Study On The Performance And Processes Of Simultaneously Nitrogen And Phosphorus Removal By Aerobic Granular Sludge At Low Temperature

The problem of water eutrophication increasingly outstanding puts forward urgent task for controlling N and P discharge. There are many problems such as the worse sludge performance and treatment effect at low temperature. In addition, sludge bulking affects the stable operation of the system to some extant. The aerobic granular sludge process, which is considered as a special biological membrane structure, has received much attention due to the unique merits of aerobic granular sludge. However, the quickly start-up and stable operation of aerobic granular sludge reactor, especially at low temperature, is still the key problem that restricts its engineering application. In this work, start-up process of aerobic granular sludge at low temperature was investigated, and simultaneous nitrogen and phosphorus removal processes, the operating stability of aerobic granular reactor at low temperature were studied systematically.The start-up properties of aerobic granular sludge reactor at low temperature were studied, in which the mixture of acetate sodium and glucose served as carbon source. The granules were obtained at 10±1℃. The obtained granules exhibited clear and slippy surface, compact structure and better sedimentation feature. In the first 35 days during the start-up stage, nitrosation activity increased and nitrification activity decreased with the effluent nitrosation ratio of 46%. The results of PCR-DGGE and microbial community ecological succession showed that the bacteria genus which had the function of PHB synthesization and denitrification. This provided foundation of the simultaneously nitrification-denitrification and denitrifying phosphorus removal inside aerobic granules at low temperature. Effect of temperature on the removal properties of ammonia and phosphorus were also studied. The results showed that the stability of granules was affected when the operating temperature increased from 20±1℃to 26±1℃. The nitrite of effluent was accumulated as the temperature increased and its nitrosation ratio was 93.9%. As a result of increased temperature (△T>6℃), the granules grandually disintegrated during the operation period of 47d after the change of temperature.The operating parameters optimization of aerobic granular SBAR at low temperature was investigated. It was found that the effluent ammonia concentration was higher when COD/N/P was 100/5/1, and the removal efficiency of ammonia was not improved obviously only by extending the cycle time. A decline of COD/N ratio was benefit to the accumulation of nitrosation bacterial inside the granular sludge at low temperature. Furthermore, it took full advantage of higher MLSS in the reactor to achieve better nitrification effect at low temperature. When the influent concentration of COD decreased from 1120mg/L to 560mg/L, the diameter of stable granules decreased from 3.4mm to 2.6mm, and MLSS also reduced from 11.0g/L to 8.0 g/L. The aeration rate is the key parameter in controlling the ratio of anaerobic and aerobic area inside granules to achieve simultaneous nitrification and denitrification, and remarkably affects the stability operation of granular reactor. In the case of Vobs lower than 0.58cm/s, the effect of N and P removal by the granular sludge reactor was better. But the excessive propagation of filamentous organisms and then decreased operating stability of granular sludge reactor. In the case of Vobs higher than 0.58cm/s, the dissolve oxygen concentration in the mixture approached to saturation. And anaerobic area inside granule was compressed and denitrification effect was restrained and dynamic consumption increased as the aeration rate increased. It behaved better simultaneous N and P removal effect in the parameters optimized single grade reactor of aerobic granule sludge and the removal coefficiency for COD,NH4+-N and PO43--P were 89.5%,98.9% and 95.5%, respectively. Meanwhile, the reactor exhibited better performance of simultaneous nitrification and denitrification at 10±1℃. The experimental data showed that nitrite and nitrate concentration in effluent were 3.2mg/L and lower than 0.5mg/L, the nitrification and denitrification efficecy was 83.3%.The denitrifying phosphorus removal proformance and processes were studied through static experiments at 10±1℃. The results showed that the PHAs components synthesized by phosphorus accumulating organisms (PAOs) in granule sludge were affected by different types of carbon source in anaerobic period, and the release and uptake of phosphate were also affected. It was indicated that the increased ratio of PHV to PHAs led to the increase of the utilization efficiency of nitrate nitrogen by denitrification PAOs. When initial COD concentration in anaerobic period was lower than 300mg/L, it can effectively keep organic substrates from flowing into the following anoxic period which influenced the efficiency of denitrifying phosphorus removal. Inhibition of nitrite to denitrifying phosphorus removal was decreased due to biology film growth manner of aerobic granular sludge. The inhibitory concentration of intrite to denitrifying PAOs was 60 mg/L. Denitrifying phosphorus removal experiments with different types of electric acceptor showed that denitrifying PAOs inside granules obtained in aerobic-anaerobic condition was 58.7% to the total PAOs, which was much bigger than that in floc sludge existed in the same operation way.In order to shorten and simplify the start-up process of aerobic granular sludge reactor, the effects of aerobic granules storation on the granule properties were studied. The recovery of removal activity for ammonia and phosphorus by granular sludge.after a longer storage period was also investigated. After 2 monthes storation, SVI increased from 32mL/g to 61mL/g, and wet density decreased from 1.036 to 1.017. When inffluent concentration of NH4+-N and phosphate were 40mg/L and 12 mg/L, respectivily, the activity for nitrification and phosphorus removal of granular sludge recovered after 11 day and 42 day cultivation in aerobic-anaerobic condition. When inffluent concentration of NH4+-N was 80mg/L, the NH4+-N can be converted completely after 27 days operation. Due to the higher pH of mixture, chemical sedimentation happened in the surface layer of granules, which limited the transmission of the substrates into granules, and the phosphorus removal activity of the granules cannot be recovered effectively, and the effluent phosphorus concentration was higher than 6.1 mg/L. Thus, pH should be kept lower than 8.0 in the activity recovery processes of granule sludge.