Variation in microbial communities colonizing horticultural slow sand filter beds: implications for filter function

The effect of microbial colonization on the function and rejuvenation of slow sand filters was investigated using culture-independent profiling. Colonization resulted in significant reduction in filter pore size, which may be important in order to fully remove pathogens, but was not associated with a specific microbial component. Communities were highly variable, and no common microbial groups were found in effective filters. Bacterial community composition was affected by sand particle size, although high levels of microbial turnover during filter maturation suggested that this was unlikely to have a major influence on community composition. The composition of microbial inoculum from a previous filter could not be maintained through a cycle of culture, storage and re-culture. Furthermore, no significant proportion of the inoculum persisted in filter maturity, and no advantages in terms of time to filter maturation or final filter efficiency were evident. These results may explain why filtration is such an effective and robust water treatment and emphasize the need for further research on the mechanisms involved in pathogen elimination.