Effect Of Polyaluminum Chloride On The Formation And Characteristics Of Aerobic Granular Sludge In Baffle Reactor

Aerobic granular sludge is a spherical or like-spherical granular sludge formed by gathering together each other of microbial cells under the effect of self-fixation,with strong adsorption ability,stable structure,fast sedimentation rate,high pollutant removal efficiency,strong hydraulic impact resistance,and other advantages,which is widely used in processing the high pollution risk of the heavy metal wastewater,organic toxic waste,etc.However,the start-up of aerobic granular sludge reactor is usually a long time-consuming,and it needs to be cultured in a reactor with a relatively high height-diameter ratio,the two factors have limited the further promotion and application of aerobic granular sludge.The height of reactor appearance is reduced through designing the baffle reactor by this study,and the groups with no addition of PAC are taken as the control group,the formation mechanism of aerobic granular sludge and its change of characteristics are respectively investigated under the condition of the low PAC,middle PAC group,high PAC group（50,100,400 mg effective aluminum/L wastewater respectively）,the main research contents are as follows:the influence of different PAC concentration on the formation process of aerobic granular sludge,the basic physical and chemical characteristics of mature granular sludge,the pollutants removal characteristics of mature granular sludge,and the microbial population distribution law of mature granular sludge.They are cultured for 70 days at the same time in the four group of baffle reactor.The study has found that the control group without adding the PAC does not form the granular sludge,and the times for the low PAC group,medium PAC group and high PAC group forming the granular sludge（the particle size>0.20mm）are 10 days,7 days and 4days,respectively,the higher the PAC dosage is,the shorter the time for the occurrence of granular sludge is.Compared with the inoculated sludge,the extracellular protein of the mature sludge in the four groups of reactors has been increased respectively by about11,40,40 and 48mg/g MLSS,and the extracellular polysaccharide has been increased respectively by about 54,39,28 and 14mg/g MLSS,indicating that the addition of PAC could promote the secretion of extracellular protein in the sludge microorganism,but is not conducive to the secretion of extracellular polysaccharide.After the 70 days of culture,except for the decrease of the extracellular protein and extracellular polysaccharide ratio in the control group,all the three group extracellular protein and extracellular polysaccharide ratio of the granular sludge formed by the other PAC reinforcement are increased significantly.In addition,all the Zeta potential of sludge in the four mature groups are increased,the more the PAC is added,the more obvious the Zeta potential is increased.Through the analysis for the four groups of mature sludge,it has been found that with the increase of PAC dosage,the sludge moisture content and incomplete coefficient are decreased accordingly,while the specific gravity,the average sedimentation rate,and the content of aluminum element in the sludge are increased correspondingly.It has been proved that the settling performance and structural stability of sludge have been enhanced by the addition of PAC significantly.It has been found from the scanning electron microscopy that the surface of control group shows a reticular structure with more filamentous bacteria,while the surface of the granular sludge formed by the PAC reinforcement does not form a reticular structure,which is dominated by the bacilli,and there is very few filamentous bacteria,and there are some 5-10μm pores in the particles,these pores are the mass transfer channels of microorganisms inside the particles.The dissolved oxygen mass transfer characteristics have decided the granular sludge denitrification ability,the dissolved oxygen in the control group without the granular sludge can directly penetrate the sludge,the diffusion depths of dissolved oxygen in the three groups forming the granular sludge are 600μm,700μm,850μm,respectively,the shallower the diffusion depth of dissolved oxygen is,the better the denitrification conditions in the group are,the study has also found that the more difficult the dissolved oxygen mass transfer is,the higher dead cells/living cells in the group are.During the cultivation process,the COD of effluent from the four groups of reactors roses slightly at the initial stage,and then shows the decline trend on the whole,this is due to the good adsorption of granular sludge to the organic matter,the sludge will absorb a lot of COD from the water in one cycle,finally,the COD of effluent from the four groups of reactors can reach A standard of grade 1 stably,the COD of effluent from the control group is higher than that of the three groups which forms the granular sludge.During the cultivation process,the NH₄⁺-N of effluent from the four groups of reactors is deteriorated in the early stage,the NH₄⁺-N concentration of effluent from the three groups with the addition of PAC gradually is decreased gradually with the developing of granulation,and finally reaches A standard of grade 1 stably,while the NH₄⁺-N of effluent from the control group does not be improved after the 70 days of cultivation.The denitrification process of control group has been restricted to the nitrification stage during the process of pollutant removal in the cycle,the specific aerobic rates of AOB in the mature sludge after the 70 days of culture are all higher than that of NOB,therefore,a certain degree of accumulation phenomenon has been occurred for all the NO₂^--N in the cycle,and the three groups of denitrification rates for the granular sludge are formed:the low PAC group>the middle PAC group>the high PAC group,the NO₃^--N of effluent has also a big difference,after the final stabilization,the total inorganic nitrogen removal rate of the four groups of reactors reaches the following percentage stably and respectively.The formation of aerobic granular sludge can improve the diversity and abundance of microbial populations,the addition of a certain concentration of PAC helps to enhance the abundance and diversity of microbial populations in the granular sludge,but the excessive addition of PAC has no obvious promoting effect on the microbial abundance and diversity,and even has an inhibition effect.The influence of low concentration PAC on the diversity of microorganism in the phylum and class levels is smaller,the influence of the middle and high concentration PAC on the diversity of microorganism in the phylum and class levels is larger,but the response to the change of concentration of PAC on the microbial genus level is obvious,the higher concentration of PAC is helpful to promote the diversity of microbial genus level,However,when excessive PAC was administered,the abundance growth of EPS producer bacteria was inhibited.The formation of granular sludge is promoted by the PAC mainly through the effects of the electrical neutralization and adsorption bridging,etc.,the sediment formed by the PAC and the impurities in water and the long chain hydroxy-complex and polymer formed by hydrolysis enter into the particle,the structural stability of granular sludge formed by the PAC reinforcement has been strengthen.