature,12h HRT and 19.0-25.5 mg/L methanol dosage. Results showed that the nitrogen and organic removal efficiency achieved by MBBR filled with PE is better than the others, and its nosZ genes community diversity and evenness is higher as well. narG and nosZ gene showed the highest abundance in PE MBBR among the MBBRs, which further explained the better performance achieved by PE MBBR. In the condition of PE as the MBBR carrier, the nitrogen and organic matter removal efficiency under 30% and 40% FR is better than that under 20% and 50%. The nosZ diversity and evenness are higher than others under 30% FR, and the narG and nosZ gene abundance increases unobviously with an increase in FR. So 30% FR is selected as the best FR.Under the conditions of 30% FR and 12h HRT, the impact of C/N ratios, i.e.4.6,6.7,8.0 and 8.4, on the nitrogen and organic removal efficiency of PE MBBR were investigated extensively. The results showed the highest denitrification rate is achieved under 4.6 C/N ratio, and the effluent COD concentration is less than 30mg/L. Under this ratio, the nosZ diversity and evenness is higher than others ratios, and the narG and nosZ gene has the highest abundance as well. Under high C/N ratio conditions (8.0 and 8.4), the reaction of dissimilatory nitrate reduction to ammonium (DNRA) may be occurred, which resulted in increased concentration of effluent ammonium and decreased removal of TN.4.6 is selected as the optimal C/N ratio.Under the conditions of PE carrier,30% FR and 4.6 C/N ratio, methanol and sodium acetate were campared as external carbon source influenced on the MBBR nitrogen and organic removal. Results showed that MBBR with methanol as carbon gets higher nitrogen and organic removal efficiency than with sodium acetate, and the MBBR with sodium acetate has far lower nosZ genes abundance than with methanol, which indicates that this denitrification process may be not complete, and it would generate more intermediate gaseous, such as NO and N2O, etc. Methanol is selected as the optimal carbon source.Under the conditions of PE carrier,30% FR,12h HRT,4.6 C/N ratio and methanol carbon source, the influence of HRT, i.e.,12h,8h and 4h, on MBBR nitrogen and organic removal performance was investigated. The results showed that HRT ranging from 4 to 12h has little effect on NO3--N removal rate. When HRT is 8h, the DOM is removed best, and narG and nosZ gene has the highest abundance for MBBR.8h is selected as the optimal HRT.Considering the nitrogen removal efficiency, organic removal efficiency and microbial characteristics comprehensively, optimum parameters of denitrification MBBR for WWTP effluent treatment are:PE carrier,30% FR,4.6 C/N, methanol as the carbon source and 8h HRT. |
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