Sand Filter Media Analysis Essay

When using sand as a filter media two important factors play a role; sand grain size and sand bed depth. Both have important effects on bacteriological and physical water quality.

Sand grain size

Most literature recommends that the effective size of sand used for continually operated slow sand filters (COSSFs) should be in the range of 0.15 – 0.35mm, and that the uniformity coefficient should be in the range of 1.5 – 3, although a coefficient of less than 2 is desirable (Schulz and Okun, 1984 {end-texte}Ref.01: Schulz, C.R.; Okun, D.A. (1984). Surface water Treatment for Communities in Developing Countries. IT, London. p.193. Available from www.developmentbookshop.com).

The sand used for a slow sand filters should preferably be preferably rounded, and free from any clay, soil or organic matter. If necessary, the sand must be washed before being used. If the raw water is expected to have high levels of carbon dioxide, then the sand must contain less than 2% of calcium and magnesium, calculated as carbonates. This is to prevent the formation of voids in the media if the calcium and magnesium are removed by solution (Huisman and Wood, 1974 {end-texte}Ref.02: Huisman, L; Wood, W.E. (1974). Slow Sand Filtration. WHO, Geneva, Switzerland. p.52. Available from WHO).

Effect of sand size on microbiological quality
Results from some studies on continiually-operated slow sand filters have shown that there is scope for the relaxation of typical values that have been used as benchmarks of slow sand filter design. One such study (Muhammad et al, 1996 {end-texte}Ref.03: Muhammad, N.; Ellis, K.; Parr, J.; Smith, M.D (1996). Optimization of slow sand filtration. Reaching the unreached: challenges for the 21st century. 22nd WEDC Conference New Delhi, India, 1996. pp.283-5. Available online here) done on coarser sand with a constant uniformity coefficient of 2, found that the treatment efficiency (for removal of bacteria, turbidity and colour) of slow sand filters was not very sensitive to sand sizes up to 0.45mm, although a slight increase in treatment efficiency was observed with decreasing sand size. They concluded that from the standpoint of removal efficiency the argument for using very fine sand is not strong.

The ideal range for the uniformity coefficient seems to vary – for example, Ellis (1987) {end-texte}Ref.04: Ellis, K.V. (1987). ‘Slow Sand Filtration’, WEDC J. Developing World Water, Vol 2, pp 196-198. recommends a uniformity coefficient in the range of 1.7 – 3, while one of less than 2.7 is preferable. In practice, it seems that sand that is both finer and coarser than the recommended range still provides acceptable results in terms of filtration in continually-operated systems (Barrett, 1989 {end-texte}Ref.05: Barrett, J.M. (1989) Improvement of Slow Sand Filtration of Warm Water by Using Coarse Sand. PhD Thesis, University of Colorado, USA.).

However, most of this research has been carried out only on continually-operated sand filter systems. In contrast, research carried out on intermittently-operated filters does seem to indicate that sand size is important.

Research done by Jenkins et al (2009, {end-texte}Ref.10: Jenkins, M.W.; Tiwari, S.K.; Darby, J.; Nyakash, D.; Saenyi, W.; Langenbach, K. (2009). The BioSand Filter for Improved Drinking Water Quality in High Risk Communities in the Njoro Watershed, Kenya. Research Brief 09-06-SUMAWA, Global Livestock Collaborative Research Support Program. University of California, Davis, USA. Available here.; 2011, {end-texte}Ref.11: Jenkins, M.W.; Tiwari, S.K.; Darby, J. (2011) Bacterial, viral and turbidity removal by intermittent slow sand filtration for household use in developing countries: Experimental investigation and modeling. Final draft of submitted paper. Dept of Civil & Environmental Engineering, University of California, Davis, USA. The draft is available here, and the published article is available at ScienceDirect. A poster submitted at a household water treatment conference in 2008 also illustrates the findings well, and is available here.) found that filters using finer sand (D10 of 0.17 mm) performed significantly better in terms of bacteria and virus removal than filters using coarser sand (D10 of 0.52 mm). More details here about their research of the effect of sand size and hydraulic loading. Research done by Logan et al (2001) {end-texte}Ref.06: Logan, A.J.; Stevik, T.K.; Siegrist, R.L.; Rønn, R.N. (2001). Transport and fate of Cryptosporidium parvum oocysts in intermittent sand filters. Wat. Res. Vol. 35, No. 18, pp.4359-4369. on intermittent sand filter columns of 60cm sand revealed that fine-grained sand columns (D10 0.16mm) effectively removed cryptosporidium oocysts under the variety of conditions examined, with low concentrations of oocysts infrequently detected in the effluent. Coarse-grained media columns (D10 0.90mm) yielded larger numbers of oocysts, which were commonly observed in the effluent regardless of operating conditions. Factorial design analysis indicated that grain size was the variable that most affected the oocyst effluent concentrations in these intermittent filters.

