Study On The Effect Of Denitrification In Slow Filter Based On Solid Carbon Source

As a water treatment technology with both physical and biological effects,slow filter technology has the advantages of low cost,simple management,stable and reliable operation,and is widely used in the fields of water supply and sewage treatment.However,this technique generally has a low TN removal rate due to the lack of available carbon sources in the treatment of low carbon to nitrogen ratio water.Biodenitrification by adding a solid carbon source is a viable technical solution to this problem.Therefore,40 mesh and 200 mesh polycaprolactone（PCL）and 200 mesh polybutylene succinate（PBS）were selected as solid carbon sources to enhance the removal of TN by the slow filter.The study investigated the effect of different solid carbon sources on improving the denitrification of slow filter cells,and analyzed the differences in the characteristics of the filter layers formed by different carbon sources,and determined the preferred addition amount of carbon source and the filtration rate of slow filter to slow filter.The main conclusions are as follows:（1）The carbon release and carbon release rate of PCL₂₀₀00 and PBS₂₀₀00 are better than the PCL₄₀.The particle size may directly affect the carbon release rate of the solid carbon source.The smaller the particle size,the stronger the carbon release capacity.The first two small particle size carbon sources provide more organic matter.The removal rates of nitrate and nitrogen in the slow filter with PCL₂₀₀00 and PBS₂₀₀00 were 95.6%and96.9%,respectively,and the effluent has no nitrous oxide accumulation.In the slow filter system with PCL₄₀0 added,the removal rate of nitrate and nitrogen is only 20%,and the effluent nitrous oxide reaches5mg/L.Due to the addition of carbon source,the growth of nitrifying bacteria is limited,and the removal rates of ammonia nitrogen in the three slow filters are below 20%.（2）The biomass in the slow filter system with PCL₂₀₀00 and PBS₂₀₀00 as the carbon source is higher than that of the slow filter system with PCL₄₀0 as the carbon source.,and the total amount and protein content of the extracellular polymer of the first two slow filter membrane layers are higher than the latter,indicating that the biomass and the biofilm stability of the slow filter with PCL₂₀₀00 and PBS₂₀₀00 as the carbon source is better than that of the slow filter with PCL₄₀0 as the carbon source.Small particle size solid carbon sources may be more conducive to the growth of microorganisms.（3）The abundance ratio of the denitrification function in the slow filter with PCL₂₀₀,PBS₂₀₀00 and PCL₄₀0 as carbon sources was 62.12%,27.58%and 28.05%,respectively.The proportion of denitrifying bacteria in the PCL₄₀0 slow filter is low,resulting in a low rate of nitrogen and nitrogen removal in the system.The dominant genus Variovorax in the PBS₂₀₀00 slow filter accounted for 27.6%,which may contribute to the removal of nitrate.（4）PBS₂₀₀ is a preferred carbon source from the viewpoints of denitrification performance,carbon release characteristics,biofilm stability,and price.（5）The time required for the stability of the slow filter tank with PBS200/quartz sand addition ratio of 1/5,1/12,1/24,respectively,is prolonged with the decrease of the amount of carbon source added,and the NO₃^--N removal rate is gradually reduced,NO^-₂-N gradually accumulates,the effluent COD gradually decreases,and the reduction of carbon source creates an environment conducive to the growth of nitrifying bacteria,resulting in an increase in the removal rate of NH₄⁺-N.When the filtration rate is increased from 0.03m/h to 0.09m/h,the removal rate of NO₃^--N is reduced,but the removal rate can still be maintained at 86%.Due to the increase of carbon source dissolution rate and diffusion rate in water,slow filter denitrification rate increases;the NH₄⁺-N removal rate increased significantly;the system TN removal rate changed little,maintaining at 67%-72%;the effluent COD gradually decreased.（6）After the slow filter of PBS₂₀₀/quartz sand is 1/24,the slow filter with the same size and no carbon source is connected in series,and the concentration of NO₃^--N in the second stage slow filter in series is 10mg/L,NH₄⁺-N is 5mg/L,and the COD removal rate is 30%when the filtration rate is 0.03m/h.Dissolved microbial metabolites and fulvic acid-containing substances in the effluent as refractory substances may lead to a decrease in COD removal rate;at the same time,the maximum TN removal rate is 18%,and the average removal rate of NO₃^--N is 26%.The average removal rate of NH₄⁺-N is15%.Only a small amount of carbon source can participate in denitrification reaction and the decrease of dissolved oxygen in water restricting the growth of nitrifying bacteria may be the reason for the decrease of NO₃^--N and NH₄⁺-N removal rates in the second-stage slow filter in series,respectively.