Town-Planning and the Small Sewage Purification Plant

Abstract

IN the percolating filter system of sewage purification, at any one works a more or less constant supply of fine solids and nutritive substances in solution is fed to the filters, and from these is built up a composite growth that tends to choke the passage. The growth is counteracted by scouring organisms, usually of varied type, but including in particular enchytraeid worms and the larvae of flies. In the main, strength of sewage determines the composition of the fauna, and the season of the year determines what forms will be dominant for the time being. Reynoldson has recently shown that obstruction and ponding occur when the fauna is restricted to one dominant form1,2. A balanced fauna is most efficient as covering all seasons of the year, with some restriction of any one species through the effects of competition3. The operator at present has no control over the composition of the scouring fauna. It follows that if there are more worms there will be fewer flies, and vice versa. The fact that there must be a large output of flies makes it very undesirable that percolating filters should be close to dwellings. For though the flies are not in that dangerous class particularly attracted to human foods, it must be remembered that they originate from highly contaminated places, that they are mostly inconspicuous and that they may fall into milk and foods. No instances of ill-health, however, have yet been traced to them, except in so far as worry is a cause of illness. Proof of direct association with disease might be impossible.