166 BACTERIAL TREATMENT OF SEWAGE 



has a capacity of 53,800 gallons, and takes the average sewage of 

 1500 inhabitants in twenty-four hours. Near the entrance is a 

 submerged wall, forming a grit chamber for the arrest of gravel 

 and coarser detritus. The remaining solid matter passes into the 

 tank itself. The inlet and outlet being below the level of the 

 sewage, light and air are excluded as far as possible. Both in the 

 sediment at the bottom of the tank and in the thick scum on the 

 surface the organic compounds are broken down and made soluble. 

 In the former position this is accomplished by anaerobic bacteria, in 

 the latter, on the surface, by aerobic bacteria. It need hardly be 

 added that these are denitrifying and putrefactive bacteria, and that 

 those at the bottom of the tank perform greater service than those at 

 the top. What are the changes taking place in the tank? On 

 every side throughout the tank innumerable small masses of organic 

 matter may be observed rising and falling. At first the masses fall 

 to the bottom by gravity ; here they are attacked by countless bacteria 

 which generate numerous gases in the small masses which are thus 

 caused to rise again to the surface ; the pressure being then reduced, 

 the gases expand and burst in bubbles, leaving the particles to sink 

 again and commence a similar cycle. Thus the sewage is rapidly 

 broken down by a process of peptonisation and digestion (anaerobic 

 hydrolysis) until all the organic matter is in solution (soluble nitro- 

 genous compounds, phenol derivatives, gases, ammonia, nitrites, etc.). 

 No rest is necessary, for the supply of organisms is unlimited, being 

 perpetually replenished by incoming sewage. It is contended, and 

 probably with some truth, that most pathogenic organisms would not 

 be able to survive in the competition which must be present in the 

 septic tank. When the liquid sewage passes out of the tank, it 

 differs from the crude sewage entering the tank, in the following 

 particulars : (a) The gravel and particular debris have been removed ; 

 (&) the organic solids in suspension are so greatly diminished that 

 they are almost absent ; (c) there is an increase of organic matter in 

 solution ; (d) the sewage is darker in colour and more opalescent ; 

 (e) compounds like albuminoids, urea, etc., have been more or less 

 completely broken down, reappearing in more elementary conditions, 

 like ammonia, methane, carbonic acid gas, and sulphuretted hydrogen. 

 These latter bodies may be in solution, or may have escaped as gas. 



The cultivation beds are five or six " filters," to which the sewage 

 from the tank flows, and by an automatic arrangement is distributed 

 to each bed in turn. Each filter may thus be full, say, about six 

 hours, and has from ten to twelve hours' rest. The depth of the 

 filtering medium is 4 or 5 feet, and is composed as follows from the 

 bottom upwards : 



(a) About 1 foot in depth of broken furnace clinker, which will 

 pass a 3-inch mesh, but not a 1-inch. 



