| With the increasing of nitrogen pollution in water and the severity of water qualitystandards, high efficiency nitrogen removal technology has been widely concerned.Biogranulation is a promising biotechnology due to its advantage of high treatmentefficiency, high biological activity, and device miniaturization. However, the stability ofdenitrifying granular sludge is relatively poor. Therefore, denitrifying granules werecultured in USB reactor. Granule characteristics were investigated. Microbe characteristicwas analysised by SEM. With removal efficiency and characteristics of denitrifyinggranular sludge as evaluation index, optimum conditions for USB operation were studiedby controlling ratio of C/N, hydraulic loading and pollutant loading. Five substrateremoval models were used to simulate pollutant removal of denitrifying granular sludge.Changes of metal and EPS content in sludge were studied during granulation process.Microbes were isolated and anlysised by molecular biological identification to investigatemicrobial community of mature granules. The formation mechanism of denitrifyinggranular sludge was discussed from aspects of physicochemical effect and microbialcommunity. Conclusions were as follows:1) Denitrifying granular sludge was developed in the USB reactor. Granule VSS/SSratio of0.34, sedimentation rate of186-217m/h, particle size of1-5mm. Microorganismsmainly distributed in the particle surface, winded by a large number filaments.2) The optimum C/N ratio was4. At this condition, the carbon and nitrogen removalrates were high, the granular sludge performance was better and the operation of reactorwas stable. Through the substrate removal kinetic analysis, Grau second-order substrateremoval model was more appropriate to describe pollutant removal of denitrifyinggranular. It was benefit to control recycling flow rate at certain degree to improve granularsludge stability and denitrification performance.3) In the process of granular sludge formation, MLSS increased from25.4g/L to47.7g/L, the curve of VSS/SS ratio went down and then became flat; EPS content of maturegrannules was1.3times that of inoculation sludge; EPS protein/polysaccharide ratio rosefrom6:1to17:1, which reflected the enhanced hydrophobic on cell surface; Ca2+inmature grannules increased by1.6times, which enhanced the bridge role of EPS amongcells; Dominant organisms in granular sludge were Ochrobactrum GX-1, Bacillus GX-2 and a Fusarium oxysporum genus fungus fungi-1.4) The formation of denitrifying granular sludge contributed to the combined action ofcalcium ion, EPS, fungi and physiochemical effect. At the transport and reversibleadsorption stage, substratum was transported near to each other and then collided byexternal physiochemical force. EPS provided extensive binding surface area and attractionsites for bacterial. Ca2+reduced electrostatic repulsion between cells through electricityneutralization; At the irreversible adhesion stage, EPS favored cells adhesion effect bycross-linking cells intensely. Ca2+bonded with electronegative substances and strengthentertiary structure of EPS; At stage of aggregation, filamentous fungi-1with large surfaceareas supplied a good platform for cells to adhere, and acted as the framework of granularsludge. Fungus winded and wrapped cell aggregations intensely, which could boost thestrength of granules greatly; At the multiplication stage, fungi-1and bacteria cooperatedwith each other, boosted the metabolism rate by enhancing the substance transformationand energy utilization efficiency, and then increased the microorganism multiplication ofgranular; At the maturation stage, multiplication of microorganism and the increase ofinorganic substance would be controlled by external physiochemical force and energylimit. EPS strengthened granular structure and favored nutrient transport. Calciumprecipitation accumulation made a good settleability and a compact interior structure ofgranules. Fungi and bacteria cooperated to complete denitrification and balancedmultiplication of granules. |
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