Removal And Migration Characteristics Of Contaminants In Bioretention Modified With Aluminum-based Water Treatment Residual

Contaminants in rainwater have a serious detrimental impact on aquatic ecosystems and water environment.Water Treatment Residual?WTR?can adsorb phosphorus and other pollutants in the runoff strongly.The use of WTR as bioretention media has demonstrated its excellent ability to remove pollutants in rainwater,especially for phosphorus immobilization.This is also a recycling method for WTR utilization.Bioretention modified with aluminum-based WTR is a promising technique for stormwater management.In this study,WTR's charactcistics was analyzed with XRD,XRF and BET methods,the chemical composition and internal micro-structure characteristics are figured out,the physicochemical characteristics such as composition,porosity,pore size distribution and specific surface area are determined.The phosphorus adsorption characteristics on WTR were tested at 25 oC.The enhanced pollutants performances of bioretention with WTR were examined with two bioretention columes,one completely filled with sand and the other filled with sand and WTR?mass ratio: 10%?.By monitoring the pollutants changes in effluent of different bioretention media layers,the pollutants removal and transfer in the bioretention is researched.Four sets of simulation bioretention box devices were designed to investigate the removal of pollutants in the conditions with and without plants in the sand and WTR modified sand media,to explore the effects of plants on pollutant removal in the bioretention.Through a series of experiments,the following conclusions are obtained:?1?XRF analysis showed that the WTR has high aluminum and iron content,with the content of Al and Fe are 6.74% and 15.9% respectively.The background value of P is very low in WTR,which is only 0.0691%.WTR's specific area is 126.44m²/g,and average pore size is 5.02 nm,mainly in the mesoporous pore voids.In WTR's static adsorption experiment to P,the theoretical P adsorption capacity on WTR was 19.01mg/g.It can be seen that the adsorption capacity of WTR is excellent,and the adsorption capacity and adsorption affinity are better than that of the conventional bioretention media.?2?During the test,the sand-WTR bioretention system show sustainable and efficient ability of the phosphorus removal.The influent phosphorus concentration of the experiment was 3.0-7.0mg/L,and the total influent was about 2600 L.Starting from the top of 20 cm at the deep of the upper media layer,the effluent TP concentrations can be maintained at between 0.01-0.04mg/L,that means that from the top of 20 cm at the base of themedia layer,TP are removed in the runoff.In contrast,the adsorption capacity of P in the sand media bioretention system is not expected.In the test,the adsorption of P from the upper to lower of themedia is gradually saturated in the sand media bioretention.Generally,the TP can be efficient removed by using the 20 cm thick sand-WTR substrate for the small-scale biological detention facility.?3?The removal mechanisms of NH₄⁺-N in the runoff by bioretention system includes the adsorption of nitrifying bacteria,nitrification and plant absorption etc.In this experiment,the proportion of NH₄⁺-N removal by nitrification was 22.5%-30%.In two group of bioretention systems,rain flows through themedia the upper 20 cm layer,NH₄⁺-N in the runoff is nearly completely removed.Finally from the upper 20 cm layer,the run off average NH₄⁺-N removal rate was 98.8% in sand media bioretention system,slightly higher than the sand –WTR media system's 94.4%.The average removal rates of NO₃^--N were 27% and 24.9% in the sandy media bioretention system and the sand-WTR bioretention system respectively.The removal efficiency of both the systems is poor and the effect is not stable.The decrease of the concentration of NO₃^--N is mainly in the following two layers.Both bioretention systems on the removal rate of TN is relatively low,in the range of 27.8-39.5%.Because of the size ofmedia,the experimental results show that adding WTR hss no obvious positive effect on the removal of TN,compared with the sand media bioretention system.?4?The influent COD concentration was 140-200mg/L,and the removal rate of COD was proportional to the thickness of media generally.Removal rate of COD by planting layer of two kinds of bioretention system is about 15%.The final effluent COD concentration of sand-WTR media and sand media bioretention system at the bottom of the intake were average 38.6mg/L and 17.1mg/L,and their average removal rates of COD reached 78.7% and 90.5% respectively.So the COD removal efficiency of the sand media bioretention system was a litter better than that of sand WTR retention system.In addition,the addition of WTR has no significant effect on the removal of COD compared with that of sand in the experiment.?5?In practical application,the TP removal of bioretention can be efficiency achieved by using 20 cm thick sand-WTR media.However,take the removal of TN and COD into account,the removal efficiency is better when using thicker media layer.At the same time,due to the influence of media's particle size,the addition of WTR in the sand media can lead to a slight decrease of COD and TN removal rate.?6?As for the results of simulation bioretention boxes,when planting plants,the average removal rate of TP increased by about 6% for the final effluent for sand-WTR media bioretention system's planting layer.But there is no big difference for the bottom layer's effluent.Because WTR's adsorption capacity of TP is strong and stable.The average removal rate of TP is also increased about 6% different plants for planting layer when planting plants of sand media bioretention,and compared with not planting plants,TP removal rate increased about 4% for the bioretention system's final effluent.?7?Compared with not panting plants,the average NH₄⁺-N removal rate of Sand-WTR bioretention's and sand bioretention's planting layer were increased by about 5%-10% when planting plants.But the final effluent NH₄⁺-N removal rate of sand-WTR bioretention was increased less than 1%,while that of sand bioretention was increased 2%.Visible when planting plants,the NH₄⁺-N removal rate of planting layer has a small increase,but the strengthening effect of the final effluent NH₄⁺-N's removal is not obvious.The average removal rate of NO₃^--N was increased by 3% in the planting layer,and the removal rate of NO₃^--N in the final effluent was increased by about 1% for sand-WTR media bioretention and 2% for sand media bioretention.The average TN removal rate of Sand-WTR bioretention's and sand bioretention's planting layer and the final effluent were all increased by about 5% when planting plants.It can be seen that TN removal was increased about 5% when planting plants.?8?The influent COD concentration was 140-220mg/L.when not planting plants,the average COD removal rate of sand-WTR bioretention's panting layer was 11.0%,and when planting plants,the average COD removal rate of sand-WTR bioretention's panting layer was 15.4%.When not planting plants,the average COD removal rate of sand-WTR bioretention's final effluent was 70.6%,and when planting plants,the average COD removal rate of sand-WTR bioretention's final effluent was 75%.So,the average COD removal rate of Sand-WTR bioretention's planting layer and the final effluent were both increased about 5% when planting plants.And the average COD removal rate of Sand bioretention's both planting layer and the final effluent were also increased about 5% when planting plants.So,COD removal was increased about 5% when planting plants.?9?Plants,microorganisms and media in bioretention system is synergistic in the removal of phosphorus and other pollutants.As an indispensable part in bioretention systems,plants in bioretention systems is more than landscaping,also it contributions to the remove of many kinds of pollutants in stormwater to some degree.