trine of slow sand filtration. Various new
processes for cleaning the sand surface have been advocated; some of
these partly destroy and others completely exterminate any semblance
of a bacterial film on the sand bed. These ideas, advanced without
any real and serious discussion of their intrinsic merits, or their
effects on the public health, are not founded on long continuous
records of such results as are necessary to establish confidence in
the final value of any of these methods.

Rapid advances along this line have been made more recently,
notwithstanding the occurrence of notable instances of trouble and
the resultant need of complete repair of filtration beds. Because of
the rough treatment of the sand surface, a penetration of organic
matter and filth into the bed had taken place. This caused deep
clogging, prevented the usual yield of water, and brought about a
lessened bacterial efficiency, due to the attempt to force water
through the filters, and because some organic matter and growths in
the lower part of the bed had furnished a breeding place for more
bacteria.

All these endeavors to reduce the work of cleaning have been
commendable, because scraping and sand handling are the items of
greatest expense in slow sand filter maintenance. Every one has been
desirous of minimizing this cost. However, as the writer will
endeavor to show, it seems that attempts along this line should be
with the idea of doing more economically, as well as efficiently,
the things which one knows will accomplish the proper results,
rather than unwisely to adopt new methods which have not been tried
for a long enough period to determine their effect on the public
health.

_Pittsburg Methods._--When first taking up the problem of design in
Pittsburg, in 1902, the writer had presented to him for
consideration and adoption, a suggestion that a certain method of
cleaning sand filters, which would involve the washing of the sand
in place (similar to that recently tried at the Jerome Park
Experiment Station, New York City), would be advisable and
economical. The decision then made has never been regretted. As this
plan involved such a complete departure from those principles which
had been well tried and had proven successful, it was believed that
it was not safe to adopt such a method on the municipal filtration
works, from which the people were to derive their drinking water.
There is more to be considered in such a problem than m