Study On The Hybrid Process Of Submerged Membrane Bioreactor For Treating Polluted Source Water

Many of the drinking water sources in China have been contaminated, with organics and ammonia as the main pollutants. On the other hand, the drinking water standards ever increases, which lead to the development of new water treatment methods with higher capacity than conventional processes. Among these advanced methods, membrane filtration technology exhibits many advantages, and been recognized as the water treatment technology of 21st century. The submerged membrane bioreactor (SMBR), i.e. the combination of membrane separation and activated sludge process, has been widely researched and applied to wastewater treatment on full-scale. However, in drinking water treatment, SMBR is a relatively new technogy, and there still are some key problems remained to be solved due to the difference of raw water qualities between wastewater and drinking water.In this paper, the natural start-up of SMBR for treating contaminated raw water was firstly investigated. Results showed that the natural start-up of the SMBR could be accomplished within about 35 days. During the long-term operation, the SMBR exhibited excellent ammonia removal efficiencies through biological nitrification, and could cope with the sudden high ammonia loads in drinking water source. However, the removal capacity of the SMBR for dissolved organic matter (DOM) is low. The resason might be that natural organic matter in drinking water source is bio-refractory in nature as a whole.Biological activated carbon (BAC) is another advanced technology for drinking water treatment. The field study showed that the influent organic content and carbon size imposed main influence on the BAC for pollutants removal; while the influence of the filtration rate, pre-aeration and backwashing is low. It was also discovered that influent orginics and ammonia were preferentially removed in the upper layer of the BAC. By comparison of SMBR and BAC under the same experimental conditions, it was found that the capacity of SMBR for organics removal is lower than that of BAC, with the main mechanism of biodegradation; while higher removal efficiency of organic matter was achieved by BAC, through the synergetic effect of adsorption and biodegradation. However, particulate organic matter was detected in BAC effluent; and the ammonia removal efficiency by BAC was proved to be lower than that by SMBR.To take the advantages of BAC for organics removal and SMBR for ammonia removal simutaneously, the hybrid process of BAC and SMBR was investigated for the drinking water treatment from polluted surface water. Rsults showed that the pre-treatment by BAC was able to remove a certain amount of pollutans, thus decrease the load of the SMBR and alleviate membrane fouling. On the other hand, the SMBR treatment could further eliminate organics and ammonia in the BAC effluent. Moreover, the membrane in the SMBR served as the final barrier, which could separate the particles almost completely. However, the hydraulic retention time and the corresponding foot-print would be increased when the BAC and SMBR were combined. Therefore, in the study, the BAC was substituted with powdered activated carbon (PAC). PAC was directly dosed into the SMBR, and the membrane adsorption bioreactor (MABR) was constructed. In the MABR, separation by the membrane, biodegradation by the microorganisms, and adsorption by PAC collectively contributed to the removal of organic matter. The MABR was demonstrated to be effective for drinking water treatment; and the HRT was substantially decreased. In the MABR, the PAC could also worked as the surpport for bacterial growth, thus enhance the capacity of the MABR for coping with impact load. MABR might be a promising technology for drinking water treatment.It was attempted to dose inorganic coagulant into the SMBR directly, i.e. the membrane coagulation (MCBR) was constructed. The investigation showed that when compared with conventional SMBR, the MCBR was not only able to reduce much more influent organic matter, but also able to eliminate almost all of influent phosphate, thus improve the biostability of the finished water. On the other hand, the ammonia removal efficiency of the MCBR reached more than 96%, indicating that direct addition of coagulant into SMBR would not adversely affect the microbial community in the bioreactor. Therefore, polyaluminium chloride (PACl) and PAC was dosed into the SMBR simutaneously in the experiments, and the integrative process-membrane coagualtion adsorption bioreactor (MCABR) was established. In the MCABR, four unit effects were identified to contribute to the water purification, i.e. separation by membrane, biodegradation by microorganisms, coagulation by PACl, and adsorption by PAC. As a result, influent organic matter was removed by about 70%. A sludge layer was found on the membrane surface in MACBR through SEM observation; energy diffusive X-ray analysis demonstrated that there was 10.4% of Al element in the sludge layer; CLSM observation showed that polysaccharides was extensively distributed on the membrane surface. Thus, it was inferred that Al hydrolysis product and polysaccharides were combined and form the net structure on the membrane surface, which was able to enhance the membrane for the rejection of DOM in the mixed liquor, especially the organic molecules of 300~3000 Da. Furthermore, the PAC layer with high density formed on the membrane surface during suction could also help to reject the DOM in the mixed liquor. Pilot study was conducted to investigate the operational characteristics of immersed hollow-fiber membrane for ultrafiltraion of polluted surface water. the UF membrane exibited excellent capacity for turbidity separation. However, the UF membrane was not good at removing organics, especially DOM. When the flux increased, the accumulation of pollutants in the membrane tank became more serious correspondingly. Therefore, the flux exerted important influence on the membrane fouling. Air bubbling could remove the sludge layer on the membrane surface, thus mitigate membrane fouling to some extent. Experiments showed that continuous air bubbling was more effective for alleviating membrane fouing under the same air flowrate. On the other hand, the higher the air flowrate was, the slower the TMP development rate would be. However, the energy depletion and operating cost should be taken into account to determine the optimum air flowrate. For the immersed membrane with the conventional configuration, small air bubble was demonstrated to be more effective for alleviating membrane fouling. Bachwashing could remove the surface foulants and in-pore foulants on the membrane simutaneously. Thus, higher cleaning effectiveness could be achieved by bachwashing. Therefore, combination of air bubbling and backwashing should be optimized in practical uses. During the long-term operation of membrane, irreversible fouling would be formed on the membrane inevitably. This paper also investigated effectiveness and mechanism of chemical cleaning of hollow-fiber PVC membrane by the combination of NaOH and ethanol for ultrafiltration surface water was discussed in the paper. SEM and AFM analyses indicated that NaOH and ethanol were able to synergetically remove the surface and in-pore fouing of the PVC membrane, thus exhibited excellent cleaning effectiveness.