| Generally, it is unsuitable for treatment of domestic wastewater containing a low C/N ratio by conventional wastewater treatment processes due to the lack of carbon source in sewage, probably leading to not meeting the A in first-grade sewage effluent discharge standard of GB18918-2002. For this, development of an economic and environmental friendly process for treating of domestic wastewater is urgent, which may provide a benefit for development of wastewater treatment technologies and enhancement of water environmental quality. Moreover, phosphorus is a non-renewable resource and the quantities of mineral phosphorus resources are rapidly decreasing in the world due to the enlargement in the industrial and agricultural activities. Recovery of phosphorus is regarded as one of effective strategies for preventing exhaustion of phosphate rock. Domestic wastewater and its treatment processes are commonly a potential point source for phosphorus recovery, which may provide a new suggestion for development of sewage treatment process linking nutrients removal with phosphorus recovery. In recent years, theories development in denitrifying simultaneous nitrogen and phosphorus removal and induced crystallization may give a new idea and direction for exploitation of new domestic wastewater treatment process linking nutrients removal with phosphorus recovery.A novel process for nutrients biological and phosphorus recovery from domestic wastewater containing a low C/N ratio was proposed in this study, based on denitrifying simultaneous nitrogen and phosphorus removal and induced crystallization, by combining biological nutrients removal (BNR) with induced crystallization (IC), BNR-IC for short here. The advantages of the BNR-IC may display:1) denitrifying simultaneous nitrogen and phosphorus was adopted here, which may alleviate the competition of carbon sources between nitrogen and phosphorus removal; 2) separation of nitrification and phosphorus removal activated sludge can successfully solve the contradiction of sludge retention time between nitrifying bacteria and denitrifying phosphorus removal bacteria; 3) a phosphorus-rich supernatant generated by denitrifying polyphosphate accumulating organisms (DPAO) in anaerobic tank was partly introduced into IC column to recover phosphorus, which can greatly reduce the phosphorus loading in the sequential biological phosphorus removal section (i.e. anoxic tank), leading to the enhancement effect of IC on BNR in the process.The enrichment of DPAO was divided into two phases, presenting anaerobic-aerobic and anaerobic-anoxic, in this study. The phosphorus (P) release and uptake capacities of activated sludge were assessed after completing the DPAO cultivation, where the sludge anaerobic P release and anoxic P uptake per MLSS were 8.47 mg P/g MLSS and 11.13 mg P/g MLSS, respectively and the nitrogen removal per phosphorus uptake was 1.08 mg N/mg P, displaying the obvious characterization of denitrifying simultaneous nitrogen and phosphorus removal. Fluorescence in situ hybridization (FISH) analysis showed that Accumulibacter population significantly increased before and after enrichment, from 9.3% of all bacteria in seed sludge to 68.9% in anoxic tank sludge and rod morphology of Accumulibacter were observed in anoxic sludge by using scanning electron micrograph (SEM). Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis showed that noticeable shift in the bacterial community structure was observed before and after the cultivation of DPAO, presenting an obvious decrease in microbial diversity.The decay characteristics of nitrification sludge and denitrifying phosphorus removal sludge in the BNR-IC process were investigated here and it was found that the decay rate of DPAO is 0.15 d-1 in the first day and 0.04-0.05 d-1 in the rest six days, suggesting that DPAO can resist to hunger by oxidation of PHB in cell for the supply of energy.In nitrification sludge, the averaged decay rate of nitrite oxidizing bacteria (NOB) for 7 days was 0.41 d-1, showing an exponential relationship between oxygen uptake rate (OUR) and time and that of ammonia oxidizing bacteria (AOB) was 0.13 d-1:, displaying a linear relationship between OUR and time.Comparison of DPAO with PAO, it was found that when DPAO is exposed to aerobic conditions, it can immediately exhibit a good phosphorus removal similar to that under anoxic conditions, while PAO can only present poor phosphorus removal when exposed to anoxic conditions; the P uptake capacity by DPAO in