Bank filtration is a type of filtration

Filtration

Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...

 that works by passing water to be purified for use as drinking water

Drinking water

Drinking water or potable water is water pure enough to be consumed or used with low risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually...

 through the bank

Bank

A bank is a financial institution that serves as a financial intermediary. The term "bank" may refer to one of several related types of entities:...

s of a river or lake. It is then drawn off by extraction wells some distance away from the water body. The process may directly yield drinkable water, or be a relatively uncomplicated way of pre-treating water for further purification.

Efficiency

Three filtration mechanisms are possible. Physical filtration or straining takes places when suspended particulates are too large to pass through interstitial spaces between alluvial soil particles. Biological filtration occurs when soil microorganism

Microorganism

A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...

s remove and digest dissolved or suspended organic material and chemical nutrient

Nutrient

A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...

s. Chemical filtration or ion exchange

Ion exchange

Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion...

 may take place when aquifer soils react with soluble chemicals in the water. Most 'normal' contaminants (microbial organisms and inorganic or organic pollutants) will be removed by bank filtration, either because they get filtered out by the sand/earth

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 of the bank, or because the passage time (which may be days or potentially weeks) is sufficient to render them inactive. Research has also shown that the removal efficiency depends not only on the contaminant, but also on the "hydraulic and chemical characteristics of the bottom sediment and the aquifer, the local recharge-discharge conditions, and biochemical processes".

There have been indications that some pharmaceutical compounds (medical drug traces from human use) may not always be sufficiently removed by bank filtration, and that in areas with substantial contamination of this type, additional treatment may be needed.

History

The process has been in use in Europe, especially in Germany along the Rhine and later in Berlin, since the 1870s. Major facilities also exist in many other countries, including the United States, where Nebraska is leading in the use of such facilities.

Wastewater applications

Alluvial soils may also be used to purify waste-water being returned to a river after use. Treated wastewater is typically discharged into a percolation

Percolation

In physics, chemistry and materials science, percolation concerns the movement and filtering of fluids through porous materials...

 pond on the alluvial flood plain rather than flowing directly into the river. Chemical filtration will cease when all reactive sites within the flow pathway through aquifer soil have reacted. Physical filtration tends to become more effective as trapped particulates block interstitial passages between alluvial soil particles; but may ultimately limit flow rates through the soil. Particulates on the river side of the soil interface may be periodically scoured away by turbulent flow. Wastewater percolation ponds may require maintenance to break up or remove a scum layer (Schmutzdecke

Schmutzdecke

Schmutzdecke is a complex biological layer formed on the surface of a slow sand filter...

) forming at the bottom of the pond. Biological filtration remains effective unless organic loading causes anaerobic

Hypoxia (environmental)

Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...

conditions within the riverbed alluvium.

External links

• BankFiltration.Org (German website promoting advantages of and research into bank filtration)