Bank Filtration

Bank Filtration (BF) has been common practice in many European cities for more than 100 years. It was originally initiated to remove pathogens and suspended solids from increasingly polluted surface waters. Increasing demand required adaptations such as sand layers incorporated in percolation ditches (SCHMIDT et al. 2003).

For drinking water production, BF is regarded as a possible pre-treatment step within the multi barrier approach in the overall treatment chain recommended by the WHO (2011). BF can potentially reduce or stabilise the microbial, natural organic matter and particulate load. It makes drinking water safer and more acceptable for the consumer, but not all undesired substances can be removed during soil and aquifer passage (SCHMIDT et al. 2003; HUELSHOFF et al. 2009, Relevance and Opportunities). The BF method prevents from shock loads and assures the removal of particles, biological contaminants and biodegradable compounds (SCHMIDT et al. 2003). Furthermore it allows for temperature adjustment.

Besides the treatment of surface water for drinking purposes, BF is also a managed aquifer recharge (MAR) technique, since the water flows through the soil and mixes with the existing groundwater from the aquifer. MAR is used to enhance the quality of treated sewage and surface water for both drinking and non-drinking purposes and prevents overuse of aquifers, saltwater intrusion and land subsidence (also see soil aquifer treatment, surface groundwater recharge and subsurface groundwater recharge).

BF is the abstraction of water from aquifers that are hydraulically connected to a surface water body, which in most cases is a river or a riverine system. By pumping from wells adjacent to a surface water body, the groundwater table is lowered and water from the surface water body infiltrates into the aquifer. This so-called bank filtrate percolates through the soil towards the production wells (e.g. a borehole) where it mixes with the ambient (or natural) groundwater (HISCOCK & GRISCHEK 2002). 

The beneficial filtration process occurs mainly in two zones: a biologically active layer where intensive degradation and adsorption processes occur within a short residence time; and along the main flow path between the surface water body and the abstraction well where degradation rates and sorption capacities are lower and mixing processes more quickly (HISCOCK & GRISCHEK 2002; SCHMIDT et al. 2004).

Due to its easy implementation and little maintenance requirements, BF is considered to be a useful drinking water (pre-)treatment method for developing and newly-industrialised countries. BF systems are particularly known for the efficient removal of pathogens, suspended solids, toxic algae, or organic trace compounds (e.g. pharmaceutical products) from surface water, all being water quality parameters of high relevance (SPRENGER et al. 2006; HUELSHOFF et al. 2009). Experience from Europe has demonstrated that BF is suitable to remove a range of organic and inorganic contaminants while an exhaustion of cleaning capacity has not been observed (SCHMIDT et al. 2003).

BF offers freshwater storage in times of heavy precipitation (e.g. during monsoon) or elevated stream run-off (e.g. snow melt from higher altitudes). Underground freshwater stocks are protected from evaporation and deterioration (JIMENES 2008). They can be stored until dry weather conditions require groundwater abstraction to meet water demand (HUELSHOFF et al. 2009). This will be increasingly useful with the more extreme weather patterns that are being anticipated with climate change and that pose challenges to developing countries, which are usually more vulnerable to water shortages or flooding (HUELSHOFF et al. 2009).

BF systems can mitigate shock loads resulting from chemical spills or defects in industrial wastewater plants. Furthermore, they can even out temperature differences between surface water and groundwater aquifers; for instance when rivers receive cooling water from a power station (SCHMIDT et al. 2003).

Because of its relatively low cost and the fact that neither high-tech nor highly skilled labour is required, BF is suitable as a water purification tool in developing countries (SPRENGER et al. 2006).

(Adapted from HUELSHOFF et al. 2009)

Several physical, chemical and biochemical processes are responsible for the improvement of water quality during subsurface passage (SHARMA & AMY 2009). The processes that remove substances in the bank filtrate include:

• Straining or filtration: The mechanical filtration process retains suspended matter in the soil, depending on pore throat size.
• Biodegradation and decay: biodegradation and radioactive decay are the only sustainable removal processes. Biodegradation is the main driver for redox processes occurring during subsurface passage and is responsible for the breakdown of dissolved and/or sediment-bound organic matter.
• Sorption and desorption: Trace elements such as iron, manganese and various heavy metals are eliminated during ground passage, mainly by sorption processes (SCHMIDT et al. 2003) and by accumulating on the surface of an adsorbent or substrata. Biological contaminants such as protozoa, bacteria and viruses are reduced by a combination of processes including adsorption to aquifer materials and inactivation (SCHMIDT et al. 2003).
• Ion exchange: In aerobic aquifers, removal is achieved by ion exchange processes at negatively loaded surfaces of clay minerals, amorphous ferric oxides and alumina and organic solid matter (SCHMIDT et al. 2003). 
• Chemical precipitation: In anoxic aquifers, the removal of metal ions is dominated by precipitation reactions with sulphide (SCHMIDT et al. 2003) when insoluble compounds form and precipitate from the solution.

The removal of biological contaminants through BF is most efficient when groundwater velocity is slow and when the aquifer consists of granular materials with high grain surface contact. Levels of many organic micropollutants can be reduced or even eliminated during both aerobic and anaerobic underground passages (MUELLER et al. 2010; MAENG et al. 2009). Attenuation is extremely dependant on the underlying redox processes (SCHMIDT et al. 2004). The purification capacity of BF schemes regarding surface-water pollutants depend on (SCHMIDT et al. 2003):

• Quality of the surface water
• Geological conditions
• Porosity of the soil (flow velocity)
• Covered distance determined by the permeability
• Hydraulic residence time of the water in the soil
• Hydraulic potential in the aquifer
• Temperature (see HUELSHOFF et al. 2009)
• PH-values
• Oxygen concentration

Despite BF being widespread in Central Europe, knowledge about processes during BF is still scarce. Especially information about the physical, chemical and biological processes of water purification, extension of the active sand filter zone, significance of bank vegetation for water purification, hydraulic permeability with its clogging phenomenon (GUENTER 2011) and influence of changing environmental conditions on the operation of BF schemes is rare (HUELSHOFF et al. 2009).

BF provides about 50% of potable water supplies in the Slovak Republic and 45% in Hungary (HISCOCK & GRISCHEK 2002). In Germany, more than 300 water works use BF and about 16% of German drinking water is produced from bank filtrate.

In Central Europe, increasing chemical pollution, high concentrations of ammonia, organic compounds and micropollutants in the river water called for the introduction of supplementary pre- and post-treatment steps to build up a multi-barrier system. Aeration or ozone may be used to oxidise iron and manganese, or to activate carbon for adsorption of and protection against more persistent contaminants.

Today nearly all water utilities situated along large rivers use granular activated carbon filters, often combined with isolation and filtration (SCHMIDT et al. 2003).