Many were the early efforts to purify sewage by passing it through mechanical filters, either stationary, joggling, or rotating, but all these attempts, it is needless to state, were entirely unsuccessful, and experience has shown that it is impossible to artificially filter sewage, except through land or carefully-prepared bed-filters of suitable materials, and even then it is necessary to first deal with the sewage by some chemical or other process, in order to get rid of the sludge before reaching the filter, as otherwise this is soon choked and rendered inoperative

It is true, however, that partially-successful purification of crude sewage has been obtained by what is known as Intermittent Downward Filtration, where a sufficient amount of suitable light land has been employed, but, in such a case, the treatment may be compared to broad irrigation. The land, however, in the former case is usually drained to a greater depth (6 or 8 feet), the drains are more frequent, and the soil (which must be of an exceptionally light nature) well ken up to receive the sewage. Where a large quantity of sewage has to be dealt with, in some cases equivalent to the sewage of a thousand persons per acre,1 the land is not cropped, an<l the surface is frequently turned over in order to revivify it. No successful downward filtration of sewage through land or other materials can take place, however, unless the filtering medium is allowed to rest at intervals, in order that air, the great restorer, may enter the pores, and oxidize or burn up the organisms which have been at work eating up and destroying the organic matters in the sewage; and very little successful purification can take place, unless the sewage has previously been deprived of its heavier and slimy ingredients by some chemical or other precipitation process, as otherwise the sludge will eventually choke not only the surface of the filter, but sometimes even the interstices, and thus render the filter totally inefficient

1 The entinent engineer, Mr.Mansergh, states that no more than the sewage of 700 persons should be put upon one acre of land drained six feet deep. (Vide Minutes of Proccedings of the Institution of civil Engineers, voL xlix. page 190.

Dr. Frankland, in the First Report of the Rivers Pollution Commission, says, with regard to the filtering power of soils: "These results show how rapidly the process of nitrification (the conversion of ammonia and animal organic matter into nitrates) takes place in the Beddington soil, and how satisfactorily the sewage is purified, even at the rate of 7.6 gallons per cubic yard of soil per diem. But when this rate was doubled, the nitrification ceased, and the pores of the soil became blocked up, so that they would no longer transmit the whole volume of sewage applied and also afford time for aeration."

The limits of this article will not permit any further reference to intermittent downward filtration through natural soils; suffice it to say that good results can be obtained if the sludge is first removed from the sewage, and time is given to thoroughly aerate the filter before a fresh application of sewage is made. Dr. Voelcker says: "A well-drained and fully-aerated soil burnt up, or, in chemical language, oxidized most perfectly, the putrescible and nitrogenous organic constituents of sewage, and transposed them into nitrates and other final products of the decomposition of animal refuse matters, products having no smell, colour, or injurious properties".1

More will be said in due course upon these points when the question of irrigation is being considered.

Filtration through artificial substances has been rarely if ever successful with crude sewage, and is employed only where the effluent water (after the solids have been precipitated or arrested) cannot be passed over or through suitable land. The system of Mr. Ernest Bell is of this kind, and may shortly be described. After the sewage has been precipitated in tanks by a special preparation of alumina and protosulphate of iron, the effluent is passed through filters containing "Magnetone", an insoluble impure magnetic oxide of iron of a very porous nature, which is said to have the property of oxidizing noxious matter during the passage of the sewage through it Fig. 426 is a section of this filter. Mr. Bell's system was tried at Salford with, the author understands, very fair results.

1 Vide Minutes of Proceedings of the Institution of Civil Engineers, vol xlix. page 191.

It is almost needless to recapitulate the various filter-beds of coke and other materials which have been tried with various success. The whole point seems to he that atmospheric oxygen should have frequent and free access to the interior pores of the filter, and where this can be achieved, it is possible to effect a considerable amount of purification. This important point of the revivifying action of oxygen will again be dealt with.

Fig 426   Section of sewage filterr composed of Sand, Gravel, and Magnetone

Fig 426 - Section of sewage-filterr composed of Sand, Gravel, and Magnetone.

The Scott-Moncrieff System consists principally in passing the sewage upwards through a filtering medium about 14 inches deep, composed of successive layers of flint, coke, and gravel. This is simple filtration, and the action is dependent upon the bacteriological theory that certain microbes, under favourable conditions, are capable of indefinite multiplication, and that these microbes or bacteria exist in all sewage, and are capable of peptonizing the solid organic matter, or in other words, that nature has provided its own means for the purification of its refuse. This important discovery will be considered hereafter.

The "Polarite" or "International Water and Sewage Purification Company's" Process is par excellence the most complete chrmical-cum-filtration process at present existing. The sewage is first of all treated with Ferozone, a patent composition containing ferrous iron salts, salts of alumina and magnesia, and finely -divided very porous magnetic oxide of iron, which is said to materially assist precipitation of the solids contained in the sewage. The effluent, after subsidence of the sewage in tanks, is taken through the polarite filter-bed, a section of which is given in Fig. 427. The effluent sewage from the precipitating tanks is allowed to flow on to the filter over the weir-boards, and passes through the filter-beds at such speed as will effectually purify and clarify the effluent1