therefore in better condition for disposal by dilution than merely settled effluent. Moreover the grosser matters which cause surface clogging of filters are removed. It is a question, however, whether septic effluent is better adapted for disposal on fine-grain filters, as the fineness of the suspended matter and absence of the surface mat which is formed on a filter when coarser matters are present result in a deeper penetration of the deposits. FIG. 67. INTERIOR OF COVERED SEPTIC TANK. A septic tank, being essentially a sedimentation tank, is constructed in much the same way. Its cubical contents should be that of from six to twelve hours flow of sewage. If larger, the effluent may be subject to undesirable anaerobic action, and it has been found also that the amount of sludge which resists reduction is increased. As the volume of sewage flow varies. from day to day, and generally increases continually as the population increases, two or more tanks should be provided, and provision made for adding others as needed. With this, arrangements should be made by which, when the flow through the tank or tanks in service becomes greater than desired, another shall come into service; and when the flow diminishes, one shall be put out of service. In perhaps the majority of plants, especially of small ones, this flexibility is not provided, but only one tank is used; but the results from many of those are far from satisfactory. Efforts have been made in some plants to minimize the taking of sludge into resuspension. At Worcester two low baffle walls divide the bottom of the tank into three equal parts, and above these are suspended baffles of scum boards, submerged a few inches. These tend to confine the most vigorous action to the first third of the tank and permit resedimentation in the last third. The largest septic tank plant that has been built in the country was that at Columbus, O., where provision was made for treating 20,000,000 gallons a day, by four tanks 56×150 feet, and two tanks 115X262 feet, each about 12 feet deep, uncovered. The tanks were divided into three sections by transverse walls. These tanks, of concrete throughout, cost $4070 for masonry, $3640 for earth work, $12,530 for sluice gates, and $2490 for other details; or about $3336 per million gallons. These tanks were changed, in 1915, to Imhoff tanks, described further on in this article. Previous to the completion of the Columbus plant, the one at Birmingham, Ala., was probably the largest septic tank plant in the country, comprising six tanks each 100X20 feet by 10 feet deep, treating about 5,000,000 gallons a day. A tank at Lake Forest, Ill., capacity 200,000 gallons per day, cost $8000. One at Delaware, O., of 100,000 gallons capacity, cost, including coke filters, $12,000. One at Lakewood, O., 300,000 gallons capacity, cost, including 625 acres of contact filters, $24,175. One at Mansfield, O., 1,000,000 gallons capacity, with 1 acres of contact filters, cost $65,547. One at Wauwatosa, Wis., handling 100,000 gallons a day, cost $5370. A tank treatment was devised several years ago by Mr. Travis, of England, known as the "hydrolytic " or Travis tank. This aims to separate the sludge from the flowing sewage, to prevent the objectionable effects upon the latter caused by gases and particles of deposited matter rising from the former, and also to take advantage of the principle of surface adhesion of colloids for removing these minute matters from the sewage. The tank, the bottom of which has the form of a flat V, is divided into three compartments by a longitudinal arch-shaped wall enclosing a lower compartment, on top of which is a vertical double wall enclosing a narrow channel and dividing the upper portion into two compartments. The arch has openings along the line of its a, channel through which sewage flows; b, slot through which sediment slides into chamber below; c, gas deflector to prevent gas rising through b into a: d, sludge digesting chamber; e, scum in gas outlets or scum chambers; f, sludge pipe, for withdrawing sludge. Irregular dashes indicate gas rising from decomposing sludge. FIG. 68.-TYPICAL SECTIONS OF TANKS FOR SEWAGE TREATMENT. junction with the V-shaped bottom and also in its crown. The outlet end of the tank has a level weir which is divided by the arch so as to apportion a definite width of weir to each of the compartments. The compartment under the arch receives the sludge through the openings in the arch, the sedimentation occurring in the other two compartments. Sewage enters the upper or sedimentation chambers only, the other compartments receiving sewage and sludge from them. There is, however, some flow through this bottom compartment and over the weir at the end, the amount being determined, as before stated, by the relative length of the weir at the end of this compartment. At Hampton, England, this section of the weir is 20 per cent of the total length. It is believed that tank and weir proportions which will cause the sewage to remain four hours in the sedimentation chambers and twelve hours in the sludge or reduction chamber give the best results. The gases formed in the sludge compartment will not reach the sedimentation chambers to interfere with the sedimentation. There is some flow through the sludge tank; probably because this was thought necessary to maintain maximum septic action. In addition to this construction, the sedimentation chambers, except in the first onefourth of their length, contain a number of vertical or practically vertical surfaces or curtain walls, on which the colloids collect by surface adhesion, to slide off in patches as the accumulation becomes sufficiently dense and weighty to detach itself from the surfaces. The V-shaped bottom of the sludge tank facilitates withdrawing of sludge through a pipe placed at the angle of the V. ART. 76. TANK TREATMENT. IMHOFF TANKS About 1909, a new kind of tank was introduced into this country, which had been developed in Germany by Dr. Karl Imhoff for treating sewage in the Emscher district, for which reason it is called the Emscher, or more commonly the Imhoff tank. In some respects it resembles the Hampton tank. Its aim is primarily to prevent gas, scum and regurgitated particles from reaching the flowing sewage from the sludge; but perhaps a more important effect is the production of a sludge less offensive and much more easily dried than that of a septic tank. The latter characteristic has been the chief cause of its popularity in this country, where it is receiving general acceptance. The tank consists of a channel through which the sewage flows, which has a sloping bottom down which the sediment slides to a slot through which it drops into a sludge-digesting chamber beneath. The slot is so arranged that gases and sludge rising from the bottom cannot pass up through the slot, but are furnished with an outlet outside the channel, where the scum collects. There is very little motion of translation in the bottom chamber. A little liquid rises through the slot as it is displaced by the falling sludge; but except for this, the effluent is practically as fresh as the entering sewage. As most of the sediment collects near the inlet end, it is customary to reverse the direction of flow every few days or weeks, so that the entire length of the tank may be occupied by sludge. The cross-section of the channel should be such as to give a velocity sufficiently low to permit of as complete sedimentation as is practicable, say about 1 foot per minute, and the length of channel such as to give a flowing-through period of two to four hours. The slope of the channel bottoms should be at least as steep as 45 degrees and the surface smooth, so that all sediment will slide at once into the chamber below. To prevent gas rising through the slot, either the channel bottom is made V-shaped and one side is carried beyond the other until its lower end is 6 to 9 inches horizontally beyond that of the other side; or a beam with an inverted V-shaped top is placed under the slot; or the slot is at the side-wall, on which is a projection extending across the slot and a few inches below it. In an Imhoff tank that is functioning properly, the sludge is digested into an inodorous humus mass, with a fairly uniform liberation of the gases caused by the splitting up of the complex molecules of the suspended organic substances into carbon, hydrogen, nitrogen and sulphur atoms. This digestion is probably effected by enzymes or liquid ferments excreted by certain kinds of bacteria. When first put into action, ordinary bacterial decomposition predominates over enzyme digestion; but as the tank" ripens," the latter becomes more effective. The sludge-digesting chamber must have a capacity for holding all the sludge that will accumulate during the period required for sludge digestion or, in cold or moist climates, that during which sludge can not be withdrawn to advantage, generally three to six months. The volume of thoroughly digested sludge (75 per cent moisture) from a system of separate sewers is |