Wastewater Treatment Efficiency Of Activated Sludge Process By Pure Oxygen Aeration And Its Corresponding Biological Characteristics

Currently, literature reports of pure oxygen aeration were most of wastewatertreatment efficiency, less in terms of the biochemical mechanisms. Relations betweensludge morphology and impact resistance, removal efficiency, as well as settleability werestill not clear. If those mechanisms were understood, it would be beneficial to enhance thetreatment capacity of activated sludge, to improve the operation parameters of wastewatertreatment. More importantly, the advancement of treatment efficiency was observed by thetheoretical support. Thus, this research was to explore the treatment efficiency in terms ofimpact resistance, removal rate and settleability, based on comparative study of pureoxygen and air aeration in sequencing batch reactor （SBR） for wastewater treatment. Thenthe mechanisms of treatment efficiency were discussed by sludge morphology. Thepreliminary results of this work included:Stabilities of the pure oxygen and air aeration system were investigated. The error ofeffluent chemical oxygen demand （COD）, ammonia nitrogen （NH₄⁺N） was defined as thestandard deviation （SD） of the average value to explain the data volatility, under1.5-4.1kgCOD.m^-3.d^-1and0.07-0.34kgNH₄⁺N.m^-3.d^-1conditions. The values using pure oxygenwere lower31%,6.0%than that using air. The SD of COD, NH₄⁺N removal efficiencyunder pure oxygen aeration were decreased about6.0%and5.5%, respectively, againstthose under air aeration.Particularly, the influent loading more than2.0kgCOD.m^-3.d^-1, the SD of COD ineffluent under pure oxygen aeration was lower30%than that air aeration. The influentloading more than0.13kgNH₄⁺N.m^-3.d^-1, the value of effluent NH₄⁺N using pure oxygenaeration was lower10%, compared with air aeration. The SD of COD, NH₄⁺N removalefficiency with pure oxygen aeration were decreased about6.0%and3.0%, respectively,against those under air aeration. The results showed that the pure oxygen aeration tank canendure high influent organic loading and have abilities to treat them.Comparing the use of pure oxygen and air to supply the aerobic condition of thesystem, the treatment efficiency of COD, NH₄⁺N was higher when pure oxygen was used.When influent loading of COD was2.9kg.m^-3.d^-1, the effluent of COD in the pure oxygenaeration tank was lower55%than that in the air aeration tank. When influent loading ofNH₄⁺N was0.18kg.m^-3.d^-1, the effluent NH₄⁺N in pure oxygen aeration tank was lower45%, comparing with air aeration tank. COD, NH₄⁺N removal efficiency in pure oxygenaeration tank against those in air aeration tank were more than22%and14%, respectively.Therefore, the results showed that the efficiencies of wastewater treatment under pureoxygen condition were better those under air aeration.The abilities of sludge settling were also investigated. The settling curves wereobtained. Based on fit for the data, the relationship between height and time of sludge settling showed that the settling heights were decreased exponentially with the settling timein bioreactors, there was R2=0.97-0.99using pure oxygen aeration, R2=0.94-0.99using airaeration （p<0.01）. Additionally, the settling velocity in pure oxygen aeration tank was3times as fast as those in air system. The results implied that the sludge settling propertiesfrom pure oxygen aeration was better than that from air aeration, improved solid-liquidseparation effect, decreased organics by reducing the turbidity in effluent. Therefore, thehigh removal efficiency was observed.According to the above results, the pure oxygen aeration process showed efficientperformances of sewage treatment. The fundamental reason originated from the adequatedissolved oxygen （DO）. The mixed liquid suspended solids （MLSS） of biomass were4.9g.L^-1under pure oxygen aeration, and the MLSS was3.4g.L^-1under air aeration.Therefore, the ability of metabolism in the pure oxygen aeration tank was more intense.The extracellular polymeric substances （EPS） secreted by microorganisms to strengthenbioflocculation in pure oxygen aeration was more1.8times compared with air aeration.The morphologies of activated sludge under pure oxygen and air conditions were observedby microscopy. It was found that aerobic granules were formed in both bioreactors. Thediameter of granules ranged from0.5mm to10mm, the mean size was about5mm.Granules were dark brown with a clear, dense, round outer shape in pure oxygen aerationbioreactor. However, the granules were yellow brown with loose structure in the airaeration tank. Large amount of milky like filaments were observed around the surface ofthese granules. In addition, the microstructure of aerobic granules with pure oxygen and airaeration were examined by Scanning Electron Microscope （SEM）. The surfaces ofgranules from pure oxygen system were settled by various microorganisms similar toCocci, Bacilli, except for Sphaerotilus natans. Nevertheless, there was only Spherotilusnatans （0.5-5μm） with sheath membrane and false branch on the surface of granules fromair aeration system.The above results showed that the physical structure of the activated sludge was moredense and compact under pure oxygen aeration conditions. Those structures aggregated themulti-species community of microorganisms, increasing the microbial biomass secretedabundant viscous EPS. Those factors should be the important reasons for enduring the highloading influent, keeping high removal efficiency, as well as good settleability.