To conserve fuel, a still can be placed on top of a cooking- pot which is used everyday, such as a rice pot. As it boils or simmers, the 'waste' heat is usefully harnessed.
Care should be taken to ensure that all pots are stable and out of reach of young children.
'''Household solar stills''' <br>
Household solar stills have not been widely promoted, yet can provide 2.5 – 3 litres per m2 surface area per day. However there is
scope to increase yields – more efficient and expensive stills (Aqua Solaris) have been tried that can increase volume to 40 litres per m2 per day. Water temperatures must be high, while the condensing surface should be as cool as possible – for this reason stills are most efficient in the early evening when water is still warm but temperature of the glass is a lot lower, and stills continue to produce water during the night.
The Watercone is a massproduced innovation that can produce 1.5 litres maximum per cone per day, but tends to be expensive. Solar stills can be constructed using local materials which are cheaper. There are key points to get right:
* Keep water temperature in the solar still as high as possible:
# Use a condensing surface with a low absorption capacity. Glass is most commonly used as it is “wettable” (i.e. water condensing in a film rather than forming droplets which reflect radiation) but should ideally be sufficiently thick to withstand rain, wind and some knocks – 1/8” or 3.2mm is adequate. Plastic should not be used due to the high temperatures.
# Keep water level in the still to between 0.5 – 2.5 cm deep only. This means you'll have less water to heat up and increased efficiency, but not too little that it will dry up.
# Insulate the base and walls of the still. This will retain the heat rather than losing it out the sides and base of the structure. Expanded polystyrene sheets 1” thick is widely available and is good for this purpose. The lining on top of this insulation will retain more heat if it is black.
# Make the solar still waterproof – the easiest way to do this is to use a liner. EPDM or butyl rubber is a good choice as they will not break up or give unpleasant taste/odour to the water.
# Lining the walls inside the solar still with a reflective material (e.g. aluminium foil) may increase reflection of heat energy but has not been tried.
* Avoid vapour leaks. Silicone applied using an application gun works well to seal the glass to the frame.
* Width of glass is normally limited to between 0.65 – 0.9 metres.
* Add 3 times the daily clean water amount each morning to flush the still – water will flow out through the overflow. Failure to do this will result in salts deposited in the still.
* The slope of the glass should be minimal. Water will run off glass even set at 1 degree tilt. As a guide, set the angle so that the distance from water level to glass is in the range of 5 – 7 cm in order to minimize air volume in the still, and to increase efficiency.
* Never allow the still to go dry otherwise it can melt the lining and insulation in the still.
Communal solar stills have been constructed in some locations. Like any other larger communal installations, there have been problems with maintenance. They may work however if they could be owned and managed by people who have a vested interest in the technology (e.g. water vendors).
For volumes of water over 1m3 per day, reverse osmosis (RO) or electrodialysis can be considered. However, for rural areas and most small towns, it will be wise to avoid more complex systems unless you can guarantee technical competence in the design, construction and maintenance of the systems, as well a supply of spare parts and chemicals. If considering this route, it is particularly important is to get a full water analysis done prior to system design, and to have design carried out by RO specialists.
In areas of shallow saline groundwater, Managed Aquifer Recharge (MAR) techniques have been used to dilute this groundwater which can then be re-extracted. Examples are “Tajamar” infiltration ponds in Paraguay and roof water recharge systems in Mozambique.
Salinization of shallow groundwater can occur due to water-logging (e.g. where irrigation is practiced) – this is because of a shallow water table and high evaporation rates. Techniques to reduce salinity include:
# Reducing water table depth. This can be done by reducing groundwater replenishment by lining irrigation canals and using improved irrigation techniques that limit volume of applied water (e.g. drip or sprinkler irrigation), but can also be done by lowering water tables through increasing groundwater discharge through better drainage or planting vegetation with high water consumption rate.
# Reducing evaporation. Deep tillage can increase soil pores and reduce capillarity, and screens/trees/mulching/high plot perimeters can improve shade and act as windbreaks to reduce evaporation rates.
Legislation can impact increasing levels of groundwater salinity – in Mozambique, the government prohibited the drilling of new boreholes in rural areas where salinity was an issue. This combined with rainwater MAR techniques proved to be a good combination at addressing the saline issue.
'''Reverse osmosis'''<br>
