Experimental Study On Shortcut Biological Nitrogen Removal For The Treatment Of Low C/N Ratio Wastewater By Biological Aerated Filter

Traditional biological nitrogen removal processes remove ammonia from wastewater by the way of NH₄⁺→NO₂^-→NO₃^-→NO₂^-→N₂ as nitrification and denitrification. Shortcut nitrification and denitrification biological nitrogen removal refers to controlling nitrification stage to the nitrite, and then direct accesses to denitrification stage. It removes nitrogen by nitrification and denitrification via nitrite accumulation. The critical factor is to prevent the formation of nitrate.This experiment was performed on biological nitrogen removal for the treatment of low C/N ratio wastewater by biological aerated filter (BAF) to investigate the influencing factors of shortcut nitrification and denitrification. The startup operation conditions of the biological aerated filter and the realization of nitrite accumulation under the midst temperature and normal temperature respectively were inspected, and the factors such as temperature, operation mode, dissolved oxygen (DO) and free ammonia (FA) on the nitrite-nitrogen accumulation were explored. The upflow and downflow performances in the integrated biological aerated filter for ammonia removal, nitrite accumulation and total nitrogen removal were compared, and the reasons for the differences were analyzed. The experiment on shortcut denitrification in a single submersed biological filter was carried out in order to achieve the denitrification performances, operating conditions, systematic parameters. A kind of economical and suitable carbon matter– the sweet potato lixivium was found and used in the denitrification efficiently.(1) Higher temperature was helpful to the realization of nitrite-nitrogen accumulation, a steady above 90% nitrite-nitrogen accumulation rate could be achieved under the condition of continuous influent, influent NH₄⁺-N 53¹01mg/L, temperature 33℃, HRT 8h, the alternating proportion of aeration to without aeration 5﹕ 3. Simultaneously, the ammonia-nitrogen removal rate could maintain above 70%. The simultaneous nitrification and denitrification phenomenon could also be observed with the total nitrogen removal rate of 60 percent or more.(2) It was feasible to achieve nitrite-nitrogen accumulation by controlling the DO, FA, and other factors under the condition of normal temperature. When the HRT was 4h, the ratio of gas to water was 15 (when DO was 3⁴mg/L), influent ammonia-nitrogen concentration was 50mg/L⁶0mg/L, the nitrite-nitrogen accumulation rate was up to 70.02% with the ammonia-nitrogen removal rate was 90.63%. The fundamental reason of nitrite-nitrogen accumulation was that ammonia oxidation bacteria competed out nitrite oxidation bacteria during the process of adapting to temperature,DO,pH and FA. Nitrite oxidation bacteria could gradually acclimate to nitrous nitrifying conditions and refreshed, so the sludge should have the acclimation and the shortcut nitrification phenomenon disappeared.(3) The performance of the upflow system was better than the downflow system in all aspects, but it might have relation to the instabilization operation of the down flow system. The upflow system could be observed short-term nitrite-nitrogen accumulation of above 50% with HRT of 8h, consecutive aeration, the ratio of water to gas of 3, returning ratio of 1 with the ammonia-nitrogen removal rate of 50%, but only maintained six days. The shortcut nitrification in the upflow system was also the results of various factors such as low gas to water ratio and intermittent aeration. The downflow system had an average of 60% ammonia-nitrogen removal rate or more, but the nitrite-nitrogen accumulation rate was only 10.37%. The function of denitrification system did not play well; the total nitrogen removal rate was quite unsatisfactory. The average rate of total nitrogen removal of the upflow system was 33.97%, and the downflow system was only 13.59%.(4) It was showed that the sweet potato was an appropriate carbon lixivium for releasing an average of 2000mg/L COD in a static experiment. The continuous experiment indicated that it was feasible to use the sweet potato lixivium as the carbon material for denitrification as long as adjusting the way for immersion and stirringing. In this way, the total nitrogen, nitrate-nitrogen, nitrite-nitrogen and ammonia-nitrogen in the wastewater had been effectively removed with the rate of 86.06%, 88.24%, 86.83%, 62.80%, and the effluent COD concentration was maintained below 50mg/L. The ratios of nitrite-nitrogen to nitrate-nitrogen obviously affected the efficiency of TN removal under the condition of the same concentration of organic carbon.The research results indicated that, it was feasible to realize a short-term shortcut nitrification and denitrification in the biological aerated filter by controlling factors such as temperature, DO, FA and operation mode, however more investigations were needed to set up a long-term steady system for shortcut nitrification and denitrification.