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[[File:Bioretnetion TP.jpg|200px|thumb]]

[[File:Bioretnetion TP.jpg|200px|thumb]]

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The mechanisms involved in, and ability of bioretention to reduce bacteria and other microbial pathogen concentrations is also an area of active research. Preliminary laboratory and field study results report good but variable removal rates for fecal coliform bacteria from biofilters and bioretention cells (Rusciano and Obropta, 2005<ref> Rusciano, G.M., Obropta, C.C. 2007. Bioretention Column Study: Fecal Coliform and Total Suspended Solids Reductions. Transactions of the ASABE. 50(4): 1261-1269. https://elibrary.asabe.org/abstract.asp??JID=3&AID=23636&CID=t2007&v=50&i=4&T=1 </ref>; Hunt ''et al''., 2006<ref>Hunt, W.F., A.R. Jarrett, J.T. Smith, and L.J. Sharkey. 2006. Evaluating Bioretention Hydrology and Nutrient Removal at Three Field Sites in North Carolina. ASCE Journal of Irrigation and Drainage Engineering. 132(6): 600-608.</ref>; TRCA, 2008<ref>. Performance Evaluation of Permeable Pavement and a Bioretention Swale, Seneca College, King City, Ontario. Prepared under the Sustainable Technologies Evaluation Program (STEP). Toronto, Ontario.</ref>). In a recent review, Clary et al. report bioretention E.coli removal efficiency of 42.5% and fecal coliform removal efficiency of 99.4% based on median inlet and outlet concentrations from 12 and 8 studies, respectively <ref> Clary, J. Jones, Leisenring, M., Hobson, P., Strecker, E. 2020. International Stormwater BMP Database 2020 Statistical Summary. https://www.waterrf.org/system/files/resource/2020-11/DRPT-4968_0.pdf</ref>. In a recent article, Peng et al. (2016) review factors influencing microbial removal and effects of design choices on treatment performance.  They found that approaches for improving the removal of microorganisms by biofilters could involve altering the grain size range and surface properties of the filter media. This could involve the use of filter media with smaller average grain sizes, the inclusion of [[Additives |additives]] (e.g., activated carbon, zeolite, or biochar) to improve filtration rates, or chemical modifications of filter media grain surfaces (e.g., with biocides) to promote microbial die-off. Including an [[Bioretention: Internal water storage |internal water storage reservoir]] was also found to improve microbial removal rates <ref> Peng, J., Cao, Y., Rippy, M.A., Nabuil Afrooz, A.R.M., Grant, S.B. 2016. Indicator and Pathogen Removal by Low Impact Development Best Management Practices. Water. 8. 600. https://www.mdpi.com/2073-4441/8/12/600 </ref>.<br>

The mechanisms involved in, and ability of bioretention to reduce bacteria and other microbial pathogen concentrations is also an area of active research. Preliminary laboratory and field study results report good but variable removal rates for fecal coliform bacteria from biofilters and bioretention cells (Rusciano and Obropta, 2005<ref> Rusciano, G.M., Obropta, C.C. 2007. Bioretention Column Study: Fecal Coliform and Total Suspended Solids Reductions. Transactions of the ASABE. 50(4): 1261-1269. https://elibrary.asabe.org/abstract.asp??JID=3&AID=23636&CID=t2007&v=50&i=4&T=1 </ref>; Hunt ''et al''., 2006<ref>Hunt, W.F., A.R. Jarrett, J.T. Smith, and L.J. Sharkey. 2006. Evaluating Bioretention Hydrology and Nutrient Removal at Three Field Sites in North Carolina. ASCE Journal of Irrigation and Drainage Engineering. 132(6): 600-608.</ref>; TRCA, 2008<ref>. Performance Evaluation of Permeable Pavement and a Bioretention Swale, Seneca College, King City, Ontario. Prepared under the Sustainable Technologies Evaluation Program (STEP). Toronto, Ontario.</ref>). In a recent review, Clary et al. (2020) report bioretention E.coli removal efficiency of 42.5% and fecal coliform removal efficiency of 99.4% based on median inlet and outlet concentrations from 12 and 8 studies, respectively <ref> Clary, J. Jones, Leisenring, M., Hobson, P., Strecker, E. 2020. International Stormwater BMP Database 2020 Statistical Summary. https://www.waterrf.org/system/files/resource/2020-11/DRPT-4968_0.pdf</ref>. In a recent article, Peng et al. (2016) review factors influencing microbial removal and effects of design choices on treatment performance.  They found that approaches for improving the removal of microorganisms by biofilters could involve altering the grain size range and surface properties of the filter media. This could involve the use of filter media with smaller average grain sizes, the inclusion of [[Additives |additives]] (e.g., activated carbon, zeolite, or biochar) to improve filtration rates, or chemical modifications of filter media grain surfaces (e.g., with biocides) to promote microbial die-off. Including an [[Bioretention: Internal water storage |internal water storage reservoir]] was also found to improve microbial removal rates <ref> Peng, J., Cao, Y., Rippy, M.A., Nabuil Afrooz, A.R.M., Grant, S.B. 2016. Indicator and Pathogen Removal by Low Impact Development Best Management Practices. Water. 8. 600. https://www.mdpi.com/2073-4441/8/12/600 </ref>.<br>

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Recent research into the role of plants in bioretention confirms they play an important roles in hydraulic and nitrogen removal performance. In a recent review of scientific literature, Dagenais ''et al.'' (2018) found that planted facilities are more effective than unplanted ones, as the presence of plants increases filter bed permeability and nitrogen removal. Plant species selection can considerably affect hydraulic and nitrogen removal performance, with root traits (e.g., thickness and depth) identified as playing important roles. They identified further research needed to test the hypothesis that native or diversely-planted facilities perform better than ones planted with exotic or fewer species.<ref>Dagenais, D., Brisson, J. and Fletcher, T.D. 2018. The role of plants in bioretention systems; does the science underpin current guidance?. Ecological Engineering, 120, pp.532-545. http://www.phytotechno.com/wp-content/uploads/2018/10/Dagenais-2018-Bioretention.pdf</ref>

Recent research into the role of plants in bioretention confirms they play an important roles in hydraulic and nitrogen removal performance. In a recent review of scientific literature, Dagenais ''et al.'' (2018) found that planted facilities are more effective than unplanted ones, as the presence of plants increases filter bed permeability and nitrogen removal. Plant species selection can considerably affect hydraulic and nitrogen removal performance, with root traits (e.g., thickness and depth) identified as playing important roles. They identified further research needed to test the hypothesis that native or diversely-planted facilities perform better than ones planted with exotic or fewer species.<ref>Dagenais, D., Brisson, J. and Fletcher, T.D. 2018. The role of plants in bioretention systems; does the science underpin current guidance?. Ecological Engineering, 120, pp.532-545. http://www.phytotechno.com/wp-content/uploads/2018/10/Dagenais-2018-Bioretention.pdf</ref>