After the raw water was treated with the flocculant, it can be filtered. With the flocculation also the phosphate, which require algae for their growth, is retained as a sparingly soluble aluminum or iron phosphate in the filter, thus preventing the growth of algae. Then the raw water flows through one (or more) filter. As filter material, for example, gravel, sand, hydroanthracite, etc. are used.
The waste water from the filter flushing can be treated in a rinsing wastewater treatment area extent that it can be given as filling water back into the circulation. The now called filtrate water is now sanitized, this is done by means of chlorination (chlorine gas, sodium hypochlorite, calcium hypochlorite) to achieve a depot effect in the pool.