| The stormwater bioretention system,identified as the prior option by the Chinese guideline on sponge city construction,has numerous benefits such as a good performance in controlling pollutant discharge to receiving water,reducing runoff and so force,among which its function to purify urban stormwater is outstanding particularly.They are hence widely adopted in various sites like squares,street side,road green belts,etc.However,their biggest disadvantage is a high likelihood of failure because of clogging that occurs primarily due to the deposition of sediments,which will be more evident and serious in mountainous regions.With Chongqing Province being established as one of the first sponge cities constructed,it's highly urgent to conducted researches on bioretention techniques applied in mountain cities.In effect,clogging is closely related to the structure of medium layer.Not only can an appropriate design significantly reduce clogging,it may also lead to a better performance in terms of the volume of stormwater treated and pollutant removal.Therefore,based on the typical climate in Chongqing,this paper shown investigated the media-layer construction of biofilters to treat stormwater with a huge content of total suspended solids(TSS),with an expectation of developing a novel construction which has a strong anti-clogging ability together with a relative good contaminant-removal capacity to provide technical support for the application of biofilters in mountainous regions.In the study presented,an investigation for the growing media combination ratio(3%,5%,7%,9% and 11%)of flyash(with the mean diameter of 43?m)and fine sand(with diameter less than 0.5mm)was carried out at first to figure out the ideal one,then based on which a comparative analyses of different surface protective layer(with depth of 120mm)was done to ascertain the best one.Finally,the running performance of the novel construction was observed under different volume capture ratio of annual runoff(VCRAR).Conclusions obtained could be outlined as below:1)In the potable-water inflow phase,the infiltration rates of biofilters with different combination ratio decreased at first,while they all became smooth at last.In addition,there were not obvious changes of hydraulic conductivity for all trial biofilters under inflow with the semi-synthetic stormwater during the experiment.The hydraulic conductivities of column filters with the rest comination ratio except to 3% were between 100~300mm/hr recommanded by relative guidelines.Compared with other investigated ratios,using the 7% to construct growing-media layer performed better generally,which has a stable structure and a relative preferable contaminant-removal performance,for instance,the median removal of TSS,chemical oxygen demand(COD),total nitrogen(TN),nitrate nitrogen(NO3-N)and total phosphorous(TP)up to 99.8%,68.6%,76.1%,71.4% and 81.8% respectively.2)The infiltration rate of biofilters with zeolite(with diameter between 3 to 6mm)or coconut coir protective layer was continually decreasing during the running cycle,while the one with the scoria(with diameter between 3 to 6mm)had a smoother hydraulic conductivity.At first it decreased constantly,but it was to stability after six rainfalls,finally up to the value 72 ± 9.9mm/hr,revealing that it has the strongest anti-clogging capacity.What's more,compared to other investigated materials,adopting the scoria to construct the surface protective layer had a preferable pollutant-removal performance,with a median removal of TSS,COD,TN,NO3-N and TP being 99.8%,72.6%,87.1%,89% and 98.1% respectively.Results indicated that it might be the best construction for the medium layer to adopt scoria constructing the surface-protective layer,and flyash together with fine sand building the growing-media layer with the combination ratio of 7/93 measured by weight.3)Under different VCRAR,the changes of hydraulic conductivity of biofilters with the novel medium-layer construction were almost similar.In the initial phase the higher the VCRAR,the huger the decrease rate of hydraulic conductivity.But it became inverse later.After running around five times,the decrease trend of infiltration rate of all trial systems disappeared and they were all to the same value.Besides,VCRAR had limited impacts on the contaminant-removal performance of the novel media-layer construction especially the removal of TSS and TP.Results showed that the novel construction has a strong resistance against the change of VCRAR.The construction along with its performance of growing-media layer and surface protective layer of biofilters was illustrated in this study presented,in accordance with which a novel medium-layer construction was proposed.This work would provide a theoretical basis and technical support in the application/promotion of urban stormwater bioretention systems in mountain regions,having a significance and practical value. |
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