The influence of temperature and salt on metal and sediment removal in stormwater biofilters

Abstract

Stormwater biofilters are used to treat stormwater runoff. In countries with cold winter climates, biofilters are subject to low temperatures which, in some cases, are combined with potentially high salt concentrations from road de-icing, potentially affecting the biofilter's performance. Since stormwater biofilters have been developed without consideration of their critical winter use, a laboratory study was carried out to evaluate the performance of stormwater biofilters subjected to low and high temperatures, with and without salt. Both factors and their interaction had a significant effect on outflow concentrations and removal percentages. Salt had a negative impact on outflow concentrations, causing lower removal percentages for (especially dissolved) metals, this impact being most pronounced for Cu and Pb. The unrealistic combination of salt with high temperature seemed to further amplify the negative impacts of salt despite the fact that temperature alone did not cause significant differences in outflow concentrations and removal percentages. Still, biofilters showed the ability to treat stormwater efficiently under the simulated winter conditions; outflow concentrations for total metals as a minimum met the class 4 threshold value defined in the Swedish freshwater quality guidelines, while inflow concentrations clearly exceeded the threshold value for class 5. The relatively coarse filter material (which is recommended to facilitate infiltration during winter) did not seem to exacerbate biofilter performance.

Pilot Scale Testing of Adsorbent Amended Filters under High Hydraulic Loads for Highway Runoff in Cold Climates

“…Additionally, [20,27] reported that metals adsorbed on soils designed for bioretention were scarcely leached due to NaCl applications and, therefore, retention under cold climate conditions is likely to be effective for the studied purpose. However, [28] detected a significant impact of salt addition on the removal of dissolved Pb and Cu, which might be due to the fact that the salt favors dissolution of organic matter which has a strong affinity of these metals. In the present study, a high release of Ni (9%-15%) from the charcoal amended filter was detected.…”

Laboratory study of infiltration into two frozen engineered (sandy) soils recommended for bioretention

“…() and Søberg et al . () followed the general guidance offered by Caraco and Claytor () and Muthanna et al . ().…”

Hot, Salty Water: A Confluence of Issues in Managing Stormwater Runoff for Urban Streams

“…Their review did not include salt as an LID concern. Other studies have found that bioretention does not retain salt (Khan et al ., ; Soberg et al ., ); that salt in bioretention systems increases heavy metal release (Soberg et al ., ); and that salt affects bacterial communities subsequently affecting the uptake potential in a bioretention system (Endreny et al ., ). Further, studies have shown salt use within a watershed can be a chronic issue for both surface and groundwater and is not limited to winter conditions (Perera et al ., ; Corsi et al ., ).…”