Low cost filter materials for efficient waste water treatment in constructed wetlands are worldwide in demand. However, information on soil physical parameters needed for efficient treatment is scarce. We investigated 5 different lava and one fluviatile operating sand filters in Germany for their differences in pore size distribution spectra, specific inner surface area and cumulative pore volume. While these soil physical parameters did have an influence on purification capacity and hydraulic conductivity, differences in mineralogy (presence of zeolites), waste water distribution systems (baffle plates versus pipes) and loading rates were the dominating factors determining filter efficiency. Lava sands convince with high purification capacities during permanent loading rates of >100 mm d(-1), average loading rates of 20 g COD m(-2) d(-1) (chemical oxygen demand) and approximate effective loading rates of up to 250 g COD m(-2) d(-1). (C) 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).