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The effectiveness of slow sand filtration for water treatment has been very well documented.
“No other single process can effect such an improvement in the physical, chemical and bacteriological quality of surface waters.” [ref.01]Ref.01: Huisman, L; Wood, W.E. (1974). Slow Sand Filtration. WHO, Geneva, Switzerland. p.44. Available from WHO
By clicking on the links on the left, you will find a wealth of information on sand filtration.
Firstly, there is a review of the differences between rapid sand filtration and slow sand filtration, since the processes involved and applications are inherently different. Also the differences between continually-operated and intermittently-operated sand filters are explained, as both systems are commonly used in humanitarian programmes.
In addition, we have drawn together detailed information on the processes at work in slow sand filtration, including an in-depth look at flow rates and the physical/mechanical and biological processes involved. Alongside this, there is an explanation of different kinds of filter media that can be used in filters, including an explanation of what is meant by effective size and uniformity coefficient, as well as why media size is important in sand filtration. In here you will find out how to do a sieve analysis to determine what kind of sand you have in your area. The effect of sand filters on water quality is reviewed, which documents much of the research done to date both on how continually and intermittently-operated sand filtration systems affect biological, chemical and physical parameters. Alongside this is a section of how to carry out water quality testing, particularly microbiological quality.
Another section explains the various types of sand filters commonly used in humanitarian projects, including how to start a successful filter project. In here you will find practical technical information that will enable you to build your own mould and concrete filters.
Lastly, a section deals with additional research that is needed to fill the knowledge gaps that exist.
Several principles of sand filtration need to be understood, and these are briefly described below. More detailed information is available on this site – just click the links to the relevant pages on the left hand side of of this page.
Definitions of rapid and slow sand filtration
Rapid sand filtration is mainly used in combination with other water purification methods. The main distinction from slow sand filtration is the fact that biological filtration is not part of the purification process in rapid filtration. Rapid filtration is used widely to remove impurities and remnants of flocculants in most municipal water treatment plants. As a single process, it is not as effective as slow sand filtration in production of drinking water. In general, slow sand filters have filtration rates of up to 0.4 m/hour, as opposed to rapid sand filters which can see filtration rates of up to 21 m/hour.
As its name suggests water in rapid filters passes quickly through the filter beds. Often, it has been chemically pre-treated, so that little biological activity is present. Physical straining is the most important mechanism present in rapid filters. Particles that are larger than the pore spaces between the sand grains are trapped – smaller solids however can pass through the filter. Rapid sand filtration removes particles over a substantial depth within the sand bed.
In contrast, slow sand filters can remove particles that are smaller than the spaces between sand grains. Slow sand filters contain very fine sand and usually function without chemical pre-treatment, such as chlorination or flocculation. The low filtration rate causes long detention times of the water above the sand and within the sand bed. This allows substantial biological activity. Slow sand filtration removes particles mainly at the surface of the sand bed.
Where maintenance is concerned, rapid sand filters are usually cleaned on a daily basis using backwashing, whenever terminal head loss is reached. To clean the filter, the flow of water is reversed through the filter bed at a high rate so that the sand bed fluidizes. This flushes out all materials trapped between the sand. In comparison, to backwash a slow sand filter bed in the same way would destroy the bio-film and disrupt the intricate inter-relationships of sand and micro-biological life. The flow rate in slow sand filters is therefore usually restored by scraping and removing the top layer of sand, which is where most clogging occurs. Hence, large quantities of backwash water are not required.
Rapid sand filters are suitable for large urban centers where land scarcity is an issue, whereas slow sand filters tend to be more suitable for areas where land is more available, since they need a much larger surface area to treat the same amount of water. Slow sand filtration is simpler to operate than rapid filtration, as frequent backwashing is not required and pumps are not always necessary.
Principles of slow sand filtration
A slow sand filter contains biological activity and is therefore often referred to as a bio-sand filter. As micro-organisms such as bacteria, viruses and parasites travel through the sand, they collide with and adsorb onto sand particles. The organisms and particles collect in the greatest density in the top layers of the sand, gradually forming a biological zone. The biological zone is not really a distinct and cohesive layer, but rather a dense population that gradually develops within the top layer of the sand. The population of micro-organisms is part of an active food chain that consumes pathogens (disease-causing organisms) as they are trapped in and on the sand surface. The uppermost 1-3cm of this biological zone is sometimes referred to as ‘schmutzdecke‘ or ‘filter cake’. Which is defined as a layer of particles deposited on top of the filter bed or biological growth on top of the filter bed. Slow sand filters are usually cleaned by scraping of the bio-film and/or the top sand layer.
Continually-operated slow sand filtration
In order to be effective, most literature insists that a constant flow of water passing through a slow sand filter is essential. This flow provides oxygen and food to the organisms that make up the ‘schmutzdecke‘ and biological zone living within the top part of the sand, which are responsible for much of the removal of disease-causing organisms. Under stagnant conditions, the biological can start to die – sometimes within several hours.
Click here to read more on biological activity in slow sand filters
Intermittently-operated slow sand filtration
Until recently, it was considered impractical to operate a slow sand filter intermittently, due to the need for a continuous supply of food and oxygen. However, Dr. Manz of the University of Calgary re-designed the traditional sand filter, making it suitable for intermittent use at a household level. This adaptation, brilliant in simplicity, consists of raising the under drain pipe back up to between 1 and 8 cm above the sand level, ensuring a foolproof method for maintaining the water level just above the sand. Manz proved that, even when water is not continually added to the filter, oxygen can still permeate into the water to reach the organisms living in the sand by diffusion across this shallow layer of standing water.
Intermittently-operated slow sand filters can be small units that easily supply enough clean water for a family. Therefore, they are particularly suited for use in low-income countries, where the majority of people rely on untreated, contaminated surface water. Find out in detail how intermittently-operated slow sand filters work.
Further reading:
Haarhoff, J.; Cleasby, J.L. (1991). Biological and physical mechanisms in slow sand filtration. In: Slow Sand Filtration. Logsdon, G.S. (ed.). pp. 19-68. American Society of Civil Engineers, New York, USA.
Links:
The Centre for Affordable Water Supply, CAWST, is one of the main champions of household bio-sand filtration. They offer trainings and have developed quality promotional materials. Their website offers a wealth of information on the bio-sand filter.
References:
Ref 01: Huisman, L; Wood, W.E. (1974). Slow Sand Filtration. WHO, Geneva, Switzerland. p.44. Available from WHO