Study On Denitrifying Phosphorus Remo Val From Domestic Sewage In A₂N-SBMBBR

With the severe eutrophication problem occurred, sewage treatment must be taken into account not only phosphorus removal, but also nitrogen removal. The A₂N-SBMBBR system which consisted of anaerobic/anoxic sequencing batch moving bed biofilm reactor (A/A-SBMBBR) and nitrification sequencing batch moving bed biofilm reactor (N-SBMBBR) was used to probe into the performance of denitrifying phosphorus on actual domestic sewage. Experiments were carried out in the A₂N-SBMBBR system to study the removal efficiency for the low COD/TN domestic sewage as well as the key effect factors, such as the influent COD concentration, nitrate and nitrite concentrations in anoxic stages. The experimental results are shown as follows:Cultivating biofilm of denitrifying phosphorus removing bacteria (DPB) had taken three stages in A/A-SBMBBR. In the end of the first cultivating period, a number of protozoa such as Ciliata, Vorticella and Epistylis presented on the biomembrance of A/A -SBMBBR through microscopic examination, which operated in the pattern of A/O. In the end of the third cultivating period, A/A-SBMBBR was capable of removing 68.78% of the influent COD and 69.02% of TP steadily, and it also removed a large quantity of nitrate, 23.91 mg/L, in its anoxic phase. Dyeing method was used to examine endocellular polymer of bacteria like PHB and Poly-p and what had been observed could be taken as the evidence to judge the process and efficiency of biological phosphorus removal. The proportion of DPB increased approximately from 11.77% to 66.07% of total phosphate accumulating organisms (PAOs) after three phase selection. The result showed this cultivating method had good performance on the selection and dominance of DPB, and finally DPB became the preponderant flora in A/A-SBMBBR.Actual low COD/TN (average value 2.98) domestic sewage was treated in the system of A₂N-SBMBBR, which was made up of cultivation successful A/A–SBMBBR and N–SBMBBR. After 50 operational cycles, the system showed stable phosphorus and nitrogen removal performance. Average removal efficiencies of COD, TN and TP were 76.24%, 72.5% and 61.86%, respectively. By the mean of studying the transformational properties of COD, ammonia nitrogen, NO₂^--N,NO₃^--N, TN and TP in an operational cycle, it was found that COD was mainly removed in anaerobic and nitrification phases, but TN was mostly removed in nitrification and anoxic phases, and the phenomenon simultaneous nitrification and denitrification was occurred in N- SBMBBR. The experiment showed that a fraction of DPB could take up phosphorus with nitrite as the electron acceptor when the accumulative nitrite nitrogen disappearing gradually in anoxic phase. The experiments, lying in the range of 250 to 450 mg COD/L, showed that nitrogen removal rate increased as COD concentration increased, whereas the phosphate removal rate decreased. Good effects of TN and TP removal rate were gained, and the system was capable of removing approximately 75.8% of the influent TN, and 72.6% TP at an initial COD loading of 250 mg/L. The nitrogen removal rate reached maximal amount, 82%, while the removal rate of TP was even lower than 20% at influent COD of 450 mg/L. It was possible that the anoxic phosphate uptake efficiency was inhibited as the system was short of electron acceptors, NO₃^--N, and the rest of organic matter was accumulated in anoxic phase.It was found that the removal efficiency of TP was enhance as initial NO₃^--N concentration increased at nitrate additions of 20 to 45 mg N/L. The terminal TP concentration nearly decreased to zero at the nitrate addition level of 45 mg N/L, while this high initial concentration of nitrate would produce the adverse effect on the effluent concentrations of NO₃^--N and NO₂^--N. For the nitrate addition of 35 mg N/L, the system had good performance on nitrogen and phosphorus removal, and effluent concentrations of TP, NO₂^--N and NO₃^--N were 0.51, 5.93 and 4 mg/L, respectively, so it was deemed to be appropriate.It was found that nitrite at low concentration levels (about 9 to 13 mg NO₂^--N/L) were not detrimental to anoxic phosphorus uptake and could serve as electron acceptor for anoxic phosphorus uptake, and the terminal nitrite concentrations were dropped to 1.76 and 0.83 mg N/L from 16.93 and 16.33 mg N/L, respectively. Exposure to higher concentration level (roughly 20 mg NO₂^--N/L) inhibited anoxic phosphorus uptake and occurred phosphorus release, and the terminal nitrite was up to 11.32 mg N/L.