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.
The evils arising: from the introduction of drains and sewers in place of the old middens and cess-pits, and the desire to prevent the waste of what was then considered to be a valuable manorial product, many years ago induced chemists and other scientific men to attempt to discover an universally satisfactory method of sewage-disposal. Many and various were the remedies suggested, and much time and a great deal of money were expended in the endeavour to find some method which would stop decomposition, and by the aid of chemicals remove or render harmless all the- organic matter and dangerous organisms contained in sewage, arrest whatever was of manurial value, throw down all the matters in suspension, and at the same time allow the effluent water to escape in a whole-somecondition. This was the problem, and up to the present time no thoroughly satisfactory solution has been found. It would be very interesting:, did space permit, to recount the various systems of deodorization, antiseptic treatment, and precipitation, which sprang into existence and lived for longer or shorter periods. Space forbids, however, and consequently a short description of a few of those which have been most prominent, and which contain some ingenious and useful points that may afford hints and information upon this most important question, must suffice.
Amongst the earliest methods, and one which at one time had a great reputation, was Mr. Sillars's "A.B.C. Process". In this system, the precipitant adopted was a mixture consisting of the following ingredients: - Alum, 600 parts; animal charcoal, 15 parts; bicarbonate of magnesia. 2 parts; Mood, l part; boned lime. 25 parts; day, 1900 parts; magnesia, .5 parts; permanganate of potash, 10 parts, salt, 10 parts; vegetable charcoal, 20 parts.
This mixture was added to and carefully mixed with the sewage in the proportion of from 4 to 10 parts of the mixture to 1000 parts of sewage, according to the nature of the sewage to be dealt with. The treated liquid was then allowed to remain in a settling tank, until the solids were precipitated as sludge to the bottom of the tank, the top sewage - or "effluent water", as it is called - waa then claimed to be sufficiently pure to pass into a river.
This process met with some success. but has not been very largely adopted, principally Owing to the large quantity of precipitant required and the excessive amount of sludge produced. The diffieulty of dealing with a large hulk of sludge will be explained hereafter. The effluent also was not everything that could be wished, for, as in all chemical processes, difficulties arose owing to the ever-varying amount and constitution of the sewage, and the dunger of what is known as "secondary decomposition " in the effluent.
Before proceeding to describe some of the "lime" processes, which are of more modern adoption, two other methods may be mentioned, namely, the Savern and Luder's.
The Suvern System consists of an admixture of chalk 100 parts, coal-tor 18 parts, and chloride of magnesium 70 parts, this mixture being used in the proportion of from 10 to 15 lbs. to every 1000 gallons of sewage, and mixed therewith and allowed to stand in precipitating tanks in the usual way. It was found that this mixture acted as a very speedy precipitant, no doubt partly in a mechanical way, but the effluent was not satisfactory.
Luder's System consisted in mixing sulphate of iron, sulphuric acid, and plaster of Paris, with sewage in various proportions according to the class of sewage to be dealt with, but was never very successful
We will now pass on to consider the treatment of sewage with lime, this material having been found to be so economical and efficient in comparison with other chemical reagents or precipitants, as to be now much more generally used than any other. Mr. Santo Crimp, M.Inst.C.E., says1:-"The purest lime should be used, i.e, that containing a very high percentage of calcium, .Strong limes, such as those produced from gray chalk, arc not nearly as efficacious as those yielded by the upper chalk (Hare lime) and by the crystalline limestone of Derbyshire and other counties. The lime should be thoroughly slaked before being used; if possible, the day's supply should be weighed out and slaked on the day preceding that of its intended use. Before being added to the sewage it must be reduced to the 'milky' condition, and this may be accomplished by means of the ordinary mortar-mill or by a lime-mixer" (which Mr. Crimp proceeds to describe and illustrate).
He then goes on to state that the usual dose is one ton of the lime thus prepared to each million gallons of sewage, but that the tendency is to reduce this quantity to the smallest effective amount, for, he says, "an excess of lime in an effluent may cause it to act as a precipitant of the suspended organic matters present in the river-water, thus producing deposits which in hot weather become exceedingly offensive ": and he instances this having happened at Leicester, where land is now being acquired to deal with the effluent, in consequence of "secondary decomposition" having been set up in the river. Chloride of lime has been added to the ordinary lime in order to prevent this; and the late Dr. Tidy stated that one-third of a grain of chloride of lime per gallon of sewage was sufficient, and that his experiencep enabled him to say that about 56 lbs. per million gallons of sewage was sufficient for a "sewage represented by 80 gallons per head of the population ".
1 Sewage Disposal Works, by W. Santo Crimp, M.Inst C.E.
It may be interesting here to give an account of three sewage-precipitation works where lime is used.
Case A. - Lime only is need, in the proportion of 1 ton to a million gallons of sewage. The lime is made into a thin cream, which is mixed with the sewage in the pump-well, and thus gets thoroughly incorporated with it. The sewage thus mixed flows into a series of twelve depositing tanks of a total area of about 1¼ acres, the average daily flow of sewage being al>out 10 million gallons. These tanks are about 6 feet deep, their cubical capacity equalling 2½ million gallons. The sewage passes a distance of 1200 feet through these tanks, and takes 2 hours to do it. The tanks are divided from each other by walls, over which the sewage flows. To show the amount of precipitation in these tanks, it may be mentioned that the first four tanks are cleaned out consecutively about every fourth day, the fifth and sixth about every seventh day. while the remaining six scarcely ever require cleansing, the precipitation having almost ceased before reaching them. The effluent from this process only contains about 2 parts of suspended matter per 100,000, and of these 2 parts one-half is organic matter result is no battel than in Case A.