Household water treatment systems
Practical Action
Rapid sand filters
Rapid sand filters use a coarse sand to remove suspended solids and water moves at the flow
rate of 4-8 m/hr. The filter uses sand of around 0.5-1.0mm particle size and is 0.6-2.0m deep.
The sand bed needs to be regularly cleaned (daily) and this is done by backwashing where
reversing the flow removes the trapped matter.
Water that has been pre-treated by sedimentation and coagulation is often treated using this
process. Alternatively a coagulant can be added to the water and filtered directly, but this will
require more maintenance of the filter and storage for the collected solids. This method is
effective at reducing turbidity and for removing iron oxides and manganese. It does not remove
pathogens from water and would need to be disinfected or passed through a slow sand filter.
The operational and construction cost, along with the need for regular maintenance are often
inappropriate for developing countries.
Slow sand filter
Slow sand filtration is a very effective method of removing particulate and suspended matter. It
can be used for groundwater containing suspended solids but is more often used to remove
organic material and pathogens from surface water.
Water stands in a tank 1m above a sand filter bed and moves down at about 0.1- 0.2 m/hr.
Different grades of sand can filter out physical impurities and they can also eliminate pathogens
as they develop a layer of algae that feeds on the bacteria. This occurs at the top of the sand bed
and is called a schmutzdecke. The schmutzdecke is effective at killing and retaining various
bacteria, pathogens and viruses, which makes it more effective than a rapid sand filter. The
sand filter will block up over time with inorganic matter but this can be removed by
backwashing. Inorganic matter can be removed through rough filtering or by using sedimentation
tanks.
These systems are simple in design and easy to construct using local materials. The use of
special piping and equipment, mechanical maintenance and imported materials can and should
be kept to a minimum, especially in hot countries where these filters work best. Operational
costs include mostly cleaning out the filter bed, which can be done manually. Chemicals do not
need to be added to the process but chlorination is sometimes used. Compared to other forms
of treatment the amount of chlorine needed for disinfection is far less.
Sand filter systems are described at length in the document Slow Sand-Filtration Water
Treatment Plants produced by Soluciones Prácticas.
Some construction guidelines are available at the following website.
http://www.biosandfilter.org/biosandfilter/index.php/item/330
Chlorination
Chlorination is a simple disinfection treatment which needs to be carried out even after slow
sand filtration. It is effective at reducing the risk of disease but limited against protozoal
pathogens and some viruses (WHO, 2008). Chlorination alone will not be enough to disinfect
water particularly if it is very turbid (cloudy), since sediments and flocs of particles can protect
pathogens from disinfection.
Chlorine is an oxidising agent. The amount of chlorine required varies with the impurity of the
water and the dose needs to be greater than the chlorine demand of the water. There will be
chlorine residual which is needed to help protect against future contamination. Chlorine is
cheap, reliable, readily available and easy to add to the water supply but it can produce a nasty
taste and can produce unwanted by-products.
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