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WIREs Water
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Meeting the criteria: linking biofilter design to fecal indicator bacteria removal

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The capture, treatment, and reuse of storm‐water runoff are win–win propositions that can lead to improvements in both human water security and ecosystem health. Although not all treatment technologies facilitate the capture, treatment, and reuse of water, biofilters do. Biofilters are engineered analogues of natural systems that use low energy, natural processes to treat stormwater. Biofilter design is closely linked to treatment efficiency. As such, specific design components, such as submerged zones (SZs: saturated, organic‐rich layers near the base of biofilters), can significantly affect contaminant removal. Of particular interest, is the utility of SZ biofilter designs for removing indicators of pathogens, the so‐called fecal indicator bacteria (FIB). FIB exist at high concentrations in stormwater, sometimes several orders of magnitude above recreational, nonpotable reuse, or drinking water standards, and have been identified as one of the primary barriers to stormwater reuse. A comparison of FIB removal values from literature indicates that SZ systems significantly enhance FIB removal (∼10‐fold) relative to other design configurations (p < 0.05). Processes that may contribute to this effect include physicochemical filtration, biofilm formation, and protistan grazing, amongst others. A high degree of synergy exists between processes, and many unknowns remain. Model frameworks developed for evaluation of similarly synergistic systems, including biofilter analogues like the vadose zone, may be useful for addressing these unknowns and informing future biofilter design. WIREs Water 2015, 2:577–592. doi: 10.1002/wat2.1096 This article is categorized under: Engineering Water > Sustainable Engineering of Water Engineering Water > Water, Health, and Sanitation
Biofilter design schematics: (a) standard biofilter configuration without a submerged zone (SZ) and (b) advanced biofilter configuration with a SZ.
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Plots showing the change in dimensionless transport rate with pore water flow velocity (m s−1) that is predicted for 1 µm colloids under classical filtration theory. Sediment grains are assumed to be (a) 150 or (b) 1000 µm, spanning the range of grain sizes in biofilter media. The diffusion (solid red), sedimentation (dashed red), and interception (fine dashed red) components of dimensionless transport are calculated as in Yao et al. Their sum is the total dimensionless transport rate (shown in black). The total dimensionless transport rate is predicted to increase with decreasing velocity. This effect is more pronounced in systems with 1000 µm sediment grains than those with 150 µm sediment grains.
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Processes affecting fecal indicator bacteria (FIB) removal in (a) biofilters without submerged zone (SZ), and (b) biofilters with SZ. (1) Pore Water Velocity: Systems with or without SZ may have different vertical profiles of pore water velocity (shown schematically to the right of each filter column). Lower velocities exist in SZ designs and facilitate FIB capture. (2) Saturation and Flow Transients: Biofilters without SZ are unsaturated throughout, while systems with SZ are saturated near the base (see saturated SZ in panel (b)). Because they are less saturated, propagating wetting fronts may release more FIB in biofilters without SZ than those with SZ (compare unattached FIB [red rectangles] along the wetting front [black line with arrows] in biofilter (a) vs (b)). (3) Fissure Formation: Biofilters with SZ have a higher moisture content during dry periods between storms. This prevents media cracking and fissure formation, which can promote FIB leaching during subsequent rain events (see fissure [black gash] and FIB leaching [red rectangles] in schematic (a). (4) Vegetation: SZ biofilters promote plant health, including rapid growth rates and root development (see differences in plant color, size, and root length in biofilter (a) vs (b)). Well‐developed vegetation can increase FIB removal via root capture and competition. Thus, more unviable FIB (white rectangles) attach to plant roots near native microbial competitors (black circles) in biofilter (a) vs (b)). (5) Biofilm: Moist SZ conditions may increase biofilm formation: higher biofilm abundance (brown and yellow streaks) is present in SZ biofilter (b). Biofilm may increase (yellow) or reduce (brown) FIB capture/mortality, depending on the species. (6) DOM Release: The carbon source in SZ biofilters can leach DOM, and reduce FIB attachment to sediment grains (see unattached FIB [red rectangles] surrounding the carbon source [brown polygon] in biofilter (b)). This problem is not expected in standard, unamended biofilter designs. (7) Protozoan Grazing: Moist conditions increase the survival and motility of protozoa, as shown via the larger number of protists (gray balls) in SZ biofilter (b). This may increase grazing pressure and FIB mortality (note the number of unviable FIB [white rectangles] contained within protozoa in (b) vs (a)). (8) Microbial Competition: Moist, carbon‐rich SZ may increase the abundance and activity of native bacteria. This can enhance competition between natives and FIB, increasing FIB mortality (compare competition schematics between biofilter (a) and (b): note the higher abundance of microbial natives [black circles] and unviable FIB [white rectangles] in SZ biofilters (b)).
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Probability density functions of mean log10 fecal indicator bacteria (FIB) reduction (fecal coliforms, Escherichia coli, and enterococci combined) in systems with (red) or without (black) a submerged zone (SZ). 95% Confidence intervals for each distribution are shown using dashed lines of corresponding color. (a) Study‐averaged FIB removal data (reported in Table ) were used for this analysis. True average log10 FIB removal (the mean of the distribution of possible means) was ∼1.9 for SZ designs and 0.9 across other designs. (b) FIB removal data from 358 individual biofilter runs were used for this analysis. These data were compiled from a subset of studies noted in Table . True average log10 FIB removal was ∼2 for SZ designs and ∼1.1 across other designs.
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A close‐up of unsaturated sediments: filter cake layer (top) and filter media layer (bottom). (a) Size‐dependent processes that contribute to particle capture. Large particles (dark brown) that cannot pass through filter media are captured in the filter cake by mechanical filtration. Small black particles illustrate classical modes of capture via physicochemical filtration. Particle (A) has been captured by sedimentation, particle (B) by diffusion, and particle (C) by interception. Physicochemical processes that are specific to unsaturated media are shown in blue. Particle (F) has been trapped at the air–water interface, particle (G) against a sediment grain by thin film straining, and particle (H) in a pendular ring of water between grains. Small red particles illustrate capture by straining in narrow pore throats (D) and wedging at grain junctions (E). (b) A subset of the biological processes affecting bacterial survival. Black particles represent bacteria grazed by protozoa (shown in grey). Particles (A) are engulfed by phagocytosis: (A‐solid) is ingested, digested, and then excreted as waste (A‐dashed). Particles (B) are grazed by a protozoan that specializes in biofilm bacteria. Intraspecific microbial competition is illustrated via particles (C) and (D). Particles (C) show contest competition, whereby a native microbial biofilm (brown and black plaque) excretes a substance that harms nearby competitors (C‐dashed) but not distant cells (C‐solid). Particles (D) show scramble competition, whereby native biofilm communities acquire nutrients (in this case nitrate) more efficiently than species D): this harms distant cells (D‐dashed), but may aid nearby neighbors (D‐solid).
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