Enhanced Simultaneous Phosphorus And Nitrogen Removal In Membrane Bioreactors

Nitrogen and phosphorus are key nutrients that result in water eutrophication. Denitrifying phosphorus removal has many advantages, such as saving of organics, decrease in greenhouse gas emission and less sludge production, in accord with the concept of sustainable development. So this technique has broad development prospects.In this research, sequencing batch membrane bioreactor(SBMBR) was operated in a periodic way to improve the denitrifying phosphorus removal ability by optimizing the operation modes and adjusting operation parameters. The performance of SBMBRs on nitrogen and phosphorous removal, the sludge characteristics and membrane fouling were examined in different operation modes at the same time.Firstly, the nitrogen and phosphorous removal abilities were investigated by comparing the AO mode of SBMBR with conventional membrane bioreactor(CMBR) under different nitrogen loads(COD/TN=28.2～3.4). The results indicated that the TP removal rate fluctuated between 14% and 96% in CMBR system. When the COD/TN of influent reduced to 6.3, there was almost no effect of TN removal. However, SBMBR system had a better performance on nitrogen and phosphorus removal at different COD/TN ratios. Even though the COD/TN of influent reduced to 6.3, the TN and NH₄⁺-N removals of SBMBR could be maintained over 65% and 90%, respectively. TP removal rate of SBMBR was approximately 90% during most of the experiment time, which were almost not affected by the COD/TN of influent. Furthermore, the sequencing batch operation mode can reduce membrane fouling.Anoxic phase was introduced into AO MBR to enhance its denitrifying phosphorus removal ability by adjusting the operation modes step by step. The nutrients removal performances of AOA, A²O MBR and double reactors MUCT-MBR were examined respectively and the capacities of denitrifying phosphorus removal of different processes were compared. The results showed that AOA MBR formed by introducing an anoxic phase at the end of the aerobic phase of AO MBR had definite denitrifying phosphorus removal ability without NO₃^-added externally. PHB was depleted by aerobic phosphorus uptake which resulted in the deficiency of inner carbon source in phosphorus-accumulation organisms(PAOs) during anoxic stage. So it cannot maximize improvement of the denitrifying phosphorus removal ability of the system. The ratio of phosphorus removal in anoxic phase to that in the whole cycle was only 28%. In order to solve the insufficiency mentioned above, anoxic phase was moved up to the end of the anaerobic stage of AO MBR with external nitrate addition, A²O MBR system was formed and the ratio was increased to over 50%, but the operation process synchronously became more complex. So the process was modified and then MUCT-MBR was tested. It was evident that denitrifying phosphorus removal phenomena occurred in both reactors of MUCT-MBR and the percentage was about 60%. In this system, NO₃^-produced in aerobic phase was utilized as the electron acceptor for denitrifying phosphorus uptake, which enhanced the denitrifying phosphorus removal ability without NO₃^-external addition.FISH technology was adopted to examine the ratio of PAOs in total bacteria. The results indicated that the ratios increased from AO, AOA to A²O MBR. Analysis results of phosphorus content of sludge showed that the phosphorus uptake ability improved too. The sludge yield of SBMBRs was distinctly lower than that of conventional activated sludge process(CAS), reduced with the improvement of denitrifying phosphorus removal ability. In A²O MBR, the yield was only 0.29 kgVSS·kgCOD^-1. The higher phosphorus content of sludge ensured its phosphorus removal performance.It was found that the aerobic specific phosphorus uptake rate of sludge was decreased with the increase of denitrifying phosphorus removal ability in the processes of AO, AOA, A²O MBR and MUCT-MBR. This indicated that the enhancement of anoxic phase was benefit for the growth of denitrifying PAOs rather than aerobic PAOs. The specific phosphorus uptake rate in the condition that both O₂ and NO₃^-existed was bigger than that in the condition that only O₂ existed. Aerobic denitrifying phosphorus uptake existed in the mixed electron acceptors environment. Three mechanisms might be included in the phosphorus removal process, that is, aerobic phosphorus uptake, denitrifying phosphorus uptake under anoxic condition and that under aerobic condition.