THE BIOLOGY OF SLOW SAND FILTERS.

S.P. Hurley and R.S. Wotton. Department of Biology, University College London, Gower Street London, United Kingdom, WC1E 6BT

Slow sand filters are used by the water industry to clean drinking water. Each filter bed consists of a thick layer of sand with the grains coated by microbial biofilms and free–living micro–organisms. Invertebrates also colonise the filters and we found large numbers of oligochaetes (> 2 × 10⁵ individuals m^-2) and chironomid larvae (> 1.8 × 10⁵> individuals m^-2), with Tanytarsini dominating the midge fauna. As the filter matures there is a build up of organic matter at the surface, necessitating the eventual cleaning of the filter and removal of the surface biological community. In many parts of the world slow sand filters are covered, to protect them from environmental extremes or biological contamination. Covered filters are characterised by a low build-up of organic matter at the sand surface as low light levels inhibit algal growth. We investigated a covered slow sand filter adjacent to the open bed we studied. The numbers of oligochaetes were similar to those in the open filter but there were very few chironomid larvae. The quality of the filtrate from the open and the covered bed were very similar and we are investigating the effect of invertebrates on filtration in both open and covered filters.