The physical and chemical properties of three different lava sands used in constructed wetlands
for municipal wastewater treatment were investigated. The aim was to identify those properties
and mechanisms that render lava sands as highly efficient filter media which could substitute conventional,
fluviatile sands. It was shown that although lava sands per se may be suitable filter materials,
the presence of zeolite minerals within the lava sands enhances the purification efficiency tremendously.
Zeolites not only increase the sorption capacity, but even more important, they are able to absorb water
in large amounts, which in turn leads to stronger swelling. The latter reduces hydrological conductivity
considerably, resulting in a longer contact time to eliminate pollutants. A simple mineralogical survey of
filter materials for the presence of zeolites may render many installations of constructed wetlands
successful.