Plastic bottles make filtration units. Cut off the bottom, push tubing through a hole in the stopper - this becomes the base. Put gravel in the bottom, sand on top. Investigate efficiency of different depths and grades of sand.
* Introduce neutralisation by adding acidsalkalis to "pool water", monitoring pH with indicator (or a probe). Titrations show chloride concentration.
* Microbes from dirty water, grown on agar, show disinfectant concentrations which inhibit growth.
* Wave patterns, interference and reinforcement are a good excuse for playing with ripple tanks.
* Recycling versus continuous replacement of water could be considered. An overflowing beaker (pool) with dirt (ink) dripped in shows the water flow to maintain "acceptable" dirt levels. Water and heating costs could be compared with the standard system.
* Class surveys of "Who pees in the pool?" lead to concentration calculations; and how long does wee Willy's urine stay in the pool if 30 litres per bather is replaced? Answer - for ever, but good mathematicians can calculate the decay curve.
* Technologists could design systems to maintain water levels, based accurately on amounts removed (or added!) by individual bathers.