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.
Under certain circumstances and with proper precautions, the discharge of crude sewage into the sea can be carried out in a satisfactory manner without danger of any nuisance, and such a disposal has much to recommend it. All towns situated on our coasts deal with their sewage in this manner, and it is only where the outfalls have been badly selected that any evils result There are many scientists who contend that this is a wasteful practice, and that the valuable manure contained in sewage should be returned to the land from whence it originally came in the form of food; but hitherto it has been found that, owing to the enormous dilution by water of the more valuable manurial products in sewage, it is more economical to dispose of sewage in the most rapid and sanitary manner and deal with the land in other ways.
The essential points to be considered in dealing with this method of sewage-disposal may be briefly stated. The outfall must be carried well below the low-water mark of the lowest known tide; otherwise a nuisance is very likely to he caused. It must also be carried to such a point that the incoming tide or wind will not bring the sewage back upon the shore, and that the sweep of any currents in the locality will not have the same effect upon adjacent coasts.
In order to obviate such a possibility, and also to ensure that the point of outlet is so selected that the sewage will always, under all conditions of winds and tides, be carried well away to sea and not coast along any neighbouring shores, very careful and complete float-observations must be carried out under all possible conditions of wind and tide. These observations must not only be made with surface -floats, but also with submerged floats at different levels, and the various tracks <»r courses which these floats take must be followed and marked upon proper charta. When the most suitable spot has been thus determined, it may be found that even then it will not be safe to allow the sewage to flow continuously, but that it must be stored in tanks and only allowed to flow at some particular level of the ebb-tide. It is almost unnecessary to add that the culvert conveying the sewage to the submerged point of outlet must cause no obstruction to the navigation along the shore, and that it must be so marked with buoys or "perches", or be so visible both by night as well as day if necessary, that no accident to boats or shipping will occur.
It is no part of this article to enter into any details of engineering construction, but it may be well to give the following description of the outfall works at Portsmouth, as they afford an excellent example of a well-designed and carefully-constructed sea-outfall, carried out under the conditions which have been enumerated. The daily dry-weather flow of the sewage of Portsmouth is about 4,500,000 gallons. The whole of the borough and surrounding district is exceedingly flat, and for some miles the land only reaches a height of a few feet above the high-water mark of spring-tides. The sewage therefore has to be pumped. The lsle of wight is opposite, and there is no promontory along the coast near to Portsmouth which could be selected for a suitable outfall. A reference to the plan (Fig. 424), however, will show that there is a large land-locked harbour, called Langstone, situated about two miles to the east of Portsmouth, which at high water is filled with an enormous volume of water, and this water, as the tide falls, rushes through the narrow channel communicating with the sea; this narrow channel naturally suggested itself as a suitable locality for the outfall. Numerous and extended float -observations confirmed this opinion, as it was found that floats of all descriptions were without exception carried well away to sea, if placed at this point about one hour after the flood-tide had turned. The plan clearly shows the value of these float-observations, as not only was a most suitable position for the outfall determined, but also the most suitable times of tide for releasing the sewage were also aseertained.
Fig. 424. - Borough of Portamouth: sewage-outfall Works. Plan showtng Boat-observations taken from the outfall.
The sewage is raised at a pumping-station about a mile from the outfall, and forced along iron rising-mains to tanks close to the point of outfall. There are three of these tanks placed side by side, with a collective capacity of 4,500,000 gallons, and covering an area of 3¼ acres. Each tank is 160 feet in length and 150 feet in breadth, constructed of cement concrete. They are arched over and covered with soil and grass. The inverts are segmental in cross section, with a longitudinal fall of I in 150 to the outlets. In order to allow the whole of the contents to be discharged within about an hour and a half of high water, the level of the inverts has been placed one foot below ordinary high-water mark.
The quick discharge of the contents of these tanks is a special feature of the scheme, as it has to be accomplished in about three-quarters of an hour. The tanks first discharge into a culvert, seven feet by six in size, from which three linen of cast-iron pipes three feet six inches in diameter are carried well into the tide way. their mouths being just below low-water mark.
The arrangement by which the large penstocks, which let the sewage from the tanks to the eulvert. are opened is very ingenious. At the moment when the discharge ought to take place, a man opens a small penstock from the top of the tank, the escaping sewage from which operates a turbine, which sets in motion the machinery by which the large and heavy penstocks are opened, thus liberating the sewage in large volume, and with a very small expenditure of time and labour. These works and outfall have been in successful operation for the past eight years, and no nuisance or trouble of any kind has been occasioned thereby. They were designed and carried out by 8ir Frederick Bramwell and Mr. Graham Harris, and are the best example of a successful sea-outfall with which the author is acquainted.
An outfall into an estuary may also be successfully carried out, where the volume of water passing out to sea is very largely in excess of the quantity of age poured into it, and where from its velocity the sewage will be carried well past the shores and away to sea, and where no obstructive banks or bars will be formed by the detritus or heavier particles in the sewage settling on the bottom. and thus perhaps causing serious obstruction to navigation or impediment to the How of water.
There are many instances of successful sewage-disposal into estuaries, the most notable being that of Liverpool, which pours nearly the whole of its sewage in a crude state into the river Mersey. The dry-weather How of this sewage amounts to about 10 million gallons in twenty four hours, but owing to the Urge volume of water entering the Mersey at each tide, and the quick velocity of the How of the ebb-tide, no trouble has ever occurred during the great number of years this method of disposal has been practised. There are twelve outfalls of various sizes diaeharging their ooutenta below low-water mark, and though the outfalls are in many cases close to the entrance! of docks, no nuisance whatever has arisen.
Outfall Into a river is to be deprecated except under very exceptional circumstances, such as when the river consists of a very large volume of water, and where the water below the outfall in not used for domestic purposes. Unfortunately, owing to the facilities and economy of thus disposing of sewage, it was almost universally the practice throughout this country when sewers first came into vogue, hut the effects produced on the rivers, rod even on the health of the inhabitants in their vicinity, were so disastrous that steps were very early taken to prevent or mitigate the nuisance thus caused. and the whole question as to the proper disposal of sewage thus began to be discussed. Un-fortunately there are still many cases of river-pollution in this country by sewage rod manufacturers' wastes, and the condition of many of the rivers and streams in this country is a disgrace to the Local Authorities who are responsible for their condition.
The Public Health Act (1848) did not deal with the question of the pollution of rivers, nor did the great Public Health Act (1875), as these acts were permissive rather than compulsory, and even the Rivers Pollution Prevention Act (1876) entirely failed in its object. The Local Government Act of 1888, which gives certain powers to County Councils to enforce the provisions of the Rivers Pollution Prevention Act, has. however, led to something being done to remedy the great evils at present existing.
Fortunately for us, Nature herself provides her own self-cleansing powers, or we should all soon suffer from our negligent uncleanliness. The action of the oxygen of the air and in the water, the absorption of organic impurities by plants, and, above all, the myriads of bacteria to which the processes of putrefaction and nitrification are due, are constantly at work purifying our polluted rivers, and rendering them again fit for the use of man: but there can be no doubt that the practice of turning our waste products into rivers - and particularly small rivers and streams--is to be greatly deprecated, as numeroi cases are on record where the germs of typhoid fever have been carried in rivers for long distances, and have led to outbreaks of the disease in places many miles away, where the water has bcen drunk. The only safe course is to adopt some method of purification on the lines hereafter laid down.