| N2O is an important greenhouse gas, with300times greenhouse effect than that of CO2. It has been confirmed that biological nitrogen removal (BNR) is one of the important ways of N2O generation. While some novel BNR processes which have economic advantages may produce more N2O than conventional BNR processes. In this paper, a new kind of aerobic short-cut nitrification granular sludge-step-fed SBR process (SFSBR) was put for ward, aiming at mass generation and emission of N2O in Nitrosofication BNR process during high ammonia nitrogen wastewater treatment.The experiments were conducted with nitrosated activated sludge as seed sludge and low carbon, high ammonia nitrogen wastewater as matrix, running conventional SBR (R1) and SFSBR (R2) at the same time. The following questions were explored:(a) the feasibility of aerobic granular sludge nitrification in SFSBR;(b) physical and chemical characteristics of granules in different modes;(c) pollutants removal efficiency, degradation pattern and kinetics;(d) N2O production situation and its main sources combined with chemical inhibition batch tests, to clarify the critical conditions for the stable operation of SFSBR, nitrosation and N2O reduction. It is of great significance for better practical application of the new technology and to achieve energy conservation. The results show that:(1) SFSBR has advantages on granulars cultivation. Small particles appeared after the reactor started up about33days in R1and50days in R2. Original granules in R1were better than that in R2, but with the changed conditions such as the shortened settling time and the improvement of aeration rate, granulation in R2was improved gradually and very stable, better than which in R1. The particle size in the R1was smaller than in R2, with average particle size of about1mm, while2～3mm in R2. Granules cultivated in R2have clear boundary, regular shape and dense structure.(2) The aerobic granules in SFSBR can achieve nitrosation, and good nitrogen removal performance. The maximum removal effiency of TN in Rland R2could reach75%and95%respectively. Under the same influent NH4+-N concentration, ammonia nitrogen degradation rate during complete nitrification status was slightly higher than during short-cut nitrification both in R1and R2, while the nitrite conversion rate had obvious differences. It noted that in short-cut condition, the activity of NOB was not as good as in nitrification state. Nitrogen removal was usually affected when adjusting the operating conditions, but quickly recovered. Only after sludge storage, recovery was slower, especially in R2, which may be associated with the frequent anoxic-aerobic alternating, that acting as environmental disturbances.(3) R2has better denitrification performance than R1before the storage of granules. During the anoxic phase, TN removal rate was7mg/(gSS·h) or more in R2, but maximum7mg/(gSS·h) in R1. After storage, although the worse recovery able of R2than R1, it had good nitrification and denitrification performance after recovery, and was able to maintain partial nitrification of a long period of time. The gradual increase of the influent ammonium concentration is an effective way to maintain stabile partial nitrification.(4) N2O emission occurred mainly during the aerobic phase, and during the anoxic phase the release quantity was few. Dissolved N2O was in low proportion, and just temporarily appeared in the reactor mixture. Its final form was releasing gas or was degradated though nitrification and denitrification, which could be ignored for having no effect on net production of N2O. N2O release during partial nitrification is higher than the complete nitrification period,2.09and0.89mg/(gMLSS·cycle) in R1,0.12and0.10mg/(gMLSS·cycle) in R2, respectively. It is obvious that the N2O emission level was significantly lower in R2than in R1, N2O-N/TN removal ratios were3～10times difference. These all show that SFSBR does have the potention on N2O reduction.(5) According to the results analysis of chemical inhibition tests, the main source ofN2O production is simultaneous nitrification and denitrification. It is closely related with the special hierarchical structure of microbial community in aerobic granular sludge. AOB denitrification is another important source. From the FISH results, it can be seen that AOB accounted for a considerable proportion of total bacteria, and it had more advantages in nitrite granules.All have confirmed that the step-feed-aerobic granules technology is good to reduce N2O production, and nitrosated aerobic granular sludge with good nitrogen removal efficiency can also be cultivated in the SFSBR. This is of great significance to strengthen advantages of the the nitrosated process which can save the carbon source and aeration rate when dealing with the high strength ammonia wastewater, and make up for the shortcoming of the high yield of N2O. |
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