River Bank Filtration
Bank filtration (BF) is the abstraction of water from aquifers that are hydraulically connected to a surface
water body, which is in most cases a riverine system. BF involves wells being established directly by
the river used for water purification. By pumping from the wells, the groundwater table is lowered and
the surface water infiltrates into the aquifer. As the surface water infiltrates into the aquifer, various
physical, chemical and biological processes take place (Figure 1). Depending on the geological conditions,
the distance between the wells and the river bank and the level of the river, water infiltration can
last a few days or several months.
The attenuation of particles, organic and inorganic compounds through BF is site specific. Depending on the
water quality targets, additional treatment may be necessary. In most cases, BF acts as a pre-treatment
step in drinking-water production, while in some cases it can serve as the final treatment step just
before disinfection (Tufenkji et al., 2002, Grischek & Ray, 2009).
BF wells can be either
horizontal (or radial collector wells - RCW) or
vertical, depending on the site conditions and the choice of the competent utility (Figure 2).
The construction of RCW is more expensive compared to vertical wells, but RCW have greater production capacity.
The formation of a clogging layer within the riverbed is important for managing BF sites. Clogging is referred
to as the reduction of riverbed permeability and is the result of the infiltration and accumulation of
both organic and inorganic suspended solids, precipitation of carbonates, iron- and manganese- (hydr)oxides
and biological processes. The recoverable amount of bank filtrate strongly depends on the infiltration
resistance of the riverbed and the technically viable drawdown of the adjacent pumping wells. An infiltration
rate of about 0.2 m3/(m2/d) is considered sustainable, based on long term experience from RBF sites in
Europe with high organic load (DOC 5 – 10 mg/L) (Grischek & Bartak, 2016).
Reduction of pollutant levels in BF is attained by physical filtering, sorption, microbial degradation, ion
exchange and precipitation. In addition, dispersion of surface water pollutants in aquifers and further
dilution of these pollutants with groundwater (where the respective concentrations are expected to be
low) help reduce the pollutant concentrations of the pumped water.
Higher temperatures enhance biodegradation and promote the removal of oxygen, nitrate and Mn/Fe-hydr(oxides);
many removal processes are redox dependent. The subsurface retention time is important for pollutant
removal. Longer retention times result in higher pollutant removal rates. Aerobic redox conditions promote
higher DOC degradation kinetics (Gruenheid et al., 2005). The interaction of the physical, hydro-geochemical
and microbiological processes involved in BF is complex and water quality changes during BF, such as
Fe/Mn dissolution, are difficult to predict (Grischek & Paufler, 2017).
Table 1: Removal of Pathogens during BF
(Source: Huelshoff et al., 2009)
(measured / estimated)
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