Nitrite Accumulation And Maintenance Measures In Biofiltration Process

Water eutrophication is more and more serious in China recently, thus the nitrogen removal has become very important in wastewater treatment. Biological method is considered an economical and effective technology which plays an important role in nitrogen removal from wastewater. A lot of nitrogen has been removed from wastewater by biological process, such as conventional activated sludge process, A/O, A2/O, oxidation ditch and SBR. However, a lot of deficiencies still exist in these treatment processes, such as low removal rate of total nitrogen, high capital investment and operating cost, poor impact resistance, and so on. Therefore, it is necessary to develop some innovational and efficient process for nitrogen removal.Shortcut nitrification-denitrification has the advantages of energy consumption, carbon required for denitrification, and lower sludge production, providing a novel technical solution to remedy the deficiencies of current nitrogen removal processes. It is considered an effective and economical biological nitrogen removal process. Nitrite accumulation in high ammonia wastewater treatment has made considerable progress. However, for low ammonia municipal wastewater, it is still diffcult to achieve the nitrite accumulation quickly and easily and to maintain the nitrite accumulation for long time.In order to discuss the possibility of shortcut nitrification–denitrification in low ammonia municipal wastewater treatment, the nitrite accumulation and it's maintain control methods were investigated in Biological Aerated Filter (BAF). Two different start-up modes were adopted in the experiment. The first BAF started with condition that water temperature was 15~25℃, ammonia nitrogen concentration was 25~35 mg/L, COD concentration was 20~30 mg/L, and filter velocity was 1 m/h, while the second BAF started under the same condition but 900 mL activated sludge was added as inoculated seed. When the mature biofilm had formed, the bioreactors were turned to control operation by changing the parameters in proper sequence, wich included pH, DO, influent load and free chlorine. The aims of this research work were to find a feasible start-up mode for nitrite accumulation and investigate the influence of different operating conditions on nitrite accumulation, and furthermore, to search the tactics of achieving stable nitrite accumulation in BAF for long time operation.The result of the experiment showed that no-inoculation start-up was more favorable for nitrite accumulation than the inoculation one, obvious nitrite accumulation was observed in the BAF and the highest nitrite accumulation rate was over 80%. The inoculation start-up was more favorable for operating and up to 90% ammonia had been removed in the third day after start-up.The result of the experiment also showed that nitrite accumulation could be achieved by controlling pH, DO, influent load and free chlorine. The most appropriate pH for nitrite accumulation is 6.5 to 7. Lower DO help nitrite accumulation and significant accumulation of nitrite was appeared when DO less than 0.5 mg/L (over 80%). Increase in filtration velocity and chlorination were also conducive to the nitrite accumulation. When free chlorine concentration was kept at 4 mg/L for 20 days, nitrite accumulation rate reached 70%.To further investigate the maintenance and control measures of nitrite accumulation in BAF, operation conditions were overall optimized in the biological filter reactor according to the results of preliminary experiment. When the influent pH was 6.5 to 7, dissolved oxygen was 1~1.5 mg/L, filter velocity was 5 m/h and the water temperature was 20~30℃, nitrite accumulation achieved quickly in BAF and maintained stably at 65%~75% for 60 days.The main purpose of the paper is discussing the possibility of stable nitrite accumulation in BAF by control operating conditions and process parameters under environmental condition. The better start-up modes for nitrite accumulation in BAF were established. The characteristics of nitrite accumulation under different operating conditions and process parameters were revealed, and then the maintenance and control measures were also presented. It was found that nitrite accumulation could be achieved by control the operation and parameters in low ammonia nitrogen municipal wastewater in BAF. The results were not only enriching the theory of shortcut nitrification-denitrification, but also of great significance in shortcut nitrification-denitrification in the biological filtration process.