Mineral coatings on sand particles actually encourage microbial activity in the rapid sand filters that are used to treat groundwater for drinking, according to a paper published ahead of print in Applied and Environmental Microbiology. These findings resoundingly refute, for the first time, the conventional wisdom that the mineral deposits interfere with microbial colonization of the sand particles. "We find an overwhelmingly positive effect of mineral deposits on microbial activity and density," says corresponding author Barth F. Smets, of the Technical University of Denmark, Lyngby.

Mineral coating develops on the filter grain surface when groundwater is treated via rapid sand filtration in drinking water production. Coating certainly changes the physical and chemical properties of the filter material, but little is known about its effect on the activity, colonization, diversity and abundance of microbiota

Until now, rapid sand filters have been a bit of a black box, says first author Arda Gülay,one of Smets' graduate students.

"In rapid sand filters, a combination of chemical, biological, and physical reactions help in the removal and precipitation of the impurities -- iron, manganese, ammonia, and methane, for example," says first author Arda Gülay,one of Smets' graduate students. In time, the sand filter grains become coated with minerals, much of which the system managers remove, periodically, via backwashing.

It turns out that the minerals form an abundant matrix around the sand particles, sort of honeycomb-like. "Bacterial cell density in these structures can be very high, and can be boosted further when extra ammonium is provided," says Smets. The bacteria are normally engaged in removal of ammonium, manganese, and other impurities from the groundwater.