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 elementsTrace elements are substances with a concentration in water of less than 100 micrograms per litre. These can be inorganic or organic trace elements that effect the quality of water. 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

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, permeabilityMeasure of the ability of a porous material, such as rock or soil, to allow gases or fluids to pass through it. and sorptionProcess whereby a solid or liquid substance takes up or holds another gaseous or dissolved substance. The term sorption refers to processes resulting in the enrichment of a substance within a phase (absorption) or on an interfacial surface between two phases (adsorption). The term is used when absorption and adsorption cannot be clearly distinguished. The sorbing substance is also referred to as the sorbent (or sorbent material). 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 elementsTrace elements are substances with a concentration in water of less than 100 micrograms per litre. These can be inorganic or organic trace elements that effect the quality of water. 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 retentionIn the field of water management, this term refers to the balancing effect of reservoirs on the outflow of flowing waters; see also ->Retention basins. 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 elementsTrace elements are substances with a concentration in water of less than 100 micrograms per litre. These can be inorganic or organic trace elements that effect the quality of water. that are degraded and the extent to which this occurs depend heavily on the oxygen-reduction environmentA redox reaction (or more precisely reduction oxidation reaction) is a chemical reaction in which electrons are transferred from one reactant to another. Such electron transfer reactions therefore involve the loss of electrons (oxidation, as in the case of combustion) by one substance and the gain of electrons (reduction) by another. In this context, the oxygen-reduction environment describes the chemical state of the surrounding medium. in the soil, or in other words, the micro-organism oxygen supply. For example, some trace elements are removed more effectively in anaerobicState involving the presence of oxygen (O2). Also used in the context of organisms that require oxygen to live, or to describe chemical reactions that require the presence of oxygen. environment, and others in an anaerobicAbsence of oxygen (O2). Also used in the context of organisms that do not require oxygen to live, or to describe chemical reactions that occur in the absence of oxygen. one. This means that locations suitable for bank filtration are those where the water can spend a while in both aerobicState involving the presence of oxygen (O2). Also used in the context of organisms that require oxygen to live, or to describe chemical reactions that require the presence of oxygen. and anaerobic areas of soil.

Regardless of the oxygen-reduction conditions, bank filtration is suitable for removing organic trace elementsTrace elements are substances with a concentration in water of less than 100 micrograms per litre. These can be inorganic or organic trace elements that effect the quality of water. such as polycyclic aromatic hydrocarbonsGroup of organic compounds. PAHs are natural components of coal and crude oil that are created during the pyrolysis (incomplete combustion) of organic material (e.g. coal, heating oil, fuel, wood, tobacco) and are thus ubiquitous in the environment. Due to their degradation resistance, toxicity and prevalence in the environment, PAHs are considered highly potent pollutants., polychlorinated biphenylsGroup of toxic substances used up to the 1980s, primarily in transformers, capacitors and hydraulic systems; PCBs were also used as softening agent in paints, sealing compounds and plastics. The substances have since been included in the “dirty dozen” group of known organic toxins, which were banned throughout the world in 2001. In addition to their chronic toxic effect (chloracne, hair loss and hyperpigmentation), PCBs are also suspected of being carcinogenic and hormonally active. (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.

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2.4.02 Adapted slow filtration

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2.4 Knowledge transfer