THE SEPTIC TANK SYSTEM.

Progress, Volume I, Issue II, 1 September 1906, Page 309

THE SEPTIC TANK SYSTEM.

The Barrhead (Eng.) septic tanks, which we illustrate herewith, were erected at a total cost of £5,77^, including the charges of The Septic Tank Coy., and were designed to serve a population of 10,000, and to purify a maximum flow of 400,000 gallons — that is, 40 gallons per day per head of population. They deal with domestic sewage and a portion of surface water only is carried direct into the river without purification of any kind. The works consist of two grit chambers, four septic tanks and eight aerating bacterial filters, all of which are built in concrete. The main out-fall sewer discharges into the grit chambers, from which the sewage passes, without being screened, into the septic tanks, m which the solid impurities of the sewage are liquefied. The effluent, thus freed of solid matter, passes out through a slotted pipe at the end of each tank into the effluent collecting channel. From this it flows through the modules and aerators to the admission valve chambers of the filters. Each of the septic tanks is 100 feet long x 18 feet wide x 7 feet deep at low water, and is entirely covered with concrete arches carried by rolled steel joists and brick piers. The four tanks when filled to low-water level contain about 312,500 gallons. The sewage in the tanks may rise 18 inches above the low-water level, an additional capacity of about 70,000 gallons being thus brought into play to enable them to cope with sudden rushes of storm water. In spite of variations m the flow to the tanks, the quantity passing to the filters is maintained constant by means of two regulating modules. Each filter is 55 feet long x 54 feet wide, and

is filled to a depth of four feet with broken furnace cimker, which constitutes the filtering material. The total filtering material area is about 2,540 square yards. Six of the filters ordinarily form the working set, the other two being held in reserve. The tank effluent is supplied to one filter at a time, and is distributed over the surface of the filtering material by a system of stoneware distributing channels laid thereon. The discharge valve is closed meanwhile, so that the interstices of the filtering material become filled with the tank effluent. The effluent remains in the filters for about one hour and a half, during which the dissolved impurities are oxidised by the bacteria attached to the filtering medium. The discharge valve is then opened, when the filtering effluent escapes, drawing down after it a supply of air into every crevice of the filter. The latter is then left to dram and aerate while the other working filters are filled in turn, after which it is again filled. The alteration of the filters is accomplished by a system of automatic gearing which works very well. The filters have been m use for six years and have not lost any of their filtering capacity and produce without fail an excellent effluent. It does not appear that the works have lost efficiency in any respect ; from the moment the superficial scum is broken the filtration goes on with the same rapidity, and no diminution in the capacity of the filters can be detected. A patch of the filtering material was recently dug up right to the bottom of the filter and it had simply the appearance of wet dirty ashes or furnace clinker, and did not appear to have been affected or broken up as a result of its six years' use. The septic tanks have continued to perform their share of purification, without interruption, and without interference of any kind. They have never been

cleaned out, and as far as can be judged, may never require to be, but the continuous working for six years is satisfactory evidence of their efficiency* It has to be borne in mind that road detritus or insoluble grit is not allowed to enter the tanks — that is caught in the grit chamber referred to. This chamber is of comparatively small dimensions, and the grit is cleaned out two or three times a year, the refuse being trifling in quantity is easily disposed of. The next question of interest is that of the character of the effluent. It has always been good enough for requirements, being cleaner than the stream into which it was discharged, and there never has been any evidence of subsequent putrefaction. It is always in appearance like clean water and absolutly without smell. Analyses made prove that there is no falling off in the efficiency of the works. The percentage of purification seems to have steadily increased, and the nitrate appears to be exceptionally good. The apparent cleanliness of the effluent is indeed quite remarkable Messrs. Tatlock & Thompson, in their last analysis, made a favourable comparison in the matter of colour with Lock Katrine water, the water that is supplied to the City of Glasgow, and that the matter in suspension is so small that it cannot be stated in figures. The works at Barrhead were constructed in accordance with the design of the Septic Tank Company, Ltd., and was the first installation m Scotland.