though with some classes of sewage the taste comes out in the crops. Live stock are also kept on some farms of this kind.

Financial Disappointments of Sewage Farming.

Advocates of this method of sewage treatment after lengthy experience had ultimately to confess that financial success in sewage farming was unattainable. Owing to the large quantity of liquids of various kinds which accompany the solid excrementitious matter, the manurial value must, of necessity, be weakened by the dilution. A much larger area of land than was at first thought necessary is thus required to deal efficiently with each million gallons of sewage; and this, owing to land values, is a serious expense in the neighbourhood of most towns, and certainly of the large ones; also inability to acquire a sufficiently large area may give rise to serious trouble. The quality of the soil is a most important factor, as unless specially suitable the land is incapable of bearing the strain put upon it, and by receiving more than it can absorb, frequently gets clogged, becoming what is known as sewage sick. The proper purification of the liquids by the bacteria in the soil which would result if less sewage were applied is thus prevented, and there is a tendency for some of it to be very far from satisfactorily cleansed, and by passing into an adjacent brook or stream in that condition may possibly pollute the waters. Sufficient land must also be acquired to give all portions of it periods of rest" from the reception of the sewage, for the purpose of aeration.

66

It must be understood in this connection that no method of treating sewage can be carried out so as to be financially successful. Of those now most usually employed, however, as is more fully shown in the comparison of cost,* land treatment is cheaper, and in every way compares most favourably with them. Bacteriologically, for instance, given suitable conditions, land treatment will very materially reduce the number of bacteria in crude sewage. Again, organic matters present therein can be as well, if not better oxidised, than in other forms of treatment.

Intermittent Downward Filtration.

Experience in various parts of the country with soils and subsoils of varying kinds proved that all could not be relied upon to realise a satisfactory state of purification in crude

* See p. 80.

water-borne sewage, and be used also for the growth of crops. Hence, what became known as the " 'intermittent downward filtration" of sewage came into use for carrying out this system. The land is prepared as regards under-drainage in a somewhat similar manner for the reception of the crude water-borne sewage as for a sewage farm. But the drainage, which in the latter case may not be deeper than two feet six inches, is usually carried deeper (about eight feet in some cases); and under most circumstances no crops are grown, or, if they are, their growth is made quite a secondary consideration. These latter circumstances permit the surface of the soil being turned over at frequent intervals to revivify it; and, further, by allowing each portion stated periods of "rest" from the reception of the crude sewage, the air is permitted free access to all parts of the soil and subsoil to a sufficient depth, and by these means it oxidizes, or, in other words, burns up those particular organisms which have been destroying the various organic matters in the crude sewage, and so preventing its efficient purification.

The results of practical and experimental works to within the last few years can be summed up in a few words, by saying that experience over a number of years in sewage farming, and intermittent downward filtration, prove that land, unless the soil is of suitable quality and a sufficiently large area is employed, is not capable of efficiently treating crude sewage, so as to ensure a sufficiently high standard of purification. But where these requirements are obtainable, land treatment has undoubted powers of removing pathogenic (disease-producing) organisms, and rendering the effluent harmless; and, further, if the solids are eliminated and the liquids only allowed on the land, a yet higher state of purification can be ensured.

For small towns, where suitable and sufficient cheap land can be acquired in a proper situation, land treatment is of the utmost value.

CHAPTER XI.

Observations at Various Sewage Farms.

The following eight sewage farms were under scientific observation for over two years on behalf of the Fifth Royal Commission on Sewage Disposal, with the object, not only of comparing the land treatment of sewage with various artificial methods, but also with the intent of making various scientific observations which it had not been possible for previous Commissions to make, because of the limitations then imposed on them by the lack of scientific knowledgeviz., the lack of knowledge as to the bacteriological and chemical effect of land treatment on crude water-carried sewage.

In the accompanying Table II. are set out several useful details in connection with the sewage farms under treatment. It will be noticed that the best class of soils are capable of dealing with a much greater proportion of sewage than the others. For instance, the difference between a loamy soil and loam mixed with clay is very apparent, as is also the case with a sandy soil. It does not follow, however, that even the best soils can satisfactorily deal with such quantity of sewage as that treated at Cambridge. That dealt with at Altrincham cannot be very efficiently disposed of, bearing in mind that the soil is largely peat, it is, however, improved by the presence of sand. At the same time, such soil should. preferably, not deal with domestic sewage from more than 450 persons per acre.

a

Land Treatment and Soil Suitability.

One of the most prominent features of sewage disposal is the high value of land treatment, provided the conditions are suitable, even in the face of artificial methods.

The Fifth Royal Commission on Sewage Disposal reported in 1908: "That where the soil is suitable, and the area of land sufficient, the organic matter in sewage can be thoroughly oxidised by land treatment."

We would remark that this confirms the views at one time very generally held. Of course, the investigations of previous Commissions obviously did not deal with the bacteriological aspect, because the work of bacteria in sewage purification was scarcely appreciated a quarter of a century ago. The exact nature of the action which takes place in the oxidation of the organic matters while passing through land (and "filters" also), is still but little known; though enough is understood to show it to be a biological process.

Let us notice the subject of soil suitability from the results of the systematic experiments conducted on behalf of the Fifth Commission at the eight sewage farms, set out in Table II. The soils were varied in character, comprising sand, gravelly, light and heavy loam, heavy loam and clay, peaty. soil and sand. Of these, dividing them broadly into three classes, light and gravelly loam seem to occupy the premier position, especially where overlying gravel and sand; heavy loam is placed in the second class; stiff clayey soil, as heavy loam and clay, in the third.

Peat, owing to its spongy nature, is difficult to drain, and is generally not suitable. When, however, it lies upon sand or gravel, the results will be more usually satisfactory. We infer that, in such a case, an acre of land should not deal (with domestic sewage only) with more than that from 450 persons.

Clay will only deal in any way satisfactorily with a proportionately small quantity of sewage, even after settlement; possibly not more than 3,000 gallons per acre per twenty-four hours. In most cases, where clay land only is available, the provision of an installation of artificial filters, following chemical or bacterial treatment, will be found less costly.