Sand bed depth

In slow sand filtration, the vertical height of the sand bed that the water has to pass through is important in terms of filtration efficiency. The reasons for this are the existence of biological activity in a sand filter, which is known to occur at depths of up to 0.5m within a sand bed, and the available surface area for mechanical filtration and chemical reactions.

The depth of the sand bed designed for a sand filter must at least reflect this zone of biological activity. In coarser sands, an increased sand bed depth is required as the depth of activity will increase.  However an increased bed depth can contribute to better filtration as greater surface area provides a more intimate contact between the constituents of the raw water, thus speeding up chemical reactions (surface catalysis).  Normally this surface area is increased by using smaller-grained sand. However, the surface area could equally well be increased by increasing the depth of the filter bed but, when it is noted that a depth of 0.6 m of sand comprising grains of 0.15 mm effective diameter presents the same surface area as a depth of 1.4 m of sand of 0.35 mm grains, it is obvious that the application of finer sand is economically a better method (Huisman and Wood, 1974 {end-texte}Ref.02: Huisman, L; Wood, W.E. (1974). Slow Sand Filtration. WHO, Geneva, Switzerland. p.52. Available from WHO).

Effect of sand depth on bacteriological quality
Bellamy et al, (1985) {end-texte}Ref.07: Bellamy, W.D.; Hendricks, D.W.; Longsdon, G.S. (1985) Slow Sand Filtration: Influences of Selected Process Variables. American Water Works Association Journal 77 (12), pp 62-66. suggested that sand height could be reduced to 0.48m with no change in bacteriological removal efficiency. However, Muhammad, et al (1996 {end-texte}Ref.03: Muhammad, N.; Ellis, K.; Parr, J.; Smith, M.D (1996). Optimization of slow sand filtration. Reaching the unreached: challenges for the 21st century. 22nd WEDC Conference New Delhi, India, 1996. pp.283-5. Available online here) concluded that most bacteriological purification occurs within the top 400mm of a sand bed.

They found that bacteriological treatment was not highly sensitive to sand bed depth (Table 3), suggesting that a continually operated slow sand filter bed could be reduced even further to 0.40m and still produce a satisfactory bacteriological quality of water. Other research confirms that the majority of biological processes occur in the top 0.4m of the sand bed (ASCE, 1991 {end-texte}Ref.08: ASCE (1991). Slow sand filtration. Logsdon, G.S. (Ed). American Society of Civil Engineers, New York, USA.). However, while this is generally true, bacteriological treatment efficiency does become more sensitive to depth with larger sand sizes because the total surface area within the filter is reduced in a sand bed with larger grains, as well as higher flow rates potentially increasing breakthrough.

Interestingly, Ferdausi and Bolkland (2000) {end-texte}Ref.09: Ferdausi, S.A.; Bolkland, W. (2000). Design improvement for pond sand filter. WEDC Water, Sanitation and Hygiene: Challenges for the Millennium. 26th WEDC Conference, Dhaka, Bangladesh, pp.212-5. Available online at www.wedc.lboro.ac.uk found adequate faecal coliform removals to below 10 per 100ml in pond filters, which only had sand bed depths of around 30cm.

Effect of sand depth on removal of Cryptosporidium oocysts
Research done by Logan et al (2001) {end-texte}Ref.06: Logan, A.J.; Stevik, T.K.; Siegrist, R.L.; Rønn, R.N. (2001). Transport and fate of Cryptosporidium parvum oocysts in intermittent sand filters. Wat. Res. Vol. 35, No. 18, pp.4359-4369. on intermittent sand filter columns of 60cm sand revealed that while grain size was the variable that most affected the oocyst effluent concentrations, the depth of sand was also important in removal, and became more important for coarser sands (D10 0.90mm). Filters with find-grained sand that were run under a variety of hydraulic loadings (4cm to 20cm) still had no oocysts deeper than the top 10-15cm of sand. In comparison, in coarser-grained sand, oocysts were found at depths ranging from 20cm (4cm hydraulic loading) to 60cm (10 and 20cm hydraulic loading). Sand bed depth therefore becomes increasingly important with coarser sands, and becomes critical when coupled with hydraulic loads of 10cm or 20cm.

