Experimental Study On Phosphorus Purification Performance Of Improved Bioretention Facilities

As a typical LID(Low impact development)technology,bioretention technology has significant effects in terms of runoff reduction and water purification.Studies have shown that fillers are the most critical factor affecting the functioning of bioretention facilities.Most of the domestic reference to foreign research experience on the improvement of fillers,and mainly stay in the laboratory stage,lack of testing of actual operational effects.The existing specifications for filler selection are mostly based on local hydrogeological conditions,so the reference is poor.In this study,through the laboratory simulation test(batch experiment,mini-column,Bioretention column)and on-site monitoring(rain garden)and theoretical analysis,the traditional bioretention filler was improved,and the improved mixed filler with high purification ability was developed and its performance was determined.Setting process parameters of various bioretention facilities,determining process parameters of typical bioretention facilities from two aspects:pollutant removal filtration and water reduction effect;monitoring the actual operation effect of bioretention facilities,evaluating and optimizing bioretention facilities design parameter.Main conclusions:(1)Through comprehensive evaluation of the physical and chemical properties,material convenience and price of a single fillerk,10 single fillers were selected for isothermal adsorption convenience and price of a single filler,10 single fillers were selected for isothernal adsorption and desorption experiments,and it was found that fly ash has the best effect on phosphorus removal.The removal rate is over 97%,followed by WTR,the removal rate is above 50%,and the desorption rate of phosphorus by fly ash and WTR is below 0.80%,so both fly ash and WTR are suitable as improved fillers for purifying phosphorus pollutants.(2)Selecting a single filler with better effect in batch test results to improve BSM and study the durability of the mixed filler.Since the particulate matter is released irregularly during the operation of the mini column,orthophosphorus is used as a standard for evaluating the adsorption capacity of the filler.It was found that the adsorption process of phosphorus on the filler showed an overall trend of fast and slow.Except for the soil,soil+sand and BSM+ 10%WTR filler,the other fillers began to reach saturation when the influent was about 13L.The influent amount was equivalent to the amount of water under the total rainfall of 5 years,and the adsorption curves were similar.The soil,soil and sand mini-column began to reach saturation when the influent water volume was about 28L,which is equivalent to 11 years of rainfall in Xi’an.WTR-modified fillers have not reached saturation with an influent water equivalent to 15 years of rainfall.(3)The results of the small test showed that the modified filler with WTR and green zeolite had obvious effect on TP removal,and the removal rate of TP by the filler with WTR was above 95%.The average removal rate of SRP by filter column added with WTR,medical stone and vermiculite was above 97%.In the two-stage experiment,the concentration of orthophosphorus effluent was basically better than that of GB surface water environmental quality.The water reduction rate of the modified filler is 1.2 to 2.0 times that of BSM.Adding WTR and coco peat can greatly improve the water volume reduction effect.According to Pearson correlation analysis,influent concentration is the biggest factor affecting phosphorus removal in bioretention systems.The filler of the filter column was tested.It was found that the content of TP and SRP increased most obviously in the filler with WTR.In general,WTR is most suitable as an improved filler for removing phosphorus pollutants from bioretention systems.(4)Compared with the traditional rain garden,the improved rainwater garden has a water reduction rate of 1.03 to 1.59 times that of the traditional BSM rain garden.the improved rainwater garden found that the water reduction rate of the medium-light rain(<25 mm)in the WTR-modified garden was basically over 53%,and the rainfall reduction below 10mm was basically over 90%.