2.4.02 Adapted slow filtration – versatile and cost-effective

Slow filtration is a near-natural, simple and cost-effective means of water treatment; in Germany, it is usually combined with other processes. It can also be performed using alternative, locally available filter materials. A number of research projects have looked at ways in which slow filtration could be technically improved and adapted to the specific conditions at locations outside Central Europe.

Slow filtration has become a well-established procedure for biological drinking water purification. The systems usually consist of an infiltration pondReservoir for surface water which is introduced to the groundwater after being filtered through gravel and sand. filled with different filter and support layers. A drainage and support layer comprising stones, gravel and coarse sand is followed by an (approx.) one metre high filter layer. The primary filter material employed in Central Europe is sand (slow sand filtration); generally speaking, the longer the water remains in the sand filter, the greater the level of purification.

However, uniform and well cleaned filter sand is not widely available. Attempts to proliferate this procedure (particularly in developing and emerging countries) requires adaptation to local circumstances – for example, the use of locally available and cost-effective filter materials. This matter has been closely examined by the IWW Water Centre (Rheinisch-Westfälisches Institut für Wasserforschung) in Mülheim an der Ruhr (overall co-ordination of “slow sand filtration” projects) and the Institute for Water Research (IfW – Institut für Wasserforschung) in Schwerte (period of study: 2002 to 2005).

Sand alternatives examined

The scientists compared the cleaning performance of sand to that of recycled glass granulate and coconut fibres – employing a range of methods at varying temperatures and filter speeds. Another sub-project addressed the question of whether the effectiveness of slow filtration is improved by adding gravel, pumice or coconut fibres to the sand filter layer.

Extreme pollutant concentrations simulated

The research was performed in laboratories and semi-industrial test facilities. To establish the procedure’s effectiveness in the face of extreme pollutant concentrations in the unprocessed water, tests were partially conducted using contaminated surface water with raised DOC and ammonium content. The researchers also employed the outflow of a sewage plant as untreated waterWater before it has been purified or cleaned, e.g. to produce drinking water. in the test facilities; this was done to determine whether slow filtration is also suitable for the treatment of water in rivers that are greatly affected by wastewater discharge but still possess some assimilative capacity.

Another area of research was the nature of microbial colonisation in slow sand filters. To optimise system performance, the Water Technology Centre (TZW) of the Deutsche Vereinigung des Gas- und Wasserfachs (DVGW – German Technical and Scientific Association for Gas and Water) developed a module for the practical mathematical simulation of slow sand filtration under different environmental conditions.

Finally, the objective of the projects “Boundary conditions for slow sand filtration, suggestions for technical modification and adaptation to regional conditions” and “Optimisation of slow sand filters by means of special protection layers and operating methods” was to combine existing slow filtration data with current results in order to create guidelines for the planning and operation of slow filtration systems (Kühn, W.; Müller, U. (eds.): Export-oriented R&D in the field of water supply and wastewater treatment – part I: Drinking water. volume 2; Karlsruhe; at www.tzw.de).

Recycled glass granulate and coconut fibres as alternatives

The research showed that both recycled glass granulate and coconut fibres represent viable alternatives to sand as slow filtration materials. Under test conditions, however, the cleaning performance of these substances was not of a sufficient level to produce drinking water in all circumstances. The experiments highlighted specific strengths and weakness of the different filter materials, which are to be taken into account in practice and potentially compensated by means of technical modifications (e.g. pre-separation or aeration).

The purification performance of slow filtration greatly depends on the temperature: the biodegradation processes become significantly slower at temperatures below 10°C, in some cases even coming to a complete standstill. At high temperatures and with a high concentration of biodegradable substances, the degradation processes result in significant oxygen consumption. Column experiments in an air-conditioned room showed that, in the selected operating conditions (filter layer thickness, operating method, filter speed etc.) and at 5 to 10°C, none of the filter materials was able to process the untreated water (high DOC and ammonium content) such that it met the drinking water regulations of the World Health Organisation (WHO) in terms of the examined chemical parameters. At 20°C, only the filtrate treated with recycled glass granulate exceeded the thresholds, while all filtrates met the WHO limits at 30°C. In these experiments, the sand-filtered water actually complied with the values stipulated by the even stricter German Drinking Water Ordinance.

Protective layer

The experiments showed that a 20 cm protective layer consisting of gravel, pumice or coconut fibres greatly increases the filtering time. Furthermore, a large portion of the particles contained in the water was retained in this layer, thus protecting the sand filter layer below and minimising agglutination. If the filtered particles are organic rather than mineral substances, they are already biodegraded in the protective layer. The disadvantage of this is the lack of oxygen further down, which – without additional aeration – has a negative impact on subsequent 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. biodegradation processes, e.g. ammonium oxidation.

The scientists assessed the ability of slow sand filters to retain degradation-resistant 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. in a separate test system featuring an activated carbonCollective name for a group of artificially produced, highly porous carbons with a sponge-like structure. This pure material is characterised by its large specific surface area (up to 300 m2/g). Activated carbon is manufactured from peat, wood, lignite, black coal or nutshells. The materials are first carbonised, during which tiny pores are created. Activated carbon adsorbs organic substances from water and the air and is thus able to clean contaminated water or polluted air. layer below the sand layer. The result: layers of sorptive materials such as activated carbonCollective name for a group of artificially produced, highly porous carbons with a sponge-like structure. This pure material is characterised by its large specific surface area (up to 300 m2/g). Activated carbon is manufactured from peat, wood, lignite, black coal or nutshells. The materials are first carbonised, during which tiny pores are created. Activated carbon adsorbs organic substances from water and the air and is thus able to clean contaminated water or polluted air. are highly effective in retaining 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. from pesticides and pharmaceuticals. As this “sandwich” method entails high construction and maintenance costs, large-scale application is not currently a realistic option. Therefore, if slow filtration alone does not provide the desired water quality, a better option would be to integrate the procedure in a system of suitable pre- and post-treatment techniques adapted to the relevant location.

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2.4.03 German knowledge export

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2.4.01 Bank filtration