Slow Sand Filtration

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.

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.

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).

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).

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).

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

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.

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.

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).

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).

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).