Before concluding these remarks on the natural purification of sewage it is necessary to draw attention to another considerable advantage which it possesses over artificial sewage treatments, and that is the reduction in quantity of the effluent, which at times is very considerable, whereas in the artificial methods such a reduction is comparatively small.
Loss of liquid by evaporation and by plant life.
Spread over a large area of land, well cropped, evaporation is very active—especially during the summer months, when the flow of water in the brook that takes the effluent is as a rule at its lowest; and, in addition to this, the growing plants further abstract a considerable amount of the liquid that finds its way into the soil, so that the quantity of the effluent may not be more than from 30
to 50 per cent. of the total quantity that was poured over the land. In the artificial treatment the evaporation is considerably smaller, and as plants are altogether absent the quantity of the effluent is probably about 90 per cent. and more of the total quantity of the raw sewage. This is a point of very considerable importance so far as the influence of the effluent upon the water in the stream that takes the same is concerned.
Although the subject of natural purification is by no means exhausted, it is now time to direct attention to artificial methods.
V. ARTIFICIAL SELF-PURIFICATION OF SEWAGE.
Enumeration of more important experiments.
A great many experiments have been made during the last ten years with artificial processes for the self-purification of sewage, and amongst the more important the following may be mentioned:
| London experiments. | |
| Sutton | „ |
| Exeter | „ |
| Manchester | „ |
| Leeds | „ |
| Sheffield | „ |
| Leicester | „ |
| York | „ |
| Hamburg | „ |