A sort of scum forms over the surface of the last and finest bed of sand or charcoal, and if this scum is not too frequently removed, though it makes the filtering slower, the water comes out purer.

On examining this scum, we find it to consist of a thick mat of our old friends, the purifying bacteria of the soil.

So that the last step of our artificial filtration is simply an imitation of nature's great filter-bed.
[17] Several streams emptying into the Ohio River from a thickly settled region are said to be actually pumped out into waterworks systems, used for drinking, washing, and manufacturing, and run back into the river again through sewers by the different cities along its banks, at such frequent intervals that every drop of water in them passes through waterworks systems and sewers _three times_ before it reaches the mouth of the stream..