Effect of sand depth on turbidity and colour removal
Although bacteriological quality of water does not improve drastically after 0.4m of sand bed, turbidity and colour removal efficiencies were found to definitely improve as bed depth increased beyond 0.4m. This shows that adsorption occurs throughout the filter column in purifying water. Consequently, a decrease in sand bed depth causes a reduction in total surface area of the sand grains and ultimately total adsorption capacity is reduced (Muhammad, et al, 1996 {end-texte}Ref.03: Muhammad, N.; Ellis, K.; Parr, J.; Smith, M.D (1996). Optimization of slow sand filtration. Reaching the unreached: challenges for the 21st century. 22nd WEDC Conference New Delhi, India, 1996. pp.283-5. Available online here)

Effect of sand depth on removal of nitrogenous organic compounds
While most bacteriological purification occurs mainly in the top 0.4m of a filter, this does not mean that there is no biological activity below 400mm. While not sensitive to sand sizes and filtration rates, biochemical oxidation of nitrogenous organic compounds was found to be dependent on sufficient sand bed depth. These compounds were not completely oxidized within the top 0.4m (Muhammad et al, 1996 {end-texte}Ref.03: Muhammad, N.; Ellis, K.; Parr, J.; Smith, M.D (1996). Optimization of slow sand filtration. Reaching the unreached: challenges for the 21st century. 22nd WEDC Conference New Delhi, India, 1996. pp.283-5. Available online here).

References:

Ref 01: Schulz, C.R.; Okun, D.A. (1984). Surface water Treatment for Communities in Developing Countries. IT, London. p.193. Available from www.developmentbookshop.com

Ref 02: Huisman, L; Wood, W.E. (1974). Slow Sand Filtration. WHO, Geneva, Switzerland. p.52. Available from WHO

Ref 03: Muhammad, N.; Ellis, K.; Parr, J.; Smith, M.D (1996). Optimization of slow sand filtration. Reaching the unreached: challenges for the 21st century. 22nd WEDC Conference New Delhi, India, 1996. pp.283-5. Available online here

Ref 04: Ellis, K.V. (1987). ‘Slow Sand Filtration’, WEDC J. Developing World Water, Vol 2, pp 196-198.

Ref 05: Barrett, J.M. (1989) Improvement of Slow Sand Filtration of Warm Water by Using Coarse Sand. PhD Thesis, University of Colorado, USA.

Ref 06: Logan, A.J.; Stevik, T.K.; Siegrist, R.L.; Rønn, R.N. (2001). Transport and fate of Cryptosporidium parvum oocysts in intermittent sand filters. Wat. Res. Vol. 35, No. 18, pp.4359-4369.

Ref 07: Bellamy, W.D.; Hendricks, D.W.; Longsdon, G.S. (1985) Slow Sand Filtration: Influences of Selected Process Variables. American Water Works Association Journal 77 (12), pp 62-66.

Ref 08: ASCE (1991). Slow sand filtration. Logsdon, G.S. (Ed). American Society of Civil Engineers, New York, USA.

Ref 09: Ferdausi, S.A.; Bolkland, W. (2000). Design improvement for pond sand filter. WEDC Water, Sanitation and Hygiene: Challenges for the Millennium. 26th WEDC Conference, Dhaka, Bangladesh, pp.212-5. Available online at www.wedc.lboro.ac.uk

Ref 10: Jenkins, M.W.; Tiwari, S.K.; Darby, J.; Nyakash, D.; Saenyi, W.; Langenbach, K. (2009). The BioSand Filter for Improved Drinking Water Quality in High Risk Communities in the Njoro Watershed, Kenya. Research Brief 09-06-SUMAWA, Global Livestock Collaborative Research Support Program. University of California, Davis, USA. Available here.

Ref 11: Jenkins, M.W.; Tiwari, S.K.; Darby, J. (2011) Bacterial, viral and turbidity removal by intermittent slow sand filtration for household use in developing countries: Experimental investigation and modeling. Final draft of submitted paper. Dept of Civil & Environmental Engineering, University of California, Davis, USA. The draft is available here, and the published article is available at ScienceDirect. A poster submitted at a household water treatment conference in 2008 also illustrates the findings well, and is available here.

Introduction

Large-scale slow sand filtering operation. Portsmouth 1927. Source: PORTSMOUTH WATER (n.y.)

