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2.4.01 A natural water filter – bank filtration

Bank filtration is a well-established and cost-effective drinking water treatment procedure in Germany: waterworks use the natural cleaning power of the soil to improve the quality of the untreated water without the use of any energy or chemicals. A research project was set up to investigate the ability of bank filtration to remove or at the very least reduce organic contamination under fluctuating conditions, the ultimate objective being to create planning and operating guides to enable this procedure to be used worldwide.

Depending on the wastewater discharge from industry, households and farming, surface water in industrialised and urban areas often contains many organic trace elements or substances produced from their degradation: pesticides, mineral oils and chemicals with hormonal effects or active pharmaceutical agents can be detected in the water. These substances must be effectively removed if the surface water is a source of drinking water.

Bank filtration involves wells being established directly by the river used to supply drinking water, which artificially lowers the groundwater level. The result is a hydraulic slope between the riverbed and wells, and the surface water trickles over the bed or the bank to get underground: dirt and contaminants are filtered out and degraded by means of natural physical, chemical and biological processes. Depending on the geological conditions, the distance between the wells and the bank and the level of the river, this can take just a few days or as much as six months.

Fluctuating conditions on site

How bank filtration works

How bank filtration works
How bank filtration works

So to what extent is bank filtration suitable for removing organic compounds when these compounds have a whole host of different chemical-physical properties? Scientists examined this question in a project called “Determination of the potential purification performance of bank filtration/underground passage with regard to the elimination of organic contaminants under site-specific boundary conditions”, which ran from 2001 to 2005. They also wanted to discover the prerequisites a site must meet for bank filtration to be a success. This is because, alongside the range of contamination and the respective concentrations in the water, hydrogeological conditions also have a major role to play – especially the composition, permeability and sorption capability of the soil, as well as climate factors such as water temperature.

Other aspects of bank filtration were examined by the Institute for Water Research, Dortmund (overall co-ordination of the “bank filtration” project), the Karlsruhe Research Centre and TU Berlin, TU Dresden and TU Hamburg- Harburg (guidelines: Kühn, W.; Müller, U. (editor) (2006): Export-oriented R&D in the field of water supply and wastewater treatment – part I: Drinking water. Volume 2. Guidelines. Water Technology Centre, Karlsruhe, ISBN 3-00-015478-7).

With the aim of creating planning and operating guides for using bank filtration in other climate zones, the project partners collated existing research results and experience gained in Germany and other countries and conducted an analysis. They filled any gaps in information – e.g. in relation to more extreme climatic conditions – with the results of true-to-life field and lab tests.

Clear improvement in water quality

The tests performed during the project showed that bank filtration can remove most (around 80%) of the organic trace elements found in the surface water, with a reduction in concentration observed at the very least in the remaining substances. Nevertheless, the behaviour of innovative substances is not 100% predictable in bank filtration as chemically similar substances occasionally react in very different ways. The project confirmed that the removal of organic compounds is attributed to sorption processes and biological and chemical degradation processes underground. This means the bulk of the reduction in contamination takes place in the “infiltration zone”, directly after the water gets into the soil. The conclusion from this is that removing the wells (20 to 400 metres) does not have a major impact on cleaning performance. Even so, the increased flow course/water retention time underground is significant as it ensures optimum effect. The project team therefore observed that many substances were eliminated to a much higher degree when the water spent longer underground.

Oxygen supply is crucial

The trace elements that are degraded and the extent to which this occurs depend heavily on the oxygen-reduction environment in the soil, or in other words, the micro-organism oxygen supply. For example, some trace elements are removed more effectively in anaerobic environment, and others in an anaerobic one. This means that locations suitable for bank filtration are those where the water can spend a while in both aerobic and anaerobic areas of soil.

A well of the vertical well gallery in Düsseldorf

A well of the vertical well gallery in Düsseldorf
A well of the vertical well gallery in Düsseldorf

Regardless of the oxygen-reduction conditions, bank filtration is suitable for removing organic trace elements such as polycyclic aromatic hydrocarbons, polychlorinated biphenyls (PCB) and many insecticides. Odours, tastes and substances affecting hormones are also degraded to a great extent. One key consideration for bank filtration in warmer climates: rising temperatures lead to increased metabolism and thus a higher conversion rate in most cases.

DVGW – Water Technology Center Karlsruhe (TZW)
Dr. Frank Thomas Lange
Karlsruher Straße 84
76139 Karlsruhe, Germany
Tel.: +49(0)7 21/96 78 15 7
Fax: +49(0)7 21/96 78 10 4
E-mail: lange@tzw.de
Internet: www.tzw.de/en/
Funding reference: 02WT0279
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