BY HARDOLPH WASTENEYS. ahi by? 
of this filter is composed of 5 feet of sand of an effective size 
of -27 m.m.s., placed over underdrains of gravel and brick. 
The method of working is as follows :—Water is fed cn to 
the filter bed by means of a gutter placed along the centre 
of the filter 2 inches above the surface of the sand, no attempt 
is made to obtain any depth of water over the surface of the 
sand bed, so that the water flows almost directly through it 
in such a manner as to leave the greater portion of the 
surface dry. By this means the growth of algae on the surface 
of the sand which takes place in ordinary continuous sand filters 
is avoided, and the periods between the times when scraping 
becomes necessary are in consequence greatly increased. 
No water is fed on to the filter for a certain portion of every 
24 hours, during which time the filter *s allowed to drain, 
and in this manner the sand bed is completely aerated every 
24 hours. Two different periods of rest have been tried for 
considerable lengths of time in this filter, namely 8 hours 
and 4 hours, the former period, 8 hours, appears so far to give 
the best results. The cessation of work for such a large por- 
tion of the time naturally necessitates a greater rate of flow 
through the filter than is the case with ordinary sand filters, 
in order to accomplish the same amount of work for the 24 
hours. Nevertheless, in spite of the increased rate, the 
reduction of colour and organic matter has been excellent, 
and the average reduction effected by this filter when working 
with a daily rest of 8 hours is distinctly better than that 
effected by any of the ordinary sand filters. The Bacterio- 
logical results from this filter are, however, not so good as 
those obtained from the ordinary sand filters, the average 
reduction effected being about 74 per cent. (Appendix IX.) 
We now come to the consideration of results obtained 
from filters of the type designed by Mr. Morry, which he 
describes as “ Biological Oxidising Beds.” Mr. Morry states 
that the idea of their construction was suggested by the 
“ Ducat ” and “* Stoddart ” beds used in sewage purification, 
and which are built up above the ground. . In brief the beds 
are composed of about 6 feet in depth of gravel, or coal, in 
varying grades ; in some of the filters these beds are so con- 
structed as to admit of side aeration, but the feature which 
is common to all is the method of delivering the un- 
filtered water on to the bed of the filter. This is done through 