Slow sand filtration has been an effective water treatment process for preventing the spread of gastrointestinal diseases for over 150 years, having been used first in Great Britain and later in other European countries (LOGSDON 2002). SFFs are still used in London and were relatively common in Western Europe until recently and are still common elsewhere in the world. The move away from slow sand filtration in industrialised countries has largely been a function of rising land prices and labour costs, which increased the cost of SSF produced water. Where this is not the case, SSFs still represent a cost-effective method for water treatment (WHO n.y.). Since these conditions prevail in many developing countries, it is a very promising technique for water purification and, therefore, the development of a sustainable water system.

Basic Design Principles

Process

The basic principle of the process is very simple. Contaminated freshwater flows through a layer of sand, where it not only gets physically filtered but biologically treated. Hereby, both sediments and pathogens are removed. This process is based on the ability of organisms to remove pathogens. In this context, it is important to distinguish slow and rapid sand filtration. The difference between the two is not simply a matter of the filtration speed, but of the underlying concept of the treatment process. Slow sand filtration is essentially a biological process whereas rapid sand filtration is a physical treatment process (WHO n.y.). To learn more about rapid sand filtration have a look at the factsheet: rapid sand filtration.

Principle of a slow sand filter. Source: WHO (n.y.)

Although the physical removal of sediments is an important part of the purification process, the relevant aspect is the biological filtration. The top layers of the sand become biologically active by the establishment of a microbial community on the top layer of the sand substrate, also referred to as ‘schmutzdecke’. These microbes usually come from the source water and establish a community within a matter of a few days. The fine sand and slow filtration rate facilitate the establishment of this microbial community. The majority of the community are predatory bacteria that feed on water-borne microbes passing through the filter (WHO n.y.). Hence, the underlying principle of the SSF is equivalent to the bio-sand filtration. While the former is applied to semi-centralised water treatment, the latter mainly serves household purposes.

Structure

As the process itself, the basic structure is very elementary. Essentially, only the filter chamber, a type of reservoir and pipes are required. The filter chamber can either be constructed as an open or as a closed box. Depending on climatic and other factors, the one or the other is more reasonable (e.g. cold climate requires a closed box since low temperatures decrease the performance of the process).

Illustration of a slow sand filter with a regulating valve and a subsequent reservoir. Source: HUISMAN (1974)

Once a SSF facility is built, only clean sand is required for occasional replacement. The sand layers are put in gradually according to their grain sizes: rather coarse grains at the bottom and fine grains at the top. The sand-bed is usually covered with one meter of supernatant water (LOGSDON 2003). As the process of biological filtration requires a fair amount of time in order to purify the water sufficiently, SSFs usually operate at slow flow rates between 0.1 – 0.3 m3/h per square metre of surface (WHO n.y.). The water thus remains in the space above the medium for several hours and larger particles are allowed to separate and settle (see also sedimentation). It then passes through the sand-bed where it goes through a number of purification processes (HUISMAN 1974).

The water requires some kind of physical pressure in order to pass the drag created by the sand layers. In terms of construction, two different types are feasible. The pressure can be built up either by pumps or gravity. While pump systems need some type of engine and a more elaborate construction, gravity systems work without any highly technological means (HUISMAN 1974).

Health Aspects

Slow sand filtration is an extremely efficient method for removing microbial contamination and will usually have no indicator bacteria present at the outlet. SSFs are also effective in removing protozoa and viruses (WHO n.y.). If the effluent turbidity is below 1.0 nephelometric turbidity units (NTU), a 90 to 99% reduction in bacteria and viruses is achieved (NDWC 2000). Yet, slow sand filtration is generally not effective for the majority of chemicals (WHO n.y.). However, it can be argued that chemical standards for drinking water are of secondary concern in water supply subject to severe bacterial contamination (WHO 1996).

Highly effective for

Somewhat effective for

Not effective for

- Bacteria

- Protozoa 

- Viruses

 - Turbidity

- Heavy metals (Zn, Cu, Cd, Pb)

- Odour, Taste

- Iron, Manganese

- Organic Matter

- Arsenic

- Salts

- Fluoride

- Trihalomethane (THM) Precursors

- Majority of chemicals

Typical treatment performance of slow sand filters. Adapted from: BRIKKE & BREDERO (2003), LOGSDON (2002) and WHO (n.y.)

Although SSFs are very effective for the removal of microbiological pathogens, disinfectants (e.g. chlorination) are often used in treatment facilities as a step subsequent to the SSF unit. Firstly for the purpose of inactivating any remaining bacteria as the final unit of treatment, and secondly, for the provision of a residual disinfectant that will remove any bacteria introduced during storage and/or distribution (WHO n.y.). Chlorine is generally added after the filter unit in order to not affect the biological process. If the water contains high amounts of natural organic matter (NOMs), e.g. surface waters in tropical regions, chlorination should be avoided due to the risk of the formation of disinfection by-products (DBPs). When attacked by chlorine radicals, NOMs form trihalomethane (THM) and other organic DBPs, which are known to be carcinogenic.

