Hydrolytic Tank.—There are, however, several types specially designed to eliminate these troubles altogether, by separating the flow of sewage through the tank from that section in which the composition of the sludge takes place. Among these is the hydrolytic tank. This tank is already well known to most engineers, in the original form designed by Dr. W. Owen Travis, and adopted at Hampton-on-Thames, but a new and improved method of construction has recently been brought out. The principle of this tank may be described as the deposition and collection of the impurities in sewage by a process of physical de-solution, the matters being separated in the order of their grossness and specific gravity, namely (a) the removal of the grosser solids by means of screens; (b) the settling of the heavy inorganic solids in a detritus chamber; and (c) the separation of the lighter solids in suspension and in a colloidal state. Finally, means are provided whereby the deposit in the various chambers may be collected and removed with facility. It is impossible in the space available to describe in full the reasons for the various details of construction which have been adopted, but the accompanying illustrations, [Figs. 34 to 42], which have been kindly furnished by Messrs. Shone and Ault, Civil Engineers, illustrate an example of the latest type of the tank. [Fig. 34] is a plan section of the tank, and [Figs. 35 to 42] are vertical sections on the lines indicated. The tank is by preference circular, as shown. The sewage is delivered from the pipe S through a screening chamber, in which the gross matters, such as rags and vegetable debris are retained on the screen A, and are from time to time removed by hand or mechanically. The sewage passes over the weir a into the first section B, which occupies about one-eighth of the circumference and is divided into two parts by the diaphragm b, [Fig. 35]. The flow of the sewage through this first section B may, by the weirs b1 and b2, be so appointed that two-thirds of it flows from the outer compartment over b1 and one-third over b2 from the inner compartment, the only entrance to which is by the opening b3, [Fig. 35], in the bottom of the diaphragm b; so that the deposition of the solids by gravity is accelerated by the flow of the one-third of the sewage into the inner part of the compartment B. The solids collect in the conical bottom part b4, [Fig. 35]. The overflows from the weirs b1 and b2, are, by the channels b5 and b6, directed to the downtake c, [Figs. 34], [35], [38 and 39], which delivers the sewage near the bottom of the outer compartment C, which latter, with the inner compartment D, forms the second section of the tank. These two compartments are divided by the diaphragm c1, [Fig. 39], having openings c2 in the lower edge. In the drawing the second section of the tank is shown divided in two parts by the wall and weirs c3, [Figs. 34] and [40], and they occupy together about seven-eighths of the circumference of the tanks. The weirs c3 are so proportioned that 85 per cent. of the liquid passes directly through the outer compartment, and 15 per cent. indirectly through the inner compartment of the first portion of the second section of the tank, into the respective compartments of the second portion of that section. It should be noted that the only passages for the flow of liquids into the inner compartment are the openings c2, [Figs. 34] and [39]; and consequently the deposition of solids is accelerated by this flow, so that they collect in the lower part, c4, [Fig. 35], of the inner compartment B. The flow through the second portion of this second section of the tank is governed by the weirs e5 and e, [Figs. 34] and [36], which weirs are shown of such proportion as to cause 70 per cent. of the liquid to flow directly through the outer compartment, and 30 per cent. indirectly through the inner compartment. The colloiders c6, [Figs. 34] and [35], are fixed vertically in the outer compartments to attract and absorb the solids in pseudo solution. It will thus be clear that 70 per cent. of the sewage flows in a direct manner through the outer compartment, and in doing so deposits practically the whole of its permanent and a considerable portion of its convertible solids. The effluent from the inner compartment D of the second section of the tank is, by the submerged channel e3, [Fig. 36], passed into the supplementary section E, which is fitted with colloiders, e1, [Figs. 34], [35], and [41]. This effluent, which has become fouled by the disturbance caused by the evolution of gases in the inner compartment of the second section, is thus submitted to a further de-solution action by absorption and other processes. Finally the outflow from the outer compartment C, of the second section over the weir e5, [Figs. 34] and [36], and the outflow from the supplementary section E, over the weir e, are passed away from the tank by a common channel, e4, [Figs. 34] and [37], whence the effluent may, for further treatment, be led to filters or on the land. The overflows from the two weirs may, however, be led away from the tank by independent channels for separate treatment. The solids, collected in the form of sludge in the lower parts of the sections, can be drawn off periodically through the pipes c7, [Figs. 34], [35], [41], and [42], governed by valves into the central chamber F, [Figs. 34] and [35], from which it may be led by the pipe f to adjoining land, or elsewhere for further treatment. The lighter solids, that collect in the form of scum on the surface of the liquid in the tank, may be skimmed off or drawn into the channels g, [Figs. 34], [38], and [40], and conducted to the central chamber F, and disposed of similarly to the sludge. The tank is, or may be, constructed of concrete, which may be reinforced as required according as it is wholly or partly above the ground. Its shape may be greatly varied according to local requirements and other considerations.
Fig. 34.
Fig. 35.