Clean and safe drinking water is a one of the essentials of life. Around the world water has become a major issue.

Distilation

Filtration

Sand and gravel filters remove bacteria and other small particles from water using simple and exessible technology.

Water testing
It is sensible to find out the state of your water source before you start trying to improve it - it may be fine! You can get a water sample tested at a laboratory which won't cost a huge amount, or get your local authority to do it for free (which they will usually do if it is your only water source). The simple filters desribed here will not remove heavy metals or chemical pollutants but should manage bacteria and particles. If your water source turns out to be highly contaminated, you could attempt to find another source or locate and neutralise the cause of the pollutant. If this isn't possible other methods of filtration may be possible but that is beyond the scope of this guide.

If the water contains sediment, it should be passed through an initial settling tank before it gets to the sand filter.

How it works
The water passes through the sand leaving suspended particles trapped in the layers. Organic sediment is eaten by microscopic organisms including bacteria and protozoans which live in the slime that forms around the sand particles. The end result should be clean water which is safe to drink - assuming there are no other types of contaminant. If the size of the grains of sand is small enough, the filter should be able to remove and most viruses and faecal coliforms (bacteria from faeces).

Construction
Sand filters can be constructed from rendered block by setting a concrete ring onto a concrete slab or a polyethylene tank. The internal sides should be constructed with a rough finish so water does not preferentially pass down the walls of the container. An opaque lid should cover the filter to prevent algal growth and stop leaves and other debris entering it. 10cm of washed pea gravel is placed at the bottom of the container, covered by a layer of geotextile and 70 cm of fine, non-silt sand. The sand must be washed prior to use to remove any remaining silt. Filters should not be smaller than 1m² (the minimal cross-sectional area), even if very few people are using it. Allow at least 0.02-0.08m² per person.

Maintenance
0.5cm depth of sand will need to be removed every few months from the top as the flow rate drops. When the depth of sand drops to 0.5m, usually after 2-3 years, it should be topped up to around 0.8m. During times of hard frost, a tap should always be kept running to prevent the system freezing over, and water from the filter should be piped to a storage tank.
If an assured supply is required, two filters should be built in parallel, so one can be used while the other is being cleaned, or if any unforeseen problems arise.

It is preferable to avoid pumping the filtered water, since energy is required to run the pump. If the water source and filter are well above the storage tanks, pumping will not be necessary. However, as header tanks will sometimes need to be situated in the roof space of a dwelling, or at least upstairs, this will not always be possible.

It should be noted that, despite their apparent simplicity, the construction and operation of sand filters must be carried out with great care. When in use they must be checked regularly. Although the time spent is small, conscientious attention to detail is essential. Filters can be satisfactorily left while users are away, but must always be checked on return.

Controlling filtration
Controlling the rate of filtration is one important aspect of effective operation. Water should pass through the filter at around 100-200 ltr/m²/hr and not less than 78ltr/m²/hr (1.3ltr/m²/min). Measurements will need to be made to determine average daily requirements for the household.
You could set up a small experiment to work this out, either by deflecting the end of the waste pipes into a dustbin or similar store or by measuring at the source (i.e. at the tap or cistern). Either way measure over one week and record your findings daily. At the end of the week work out a daily average.

Using these two pieces of information, the size of the water storage requirement can then be calculated. The flow into the filter can be adjusted using a valve. In addition, a ball valve can be used to maintain the water level.

The water flow through the filter must be maintained contin-uously. With a constant supply this can be achieved simply by fitting an overflow to the storage tanks. If the water source is limited or intermittent the filtered water can be pumped back to the top of the supernatant (the water on top of the sand) when necessary, but this does require pumping and control gear.

Further treatment
Sand filters provide good quality drinking water and many people use them successfully. However, such a system is not guaranteed to be 100% effective in its removal of bacteria all the time, since complete absence of lapses in maintenance cannot be assured. If water is to be used for the public and subject to Environmental Health checks, or if very young or old people use the supply, it would be sensible to install an inline particle filter followed by an ultra-violet filter to kill off any remaining bacteria.

Further reading

• Centre for Alternative Technology, Tipsheet 16 : Slow Gravity Sand Filters
• Water Treatment and Supply Resource Guide, £1.50 plus 80p p+p; details of suppliers, manufacturers, organisations, and further sources of information.
• Safe to Drink? The Quality of Your Water, material on water supply, treatment, health, rainwater collection, bottled and filtered water, sand filters and other systems. CAT

All should be available mail order from the Centre for Alternative Technology.