Figs. 50, 50a, and 50b illustrate the plan of the Burnham Sewage Installation, with longitudinal and cross sections*.

Continuous Flow and Settlement Filtration.
Clifton.

(Barton-upon-Irwell Rural District Council.)

There the strong slop-water domestic sewage, mixed with soft moorland water, from some 2,000 persons, is treated by continuous flow settlement, followed by continuous flow filtration through furnace clinker and cinder, clay-soil gravel and sand. The settling tank has a capacity of 4,875 gallons, and receives the whole of the sewage, both day and night, except when stopped for sludging, which is done about every five weeks owing to the sludge beginning early to ferment. This takes one man some sixteen hours, and during that time the crude sewage is sent direct to one of the four filters. The filters are fed in a very simple way, and, owing to the fine material on their surface, no mechanical distributor is used, but the effluent is delivered from one or more points direct on to the surface of the bed in use.

Owing to the fact that the suspended solids in the incoming sewage are retained by the fine surface material, from which they are periodically scraped, and that only some 15 gallons per square yard are treated thereon per 24 hours, the system is looked upon as satisfactory, given skilful management, for a small community.

The system is considered to have a peculiar interest of its own, both in its chemical and bacteriological aspects, owing to the fact that it is an intermediate one between land filtration and artificial treatment.

General Arrangement of Installation.

Figs. 51 and 51a will perhaps clearly illustrate an average installation for a small town. The system is on gravitation principles. We have first the detritus tank, which should be fitted with suitable screening and inlet and outlet valves. Thence the sewage passes into the bacterial or other sedimentation tank, from which the effluent passes through a measuring chamber, fitted with some sort of measuring

* Reproduced by kind permission of Mr. A. Gladwell, the Engineer and Surveyor.

valve, and thence the liquid is distributed over the filterbeds by means either of a stationary or a revolving distributor, the latter actuated either automatically by the "head "" of incoming tank effluent, or by the use of some motive power.

CHAPTER XLII.

Present Position of Knowledge of Sewage Treatment.

From what has been said in these pages, it will have been seen that the scientific treatment of sewage is still in many respects in its infancy. Valuable as have been the results attained by artificial methods, considerable darkness still overshadows them. Particularly is this so in relation to the work carried out on contact beds and percolating filter beds. Up to recently it was concluded that the work of nitrification was performed solely by bacteria; but now, although but little has yet been discovered as to the manner in which the ammonia, carbon dioxide, etc., are produced in the early stages of fermentation during the breaking down and liquefying of the organic solids in suspension, it has come to be realised that there are associated with the littleknown species of purifying bacteria certain less-known kinds of insects, worms, larvæ, etc.

Although but little is known as to the division of labour, it is evident that they share between them the process of nitrification.

Mr. F. Wallis Stoddart, F.I.C., has been good enough to send the writer (May, 1911) the conclusions to which he has come as the result of a series of laboratory experiments, conducted continuously for more than a quarter of a century (in his own words) "to throw light upon the changes occurring in the sewage filter," and "to ascertain the manner in which the process of nitrification can be brought to the highest degree of activity, and the extent to which it is affected by conditions such as temperature and concentration, which occur naturally in the course of sewage treatment."

"An inclination has been shown recently,' continues Mr. Stoddart, especially by those investigators who attribute a chief share in sewage purification to physical agencies to belittle the practical importance of nitrification.

"The evidence, however, is overwhelming that, useful as the preliminary partition of sewage constituents may be, the final disposal of the putrescible matters is entirely the work of living organisms, and, in the main, of bacteria; and,

further, that this is especially true of the soluble impurities contained in the clarified sewage which is more particularly the seat of the process of nitrification."

As regards the work of living organisms in the process of oxidation, this confirms what the author had already written some months before the receipt from Mr. Stoddart of the results of his experiments.

It is not intended to set out here in any detail the means by which he came to the conclusions arrived at; but one or two of the "conclusions as to sewage disposal

usefully set down.

may be

Mr. Stoddart refers to the fact that the preliminary treatment should serve a two-fold purpose (1) the elimination of the solids; and (2) the prevention of the formation of gelatinous and other matters in the effluent when on the filter, which tend to settle in and clog the interstices of the filtering media, and thereby impede the process of aeration, which is so essential to the efficient working of the filter.

He contends that "the application of the liquid to the filter should be by drops or very fine streams, and should never be interrupted. An intermission of even ten minutes appreciably reduces the efficiency of the filter." And thence tippers, mechanical sprinklers, and all such appliances as cause interruption in the flow of liquid lower the efficiency of the filter.'

66

The writer opines that this assertion will form, in due course, a much-debated and debatable point.

"The liquid should be applied to the medium immediately after comminution to get the maximum effect,” he states, adding that it is a mistake to place the filtering medium much below the comminuting apparatus with the idea of promoting aeration.

"In practice for mechanical reasons, it would seem prudent not to exceed a depth of 12 feet, but this figure should be approached as nearly as possible."

General Conclusions of the Fifth Royal Commission.

The conclusions of the Fifth Royal Commission on Sewage Disposal are in many ways disappointing. It is possible that too much has been looked for as the results of their very extended labours, but, so far, these results were summarised in their Fifth Report issued in 1908, as follows:

"We are satisfied that it is practicable to purify the sewage of towns to any degree required, either by land treaiment or by artificial filters, and that there is no essential difference between the two processes, for in each case the purification, so far as it is not mechanical, is chiefly effected by means of micro-organisms.

"The two main questions, therefore, to be considered in the case of a town proposing to adopt a system of sewage purification are, first, what degree of purification is required in the circumstances of that town, and of the river or stream into which its liquid refuse is to be discharged; and, second, how the degree of purification required can in the particular case, be most economically obtained.'

One result of this conclusion has been the very great modification of the requirements of the Local Government Board as regards the construction of sewage disposal installations.

The decision as to what course should be pursued to deal with the various classes of sewage resulting from the admission thereto of the numerous manufacturing waste liquors is awaited with much interest; as is also the Commissioners' decision as regards the contamination of edible shellfish layings.