anoxic was slightly less than that in aerobic, also demonstrating that Accumulibacter has, at least, two different types:one is capable of not only aerobic phosphorus uptake by using oxygen as the electron acceptor, but also anoxic phosphorus uptake by using nitrate, namely DPAO, and the other only using oxygen instead of nitrate as the electron acceptor for phosphorus removal. According to the PCR-DGGE analysis, shift in microbial community occurred greatly depending on the electron acceptors in this study and Azonexus.spp、Acidovorax、Rhodocyclus and Thiothrix.spp were found in both aerobic P removal sludge and anoxic P removal sludge, probably belonging to DPAO.According to the chemical and PCR-DGGE analysis, C/N and C/P ratios in the influent had an important effect on nitrogen and phosphorus removal efficiencies and bacterial community structure. Increasing the influent C/N and C/P ratios can enhance the nitrogen and phosphorus removal efficiencies and decrease the bacterial diversity of activated sludge, contributing to the growth of DPAO and elimination of other microorganisms. Results obtained here showed a good nutrients removal performance was achieved at 6.3 of C/N ratio and 23.3 of C/P ratio, where P and TN removal efficiencies were 94.0±2.4% and 90.5±4.3%, respectively.Phosphorus from wastewater can be effectively recovered using the induced HAP crystallization, where the reaction belongs to pseudo first order reaction with the activation energy of 5.96 KJ/mol, indicating fast reaction rate as well as less influence of temperature, thus enhancing the practicability of the method. The induced crystallization reaction was obviously affected by the influent pH and a high P recovery rate is found when given pH of above 8.5 and even ranging 8.5-10, which can be easily reached by spraying air into the IC column.The effect of side stream ratio (SSR) on the nitrogen and phosphorus removal and phosphorus recovery efficiencies in the BNR-IC process were assessed in this study to investigate the optimum removal and recovery performance. It was found that SSR had an important influence on the nitrogen and phosphorus removal, phosphorus recovery and microbial community structure as well and side stream P recovery obviously enhanced the subsequent biological P removal as a result of a decrease in loading of biological P removal. Results obtained here showed that a good P removal and recovery performance was achieved when SSR was 35%and the P concentration in the effluent did not meet the discharge standard of GB 18918-2002 when SSR was 15% and 25%. However, excessive high SSR (more than 35%) would have a negative effect on the subsequent biological P removal in the BNR-IC system. Therefore, it is important to note that the mechanism of biological phosphorus removal should not be damaged when selecting the SSR in practice. Moreover, PCR-DGGE analysis showed an increase in SSR can cause an increase in microbial community biodiversity.Effectiveness of the BNR-IC process by monitoring carbon, nitrogen and phosphorus removal performances was assessed here and the crystallization product was analyzed by using SEM and EDS. When the COD, TN, NH4+-N and P concentrations of 239.2-259.5 mg/L, 39.6～43.8 mg/L,38.2～41.8 mg/L and 8.72～11.40 mg/L, respectively, belonging to low C/N sewage, were given in the influent, those in the effluent were 15.2～21.6 mg/L,8.5～9.6 mg/L, 3.6～4.7 mg/L and 0.31-0.49 mg/L, meeting the A in first-grade sewage effluent discharge standard of GB18918-2002. The COD removal efficiency in anaerobic tank was significantly higher than that in other tanks and NH4+-N removal mainly occurred in the aerobic tank. NO3--N concentration in the effluent was more than that in the influent, presenting 4.87 mg/L and 0.58 mg/L, respectively. In the BNR-IC system, phosphorus in the influent was removed mainly by DPAO and induced HAP crystallization, where the phosphorus total removal efficiency was 95.9% obtained here and 71.5% of that was completed in the IC column, suggesting the advantages of the BNR-IC in the phosphorus recovery. Analyses of SEM and EDS, moreover, also demonstrated that the surface of seed crystal (calcite used here) was completely covered by hydroxyl calcium phosphate (HAP) produced during the induced crystallization process to recover phosphorus. Moreover, the post-aeration tank can remove any surplus carbon, nitrogen and phosphorus, thus contributing to enhance the water quality of effluent. |
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