All SuDS address water quality improvement through a combination of pollutant removal mechanisms including (Scholes et al. 2008):
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• Settling
• Filtration
• Volatilization
• Adsorption
• Flocculation
• Precipitation
• Ion exchange
• Photolysis
• Plant and algal uptake
• Microbial degradation
Passive filter drain systems most importantly rely on fundamental hydraulic, geochemical and biochemical processes, including sedimentation of particulates, adsorption and precipitation, and biological assimilation for the remediation of pollutants (Huisman and Wood 1974). However, in all systems the actual (bio)geochemical processes are inherently complex; multiple removal mechanisms occur simultaneously and are specific to the pollution type and loading experienced. Moreover, much of what we assume occurs in a SuD systems is based on our knowledge of other filtration systems (such as potable and wastewater), rather than a detailed study of SuDS. Thus, for SuDS filters, our understanding of the underpinning processes and their relative importance remains poor, potentially leading to inefficient operation due to non-optimal design. In general, the succeeding paragraphs summarize the mechanisms believed to be the most important in filtration based SuDS.
Physical-chemical removal of pollutants is thought to occur in two steps. First, physical transportation mechanisms bring particles and dissolved contaminants in contact with filter media for possible subsequent removal from solution. These physical forces include (Huisman and Wood 1974):
• Straining or screening - acts to intercept and retain large particles within pores of the media, mostly at the surface
• Sedimentation - the act of particles settling on and in between the media
• Inertial and centrifugal forces - a force of gravity from a larger particle that acts to pull a particle from the water flow
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• Diffusion - the act that brings particles and dissolved phases into contact with the media
Once transport mechanisms bring pollutants in contact with the filter media, the following attachment mechanisms hold particles and dissolved phases to the surface and can be regarded under the general term of adsorption (Huisman and Wood 1974):
• Electrostatic attraction – attraction between two opposite electrical charges, mainly an attraction force holding particles and dissolved phases to the surface of the media but can also contribute to transportation mechanisms
• Van der Waals forces – weak mass attraction that draws particles and dissolved phases from water and holds to media surface, more effective as an attraction mechanism but can also contributes to transport mechanisms
• Adhesion – deposition and adherence of particles and pollutants to sticky gelatinous film of biological growth (schmutzdecke in slow sand filtration, biofilm in other aqueous environments such as a filter drain)
Victor Goldschmidt first put forth the concept of metal adsorption to mineral surfaces in the 1930’s when lower than expected heavy metals were observed in seawater and experimentation showed uptake by iron and manganese oxides (Bradl 2005). Many geochemical factors influence the adsorption of heavy metals onto mineral surfaces, most importantly, pH, ionic strength, metal speciation and competition.
Another potential mechanism of heavy metal removal that may occur in filter drains is precipitation. Precipitation occurs in aqueous systems when a change in geochemical conditions occur which cause the aqueous system to become supersaturated with respect to an insoluble solid phase, often in the form of hydroxides (the most common form for precipitated metals at low temperature), chlorides, sulfates, carbonates or sulfides (Bradl 2005; Kurniawan et al. 2006).
Supersaturation is commonly reached by a shift in pH, Eh or an increase in concentration of dissolved constituents. Precipitation is generally considered an irreversible process
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Other possible immobilization mechanisms within runoff include metals undergoing complexation with natural organics such as humic and fulvic acid, or synthetic agents such as EDTA, and is possible in SuDS filter drains due to the natural occurrence of dissolved organics ubiquitous to aquatic systems and pollutants in runoff (Schlesinger 1979).
Finally, due to the typically hydrated environment that filtration based SuDS offer, it is possible that biological growth in the form of biofilms may occur and thus contribute to metal removal. The role of biofilms in pollutant removal by SuDS has yet to be examined in detail. However, based on other non-SuDS based examination of bacteria-metal interactions, possible biological mechanisms for metal removal in a filter drain include:
• Adsorption and adhesion to cell surfaces and extracellular polymeric substances (EPS) (Beveridge and Murray 1980; Mittelman and Geesey 1985; Mullen et al. 1989; Fein et al. 1997; De Philippis et al. 2001;
Konhauser 2007)
• Active internalization of metals for cell function (Konhauser 2007)
• Accumulation in naturally occurring biofilms (Meylan et al. 2003; Serra et al. 2009; Ancion et al. 2010)