Whilst it would, therefore, appear to be correct to say, generally, that closed septic tanks afford no material advantages over open ones, so far as the purification of the sewage is concerned, they may become necessary in special cases, when the smells emanating from open ones might create nuisances in crowded neighbourhoods.

The following remarks refer, therefore, equally to open as well as to closed septic tanks, and no distinction will be made between them.

The work in the septic tank is chiefly done by obligatory anaerobes.

(c) Explanation of Process.—Although the processes taking place in septic tanks are at present but imperfectly understood, they may be said to be in the main due to anaerobic micro-organisms, i.e. due to such micro-organisms which carry on their life’s work in the absence of oxygen. They split up or peptonise the organic compounds in the absence of air, and the group of changes brought about by them has been termed “anaerobic fermentation” or “putrefaction.” During the same, it is claimed that a considerable amount of the sludge retained in the tank is liquefied or destroyed, and that the rest becomes so changed as to be denser than ordinary sludge, and to contain less moisture.

Dissolved matters entering and leaving the septic tank.

Concerning the amount and nature of the dissolved matters entering and leaving an open septic tank, the following is taken from the Manchester report for the year ending March 27, 1901:—​

Manchester observations.

"A series of determinations have been made of the amount of dissolved matter entering and leaving the tank, by evaporating known volumes of the sewage and effluent after filtration through paper and weighing the solid residue.

"An average of six determinations (confirmed by similar observations in connection with the closed septic tank) gave the following results:—​

Raw Sewage.
Dissolved matter, grains per gallon.		Open Septic Tank Effluent.
Dissolved matter, grains per gallon.
Mineral.Organic and Volatile.Total.Mineral.Organic and Volatile.Total.
30·825·055·8
33·033·066·0
Reduction, in per cent.
6·6724·2415·45