Second step: Filter

When people think of "filters," they usually think of physical filters - screens that contaminants can't pass through, which removes them from the water. (A coffee filter is a physical filter, for example.)

The Bozeman water treatment plant, however, uses chemical filters: chemicals react with contaminants and allow them to be removed.

The second step in treating this water is to add a chemical called a flocculant, which binds to bacteria and other organic contaminants, and then to run the water through deep beds of crushed coal. The contaminants (now called floc) chemically adhere to the carbon in the coal and thus are removed from the water.

The coal beds are rinsed clean every so often by backwashing. Water is run backward through the coal bed, which sweeps the floc up and out into a sludge pond. The floc ends up in a "mud field" outside the plant, where freezing forms basketball-size crystals of the chemical! Eventually the flocculant is recycled.

The coal beds need to be renewed every year or so, and it takes a few train cars of coal to do the job. The energy required to mine coal, to process the floc chemicals, to transport them to Bozeman and to recycle or dispose of them are all part of the ecological footprint for water of the citizens of Bozeman, Montana.

First step: Settling  Thrid step: Chlorine