Elimination of viruses, bacteria and protozoan oocysts by slow sand filtration

Abstract

The decimal elimination capacity (DEC) of slow sand filters (SSF) for viruses, bacteria and oocysts of Cryptosporidium has been assessed from full-scale data and pilot plant and laboratory experiments. DEC for viruses calculated from experimental data with MS2-bacteriophages in the pilot plant filters was 1.5 – 2.0 log. E. coli and thermotolerant coliforms (Coli44) were removed by full-scale filters and in a pilot plant filter with 2 - 3 log. At full-scale, Campylobacter bacteria removal was 1 log more than removal of Coli44, which indicates that Coli44 is a conservative surrogate for these pathogenic bacteria. Laboratory experiments with sand columns showed 2 - 3 and >5 - 6 log removal of spiked spores of sulphite-reducing clostridia (SSRC; C. perfringens) and oocysts of Cryptosporidium, respectively. Consequently, SSRC is not a good surrogate to quantify oocyst removal by SSF. Removal of indigenous SSRC by full-scale filters is less efficient than observed in the laboratory columns probably due to continuous load of these filter beds with spores, accumulation and retarded transport. It remains to be investigated if this also applies to oocyst removal by SSF. The results additionally showed that the schmutzdecke and accumulation of (in)organic charged compounds in the sand increased the elimination of micro-organisms. Removal of the schmutzdecke reduced DEC for bacteria with ±2 log, but did not affect removal of phages. This clearly indicates that besides biological activity, both straining and adsorption are important removal mechanisms in the filter bed for larger micro-organisms than viruses.