Study On The Technology Of Integrated Expanded Bed Combined Biological Filter For Rural Sewage Treatment

With the continuous development of the rural economy of our country and the improvement of rural living standards,the environmental problems caused by rural domestic sewage have become increasingly serious.An integrated expanded-bed combined biofilter treatment technology was designed for the characteristics of scattered and small amount and big change of domestic sewage in rural areas.This technology is made up by three-level in series:a down-flow expansion bed biofilter and an up-flow fluidized bed aerated biofilter and the down-flow expansion bed biofilter.This technology combines the characteristic of pre-denitrification and post-denitrification,and has the advantages of low energy consumption,simple operation and management,and not easy to block and so on.This study mainly investigated the influence factors of the pretreatment effect and denitrification efficiency in the first-stage,the nitrification efficiency and operating parameters in the second-stage nitrification tank,and the functions of chemical phosphorus removal and deep nitrogen removal in the third-stage expanded bed.The optimization of operating parameters and the development and promotion are of practical significance for the integrated equipment.The main results are as follows:(1)The first-stage down-flow expansion bed has good pretreatment function.In the case of no pre-sedimentation tank,the first-stage filter tank can intercept a large amount of suspended solids,and at the same time,the increase of organic concentration makes the enhancement of the denitrification function in the anoxic tank,and that the backwash cycle is shortened to 72h,which is still higher than the backwash cycle in the common aeration biofilter,and it indicates that the first-stage down-flow filter tank can be used as a pretreatment unit and denitrification filter tank at the same time.(2)The height of the filter material layer of the first-stage down-flow expansion bed biofilter has a great influence on the denitrification effect.The greater the height of the filter layer is,the longer the hydraulic retention time is,and the better the denitrification effect is.The increase of the height of the filter material layer has little effect on the head loss,and the treatment effect can be improved by increasing the height of the filter material layer.When the hydraulic load exceeds 6m~3/(m~2·h),the concentrations of COD and TN in the anoxic pool increase significantly.When the dissolved oxygen concentration at the inlet end of the anoxic pool is 1.8 mg/L,the removal rate of TN is the highest,and the backwash has little effect on the denitrification efficiency,and the treatment efficiency can be restored in a short time.(3)When the volume ratio of anoxic section and aerobic section is 1:1.5,the aerobic tank has good nitration efficiency,the removal rate of ammonia-nitrogen is 84.4%.Continue to increase the volume ratio,the increase of ammonia-nitrogen removal rate is less.When the hydraulic load is 4 m~3/(m~2·h),the air-water ratio is 3:1,the nitrification effect in the aerobic pool is better.The capacity load has a certain influence on aerobic tank.The nitrifying bacteria's loading capacity of shock resistance is obviously weaker than that of heterotrophic bacteria.In order to ensure the stability of effluent water quality,the organic load should be controlled in 1.5~3kg/(m~3·d)and ammonia-nitrogen load is 0.45~0.95kg/(m~3·d).(4)The biological treatment effect of the integrated expansion bed composite biological filter is poor,and ferric chloride is used as a flocculant for post-removal of phosphorus.When the dosage is 60 mg/L,the effluent TP can stably reach to less than 0.5mg/L.Ferric chloride requires a certain amount of time for the micro-flocculation reaction in the third-grade filter pool.When the filtration rate is 4~6m/h,the thickness of the filter material layer should be at least 3 m.When the third-stage filter pool is used as the deep denitrification unit,the carbon-nitrogen ratio of the best dosing is 5:1,the average TN concentration is 11.68 mg/L,and the effluent COD is less than 50 mg/L.(5)The water temperature is 23~25°C,hydraulic load is 4 m~3/(m~2·h),reflux ratio is150%,gas-water ratio is 3:1,when adding 60mg/L of ferric chloride to the outlet of the second-stage filter pool.The removal rates of COD,ammonia-nitrogen,TN,SS,and TP in the integrated expansion bed composite biological filter were 90.2%,92.2%,68.2%,98.6%,and 90.0%,respectively.The various indicators basically reach the first-level Class A standard of“pollutant discharge standards of urban sewage treatment plants”(GB18918-2002).