mportant item of expense in the operation
of a slow sand filter. Quite recently a charge of $1.50 per cu. yd.
for sand scraping, transportation to sand washers, washing, and
restoring to the filter, was not considered exorbitant, but the
improved methods developed during recent years at Washington,
Philadelphia, Albany, and more recently at Pittsburg (at all of
which places hydraulic ejection plays an important part), have
shown the feasibility of reducing this figure by nearly, if not
quite, two-thirds.

The practice observed at Washington of raking over the surface of
the sand layer when the available head becomes exhausted, in order
to avoid the cost and loss of time necessitated by shutting down the
filter and scraping off the surface layer, is unquestionably one of
the most striking advances in slow sand filter operation in recent
years. In rapid sand filter operation, to prolong the period of
service between washings, agitation of the filter surface has been
used to advantage for many years. The full value of surface raking
may not be generally appreciated, but the results which have
followed a trial of this procedure at Washington, Philadelphia, and
Pittsburg have shown that the output of filtered water between
scrapings may be doubled or trebled thereby, with no injury to the
filter itself or to the quality of the filtered water. The cost of
raking over the surface of a 1-acre slow sand filter unit is less
than $10 at all the above-mentioned places, which fact in itself
shows the great saving in money and time effected by periodically
substituting surface raking for scraping. Under ordinary conditions
it has been found that a filter can be raked to advantage at least
twice between scrapings.

In the case of filters thus raked, a deeper penetration of suspended
matter into the sand layer is inevitable, but at Pittsburg, as at
Washington, such penetration does not extend more than about 2 in.
below the filter surface. When the filter is finally scraped, a
deeper layer is removed, of course, but it is clearly more
economical to remove a deep layer at one operation than to remove
separately several thinner layers of an equal total thickness.

The lost-time element is an important one, and at Washington this
was the main reason for trying surface raking. It became necessary
to increase the output of the filters, and the ordinary scraping
consumed so much time that the sand-handling force was increased,
working d