Bacterial diversity of biofilms in slow sand filters

Slow sand filtration has been used for the last 200 years as part of drinking water treatment. The process has been mainly optimised through modification of operating condi-tions without investigation of the biological processes which take place in the filter. Bacteria, which are at the start of the food chain, are essential for the cleaning efficiency of the slow sand filter. To date, these have been investigated using only culture based methods. The aim of this work was to obtain an overview of the diversity and composition of the bacterial population present in the upper active layers of slow sand filters. The stability of the bacterial populations were investigated in test filters which were operated under suboptimal conditions (high concentrations of particulate matter, DOC and bacteria in the inflow water, water temperatures from 20°C to 30°C, use of glass granulate and coconut fibre as alterna-tive filter matrix). The molecular biological investigation of the bacterial populations indicated that slow sand filter biofilms are composed of multispecies communities which, through their diverse metabolic abilities, are able to sequentially degrade the available nutrients. The dominant bacterial classes are those which are known to be present in similar habitats. One of the main bacterial classes detected in slow sand filters are the Proteobacteria, in particular the beta subclass. Both qualitative analysis using ARDRA and quantitative analysis using FISH showed that the autochthonal bacterial population in mature slow sand filters is relatively stable and only slightly influenced by alterations in the operating conditions. Although, at op-erating temperatures of 20°C to 30°C, and with sufficient nutrients available, the retained hygienically relevant bacteria and Escherichia coli are capable of growing in the filter matrix and dominate the natural, non pathogenic bacteria. The slow sand filter Schmutzdecke is the location with highest bacterial cell density and highest biological activity whereas from depths of 5 cm onwards the bacterial cell number and activity in the rest of the filterbed are constant. The retention capability of experimental filters for hygienically relevant microorgan-isms showed that, from a microbiological perspective, slow sand filters are also suitable for drinking water preparation under suboptimal operating conditions. Apart from sand, it is also possible to use glass granulate and coconut fibre as filter matrix, although a heat treatment is recommended to kill all hygienically relevant microorganisms contaminating the raw material. The detection of indicator organisms through culture based techniques, as stated in the Drinking Water Ordinance 2001, is time consuming and does not encompass all the pathogenic organisms present in the sample. In the present work, a fast and sensitive tech-nique, called Fluorescent Microcolony Hybridisation (FMH), was optimised for the detection of two indicator organisms, enterococcen and Enterobacteriaceae/coliform bacteria