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In most developing countries clean drinking water is scarce. Large parts of the population rely on contaminated water and the incidence of waterborne disease is high. The cost of unsafe drinking water is huge in terms of human suffering and severely hinders sustainable development.
Slow sand filtration is an appropriate and low-cost method for household water treatment, and is proven to reduce diarrhoeal incidence. The method offers a household solution that is able to produce greatly improved drinking water from polluted surface water.
In the industrialized word, small numbers of bio-sand filters are sold to the owners of holiday homes or cabins in remote areas. Large, complicated and computer-controlled units are used by small industries, town water supplies or agricultural enterprises. However, in its simplest form, the bio-sand filter is almost exclusively promoted for use by individual families living in developing countries. Obviously, the slow sand filter doesn't provide a solution for every water problem. However, its potential is amazing and it is worth considering how the technology can be best implemented, such as through:
Slow sand filtration as a single treatment process is extremely effective. It is ranked second only to desalination / evaporation in terms of pathogenic, chemical and physical improvement of water. However, this data concerns continually operated systems.
In general, slow sand filters have filtration rates of up to 0.4 m/hour. 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.
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.
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.
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 accross 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.
Showing great potential, the specific performance of the intermittent household filter has been researched, but only through limited studies. Since some processes involved in continuous and intermittent sand filtration are inherently different, more research is therefore needed on the efficacy of the intermittent filter. This is especially important considering the large numbers of people already using household filters throughout the developing world.
Any inert, durable granular material of the correct effective size and uniformity coefficient can be used for slow sand filtration. Mostly, this takes the form of sand, which is readily available in most environments, but there are other alternatives where sand is not readily available.
BushProof's website biosandfilter.org provides further information on the filter media for sand filtration. Firstly there is an explanation of why the size of the filter media is important at all. The terms effective size and uniformity coefficient are explained, alongside a worked example of completing your own sieve analysis to determine what kind of sand you have in your area. Following that there is a section on what specific grain size is recommended for sand filtration in general, and the effect of different sand sizes and sand bed depths on parameters such as pathogen removal and colour improvement. There are various methods of enhancing the performance of the sand media in terms of water quality, flow rates and corrosion control, and alternatives to sand as a filter media. Lastly there is an explanation of the role gravle plays in the sand filter.