The Study On Bioretention Treating Effluent From Septic Tank And Methods To Enhance Its Pollutant Removal Efficiency

Septic tanks are the main measures of collecting and treating domestic sewage,especially in natural villages,but the septic tank treatment ability is limited and its effluent can cause water pollution if it is directly discharged into the river.The use of simple treatment facility the nearby ditch or local farming is one of the feasible ways to solve the problem of scattered domestic sewage in rural areas.In this experiment,the treatment efficiency of bio-retention pond on septic tank effluent was studied,and the method of improving water quality by optimizing hydraulic loading rate was discussed.The main work and results are as follows:(1)The results of wastewater experiment show that under the condition of 0.7m3/m2·d,the average influent concentrations of CODcr,NH4+-N,LAS,TN and TP are 140mg/L,66mg/L,3.97mg/L,73.24mg/L,3.1 mg/L respectively,the average effluent concentrations are 12mg/L,0.625mg/L,0.195mg/L,71mg/L,2.4mg/L.The treatment efficiency of CODcr,NH4+-N and LAS is very good,the effluent concentration reached the"urban sewage treatment plant pollutant discharge standards"(GB918-2002)in the A standard requirements.All pollutant indicators meet the requirement of "Irrigation Water Quality Standard" when irrigating vegetables(hydienics indicators was not measured).(2)The experiment of different hydraulic loads show that the hydraulic load has the greatest effect on the removal efficiency of TP,and the removal rates are 26%and-37%when the hydraulic load reach 0.3m3/m2·d and 1.0 m3/m2·d respectively.The removal refficiency of TN decreased with the increasing of hydraulic loads,and the removal rates range from-1%to 11%.The removal rate of CODcr ranges from 84%to 93%.The removal efficiency of NH4+-N and LAS remain high regardless of the changing of hydraulic loads.(3)The experimental results show that when the hydraulic load reaches 0.5 m3/m2·d,the effective volume ratio(e)and hydraulic efficiency(?)of the retention pond reach the maximum,the water divergence(??2)is the smallest.At this state the biological retention pond system is conducive to the removal of pollutants.(4)When the hydraulic load is 0.5 m3/m2 d,The influent concentration of CODcr,NH4+-N,LAS,TN and TP were 149 mg/L,66.05 mg/L,4.2mg/L,71.75mg/L and 3.15mg/L,respectively.The average effluent concentrations were 14.25 mg/L,0.07mg/L,0.17mg/L,55.5mg/L and 1.8mg/L respectively.The removal efficiency of LAS,NH4+-N,TN and TP were 99.9%,96.4%,22.5%and 43%,respectively,which were higher than those achieved in the first four hydraulic conditions.The CODcr removal rate was 90%(93%);The experimental results show that the tracer experiment can be applied to optimize the operating parameters of the biological retention pond.The results show that the biological retention pond can be used as a simple processing unit for sewage in rural areas.Through the above research,we have strengthened the understanding of the field of using bio-retention pond,and provides new ideas and methods for studying how to improve the treatment efficiency of bio-tention pond.