254 SEWAGE AND ITS PURIFICATION 



for the treatment of a portion of the sewage of Exeter, about 

 50,000 gallons, from 1,500 to 2,000 persons, on the combined 

 system. The tank is cemented watertight, and banked below 

 the ground to keep it from changes of temperature, the top 

 being arched over and covered with turf, so that light and air 

 are excluded. The raw unscreened sewage enters by two inlets 



5 feet below the surface, in order that the entry may be quiet, 

 so as not to disturb the bacterial layers, also that air may not 

 be carried in, nor any gases escape back to the sewer. After 

 passing through a " grit chamber," 10 feet deep by 7 feet long, 

 and of the same width as the tank (18 feet), the sewage flows 

 over a wall submerged i foot below the surface into the main 

 portion of the tank, which is 56 feet 10 inches in length, 7 feet 



6 inches in depth, and 18 feet wide, its capacity up to the level 

 of the liquid being 53,800 gallons, or approximately a day's 

 supply. Hence the transit of the sewage is ordinarily very 

 gradual, averaging about twenty-four hours in the tank, so as 

 to give ample time and quiet for the changes. The Local 

 Government Board at one time asked for a septic tank capacity 

 equal to one and a half times the dry-weather flow. I believe 

 that this is in excess of what is necessary, even when storm- 

 water up to six times the dry-weather flow passes through the 

 tank. 



Thus the whole sewage becomes mixed and averaged, and 

 the bacteria have a chance of working during the passing 

 through the 65-ft. length of flow, which the sewage traverses 

 at the rate of a little more than 2 feet per hour. No obstruc- 

 tion is present, and the entire space is available, differing from 

 what we have seen of tanks partially filled with stones or coke. 

 In the latter the dimensions must either be larger in proportion, 

 or the sewage must pass at a greater rate, the bacteria also are 

 not so freely distributed through the liquid. From the inspec- 

 tion chamber it is seen that a leathery scum from 2 to 6 inches 

 thick, according to the position, collects on the surface and 

 renders the whole anaerobic. Below this is a zone of fermenta- 

 tion, in which the sewage is mainly clear, but bubbles of gas 

 keep the liquid in a state of quiet admixture. At the bottom 

 of the tank there is a layer of dark peaty deposit, so small in 

 amount that during a period of one year's working it did not 

 require to be removed, and after three years without clearing 

 the sediment was less than 4 feet deep. The insoluble organic 

 matter was gradually broken up, while the inorganic substances 



