[Illustration: A PLAN OF SEPTIC TANK AND FILTER-BEDS
The _septic tank_ is a large underground vault of cemented brick, having a capacity of thousands of gallons, according to the population. That at Exeter 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, seven feet from the entrance and twelve inches below the surface of the liquid in the full tank. Within this are caught, by gravity, gravel and such-like deposits. The remaining solid matter of the sewage becomes deposited in the tank itself. 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 anaërobic bacteria, in the latter on the surface by aërobic 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. When the liquid sewage passes out of the tank it differs from the crude sewage which enters the tank in the following particulars: (_a_) The gravel and particulate débris have been removed; (_b_) 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 albuminoid ammonia, urea, etc., have been more or less completely broken down, and reappear in elementary conditions, like ammonia, methane, carbon dioxide, and sulphuretted hydrogen. These latter bodies may be in solution or may have escaped as gas.
The _cultivation beds_ are four or five filters, to which the sewage from the tank flows in such a manner as to produce a weir. By an automatic arrangement the fluid is distributed to each filter in turn. When the second filter is full the first is discharged, and remains empty during the time that the third and fourth are being filled. Each filter is thus full, say, about six hours, and has from ten to twelve hours' rest. These filter-beds (at Exeter) have an area of eighty square yards and a depth of five feet; collecting drains are laid on the bottom of the filters, joining main collectors, the latter terminating in discharging wells. The filtrant is broken furnace clinker or broken coke.