A natural alternative to the chlorine with no chemical treatment. This natural environement pool is the pool of the future. It is naturally heated. The price of this pool is around per meter square. The cost for one year for electricity and for the water.
An alternative to chlorine pools, the BioTop Natural Pools use plants to keep water clean and clear. Wikipedia has some more info on natural swimming pools as well. These could make a great addition to a permaculture design site.