A Simplified Method
for the Purification of Comparatively
Small Amounts of Sewage and
Industrial Wastes
A.   PASVEER, Chemist Bacteriologist
Research Institute for Public Health Engineering, T.N.O.
The Hague, Holland
The purification of waste waters from villages, small residential communities and small industrial works, dairies, etc., is a great problem. The
costs of building and running a small purification plant are comparatively
very high, so high that in a large number of cases the needed plants cannot
be built.
In the last few years the Institute of Public Health Engineering T.N.O.
has been engaged in research to find a method whereby small amounts of
sewage can be purified at such a cost that the method may find general
adoption.
PRINCIPLE OF THE SIMPLIFIED SYSTEM
This research has led to a greatly simplified purification technique, of
which in first instance the principle is given in a condensed form. The
method consists in the main of a single process, viz. , that of oxygenation in
an oxygenation tank of the simplest possible construction, usually an earthen
ditch. The sewage is not subjected to any preliminary treatment. By intensive aeration it is possible to achieve the result not only that the sewage is
fully purified, but also that the fresh sludge carried by the sewage and the
sludge formed in the purification process are mineralized to such an extent
that the surplus sludge can be dried without causing any objectionable odors.
This means that it is not necessary to have a sludge-fermentation tank. By
selecting a suitable working method it is furthermore possible to avoid building a secondary sedimentation tank with a return system.
The "basic form" of plant is a ring-shaped circuit or ditch, Figure 1.
Mounted in this ditch is an aeration rotor B which provides for oxygenation as
well as circulation. In this way the mixture of sewage and floe in the circuit
is made to pass the aeration rotor regularly at brief intervals during which it
becomes aerated.
For domestic sewage the capacity of the aeration tank is 300 1 per inhabitant equivalent (54 g BOD). The crude sewage entering at A becomes
diluted with the large amount of purified sewage which is present in the
ditch. The removal of the water, after it has undergone complete biological
purification, is effected in such a manner that all the floe (the sludge) is retained in the plant and a clear effluent is drawn off at C. The quantity of
suspended matter in the plant constantly increases, since the supplied fresh
sludge and the sludge that is formed in the plant are kept in the installation.
Due to the large volume of the aeration tank and due to the fact that the
concentration of suspended solids in the oxidation ditch is kept fairly high,
the total amount of floe in the plant is 10 to 30 times as great as that existing
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