Lab Scale Study On Urban Landscape Water Receiving Tail Water From Wastewater Treatment Plant (WWTP)

Effluent achieving 1A standard of the wastewater treatment plant is still far inferior to that of the surface water V-type water quality standard,thus,tail water has become one of the most important point pollutions in city.The currently used "tail water diversion way" is gradually thought to be inconducive to city water circulation.Tail water discharging problem has not been solved properly.On the other hand,in addition to being able to adjust microclimate,improve biodiversity and beautify the landscape,urban landscape water can also play a role of dilution and degradation of water pollution.When the external pollution control methods are meeting more and more limits,water purification can be thought as an important breakthrough in water pollution prevention and control.After receiving tail water,the degradation and purification effect of city landscape water is closely related to supplement ratio of tail water,supplement frequency,hydraulic retention time(HRT),river ecosystem and other external conditions(such as T,DO,etc..).Therefore,it is of great significance to study how to make full use of natural conditions of landscape water to improve its purification ability,controlling tail water pollution,and as far as possible not to damage the original function of the landscape water.This paper constructs a set of simulated city landscape water receving tail water from WWTP by lab experiment.Taking NH3-N,NO3--N,NO2-N,TN,PO43--P,TP,CODMn,as main analysizing indicators,the removal effect of different pollutions in simulation system under different HRT(1d,3d,5d),different supplement proportion of tail water(50%,33.3%,25%)and different sediment types(sand,sediment,ceramsite)is studied.On this foundation,taking N,P,organic matter and other water quality standards for reference,effluent quality under various control conditions is discussed.The results are as follows:(1)removal rate of pollutants in simulation system:?During this experiment,the removal rate of NH3-N,TN,TP and CODMn is(28.0%±16.4%)?(48.8%±15.8%)?(7.1%±3.8%)?(25.9%±5.8%)?(15.3%±7.0%)?(58.0%±2.9%)?(3.6%±2.6%)?(29.4%±7.9%),separately;?HRT has a significant effect on removal of TN and TP,whose removal rate under 3d and 5d retention time are significantly higher than that of 1d(P<0.05).When HRT is 3d,the removal rate of NH3-N,TN,TP,CODMn is 35.5%?46.2%,18.3%?24.3%,30.5%-38.6%,10.2%?22.5%,separately.There's no significant difference of removal rate between 3d and 5d retention time.Different sediment types can significantly effect the removal of pollutants in simulation system.Sediment has good effect to remove NH3-N and TN,while ceramsite has an advantage at removing TP and CODMn.With the extension of HRT,the effects of different sediments removing TN and TP become significantly different.Under a certain extent,increasing initial concentration can improve the removal rate of pollutants,but the effect is not significant.In this experiment,when the tail water and river mixed with ratio of 1:1,all pollutants reach a higher removal rate respectively.?The effects between three factors:a.removal of NH3-N:supplement proportion(significant)>sediment type(significant)>HRT;b.removal of TN:sediment type(significant)>HRT(significant)>supplement proportion;c.removal of TP:HRT(significant)>supplement proportion>sediment type;d.removal of CODMn:HRT>sediment type>supplement proportion;(2)effluent quality of simulation system and water quality standards:?Under 1:3+HRT(3d)+ sediment conditions,the lowest effluent concentration of NH3-N is 1.71mg/L.Under 1:3+HRT(5d)+ sediment conditions,the lowest effluent concentration of TN is 6.31mg/L.Under 1:3+HRT(3d)+ ceramsite conditions,the lowest effluent concentration of TP is 0.25mg/L.Under 1:2+HRT(5d)+ ceramsite conditions,the lowest effluent concentration of CODMn is 5.40mg/L;?When influent concentration is high,effluent concentration is high,thus,the supplement ratio of tail water is the main factor affecting the final effluent quality.Effluent under all conditions in this simulation system can meet the GB 18921-2002 standard(river type).While compared with the surface water V-type water quality standard,only the effluent of CODMn,or NH3-N and TP at the lower ratio(1:3 or 1:2)can reach the standard.As for TN,under this test condition,it can not meet the requirement of V water environment quality standard;?The simulation system is able to reduce the tail water pollution,but from the perspective of receiving water,whose initial concentration of NH3-N,TN,TP,CODMn is 1.26mg/L,3.43 mg/L,0.07 mg/L and 6.04 mg/L,adopting the minimum mixing ratio(tail water:water=1:3),except that CODMn can be reduced to the initial concentration under low proportion supplement and stays for 5d in the system,further treatment steps are needed in order to reduce the concentration of N and P to the initial concentration of receiving water.In short,the simulation system has a certain effect of main pollutants removal in tail water.HRT can affect water quality improvement,within a certain range,the longer the HRT,the better removal of pollutants,and staying 3 days is appropriate.River sediments have different effects in removing N and P.Although different proportion of tail water supplement shows no significant effect on pollutants removal rate,from the perspective of water quality standards,and considering different volume of receiving water,the total amount of tail water must be kept in a certain proportion.In this experiment,better result shows up when the ratio of tail water and landscape water is 1:3.Continuous replenishment of tail water will cause an increasement in water pollution index of receiving water.TN is the key control index in tail water replenishing in landscape water.