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encourage the growth of micro-organisms in filters constructed
of various materials, which have the power of converting
("nitrifying") organic matter into harmless chemical compounds
and mineral salts, and the action is so far-reaching that the
insoluble organic matter (sludge) is also broken up and
changed into soluble compounds. The system at Exeter has,
in addition, an arrangement to encourage the growth of other
organisms, which can live only in the absence of air, the effect
being to hasten the putrefactive processes and to split up and
render soluble the bulk of the suspended matters. This
arrangement is used as a preliminary to the filters, and is
carried out in what is called a "septic" tank—that is, a
receptacle to which air is not admitted, and into which the
sewage flows in a crude state.
Although both these methods are still undergoing the
test of trial under practical conditions, yet they have already
been so long under observation that their future prospects
seem to be hopeful, and, if success is ultimately ensured, it is
probable that they will be largely adopted; for the difficulties
entailed in other schemes, by reason of the suspended matters
and "sludge," will disappear, especially with the Exeter plan,
which has also the additional advantage of being automatic
in action, so that the expense of labour is almost abolished.*
VII.—HOUSING OF THE WORKING CLASSES.
A table on page 40 of the last Annual Report sets forth the
results of the action taken in respect of certain houses which
had been represented to be unfit for habitation, and the
following is a summary of the present condition of those which,
at the time of that report, had not been dealt with.
* It is now announced that a Royal Commission is to consider these
biological methods of sewage purification.