An evaluation of the treatment performance of a ten year old stormwater biofilter in Sweden
Abstract: Urban runoff may be contaminated with, for example, metals, nutrients and sediment. Generally, such runoff enters waterways and oceans without any type of treatment. Bioretention systems can be used to protect the aquatic environment since one of their main objectives is to remove pollutants from stormwater. This study presents data on one of Sweden´s first biofilters, constructed 10 years ago in the municipality of Tyresö. The main aim of this thesis is to evaluate the treatment performance of an operational biofilter through a comparison of inlet and outlet stormwater quality. The objectives include: Determine the concentrations of selected heavy metals and nutrients in stormwater at the inlet and outlet of the biofilter Identify if, and if so how, heavy metal concentrations and nutrient concentrations change after treatment Assess the condition of the biofilter in relation to particle size distribution and constitution of the filter media compared to data generated in an earlier study Ten sampling campaigns were conducted in 2015 involving the collection of stormwater at the inlet and outlet. Rainfall data was also collected for each event. Samples were analysed for selected metals (lead, cadmium, copper, zinc, nickel and chromium) and nutrients (nitrogen and phosphorous). At the end of the sampling campaign, samples of the biofilter’s filter media were analysed for particle size distribution. Data from a previous study conducted in 2013 was also used in this thesis to provide a baseline data set for comparison. The event mean concentrations of most monitored substances were lower in the outflow than inflow, except for chromium which typically showed an increase. Calculated removal efficiencies show considerable variation, with low removal efficiencies in comparison to previous studies reported in the literature. The low removal performance may be explained by the relatively low pollutant concentrations in both inflowing and outflowing stormwater. If the inflow concentrations are close to the “irreducible concentrations” (Cirr) of a stormwater facility, no further reductions are likely. There could even be negative removal as even small variations in concentration can generate (translate into) substantial negative increases. Comparison of the current data set with the results of the study conducted in 2013 indicate that the treatment performance of the biofilter has not changed suggesting that, despite the colder climate, biofilters can continue to remove pollutants from stormwater after a ten year period of operation. Analyses showed that the filter media has the same particle size distribution as when the biofilter was constructed. Deeper samples of the filter media have lower metal concentrations than those reported in samples collected at the surface of the biofilter.
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