Construction, Operation & Maintenance

Construction

Simple small-scale slow sand filter made out of plastic. Source: GLOBAL GIVING (2011)

A SSF consists of a box, often made of concrete in which a bed of sand is placed over a layer of gravel and perforated pipes. These pipes collect the treated water (VEENSTRA & VISSCHER 1985).For community use, filter chambers can also be made out of brick or ferro-cement (BRIKKE & BREDERO 2003). Recently, also plastic boxes have been used as filter chambers.

More elaborate slow sand filter constructed with a massive concrete filter chamber.Covered community scale SSFs at Nyabwishongwezi Water Treatment Plant, Umatara, Rwanda. Source: THAMES WATER & UNIVERSITY OF SURREY (2005)


The simple design of SSFs makes it easy to use local materials and skills in their construction (HUISMAN 1974). Due to the simplicity of construction, SSFs can be built by experienced contractors, or by communities with external technical assistance (BRIKKE & BREDERO 2003). Basic hydrological equipment like valves and measurement devices become necessary only if the facility is rather large.

Foundation of a slow sand filter. Source: EWB (2010)

Operation & Maintenance

For a SSF to be effective, it must be operated and maintained properly. If topographic circumstances allow the water to flow through gravity during the whole process, no pumps and thus no electricity is required. However, the flow of water must be maintained at a rate between 0.1–0.3 metres per hour. This provides a stable flow of nutrients and oxygen to the microorganisms in the filter and gives them time to treat the water. After several weeks to a few months, the population of microorganisms may get too dense and start to clog the filter. If flow rates are too low, the filter must be drained and the top layer of the sand scraped off, washed, dried in the sun, and stored. After several scrapings, the cleaned and dried sand is added back to the filter, together with new sand, to make up for losses during washing. Every two months, all the valves must be opened and closed to keep them from becoming stuck, and any leaks in the system must be repaired immediately (BRIKKE & BREDERO 2003).

Tayakome’s village water committee cleaning their slow sand filters. Source: GLOBAL GIVING (2011)

SSFs can be operated and even monitored by communities, provided the caretakers are trained well. It takes a caretaker less than one hour a day to check whether the filter is functioning properly and to adjust flow rates. Several people can clean a filter unit in only one day, but it is important that hygienic measures are observed constantly. If the filter is well-designed and constructed, hardly any repairs of the filter tanks and drainage system will be necessary, although the valves and metal tubing may need occasional attention. If water test kits are available, water quality can be easily monitored without special training. Nevertheless, a SSF for community use requires considerable organisation for scraping and re-sanding the filter units. A local caretaker will have to be trained. Apart from extra sand, some chlorine and test materials, very few external inputs are needed. With proper external assistance, water organisations can manage their water treatment independently (BRIKKE & BREDERO 2003).

Costs

Construction Costs

Construction costs strongly depend on local conditions. Since SSFs demand rather large land areas but low input of construction materials, the capital costs primarily consist of wages and costs for land acquisition. The cost of imported materials and equipment may be kept to almost negligible proportions (VEENSTRA & VISSCHER 1985). Therefore, water purification through a SSF is very economical in areas where labour costs are low and land is not a limiting factor.

Operation & Maintenance Costs

Operational costs are incurred almost solely from the cleaning of the filter beds. No chemicals or other materials are needed for the process. No compressed air, mechanical stirring, or high-pressure water is needed for backwashing. There is thus a saving not only in the provision of plant but also in the cost of fuel or electricity (HUISMAN 1974).

At a Glance

Working Principle

Freshwater flows through a sand-bed with a thin layer populated by microorganisms. Hereby, the water gets purified through various biological, physical and chemical processes.

Capacity/Adequacy

Primarily small, rural communities due to large land requirements (WATER FOR THE WORLD n.y.)

Performance

Removes turbidity, protozoa, pathogens, viruses and heavy metals. 100–300 litres per hour per square metre of surface (HUISMAN 1974)

Costs

100–300 USD per square metre (BRIKKE & BREDERO 2003)

Self-help Compatibility

Very high

O&M

Simple, low costs

Reliability

Very high if properly operated and maintained

Main strength

Simplicity; can be constructed, operated and maintained by the community; often no need for pumps/electricity

Main weakness

Large land requirements; excessive turbidity (>30 NTU) in the fresh water can cause the filter to clog rapidly (BRIKKE & BREDERO 2003)

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