The Crossness Sewage Precipitation Works.
Our English contemporary, Engineering, gives an interesting description of the Metropolitan Sewage Works at Crossness, where important enlargements and modifications of the existing works are now being earned out. The old works consist of a covered reservoir having an area of about six acres, and capable of containing about 24,000,000 gallons of sewage. This reservoir is divided into four compartments, any one of which can be filled or emptied independently of the others, and the whole four are connected by a culvert with the main sewer from London. This sewer is about seven and one-half miles long and eleven feet six inches in diameter. All the sewage of London, south of the Thames, which has about 2,500,000 inhabitants, is collected in this main, and the average daily flow through it is about 75,000,000 gallons, but in rainy weather these figures may be doubled. Depending on the state of the tide, the sewage is at present turned directly into the river or else into the reservoirs, from whence it is pumped into the river, in either case being totally untreated. The culverts leading from the main sewer to the reservoir and pump wells are somewhat peculiar, three separate culverts being arranged one above the other. This plan was adopted in order that one foundation might serve for all three culverts, as the ground is water-logged and good foundations can only be obtained at a depth of twenty-three feet below the marsh level. The main pumps are eight in number, but in average weather four only are worked. There are eight barrels in all, twelve feet in diameter, and there are four rams, each four feet three inches in diameter by two feet three inches and four feet stroke to each pump barrel. These rams are worked from the beams of four large rotary beam pumping engines erected by James Watt & Co. at Crossness about 1856. These engines have each a single cylinder forty-eight inches in diameter by nine feet stroke, and at eleven revolutions per minute indicate about 250 horsepower, using steam at a pressure of forty pounds per square inch. At this rate of working the two pumps driven by an engine lift about 100 tons of sewage per minute. The suction valves to the pumps are situated at the bottom of the barrel and are of iron faced with leather; the opening through a single valve measures eighteen inches by nine inches. The de livery valves are at the side of the barrel and measure eighteen inches by twelve inches each. In addition to the above pumps two auxiliary pumping engines are also ready in case of emergency. These engines were originally Great Western broadgauge locomotives, which have been converted to drive a couple of centrifugal pumps with horizontal fans five feet in diameter, and each is capable of raising 40,000,000 gallons of sewage a day. One of these engines is always kept fired up, ready to start at a moment’s notiee should the necessity occur. As already mentioned, the sewage is at the present time turned into the river totally untreated, but the works now in progress have been designed in order to allow of some purification of the sewage before it is passed into the river. The method to be adopted is to add four grains of lime and one of iron to each gallon of sewage. This precipitates a large proportion of the solid matter, and the supernatant liquid is then to be decanted off into the river. The quantities of lime and iron seem small, but it is said that an increase of either the lime or the iron would cause a further reaction to take place after the purified sewage had been passed into the river. Moreover, one grain of lime per gallon to the London sewage means on an average ten tons of lime per day, so that the total quantities of lime and iron to be dealt with are really very large. The new reservoirs now being constructed for this purpose cover an area of about two and one-half acres, and will be capable of holding about 8,000,000 gallons of sewage. The reservoir is being built in two stories, the upper one being a setting tank, in which the precipitation of the sludge after the addition of the chemicals is to take place, whilst the lower tank is simply for storage purpose when the state of the tide is unfavorable. The liquor is decanted from the storage tanks by a very ingenious device much resembling the laths of a Venetian blind. These laths are pivotted at one of their edges, and as the sewage rises in the chamber a float sweeps up the face of what we may ca’l the blind, and makes each lath in succession become vertical, in which case it makes a joint with its neighbor immediately above it. Flow from the chamber, therefore, can only take place over the edge of the lath immediately above the float, and as the float sinks, each lath in succession falls over into an inclined position, leaving a gap between it and its predecessor for the sewage to flow through. It will thus be seen that it is always the upper layers of the liquor which escape into the culvert, and the sludge at the bottom is not disturbed. This sludge, even when thoroughly settled, contains about 90 per cent of water, and can therefore be pumped without difficulty. It is intended to pump it on board steamers specially designed for the purpose, which are to convey it down the river for about forty or fifty miles, and there deposit it in the waterway. Each steamer will carry about 1000 tons of sludge, and can be filled through one fifteen-inch pipe in about one hour. The cost of thus disposing of the sludge is said to be about 5d. to 6d. per ton, whilst it was found that pressing could not be done at Crossness for less than 3s. 6d. per ton. As upwards of 3000 tons of sludge will daily have to be dealt with, it will be seen that there were good reasons for not adopting the presses in the absence of a satisfactory market for the product.
—A communication has been received at Marlboro, Mass., from the Boston water board, which would indicate that the city is willing to pay such sum toward the sewerage system as the town authorities think best. The work of building the Marlboro system will, it is expected, be commenced soon, as a sub-committee has been chosen to obtain the price of iron pipe. The sewerage farm will comprise about sixty acres of land, and the total cost of the entire system will probably be about $120,000.