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.
A residuum which is technically known as "sludge" remains, as already stated, in all sewage-settling tanks after chemical treatment, and the ultimate disposal of this offensive.slimy semi-fluid material is by no means an easy matter. The amount of sludge produced from a given quantity of sewage is naturally wry varied, according to the quality or consistency of the sewage and the description and amount of the chemicals used in the process. For instance. it appears that the amount of sludge produced daily at Birmingham from the sewage of a thousand persons is nearly a ton (a cubic yard of sludge weighs about 16 cwts.), whereas, for the same number of persons at Chiswick. the amount of sludge is about a ton and a half, and at Leeds only a third of a ton.
The manurial value of sludge in its crude state is negligible, owing to the exeees of water it contains (about 95 percent), but when dried it is said to be worth about as much as ordinary farmytid manure, weight for weight; consequently all serious attempts to deal with this material have ben in the direction of eliminating as much of the moisture as possible.
At some sewage works the preliminary stcp in the separation of the liquid consists in running the sludge upon roughly-contrived filter-beds, composed of ashes screened from ordinary house-refuse; after a partial drying the sludge is mixed with more ashes, and when sufficiently hard and dry this "compost", as it is called, is carted on to the land and dug in as manure. This method is, however, very tedious, as in damp or wet weather the drying by evaporation is much retarded, and the handling and cartage also become expensive items.
At Ealing, near London, the sludge is mixed with the house-refuse and burnt in an ordinary destructor, the residuum being an innocuous and inoffensive "clinker". This method was also adopted at Salford, but the process is liable to produce offensive fumes, which must be specially dealt with.
At the Birmingham sewage-farm the sludge is simply dug into the land, whilst at Crossness on the Thames, where a large proportion of the London sewage is dealt with, the sludge after a partial natural drying is pumped into special hopper steamships and carried out to sea, where it is discharged into deep water; this latter method has also now been adopted at Salford.
The more modern method, however, of dealing with this necessary evil of all chemically-treated sewage, is to pass it through a "filter-press". The plant necessary in this case is a steam or other engine, working an air-compressor of such capacity as will compress the required amount of air to a pressure of about 100 lbs. on the square inch. The filter-press is usually made of vertical east-iron plates, with recesses on each face and projecting rims, so that when pressed together there is a space between. The surface of each plate is furnished with cloths of jute, hemp, canvas, felt, or some such material, acting as a filtering medium.
Fig. 425 shows the general appearance of a filtering-press of the pattern supplied by Messrs. Manlove & Alliott of Nottingham. The sludge is forced through the centre of the fixed end into the chambers between the plates, where the pressure is maintained until nearly the whole of the moisture has been forced through the firtering-pads and flows out by openings at the lower edge of the plates. When water ceases to flow, the hand-wheels are loosened, and the end frame is moved by the piston acted on by the compressed air, and the plates are separated one from the other by sliding them along horizontal shafts. The sewage-eakes, which have thus been formed between the pairs of plates, drop out.
as the plates are moved, into a truck or other receptacle placed under the press, and are removed to a suitable shed for sale as manure. The press is again dosed, and the process of pressing resumed. Such a press as has been described is capable of turning out ham 20 to 25 tons of cake per diem, at a cost of about 10d. per ton, the cake containing only about 50 per cent of moisture. About 5 tons of sludge can be pressed into one ton of cake, and this if dried and pulverized can be sold for about 2s. per ton. These figures will of course vary with the description of sewage dealt with and the chemicals employed, but it may be well to point out that it is generally found necessary to add from 3 to 5 per cent of lime to the sludge before pressing it, in order to prevent subsequent decomposition.
Fig 425 - Elevation of Manlove and Alliott's a Fillter-press for Sewage Sludge.
In concluding these remarks upon the disposal of sludge, the author will describe the methods at present adopted for the disposal of sewage sludge at Manchester. The present production of sludge is about 250 tons a day, but when the whole of the sewers are connected with the main drainage system, the amount will be about 510 tons per day, and ultimately, owing to increase of population, about 624 tons. When the sewage has been run continuously through a precipitation-tank for a period of from two to three days, - varying according to the weather and the character of the sewage, - the penstocks are dosed, and the sewage in the tanks is carefullv decanted by means of floating arms, the sludge being left at the bottom of the tank. Valves connecting with the sludge-well are then opened, and the sludge gravitates, with the assistance of manual labour. into the well, from which it is pumped into the sludge-pits adjoining the lime-house. About two per cent of fresh lime is added to the sludge by hand as it passes to the pits, in order to break up the glutinous matters it contains and render it fit for pressing.
The pressing is performed in the following manner: - A valve is opened at the bottom of the sludge-pit, and the sludge is admitted into one or more of the vertical iron sludge-rams, which are placed under the press-house, and may be compared to the modern aerated-water syphon. When the rams are full of sludge, air under pressure of about 90 lbs. per square inch is admitted and forces the contents into the filter-presses, which are placed on the first floor of the building. Each press contains 44 ribbed vertical plates, 41 inches square, resting on a horizontal frame, upon which they are free to slide. These plates are covered on both sides with coarse cloth, specially manufactured for the purpose. The sludge under pressure is admitted between the cloths, through which the moisture percolates and passes away to a covered tank by openings at the bottom of the press, the solid portions remaining between the cloths. The presses are opened and closed by hydraulic pressure. Each pressing takes about 55 minutes, and leaves a deposit of cake about an inch and a half in thickness between the cloth-covered plates.
The present filter-presses, eight in number, produced during the year 1895 an average quantity of 738 tons of pressed cake per week, at a cost of 2s. 11 .9d. per ton of cake produced, equal to 9.7d. per ton of wet sludge. The cost of pressing is based upon the returns of the manager. For some time after the works were put in operation, the cost was at the rate of 3s. 6.6d. per ton of cake produced, equivalent to 11.5d. per ton of wet sludge. The weight of cake produced by each pressing is about l½ tons. The liquid passed off from the presses equals about fifty per cent of the bulk, or about three-fourths the weight of the wet sludge. This liquid contains a large proportion of polluting matter, and is pumped back to the precipitation-tanks for re-treatment. The weight of wet sludge resulting from one million gallons of sewage is about 24 tons, or 6.5 tons of cake, the proportion of wet to pressed sludge having varied between 3.71 to 1 and 3.25 to 1, since the opening of the works in January, 1894. It is difficult to assign any definite reason for this variation in the consistency of the sludge, but it may possibly arise from the admission of trade-refuse into the sewerage-system through old sewers recently connected. Furnace-ashes are at times brought down with the sewage, and the bulk of this material is occasionally so great that it can be separated from the sludge and removed to a tip without pressing. The estimated future production of wet sludge is 624 tons per day, equivalent to 168 tons of cake.
The cakes are dropped from the presses through a hopper into wagons, and conveyed to a tip formed by the disused bed of the river Mersey, which formerly passed through the works. A proportion of the pressed sludge, amounting to about 17.598 ton.- per annum, or an average of from 40 to 50 tons per day, is at certain periods of the year removed by farmers in the locality.
It is stated that the present tip, formed by the old river wall, will at the present rate he shortly filled up, so that the Corporation of Manchester are proposing to purchase a steam barge, of about 750 tons carrying capacity, for the purpose of conveying the sludge in a wet condition to sea about 30 miles from the entrance of the Ship Canal, or a total distance from the works of about 60 miles, at a cost of nearly 9d. per ton of wet sludge.
Such are the difficulties connected with the sludge question, that "bete noir" of all precipitation-systems, and we will now proceed to discuss some other methods of dealing with the disposal of sewage.