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It will be noted that all the beds showed a very slight but still an appreciable improvement
as regards removal of dissolved oxidisable matter, presumably as the result of the magnesium
carbonate treatment. Thus the increase in purification in the different effluents was 3.5 per cent.;
0.7 per cent.; 5.2 per cent.; 0.7 per cent.
The figures as regards oxidised nitrogen were more encouraging. Thus, with the exception
of the secondary coarse bed (series A), where there was a decrease equal to 2 per cent., all the other
effluents showed a definite increase of nitrates amounting to 19, 33, and 18 per cent. respectively
as regards the effluents from the primary coarse bed (series A), the primary coarse bed (series B),
and the secondary fine bed (series B).
In a sense there was a double gain as a result of the treatment, because the effluents
contained a slightly smaller amount of putrescible matter and also carried with them into the
river a greater amount of an oxidising substance.
But while the results are of considerable interest from the scientific point of view as
affording some confirmation of theoretical considerations, it can hardly be said that they are of
equal importance from the practical point of view. They do not, in short, indicate the advisability
of continuing the addition of magnesium carbonate as a measure of practical utility.
In the second series of experiments Mr. E. Brooke Pike added, as well as magnesium carbonate,
a like quantity (viz., .001 per cent.) of sodium carbonate.
Without entering into detail, it may be said that the results were no longer satisfactory, and
indeed showed a general decrease in respect of nitrate production and of removal of dissolved
oxidisable matter. The amount of sodium carbonate added was presumably too small to inhibit
bacterial growth, and it is difficult to explain the falling off in the results, unless it be assumed that
the sodium carbonate actually increased the vital activity of the micro-organisms in general, but
in particular of the de-nitrifying germs and those specially concerned in producing putrefactive
changes.
In conclusion, it may be said that further and more prolonged experiments seem desirable
with magnesium carbonate and calcium carbonate, as well as with other substances. These might
be tried in varying amounts and added continuously or intermittently. The experiments, of
course, which have been described were merely of a tentative character. They may serve, however,
to indicate a line of inquiry, not devoid of scientific interest and possibly of practical importance,
which has hitherto been neglected.