Biofilters for Stormwater Harvesting: Understanding the Treatment Performance of Key Metals That Pose a Risk for Water Use

The concept of recycling nontraditional water sources like stormwater is becoming increasingly popular as the quality and quantity of natural water resources continue to be depleted. A practical method of treatment has not been set in play, however, because affordable technology is yet to deliver promising results. There have been good results for biofiltration systems, specifically in the treatment of polluted stormwater discharges that enter receiving waters and affect aquatic health. Biofilters have effectively removed pollutants and harmful chemicals to meet aquatic health standards and protect the ecosystems of receiving waters; however, very little study has been done to address the capacity of biofiltration for treating water to the standard of human use (and consumption). The behavior of iron—its impact on taste and color—are of particular concern. Feng et al. (2012) constructed multiple biofiltration systems—varying the plant species used (C. appressa, D. revolute, M. stipoides, L. brownii, and M. ericifolia), filter media types and depths (300, 500, and 700 mm), inflow volumes (50, 25, and 12.5 L ), and concentrations of pollutants—to optimize the removal of iron and other toxic metals and pollutants so that the required drinking water standards may be met. They found that vegetation and filter type were significant to the treatment of metals, and that larger filter media depths increased the outflow of toxic metals; they concluded that biofilters can be developed as viable water treatment systems that remove iron and meet the required drinking water standards.—Genevieve Heger
            Feng, W., Hatt, B. E., Fletcher, T. D., Deletic, A. 2012. Biofilters for Stormwater Harvesting: Understanding the Treatment Performance of Key Metals That Pose a Risk for Water Use. Environ. Sci. Technol. 5100–5108

            Feng et al.constructed several biofilter columns using various plant species (C. appressa, D. revolute, M. stipoides, L. brownii, and M. ericifolia), filter media types and depths (300, 500, and 700 mm), inflow volumes (50, 25, and 12.5 L), and concentrations of pollutants. To simulate storm events of the area, synthetic stormwater was prepared and volumes equivalent to 4.53 mm rainfall depths (1 in 4 month average for Melbourne) were dosed into the constructed columns twice a week between June 2006 and April 2007. Approximately every seven weeks, the outflow samples from each column were collected and analyzed for concentrations of metals (Fe, Pb, Cu, Zn, Al, and Cr), total suspended solids, and nutrients. The hydraulic conductivity of each column was also measured, and the results were estimated based on Darcy’s law. The data were analyzed by comparison with reference to Australian guidelines of for drinking water (as well as for irrigation and protection of aquatic ecosystems).  Tukey or Tamhane’s posthoc tests were used to determine the significance difference of treatment performance between columns (e.g. filter media depth). Lastly, this model by Johnson et al. was used.
ln(Co/Ce
1) = knM2rmediax/ 1E6 Cou * BV
[Where Co is the inflow concentration of pollutants (mg/L), Ceis the outflo concentration (mg/L), k is the rate constant (1/day), NMis the capacity of th filter media used (mg pollutant/g media),
rmedia is the saturated density of the filter media (g/m3), x is the filter depth (m), u is te unit loading rate (m/day), and BV is the total volume treated divided by volume of media.]
The measured (Co/Ce
1)/ Co  value was plotted against the measured hydraulic conductivity to examine the impact pollutants had on outflow concentrations.
            In summary, the authors found that biofiltration systems pose treatment technology for delivering safe water into surrounding aquatic ecosystems, and meet the majority of Australian drinking water standards. Outflow concentrations of iron (0.3mg/L), and the removal of aluminum never seemed to reach the drinking water standard.
The variation of each component in the column proved a certain significance according to the data collected. Vegetation made significant difference in metal removal—C. appressa demonstrated good removal of iron, whereas M. ericifolia did not. Filter media type and depth also played a significant role in metal removal—vermiculate and perlite showed the best removal of zinc, where mulch and compost demonstrated great removal of aluminum and chromium, and it was concluded that larger media depths resulted in higher outflow of metals. Testing biofiltration performance under laboratory conditions is not entirely reflective of  of the system; however, it provides valuable insight and excellent evidence for further experimentation.

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