Research On Deep-water Type Membrane And Algal-bacteria Symbiotic System For Farmland Drainage

As the major carrier of agricultural non-point source pollution,farmland drainage instantaneous discharges runoff with high volume,unstable water quality,and discordant carbon/nitrogen ratio,which make it difficult to treat with conventional bio-ecological technology.In this research,a deep-water type membrane-bacteria-algae symbiotic reactor was designed.The performance of the membrane-bacteria-algae symbiotic system on pollutants removal was investigated.The optimal operating conditions of the system under different Hydraulic Retention Time(HRT),different velocity of hydraulic stir,and different stir time were determined.The dominant nitrogen removal mechanism of the system was explored;the algal water separation performance and the fouling characteristics of the stainless-steel membrane were investigated.Results indicated that:1.Membrane-bacteria-algae symbiosis system shows a satisfactory effect on farmland drainage purification.Under the condition of 6d of HRT,0.2m/s of hydraulic flow velocity,16 h of flow time,6.98 mg/L of average total nitrogen(TN)mass concentration of inflow,and 0.63 mg/L of average total phosphorus(TP)mass concentration in inflow,the system could reduce the average mass concentration of TN and TN in outflow to 0.45mg/L and 0.03mg/L,respectively,which can meet the Class ? Standard of Surface Water.However,the system barely has an effect on chemical oxygen demand(COD),further measures are required in order to lower the COD in the outflow.2.Under the condition of 8.20 mg/L of average TN mass concentration in inflow and 0.83 mg/L of average TP mass concentration in inflow,test the treatment effect of the system in different HRT,which are 6d,4d,2d,and 1d.When HRT is 2d,the system could reduce the average mass concentration of TN in outflow to 0.94mg/L,which can meet the Class ? Standard of Surface Water;the average mass concentration of TP in outflow can be reduced to 0.03mg/L,which can meet the Class ? Standard of Surface Water.Consider the pollutant treatment effect and practicability,the appropriate HRT of the system should be set as 2d.3.Under the condition of 7.55 mg/L of average TN mass concentration in inflow and 0.56 mg/L of average TP mass concentration in inflow,test the treatment effect of the system in different hydraulic stir velocities,which are 0.1m/s?0.2m/s and 0.3m/s.When the hydraulic stir velocity is 0.2m/s,the system achieved the best effect on TN and TP reduction.The average mass concentration of TN in outflow can be reduced to 0.67mg/L,which can meet the Class ? Standard of Surface Water;the average mass concentration of TP in outflow can be reduced to 0.02mg/L,which can meet the Class ? Standard of Surface Water.Therefore,the best hydraulic stir velocity for the system is 0.2m/s.4.Under the condition of 6.98 mg/L of average TN mass concentration in inflow and 0.63 mg/L of average TP mass concentration in inflow,test the treatment effect of the system in different stir time,which are 8h,12 h,and 16 h.When the stir time is 16 h,the system achieved the best effect on TN and TP reduction.The average mass concentration of TN in outflow can be reduced to 0.45 mg/L,and the average mass concentration of TP in outflow can be reduced to 0.03mg/L,which can both meet the Class ? Standard of Surface Water.Therefore,the best stir time for the system is 16 h.5.Dominant nitrogen removal mechanisms in the system are the absorption of nitrogen by algae and nitrification/denitrification.Algae store excess nitrogen in cells under nitrogen-rich conditions,at the meantime,the change of molecular weight distribution indicates that algae begin to change from suspended state to attached state or aggregated state,which causes the concentration of suspended algae and TN in the mixed solution decreased significantly.The change of dissolve oxygen(DO)and oxidation-reduction potential(OPR)in 24 h show that part of nitrogen in the system is removed by nitrification/denitrification process.6.Stainless-steel membrane shows a satisfactory effect on algae interception,and the intercept ratio is 94%.In this research,HRT is the main factor that affects the membrane fouling rate,long HRT can effectively mitigate membrane fouling rate.The algae mainly form a dense layer of bacteria and algae on the surface of the membrane.The cleaning method is off-line cleaning.After cleaning,the pure water flux can be restored to 96% of the initial value.