Research On Nitrogen Removal Efficiency Of Three-Phase Biological Fluidized Bed With Four Step-Feed Process

Step-feed nitrogen removal process, as a new sewage treatment technology, was concerned by scholars domestically and abroad in recent years. The idea of step-feed nitrogen removal process combined with biological fluidized bed was introduced in this study. In fact, there were many advantages, such as high nitrogen removal efficiency, saving the return facility, no external carbon source, improving the system resistance and increasing the concentration of activated sludge. The nitrogen removal mechanism and nitrogen removal efficiency of fluidized bed step feed process were studied in a deep and scientific way. This process could be used in the reconstruction project of original wastewater treatment plant and construction of new plant because of its high nitrogen removal performance and simple and flexible operation.The removal efficiency of three-phase biological fluidized bed step-feed process was studied. This process had good removal ability of nitrogen and organism. As a result, the experiment result showed that when the influent water quality and the operating conditions were equivalent, the denitrification efficiency of the three-phase biological fluidized bed with four step-feed process was higher than the fluidized bed A/O process. When the influent proportion of each segment was equal, the system showed a well nitrogen removal efficiency with the change of influent C/N ratio, HRT, carbon type and sludge return ratio. The removal rate of TN reached up to 88.9%. It showed that the simultaneous nitrification and denitrification phenomenon happened in the aerobic zone. The nitrogen removal mechanism was studied from the way of influent and biofilm.The influence of flow distribution on nitrogen removal performance was studied. Through theoretical derivation, the relationship of C/N radio and the best influent split ratio was concluded. When the C/N rate was 4, 6, 8, 10 and 12, the best influent split ratio was 0.59, 0.88, 1.18, 1.47 and 1.76. The theoretical nitrogen removal rate under best influent split ratio was calculated. The advantages of best influent split ratio was analyzed from the theoretical and experimental point of view. The influence of influent split ratio on nitrogen removal, nitrification, denitrification and nitrification rate of aerobic zone was verified. Under different influent split ratio, ammonia nitrogen removal rate could reach to 93.4% or more. With the influent split ratio increased, the TN removal rate also increased, up to 92.6%. Under different influent split ratios, the nitrification rate was lower along the trend, and the greater the influent split ratio was, the faster the nitrification rate decreased. Under low influent split ratio, denitrification would be affected by carbon source. On the other hand, under influent split ratio, the effluent nitrate-nitrogen was mainly from the last aerobic zone. Therefore, the application of higher influent split ratio helped to improve the denitrification efficiency. Influence factors such as volume ratio, sludge return ratio, DO concentration and carriers filling rate were also investigated. The results showed that the flow distribution played a significant role in nitrogen removal performance. When the best influent split ratio was adopted, denitrification could be enhanced and TN removal efficiency could be improved by maximizing the share volume of anoxic zone. Sludge return ratio was not significant for the nitrogen removal performance of the system. Based on the perspective of low-carbon energy, 50% was appropriate. DO concentration in aerobic zone was controlled in 1.5～2.0mg/L to avoid denitrification environment being destroyed. Take the operating results and one-time investment cost into consideration, 30% of the filling ratio was more appropriate.By using microscopy and SEM technology, Microbial composition and morphology of activated sludge and biofilm during start-up stage and formal running stage. Microbial characteristic and mechanism of microbial immobilization were analyzed. The micro-organisms of the process showed good treatment effect and strong resistance to shock load. The sludge production was in the range of 0.136～0.311kgMLSS/kgCOD, that was lower than which of conventional activated sludge process.Finally, based on the classic dynamic model of activated sludge, substrate hydrolysis kinetics, nitrification kinetics and denitrification kinetics of three-phase biological fluidized bed step-feed process were studied. Dynamic model of SND was established. Attachment microorganism retention time was proposed and its expression was derived.Based on the above results, three-phase biological fluidized bed step-feed technology can be applied to actual projects. System operational parameters was adjusted to achieve the best working conditions. Under the condition of minimum cost, make sure the effluent quality meet the national emission standards.