In connection with this it is of importance to point out that not the fresh sewage which is poured on the surface of the land will at once pass into the lower layers, but a portion of the old sewage, which up to then was stored in the pores and is now displaced by the fresh discharge, so that the fresh raw sewage is retained and only purified sewage allowed to escape into deeper layers, which means that in its downward movement all sewage undergoes purification. Were this not the case the raw sewage might reach the effluent drains.
It appears time now to examine somewhat more closely the processes of decomposition and the products elaborated therein.
Factors that influence the process of decomposition.
Concerning the factors which have a favourable influence upon this process, some of them, such as permeability for air, high retentive and absorbing powers, have already been mentioned, and to these can be added moisture and warmth, the latter of which are always present in sewage.
Advantages of a systematic underdrainage.
One word here concerning the systematic under-drainage of the subsoil. Its chief function is, of course, the carrying away of the effluent water and by doing so to prevent the formation of a swamp, but after the land has done its work, and during so-called periods of rest, the under-drains act as ventilators of the subsoil and thus make it artificially more permeable for air, with the result that a drying-up action is set up and oxygen supplied for micro-organic life. For the purpose of improving the ventilation of the soil it may become advisable in places to connect the upper ends of the drains with a short upcast shaft. The mouths of the drains should always discharge above water so as to allow of a free circulation of air.
Micro-organisms that carry on the work of splitting up and converting organic compounds.
The work of splitting up and converting the organic compounds is primarily carried out by micro-organisms such as yeast fungi, mould fungi, algæ, protozoa and even by higher forms of life such as earthworms and insects.
To what extent in addition to these other agencies take part in this bio-chemical process is not yet fully elucidated.
Decomposition and putrefaction most complicated processes.