In many cases it is desired to purify the effluent still further, either because of lack of means for final disposal or because the effluent would contaminate the water into which it is discharged. In such cases the plant will consist of the septic tank, an anaerobic filter and a filter ditch or sand-bed filter. The effluent from such a plant will be clear sparkling water that might be mistaken for spring water.
The design and construction of sewage-disposal plants has been made a subject of investigation in a number of State engineering experiment stations. In addition, manufacturers of cement have prepared descriptive literature that is sent gratis on application. These bulletins contain detailed information as to the working properties and construction of private plants to suit the various conditions of disposal. The following is taken by permission of the Universal Portland Cement Co. from their bulletin on “Concrete Septic Tanks.”
Fig. 156.—Septic tank. This shows the construction as if cut away along a center line following its length, also a section of the siphon chamber and a plan of the whole construction.
Fig. 156a.—Photographic reproduction of a concrete septic tank, similar to that of Fig. 153. The tank requires only the cover to make it complete.
“The design in Fig. 156 shows a septic tank as it would appear if partly cut away to expose the interior to view, and as if cut in half along a center line following its length. This type will be found to operate effectively where final disposal is accomplished by sub-surface irrigation. This system once started is self-operating due to the siphon shown in the second, or right-hand compartment, which at regular intervals empties the contents and discharges them into the line of tile from which the liquids leach out through joints into the soil. In a tank constructed as shown in the design mentioned, it is very important to use a siphon to empty the second compartment at intervals instead of allowing a continuous outward flow of contents, because of the tendency for drains to become clogged when liquids are constantly trickling through.
“The size of tank required for residence use depends upon the quantity of sewage to be handled in the first chamber during a day of 24 hours; therefore, this compartment should be large enough to contain an entire day’s flow. This frequently amounts to from 30 to 50 gallons per person per day, so the required capacity can readily be computed from these figures, although it must be remembered that the required depth for the tank should be figured from the top of the concrete baffle wall or partition which separates the first and second compartments. Another point to bear in mind is that the width of the first compartment should be about one-half its length.”