SEWAGE DISPOSAL. The following methods are utilized for the ultimate disposal of sewage:— (a) direct discharge into the sea or tidal estuary, (b) treatment on land, (c) treatment on artificial beds, (d) aeration and agitation, (e) tank treatment. Methods (b), (c), (d) and (e) are used inland and are necessary before discharge into a stream. Since the term sewage will include trades wastes, any treatment will have to allow for their presence, and this always increases the difficulties of the problem. The quantity received at a disposal works will vary considerably throughout the day, but is very different in different towns. Rain water and infiltration water will also increase the supply at irreg ular intervals.
at Sea.—This method is obviously cheap, simple and complete. No treatment should be required, the dilution being so great as to render the sewage harmless. Care must be taken, how ever, to choose the site of the outfall so that no sewage is brought back onto the shore and there is no possibility of the pollution of shell fisheries. The sewage should be carried rapidly out to sea at all states of the tide and no nuisance created by the solids being left on islands or mud flats. Such a method of disposal will probably include the rainwater, and the outfall sewer may have to become a tank at flood tide. If possible, this should be avoided, as otherwise the open end will have to be fitted with a flap valve or penstock to prevent the entrance of sea water. The outlet should, if possible, be always below low water level.
towns are compelled in Great Britain to adopt some form of purification as the Rivers Pollution Act forbids the entrance of raw sewage into streams. Purely chemical methods are now obsolete, reliance being to a great extent placed on bacteria. Such organisms live habitually in the upper portion of the soil, and require air for their purifying proc esses, and hence the first of the modern methods was to turn the screened sewage onto or under suitable land. Clayey and heavy soils are quite unsuitable, the best for the purpose being of a light loamy character. As soils vary very much in their suitability, the proposed site of disposal must be carefully chosen, and must afterwards be given frequent periods of rest for a duration of four or five weeks at least to prevent "sewage-sickness." The proc
ess of "broad-irrigation," i.e., flooding the land with sewage at intervals, may now be considered obsolete. "Surface irrigation" is often quite suitable for small rural communities and isolated institutions or houses. The sewage is distributed by carriers which are often only furrows in the ground at anything up to 3o feet apart, and the liquid overflows into the land or soaks through the sides and bottoms of the channels. The quantity treated varies considerably, being from 2,000 to 8,000 gallons per acre per diem, according to the quality of the sewage and the suitability of the land. The effluent is either naturally absorbed by the land, or un der drains may be installed for the purpose. Evaporation will play a considerable part in this means of disposal, a dry hot climate much increasing the amount treated per acre. Certain crops such as rye-grass, may be grown with profit.
The purifying process, due to bacterial action in the upper layer cf the soil, can be much more successfully and rapidly accomplished with greater economy of space, by utilizing artificial filters, working either intermittently or constantly. The first type used were on the intermittent contact or "fill-and-draw" principle, i.e., the filter beds were filled with the liquid, after suitable screening and sedi mentation, so as to remove, as far as possible, the solid constitu ents. On common forms of screens will be caught paper, rags, orange peel and similar matter. A screening chamber will also remove large quantities of the mineral grit, which is, however, much less than formerly owing to the decreasing popularity of water-bound macadam roads. It is usual to allow a capacity in detritus chambers of at least of of the dry weather flow and
there should be at least two to allow of cleaning. If this cannot be done, some form of dredger or worm conveyor should be provided. Disposal works are not designed to deal with more than three times the dry weather flow, the remainder of the flow, up to six times, being treated in storm-water tanks.