Sewage Disposal

The royal commission on sewage disposal, however, states that it is generally only necessary to provide filter capacity up to times that required for dry weather flow, hence it publishes a table for three strengths of sewage, giving areas desirable for "sprinkling" filters and "contact" or "fill and draw" filters. (Par. 293, p. 209 of Report.) Sewage may be roughly classified by the amount of oxygen absorbed in parts per z oo,000 from per manganate of potash at 80° F after four hours. From 7 to 8 parts may be considered weak, from i o to 13 average, and 17 and over "strong" sewage. Any flow over the six times may generally be passed direct to the streams which must be done before reaching the works.

Sedimentation Tanks.

These are usually on the continuous principle and consist of at least two tanks, the total capacity being not less than i o to 15 hours dry weather flow. Fig. i shows a common type of rectangular tank, of width about one-third to one-fourth the length. The speed of travel through the tank should be slow, not more than one inch a minute, and so that the "detention period" is just sufficient to remove all solid matter possible. The tanks are usually emptied by a "floating arm" (see fig. 1) so as to draw the water from the surface, and the sludge is either removed by hand when the tank is emptied or passes to some form of pump. Vertical tanks may be used, is valuable or land very expensive. They have the disadvantage of entailing considerable excavation and greater cost of con struction. The liquid may be allowed to then pass to filters as it should contain little more than colloidal matter. These filter beds consist of shallow tanks filled with suitable rough material such as slag, coke, clinker, etc. Hard stones such as granite are too liable to polish and so form an unsuitable medium for bac terial culture. The beds are worked in cycles, each cycle consist ing of about z hour filling and the same time emptying, 2 hours standing full and 4 hours rest, the bulk of the purifying action taking place during the rest period. The beds are usually about 3 feet deep to ensure internal aeration and are never more than 6 feet. About 5o per cent of the space is available initially for the liquid, but this rapidly reduces to about 3o per cent after use. The filling and emptying may be done by hand with pen stocks or automatically by syphons or the liquid may be dis tributed by surface channels, and it is usually considered that the economical limit in size is one acre. The usual size of the broken material is about 2 in. cube, but may be larger or smaller.

The continuous type of sprinkling or percolating filter is now more common, being more economical in space and time but needing some form of distributor. The commonest form is the rotary shown in fig. 2, the beds being then circular or hexagonal. If the beds are above ground the walls are frequently omitted and consist of large, uncemented blocks of the clinker. In the type shown the effluent leaves the bed by a false floor of tiles or pipes connecting to a drain, but in the elevated type the effluent is generally collected in an open channel surrounding the bed.

Such filters are usually 6 feet in depth and the material is usually not larger than 3 in. cube. It is said that they will treat

at least 300,00o gallons per acre when 5 feet deep, but the amount depends on the size of the medium and the amount of the sus pended solids. Various other ingenious methods of distribution are in use, but fixed sprinklers with nozzles are now obsolete in Great Britain.

Before the treated liquid can be finally discharged to a stream, it is generally necessary to further pass it through what are known as "humus" tanks or filters. This will be due to the effluent still containing organic matter which has escaped the purifying action of the bacteria on the previous filters, and there will also be a proportion of the filtering medium broken down by the passage of the liquid. These tanks are usually merely repetitions of the sedimentation tanks on a smaller scale, having a capacity equiv alent to 4 hours dry weather flow. Shallow sand filters about 12 in. deep have also been used and are considered to be able to deal with not more than 50o gallons per sq.yd. per day. These latter will, however, need constant attention.

The latest methods of treatment utilize the power of air and agitation to increase the bacterial action, it having been also dis covered that sludge once treated by air can be utilized to inocu late the untreated sewage after settlement, and to much accelerate the purification. Fig. 3 shows in diagrammatic form the essential feature of the "activated sludge" process, the sewage passing along channels fitted with porous "air diffusers" in the bottom or at the side, and also often being given some form of spiral motion to utilize any surface aeration. Many experts consider that much of the purification is due to the mechanical agitation on the surface by the bursting of the air bubbles, and hence sys tems causing surface agitation are in growing use. In this method a horizontal agitator draws up sewage by the central tube and throws it to the sides of the tank, thus aerating and agitating at the same time, the liquid finally escaping of ter several revolu tions. In the Sheffield system of "bio-aeration" the liquid travels a long length of narrow channel, being agitated at regular inter vals of about 26o feet by skeleton paddle wheels as shown in fig. 4. In these aeration and agitation methods a final settling tank or tanks is required before the liquid is ultimately discharged to the stream. The amount of air used for diffusion is about cu.ft. of free air per gallon of sewage, but this includes air used in the air-lift pumps, which are often utilized to return a portion of "activated" sludge to the commencement of the process. The area of the diffusers varies from to y that of the channel floor. The air pressure is usually between 5 and io lb. per square inch, specially designed air-compressors being now manufactured. The diffuser area will be smallest for purely domestic sewage and largest for strong sewage with a large proportion of trade waste. The detention period in the channels varies from about 3 hours at least for purely domestic sewage, to as high as 20 for sewage with much trade wastes, the latter being very liable to injure the process. The diffusers are of fine concrete or silica sand and are contained in metal or preferably concrete boxes.