The Study On The Realization And Performance Of Denitrifying Phosphorus Removal In A~2/O Process

In order to accelerate the investigation development of A₂/O process, in particular to accelerate the application of A₂/O process for the low C/N wastewater treatment, the continuous model dynamic experiment operated with artificial municipal wastewater was carried out. The feasibility of introducing denitrifying phosphorus removal to A₂/O process and the effect of different operational factors and influent characteristics on the A₂/O system were studied systematically for the first time,simultaneously the variation of pH and ORP in this system were systematically investigated,and further to verify the feasibility of ORP as a process control parameter. Based on this, the feasibility of introducing shortcut nitrification to A₂/O process where denitrifying phosphorus removal existed by low DO control method, and the effect of nitrite on denitrifying phosphorus removal was investigated simultaneously.After stable operation of denitrifying phosphorous removal in A₂/O process, firstly, the effect of influent COD/TN on the nitrogen and phosphorus removal was discussed. The results showed that the A₂/O denitrifying phosphorus removal process has potential for the domestic wastewater treatment with a low influent COD/TN ratio. The COD/TN of the municipal wastewater ranged among 4⁷ in common, so the A₂/O denitrifying phosphorus removal process has a good prospect for the municipal wastewater treatment.On the other hand, it can be found optimizing anoxic biological phosphorus removal in the A₂/O system was dependent on some parameters: COD/TN ratio, sludge retention time (SRT), MLSS, sludge return ratio and internal return ratio. The influent COD/TN ratio around 6.45, the SRT of 15 days , the sludge return ratio of 60%, the internal return ratio of 300%, MLSS around 3000³800mg/L (MLSSanaerobic:MLSSanoxic:MLSSaeration≈1:1:1),were determined as optimal for the phosphorus and nitrogen removal in the system. Based on results of experimental research and analysis, the dynamics models of denitrifying phosphorus removal in A₂/O process were established.Furthermore, the influence of volume ratio of anoxic zone to aerobic zone on denitrifying phosphorus removal in A₂/O process was investigated. The results showed when the volume ratio of anoxic zone to aerobic zone increased, the effluent quality of the system got better. However, when the volume ratio of anoxic zone to aerobic zone increased, internal recirculation ratio shall be adjusted, otherwise the nitrate concentration would not be enough in anoxic zone, and phosphorus would be released again.It was found that absolute value of ORP was somehow related with the phosphorus release and uptake. With online monitoring ORP variation, the degree of the phosphorus release and anoxic phosphorus uptake can be indicated correctly. Furthermore the nitrate concentration of the final lattice of anoxic zone was somehow related with the ORP of this lattice, the higher the nitrate concentration, the higher the ORP, therefore it was possible to estimate whether the internal recirculation ratio was enough or not dynamically with online monitoring ORP variation, consequently the judgement—whether to increase internal recirculation ratio or not, could be made. So, the control of ORP combined with DO will stabilize the operation of A₂/O system. The measured pH can reflect the reaction state, but it is not considered as a control parameter in the A₂/O system.The shortcut nitrification was successfully achieved by controlling DO among 0.5～0.7mg/L in A₂/O process, NO₂^-/(NO₂^-+ NO₃^-) went up to 89.8%, the effluent quality met the national wastewater discharge standard CLASSâ… . The long term results showed that the denitrifying phosphorus removal could be remained and good phosphorus removal efficiency could be achieved when the shortcut nitrification existed in A₂/O system, but the percentage of denitrifying phosphorus removal in total phosphorus removal reduced to 44%. The study on the effect of nitrite on phosphorus removal showed that the nitrite concentration involved in this research (<5mg/L) had no effect on denitrifying phosphorus removal. It has been approved that nitrite can be used as the electron acceptor for denitrifying phosphorus uptake, the nitrite concentration involved in this research had effect on aerobic phosphorus uptake rate, it had inhibitory effect on aerobic phosphorus uptake, and it was analyzed that nitrite had harmful effect on PAO.