Controlling Stormwater Quality with Filter Soil—Event and Dry Weather Testing

Performance of two contrasting pilot swale designs for treating zinc, polycyclic aromatic hydrocarbons and glyphosate from stormwater runoff

Swales are a widespread stormwater management solution to reduce pollutant concentrations in runoff. An innovative pilot facility was constructed to evaluate the treatment efficiency of the two main types of water-quality swales, i.e. standard swales and filtering swales. Using stormwater roof runoff, without any additions or spiked with organic micropollutants, 12 runoff simulation runs mimicking frequent storm events were discharged longitudinally or laterally over the pilot swales. The performance of each swale was assessed for 4 micropollutants, i.e. zinc (Zn), glyphosate, pyrene and phenanthrene. These substances were mainly found in the dissolved phase of the stormwater runoff used to supply the pilot swales. The standard swale, constructed from a silt loam soil, partially managed stormwater runoff by infiltration. Micropollutant concentration reductions were higher in the infiltrated water (35-85%) than in the overflow (-13-66%). The filtering swale, made of a sandy central part bordered by silt loam embankments, completely managed stormwater runoff by infiltration, providing high micropollutant concentration reductions (65-100%). Mass load reductions were higher for the filtering swale (67-90% for Zn and ≥89% for organic micropollutants) than for the standard swale (33-73% for Zn, 19-67% for glyphosate and ≥50% for both pyrene and phenanthrene). For both swales, lateral inflow was often associated with significantly higher concentration and mass reductions than longitudinal inflow. Consequently, when designing swales for the treatment of micropollutants, practitioners should preferentially promote filtering swales and installations providing lateral diffuse inflow over the facility.

Analysis of swale factors implicated in pollutant removal efficiency using a swale database

Swales are traditional basic open-drainage systems which are able to remove stormwater-borne pollutants. In spite of numerous case studies devoted to their performances, parameters influencing the reduction of pollutant concentrations by swales remain elusive. In order to better characterize them, a database was set up by collecting performance results and design characteristics from 59 swales reported in the literature. Investigations on correlations among pollutant efficiency ratios (ERs) indicated that total trace metals (copper (Cu), zinc (Zn), cadmium (Cd), and lead (Pb)), total suspended solids (TSS), total phosphorus (TP), and total Kjeldahl nitrogen (TKN) exhibited many cross-correlated ERs. High ERs were observed for pollutants including a particulate form such as TSS (median ERs = 56%) and total trace metals (median ERs ≥ 62%), suggesting that these pollutants are efficiently trapped by sedimentation in swale bed and/or filtered within swale soil. Medium to high ERs were found for dissolved trace metals (median ERs ≥ 44%), whereas ERs for nutrient species were lower (median ERs ≤ 30%). The inflow concentration was identified as a major factor correlated to ER for most pollutants. For some pollutants, there is also a trend to get higher ER when the geometrical design of the swale increases the hydraulic residence time. Overall, this database may help to better understand swale systems and to optimize their design for improving pollutant removal.

Evaluating the Capability of Grass Swale for the Rainfall Runoff Reduction from an Urban Parking Lot, Seoul, Korea

This field study elaborates the role of grass swale in the management of stormwater in an urban parking lot. Grass swale was constructed by using different vegetations and local soil media in the parking lot of Mapu-gu Seoul, Korea. In this study, rainfall runoff was first retained in soil and the vegetation layers of the grass swale, and then infiltrated rainwater was collected with the help of underground perforated pipe, and passed to an underground storage trench. In this way, grass swale detained a large amount of rainwater for a longer period of time and delayed peak discharge. In this field study, various real storm events were monitored and the research results were analyzed to evaluate the performance of grass swale for managing rainfall runoff in an urban area. From the analysis of field experiments, grass swale showed the significant rainfall runoff retention in different rain events. Grass swale markedly reduced total rainfall runoff volume and peak flow during the small storm events of intensity about 30 mm/h. From the analysis, on average rainfall runoff retention from the grass swale was found around 40 to 75% during the various small rain events. From the results, we can say that grass swale is a stormwater mitigation practice which can help avoid flash flooding problems in urban areas.

Method for assessment of stormwater treatment facilities - Synthetic road runoff addition including micro-pollutants and tracer

Stormwater treatment facilities (STFs) are becoming increasingly widespread but knowledge on their performance is limited. This is due to difficulties in obtaining representative samples during storm events and documenting removal of the broad range of contaminants found in stormwater runoff. This paper presents a method to evaluate STFs by addition of synthetic runoff with representative concentrations of contaminant species, including the use of tracer for correction of removal rates for losses not caused by the STF. A list of organic and inorganic contaminant species, including trace elements representative of runoff from roads is suggested, as well as relevant concentration ranges. The method was used for adding contaminants to three different STFs including a curbstone extension with filter soil, a dual porosity filter, and six different permeable pavements. Evaluation of the method showed that it is possible to add a well-defined mixture of contaminants despite different field conditions by having a flexibly system, mixing different stock-solutions on site, and use bromide tracer for correction of outlet concentrations. Bromide recovery ranged from only 12% in one of the permeable pavements to 97% in the dual porosity filter, stressing the importance of including a conservative tracer for correction of contaminant retention values. The method is considered useful in future treatment performance testing of STFs. The observed performance of the STFs is presented in coming papers.

Selective removal of heavy metal ions by disulfide linked polymer networks

Heavy metal contaminated surface water is one of the oldest pollution problems, which is critical to ecosystems and human health. We devised disulfide linked polymer networks and employed as a sorbent for removing heavy metal ions from contaminated water. Although the polymer network material has a moderate surface area, it demonstrated cadmium removal efficiency equivalent to highly porous activated carbon while it showed 16 times faster sorption kinetics compared to activated carbon, owing to the high affinity of cadmium towards disulfide and thiol functionality in the polymer network. The metal sorption mechanism on polymer network was studied by sorption kinetics, effect of pH, and metal complexation. We observed that the metal ions-copper, cadmium, and zinc showed high binding affinity in polymer network, even in the presence of competing cations like calcium in water.