Last updated: June 1, 2014
Water can be pasteurized in black pots or jars in solar cookers using a batch process where once the water has reached pasteurization temperature, the jars and pots are removed from the cooker and allowed to cool down before drinking. A flow-through pasteurization device, however, can pasteurize more water in a given time period for the following reasons:
- The hot pasteurized water exits the cooker into a simple heat exchanger where much of its heat is transfered to the cold water that is entering the cooker.
- The thermostatic valve opens precisely when pasteurization temperature has been reached (with some safety margin). This means that the water is never heated to a higher temperature than is necessary to make it safe to drink.
Water flows through a solar collector, one end of which is connected to a thermostatic valve (such as one taken from an automobile radiator system) and the other to a storage tank for the untreated water supply. This storage tank also contains a sand/gravel/charcoal filter that does the preliminary filtering. The thermostatic valve opens at the correct temperature, allowing the pasteurized water to drain out of the tubing and into a second storage vessel for treated water. As the treated water drains from the solar box cooker, contaminated water from the storage tank automatically refills the tubing. Once this cool water reaches the valve the valve shuts and the pasteurization process begins anew.
This flow-through device has several additional advantages over the simpler batch processes. First, potable water becomes available throughout the day as new increments of treated water are added to the clean storage vessel. Second, this type of unit can adapt to variable solar conditions which takes the guesswork out of filling the jugs in a batch process. This is also a totally automatic process, freeing time for other chores and decreasing the likelihood of an accident occurring when transferring water in and out of a batch unit.
Dramatic improvements can be achieved by recycling the heat in the outgoing pasteurized water. Once the water has been pasteurized and released the energy in this water can be used to preheat the incoming water.
A simple device which accomplishes this preheating is a counter-current heat exchanger. The hot water flows on one side of a metal plate, while on the other side of the plate cooler fluid flows in the opposite direction. The energy from the hot water is transferred to the cold water, thus preheating the incoming contaminated water by lowering the temperature of the outgoing pasteurized water. There are many ways of building a counter-current heat exchanger. In the initial trials, for an increase in cost of about 15% the heat exchanger provides about a 400% increase in water output.
NOTE: Care must be taken that no non-pasteurized water is allowed to flow into the "clean water" area. All it takes is one drop of water that has not reached 149F and you no longer have "clean water".
News and recent developments
- August 2011: Solar Pool heaters proposed for solar water pasteurizer using principals of a already functioning solar hot water heater using solar pool heaters. When building the PlayaLindaHotel.com, hotel in Mexico for their future retirement, in 2008,Ken Graham purchased Solar Pool heaters to use for heating the hotels water (not the pool), as far as I know this is the first application of solar pool heaters for hot water heating, not involving swimming pools. Ken Graham finally installed the system in 2010. A free solar calculation program from the Canadian Government http://www.retscreen.net/ was used to calculate the size required and proved accurate. The water attains a temperature of 60 degrees Celcius in the Gravity loop by the time it gets to the top of the 12 foot collector, before entering the 2000 litres of storage, however if the valve entering the water storage tank is turned off the water can easily boil so venting to atmosphere is necessary for safe operation(thus proving water pasteurization is easily attainable). The problem is not the heating but having a good inexpensive thermostat available off the shelf. Various manufacturers of Solar Pool heaters are mentioned here http://www.getwithgreen.com/2007/08/05/swimming-pool-solar-pool-heaters/ at least one if not all pool heaters claim to be designed of materials that can survive water freezing, it remains to be seen if all are food grade compliant, there exists a huge untapped market worldwide for both hot water heating systems and solar water pasteurization. Solar Pool Heaters longevity is measured in decades, warm countries often use storage tanks on their roof so the cost of the solar system can easily pay for itself in less than a year. Even if not using a Solar pool system for water pasteurization but only for hot water heating the addition of a thermostat for availablity of safe water during a disaster would be minimal. - Ken Graham
- May 2010: EPA Water Disinfection Project A grant awarded by the Environmental Protection Agency will provide funding for the research and development of a solar water pasteurizer.
- Sun and water: an overview of solar water treatment devices (including the PAX flow-through pasteurizer) - Trudy Rolla