Research On The Chemical-aid Phosphorus Removal In Biological Nutrient Removal Technology

Eutrophication is becoming a severe problem in China, which seriously threatens the safety of drinking water. In order to meet the requirement of sewage discharge standard, chemicals are used to reinforce the phosphorus removal effect, which will gradually be applied in real wastewater treatment plants. However, information on the chemical-aid phosphorus removal process is still scarce. It is necessary to conduct a comprehensive study about this process.In this research, the phosphorus removal effect was investigated with real municipal wastewater using 5 kinds of chemicals:FeCl3, FeSO4, AlCl3, PAC and PSF. The optimal chemical will be choosed to be added to inverted A2/O reactors for pilot test, through which the optimal location and dosage were determined. The mechanism of the chemical-assistanted process was also explored. In addditon, the effect of chemical on sludge digestion was investigated by sludge digestion trial.The results of chemical optimization show that iron series have better phosphorus removal effect than aluminum series, and ferrous sulfate is the optimum chemical to remove phosphorus. When the molar ration of Fe/TP is 1.2, the TP content can reach 0.48mg/L, which can reach the A-level discharge standard (GB18918-2002); When the molar ration of Fe/TP is 1.6, the removal rate of PO43-can reach as high as 96.37%,when TP content reduced to 0.38mg/L;the TP removal efficiency of the chemical-aid process is improved mainly through improving the removal rate of PO43-. Taking the economy and available indicators into account, the optimum chemical to remove phosphorus is ferrous sulfate.The results of location optimization show that the end of aeration tank is the optimal location. When FeSO4 is added with the Fe/TP molar ration of 1.6:1,the removal rate of TP and PO43-is respectively 4.19% and 3.86% higher than that added in the end of anaerobic, however, the removal rate of CODcr,TN,NH3-N and SS have less different when added in different locations.The results of dosage optimization show that when Fe/TPis less than 6, the removal rate of TP is increased along with the dosage increased,from76.07% to 90.06%;When the molar ration of Fe/TP is greater than or equal to 1.6, the TP removal rate stabilized at about 90%,and the effluent TP is below 0.5mg/L,which can meet the A-level discharge standard for municipal wastewater treatment plant (GB18918-2002).When the molar ration of Fe/TP is 1.6, the removal rate of CODcr, NH3-N,TN and SS is75.54%,93.48%,34.25% and 92.19% respectively, which can reach the A-level discharge standard without TN;When the molar ration of Fe/TP is greater than 1.6 the removal rate of CODcr, NH3-N,TN and SS have not increased. The effluent total iron is increased along with the FeSO4 dosage increased. When the molar ration of Fe/TP is below to 1.6, the effluent total iron can reach the surface water quality standards (GB 3838—2002).Taking the removal effect of phosphorus and other water quality indicators into account, the optimum dosage is that the molar ration of Fe/TP is equal to 1.6.The results also show that the removal effect of phosphorus is obvious when FeSO4 is added. When the molar ration of Fe/TP is 1.6, the effluent TP can reduced to 0.46mg/L, which can reach the A-level discharge standard, however, the effluent TP in pure biological process is 1.02mg/L;The iron salt content decreased gradually along the process, and pure biological process has the lower iron salt content. When the effluent TP reach the discharge requirements, the effluent total iron content in chemical-aid phosphorus removal process is 0.27mg/L,which can reach the surface water quality standards (GB 3838-2002).The results of sludge digestion show that aerobic digestion has better effect on quantitative reduction than that in anaerobic digestion. The sludge in pure biological phosphorus removal process has better effect on quantitative reduction than that in the other process. The phosphorus release phenomenon occurred in the processes of two kinds sludge digestion, and the sludge in pure biological phosphorus removal process has greater phosphorus release, and it's supernatant can contain TP 3.13mg/L in anaerobic digest; When in anaerobic digest, it's supernatant can contain total ferric 5.97mg/L.