| Expanded Granular Sludge Bed (EGSB) reactor exhibits many advantanges such as better mass transffer, higher treatment efficiency and more resistance in shock load compared with conventional anaerobic reactors. EGSB reactor has a good adaptability for the changes of influent quality and operating condition, namely stable treatment can be obtained for both high and low strength wastewater even at low temperature(15-20℃) or low oxygen. Microaerobic condition is the transitional state betweent anaerobic and aerobic conditions. Microaerbic bio-treatment can remove organics and nutrients simultaneously with a high energy efficiency and low yield of excess sludge. This paper focuses on the performance of EGSB reactor in microaerobic condition, Study included the cultivation of granular sludge, removal efficiency of COD and nutrients as well as the related factors that influence the operation effect.First, granular sludge was anaerobicly cultured in EGSB reactor. The feed water was simulative wastewater(with COD of 4001650mg/L) and the inoculum was digested sludge mixed with a little disintegrated granular sludge. Sludge granulation was rapidly achieved with suitable upflow rate and step-up volumetric loading rate(VLR) at 25℃. As granules formed, the feed water was changed to domestic wastewater. During the three more months operation, temperature was 13-26℃and influent COD was 144-618mg/L, the hydraulic retention time (HRT) and VLR of COD were set at 0.6-1.6h and 2.5-19.6 kg/(m3·d) respectively, and the average COD removal rate reached 80% with effluent COD below 100mg/L during stable stage. Most of the anaerobic granules were 0.43-2mm in diameter, with sedimentation velocity of 14-95m/h, and the specific methanogenic activity ranged in 0.34-0.45LCH4/(gVSS·d). The anaerobic granules were compact with bacillus and coccus colonies inhabited at different zones. It was observed that temperature, hydrodynamic shear conditions, HRT and VLR had influence on running efficiency and sludge property. Either temperature down below 15℃or the average shear rate increased above 32.1×10-3Pa accelerated degranulation thus deteriorated the effluent quality. Nevertheless, at above 15℃, better granular activity and higher treatment efficiency were obtained by shortening HRT and increasing VLR.Then miceoaerobic operation was carried out based on EGSB reactor. The experimental water was still domestic wastewater and the operating temperature was 18-27℃. By increasing the dissolved oxygen gradually in the reactor, microerobic granules developed from anaerobic ones. After startup, the microaerobic granules had stable characteristics and much enhanced decontaminating capability. When DO, HRT, reflux ratio and VLR of COD were set at 0.3-0.5mg/L, 8h, 8:110:1 and 0.4-1.9kg/(m3·d), respectively, the average removal rate for COD, ammonium, total nitrogen, phosphorus and SS were 94%, 83%, 81%, 36% and 89%, respectively.Study showed that pollutants removal was related to factors such as DO, HRT, reflux ratio, sludge concentration and sludge loading. The optimum parameters were proved to be: DO 0.3-0.4mg/L, HRT 8h, reflux ratio 8:1, MLSS 11-16g/L and COD sludge loading rate 0.15kgCOD/(kgMLSS·d). Under these conditions, the average removal rates of COD, NH4+-N and TN were 93%, 84% and 82%, respectively.Nitrogen was effectively removed through simultaneous nitrification and denitrification in microaerobic condition. Both the microaerobic environment and high sludge concentration benefited pollutants removal. As compared with the activated sludge techniques for nitrogen removal, this process had the features of shorter HRT, higher volumetric loading, and better decontaminating effect, cutting down 34% aeration and three-quarter excess sludge yield.Both DO and sludge loading influenced the granular property in microaerobic condition. When DO level was below 0.4mg/L and sludge loading rate was less than 0.12kgCOD/(kgMLSS·d), the granules were compact with good settling property and were activitive in methanogenesis and nitrogen removal. There were less filamentous bacteria in granules since the microbial groups distributed were relative abundant. However, increasing in either DO level or sludge loading resulted in worse settlement, accelerated degranulation and diminished methanogenesis. It was observed that more filamentous and disintegrated granules appeared when the average sludge loading of COD rose to 0.18kgCOD/(kgMLSS·d).It was the diversity in microbial groups and metabolic pathways that improved treatment effect since kinds of populations dominanted in granules and different part of sludge bed. Kinetics analysis showed that the optimal DO was 0.41mg/L for simultaneous nitrification and denitrification to remove nitrogen. On the other hand, model of oxygen transfer was built to simulate oxygen distribution in granule based on simultaneous nitrification and denitrification. Moreover, the influence of internal diffusion on aerobic reaction within granule was evaluated. According to the calculating results, either lower DO or larger granule would prevent oxygen from penetrating thus aerobic reaction was much effected by internal diffusion. Simulating results were then applied to evaluate the oxygen consumption in different conidtion with well agreement between experiment and prediction.The present study cultured excellent microaerobic granules from anaerobic ones in EGSB reactor by controllong operating parameters, which provided reference for the practical operation and regulation.
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