Study On Nitrogen Removal Performance Of An Incorporate Airlift Inner Circular Aerobic Anoxic Biofilm Reactor

For conventional nitrogen removal processes, nitrification and denitrification are carried out in separated tanks or in a single reactor where aerobic and anoxic conditions are formed sequentially. Such processes need power pump to realize the recirculation of nitrified wastewater that is needed in nitrogen removal. The investment and operation cost is high, and the land occupied is large. As to these problems, a novel incorporate airlift inner circular biofilm reactor is designed and fabricated which is applied as a single treatment unit for carbon and nitrogen removal of domestic wastewater. Nitrification and denitrification are accomplished respectively in different zones of the reactor. Aeration not only supplies dissolved oxygen needed in nitrification but also impulses the recirculation of nitrified wastewater, which economizes energy needed in nitrogen removal.This paper studied the influence of different configurations of the reactor on nitrogen removal firstly. According to the above results a preferable two zones configuration reactor was selected. Secondly the influence of operation temperature,aeration rate and influent COD/TN on nitrogen removal was studied. The feasibility of nitrogen removal via short-cut nitrification and denitrification principle in this reactor was studied. The nitrogen removal mechanism of the reactor was analysed lastly. The main content and results are as follows.(1) Nitrogen removal efficiencies of the reactor under different configuration forms were studied and compared firstly in this paper. The results demonstrate that DO distributing is different in level direction in the three zones reactor and in the two zones reactor that makes nitrification and denitrification carrying out in one reactor simultaneously. TN removal efficiency of the two zones reactor is higher than that of the three zones reactor at a certain operational conditions because of the increasing nitrifying zone volume. 0.080 kgTN/(m³Â·d) of TN removal loading and 0.53 kgCOD/(m³Â·d) of COD removal loading are attained in the two zones reactor treating simulated domestic wastewater. Fluorescence in situ hybridization analysis indicates that ammonia oxidizing bacteria percentage of buffering sludge increases which is the main reason for higher nitrifying efficiency of the two zones reactor. The above experiment results show that nitrogen removal performance of the two zones reactor is better than that of the three zones reactor. Therefore two zones configuration form is the preferable form of the reactor. (2) The influence of influent COD/TN and aeration rate on nitrogen removal was studied. Denitrification is the restrictive process and nitrogen removal loading is 0.052 kgTN/(m³Â·d) under lower COD/TN (2.0). When the influent COD/TN is 3.5 0.069 kgTN/(m³Â·d) removal loading is achieved through adjusting aeration rate. Under a higher influent COD/TN 6.2, TN removal loading is 0.080 kgTN/(m³Â·d). Under a definite influent COD/TN the increasing aeration rate not only results in the increasing of DO concentration in anoxic zone that is adverse for denitrification but also accelerates the recirculation of nitrified wastewater that is favorable for more NO₃^--N to accomplish denitrification. When aeration rate increases from 850 L/h to1100 L/h, the total influence is not high and positive and negative effect keep balanced. When aeration rate increases to a definite quantity (1400 L/h), denitrification and nitrogen removal are both influenced adversely.(3) The baffle height between buffering zone and anoxic zone determines circulation area of nitrified wastewater. The circulation area influences the circulation quantity of nitrified wastewater and DO concentration of anoxic zone. So the influence of the circulation area between buffering zone and anoxic zone on nitrogen removal was researched. The circulation quantity of nitrified wastewater increases through increasing the circulation area under a certain aeration rate, which is favorable for nitrogen removal. But DO concentration of anoxic zone may increase which is disadvantage for denitrification and nitrogen removal. Under a definite aeration rate (850 L/h) when the circulation area increases from 150cm² to 300 cm², denitrification is accelerated, TN removal loading increases from 0.059 to 0.065kgTN/(m³Â·d). But when the aeration rate is higher (1100 L/h) the increasing of the circulation area makes DO concentration of anoxic zone increasing that is adverse for denitrification and nitrogen removal.(4) The united influence of temperature and organic carbon on nitrogen removal performance of the reactor was researched. Under a definite influent organic loading NH₄⁺-N and TN removal efficiencies increase with operational temperature increasing. Under a definite operational temperature nitrification efficiency decreases and denitrification efficiency increases with influent organic loading increasing. At a lower temperture (8℃), 95% NH₄⁺-N removal efficiency and 0.086 kgNH₄⁺-N/(m³Â·d) removal loading are attained with no organic carbon in influent. But TN removal efficiency is low which was only 10% because of no organic carbon in influent used for denitrification.(5) Nitrogen removal via Short-cut nitrification denitrification in this reactor was studied. Short-cut nitrification is realized through adjusting influent alkalinity at lower temperature (8 ℃) when the influent TN is 68⁸0 mg/L in this reactor. 80% of nitrite accumulating rate is attained. The average NH₄⁺-N removal efficiency is 88% and removal loading is 0.075 kgNH₄⁺-N/(m³Â·d). Fluorescence in situ hybridization of nitrite oxidizing bacteria indicates that the activity of nitrite oxidizing bacteria is restrained and the percentage of nitrite oxidizing bacteria does not decreased a lot when short-cut nitrification carrying out in the reactor. When the influent TN is 114-146 mg/L nitrogen removal via short-cut nitrification denitrification is realized through adjusting influent alkalinity and aeration rate at 29℃in this reactor. NO₂^--N/NO_x^--N of the aerobic and buffering zone are 77% and 79% respectively. TN removal efficiency is 72% and removal loading is 0.130 kgTN/(m³Â·d). Fluorescence in situ hybridization analysis indicates that the reason of short-cut nitrification denitrification carrying out in the reactor at 29℃is the decreasing of nitrite oxidizing bacteria percentage resulted from the restrained activity of nitrite oxidizing bacteria by free ammonia for a long time.(6) The nitrogen removal mechanism of the reactor was studied through measuring various forms nitrogen concentrations and the nitrification and denitrification activities of sludge from different zones. Nitrification prevails in the aerobic zone of the reactor, which results in lower concentration of NH₄⁺-N and COD and higher concentration of NO₃^--N in this zone. On the contrary, denitrification prevails in the anoxic zone, which results in higher concentration of NH₄⁺-N and lower concentration of NO₃^--N in this zone. Sludges from different zones of the reactor have both the ability of nitrifying and that of denitrifying. The nitrification activity of sludge from aerobic zone is higher and the denitrification activity of sludge from anoxic zone is higher. Nitrogen removal process of the reactor is realized by the nitrification and denitrification that predominated in different zones of the reactor and simultaneous nitrification and denitrification that occur in each zone resulted from the disproportion of wastewater and DO.