This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
We will now turn to a recently-discovered process, which is almost a complete reversal of all the methods hitherto applied to sewage, for instead of endeavouring to arrest decomposition, the object of the Septic System is, as its name implies, to favour decomposition or putrefaction, and for this purpose the sewage is confined in tanks, from which all light and as much air as possible are excluded, so that thorough and complete putrefaction may take place. In this way, the microbes of decomposition are encouraged, and they speedily multiply, and destroy the solid and flocculent matters contained in the sewage. The effluent from these tanks is then passed downwards through ordinary filter-beds of sand and coke-breeze, and is said to emerge therefrom perfectly clear and free from all decomposable obnoxious or organic matter; in fact, pure drinking-water. This remarkable discovery is the invention of Mr. Donald Cameron, the City Surveyor of Exeter, and the following description of the principles of the system, and of the requisite apparatus, is an almost verbation reproduction of Mr. Cameron's own words. The apparatus is shown in Plate XV.
Method of sewage . treatment.
Number of inhabi tants.
Cost per head per annum of sewage-dis-posal. including in-terest on cost of construction taken at 4 per cent per annum.
Analysis or EFFLUENTS.
Grains per gallon.
Parts per million.
The effluent is about one-fourth the quantity of the sewage.
£1, 1s. 9d.
1s. 4.3 d.
Intermittent downward filtration.
There are patches of clay on the sewage
No effluent in summer months.
Precipitation with alumino-ferric and filtration through prepared filters, 800 gallons per square yard per 24 hours.
The filters are now too small, as they require to stand recuperative purposes; 67 w.c.'s dis-charge to these works.
I>itto, filters work at the rate of 200 gallons per square yard per 24 hours
(soluble, 29.0; insol., 10.6)
The effluent is bright and sparkling, and does not decompose.
1 From a paper on "Sewage Disposal of Colliery Villages", by John Edward Parker. A.M.I.C.F., read at the Sanitary Congress of the Sanitary Institute held in Newcastle on Tyne in September. 1896.
It is significant that no difficulty seems to have arisen as to the disposal of sewage until artificial means were resorted to for the purpose. In rural districts, refuse has been largely used as manure. The larger towns, however, have steadily poured their sewage into the nearest streams, the volume of sewage often bearing a high proportion to that of the stream. In spite of the constant pollution to which they have been subjected, the rivers of Great Britain have on the whole maintained a surprising degree of purity; indeed, it may well be doubted whether, if sewage had been the only polluting matter, the question of rivers-pollution would have assumed anything like its present gravity. But the pouring of large quantities of manufacturing refuse into our rivers brought matters to a climax. The duty of maintaining a river free from pollution of every kind is now clearly recognized, and indications are not wanting that the laws prohibiting such pollution will be stringently enforced in the near future.
The contract between the old natural way and the new artificial systems of disposal is a remarkable one. The former was no doubt rough and iv. and the pouring of crude sewage into a river does not commend itself to the educated sanitary conscience. Yet the results were not wholly unsatisfactory, for it has been abundantly demonstrated, both by ordinary observation and by scientific tests, that the sewage so poured into a river is, in many cases, com-pletely destroyed within a few hours.
Until very recently, the processes by which decaying animal matter gradually disappears were little known and less understood. It was known that chemical changes occurred, but how or why was beyond the chemist's power to explain. It is now. however, recognized that many of the chemical changes which take place in organic matter are closely bound up with the life-history of micro-organsms either animal or vegetable, generally known as microbes. There is no doubt that the disappearance of solids from sewage passed into a stream is brought about by micro-organisms. These feed on the organic matter and excrete it in a new form, its chemical composition as a rule being rendered simpler by the change. As a general rule, each species of micro-organism is poisoned or killed off by its own products; but the life-products of one species will generally serve as food for another. The breaking down of the solids in sewage thhs forms a long chain of operations, though often accomplished in a marvellously short space of time.
In the Septic system no chemicals are employed, and there is no "treatment" of the sewage in the ordinary sense of the term, its purification being accomplished entirely by natural agencies,
The Septic Tank itself is merely a receptacle designed to favour the multiplication of micro-organisms, and bring the whole of the sewage under their influence. To this end the tank is of ample size, though not larger than would be necessary with chemical precipitation, and covered so as to exclude light, and, as far as possible, air. The incoming sewage is delivered below the water-level; and the outlet also is submerged with the twofold object of trapping out air, and avoiding disturbance of the upper part of the contents of the tank. On entering the still water of the tank, the solids suspended in the sewage are to a great extent disengaged, going either to the bottom or to the surface, according to their specific gravity. In the absence of light and air, the organisms originally present in the sewage increase enormously, and rapidly attack all the organic matter. By their action the more complex organic substances are converted into simpler compounds, and these in turn are reduced to still simpler forms, the ultimate products of the decomposition in the tank being water, ammonia, and carbonic acid and other gases. Other nitrogenous compounds may also be present, but they will all be soluble in a slightly alkaline solution, a condition which obtains with every norma] sewage.
No sludge is formed. Examination of the bottom of a tank which has been in use for six months, reveals only a thin layer of black earthy matter, the burnt-out ash of the solids of the sewage together with the mud and grit brought down by storm-water. So far as accumulation at the bottom is concerned, it would seem that a tank may be used for an indefinite time without requiring to be cleared. The larger part of the solids in the tank are found at the top, where a somewhat tenacious scum soon forms, consisting of the lighter solids in pro of decomposition. The intensity of the action going on is evidenced by the large bubbles of gas, which everywhere break through the scum. Here is probably the chief seat of the bacteriological action, by which the solids are eventually thrown into solution. As soon as most of the organic matter in a solid substance is dissolved, the ash falls to the bottom, where decomposition continues its work. Presently a bubble of gas is formed, which buoys a fragment of ash and brings it again to the under side of the scum. The bubble soon becomes disengaged, and the ash falls again to the bottom. There is thus a constant interchange between the upper and lower layers of the tank, whereby its solid contents are brought under the most favourable conditions for rapid decomposition and solution. After a tank has been a short time at work, the scum increases in thickness very slowly. In one case, after thirteen months' work, the scum was only a few inches thick.