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Safe drinking water is one of the most important issues of development work, especially among low-income families. Slow sand filtration is an appropriate and low-cost method for household water treatment, and is proven to reduce diarrhoeal incidence. Already promoted by several humanitarian aid organizations, e.g. tens of thousand of bio-sand filters are now in use throughout the developing world. The bio-sand filter is the only filter specifically designed for use in the bush, by poor, uneducated users, and it has the great advantage of not requiring the recurring costs associated with other (chemical) household water treatment methods. Field studies have proved that rural people are keen to purchase these filters once they have understood the benefits, and that the filters continue to produce clean water for many years afterwards. Suitable for use at household level, the bio-sand filter produces safe drinking water from biologically contaminated water, such as from open wells and rivers. The potential for this technology is enormous: 1.1 billion people lack access to safe water – yet all drink something everyday. In most cases, a bio-sand filter dramatically improves the quality of their contaminated water. This prevents disease, saves lives and reduces poverty.
Bio-sand filters can be made in very many ways. Most common is the use of concrete, for which a metal mould is required.[1] However, the use of metal oil drums, plastic bins or pre-fabricated or ready made plastic filters is also possible. Each version has its own application and with the following information you can easily find out which filter model suits your needs.
It is also possible to make bio-sand filters from plastic drums or even old fuel barrels. This can be a good option in areas where it is not practical to produce concrete filters, for instance because cement and gravel are difficult to obtain. When making drum filters, keep in mind that it is important to keep a sand column of at least 50cm within the filter, to ensure that proper filtration takes place.
In previous field programmes, the sand column in filters made from old fuel drums was kept at 55cm, underlain by 5cm of fine gravel and 5cm of coarse gravel. This allowed for sufficient sand depth coupled with enough water storage space above the outlet level. To make one drum filter using the above sand column dimensions, while allowing for a 5cm layer of standing water above the sand layer during pause time, required the following tools and materials:
To make a filter using a fuel drum, the bottom of the drum needs to be cut off first of all - this will act later as a lid. The edge must be filed, after which the inside must be washed well using washing powder.
Every old fuel drum normally has 2 openings - a large one (2") and a smaller one (3/4"). The larger one should normally be watertight since it has an o-ring, but this needs to be checked since the rubber may be expired. If it is, seal the thread using plumbing tape.
Tip: at this stage, it is much easier to assemble the pipe in two parts - the first part consists of the nipple-elbow-nipple that attaches to the drum itself. You should put these 3 parts together first, and then screw it onto the drum. Secondly, you should put the rest of the parts together first, and then attach it to the piece already on the drum - to do this requires a windmill-type action. Putting the pipes together in this way avoids overtorquing the threads and is relatively quick.
The small section of mosquito mesh should be placed inside the drum over the outlet hole, to avoid any gravel pieces from blocking it. The drum should be placed out of direct sunlight, and a small hole needs to be dug into the ground to accommodate the u-bend of the galvanised pipe where it exits the drum. The filter then needs to be filled with coarse and fine gravel, and then sand.
Note: it is exceedingly important to wash the sand and gravel prior to placing it in a filter. The waste water should run clear before the media is used. The drum also needs to be half-filled with water after the gravel is placed inside, so that air pockets do not form in the sand layer.
The diffuser plate should be made from rustproof material. Previous versions have used aluminium dinner trays and plastic bowls. The important thing is that there are sides to the diffuser plate that avoid water splashing onto the sand layer. This plate is then suspended above but close to the water layer using the galvanised binding wire. This wire is attached at 3 points on the plate, and bent over the edge of the drum, where it sits easily and can be removed quickly.
Plastic drum filters are made in a similar way, but often plastic pipe is used instead of galvanised plumbing. It is also often internal rather than external, and exits the filter at the pause time water level. Here it can connect to a threaded plastic connection that in turn connects to a metal flange fitting. One common problem with plastic filters is that the pipe needs to penetrate the side wall, requiring this well-made flange that needs to be properly bolted to the plastic.
Flow rate in a sand column is proportional to the cross-sectional area of the sand and the pressure head (hydraulic loading) of water on top of the sand. Flow rate is also affected by the length of the sand column, as well as by the properties of the fluid (viscosity, density and raw water quality) and the sand characteristics. For example, colder water should result in a slower flow rate, and over time, a higher turbidity raw water can affect flow rate by clogging the sand pores in the top centimetres of sand. In the same way, porosity and specific yield, which are both dependent on the type of sand in the filter, can both affect the hydraulic conductivity – that is, how much water passes through an area of sand in a particular time. Increasing the surface area or hydraulic loading, improving the raw water quality prior to filtration, using a filter in the tropics as opposed to cold climates, decreasing the sand height or changing the sand type to a coarser sand can all result in a higher flow rate. A drum filter will therefore give a higher flow rate than the concrete filter due to the increased surface area available.
When the flow rate becomes unacceptably low, the filter can be cleaned by removing accumulated dirt from the top few centimetres of sand. To backwash a slow sand filter bed 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. Various methods are possible, but all disturb the biological layer, which results in less effective filtration for some time. Some methods however are less disruptive than others.
The recommended cleaning method is called 'wet harrowing'. This is done by filling the filter with a bucket of water, after blocking the spout (for instance using a cork). Following removal of the diffuser plate, water is slowly swirled around by hand inside the filter. Try not to touch the sand while doing so. The movement of the water loosens accumulated dirt, which comes into suspension. This muddy water can then be carefully decanted, using a cup. The process is repeated until most dirt has been removed. Remove the cork and the flow rate should have increased dramatically.
An evaluation of a bio-sand filter project in Kenya found that some filter owners clean their filter out of routine, rather than because of blockage or inconveniently reduced flow rate. This poor practise, because such filters unnecessarily produce water of less than perfect quality, since their bio-layer is disturbed more often than necessary. If this is the case, better teaching of correct maintenance could therefore be an effective and simple method to improve the quality of water produced by a large percentage of filters. Sometimes reducing flow rates are a seasonal problem. For instance, during the rainy season water can be much more turbid. It seems that during this time, blockages can occur in the filters every few weeks.
It is important to remember that a ‘dirty’ filter actually can produce water of better quality. Due to a reduced flow rate better filtration takes place, while there is an increased contact time with a mature biological layer. Cleaning should therefore only take place when the outflow of water has become inconveniently slow. While a bio-sand filter ideally produces 1 litre per minute, filling a 20-litre jerry can in 2 hours can still be very convenient to an African family, even though this equals a flow rate of only 0.16 litres per minute.
In some projects it is found that a majority of households will either forget or ignore cleaning advice. In such cases, bio-sand filters cannot be considered a 100% failsafe method of water purification despite their potential, but rather as a ‘better-than-nothing’ interim method of water treatment. This highlights the need for intensive teaching methods. It is likely that better information, teaching of cleaning methods, or improved or more frequent follow-up will lead to much better results.
There are several factors to consider before starting a bio-sand filter project. The most important considerations are discussed here. They apply to both micro and medium sized projects; however the design of a mass-introduction on a large scale falls outside the scope of this discussion.
