and the Storage of Water. 
51 
for a town supply was that which contained from 10" to 15° of 
hardness.* 
Filtration. —Organic matters held in solution or suspension are 
highly injurious to health ; no more effective source of disease 
of the very worst type exists than water contaminated with 
animal refuse and sewage. No system of mechanical filtration 
will effectually remove this. The Rivers Pollution Comniis- 
sioners are even of opinion that the process of oxidation neces- 
sary to destroy the soluble organic matter present in polluted water 
is one of such extreme slowness, that the hitherto prevalent idea 
as to the purifying effect of running water is untrustworthy ; 
and that there is no river in this country long enough to purify 
water thus contaminated sufficiently for drinking purposes. 
Fortunately, in the pores of an open soil oxidation goes on very 
rapidly, especially when assisted by growing vegetation, and it 
completely removes all noxious matter. 
Mineral matters held in suspension, although of an innocuous 
character, diminish the brightness of water and impart a repulsive 
appearance to it. Filtration through sand is therefore requisite 
where the supply is drawn from brooks and water-courses. 
Slow filtration removes the suspended impurities and also assists 
in the oxidation and removal of organic matter in solution. 
The filter-beds generally in use are composed of sand and gravel, 
the amount varying according to the quality of the sand and of 
the water to be filtered, the average being about 2 feet of sand, 
() inches of fine gravel, and 6 inches of coarse gravel. The 
beds are made in duplicate, to allow of one being cleansed and 
oxidised while the other is in use. The gravel is only intro- 
duced to support the filtering medium, the sand, and to allow of 
the filtered water being drawn off without disturbing it. Below 
the filtering material the water drains off by means of perforated 
tubular pipes stretching across the beds and communicating with 
a central inclined channel. The head of water used to work the 
beds is -generally about 2 feet, and the same depth of water is 
kept on the top of the sand. The filtration through sand should 
not proceed at a higher rate than 6 inches of descent per hour, 
and this will allow about square yard for every 1000 gallons 
filtered in 24 hours. The sediment deposited on the surface of 
the sand requires to be scraped off frequently in summer time, 
and less often in winter. From a quarter to half an inch of 
sand is taken off each time with the sediment, the sand being 
replaced when the layer is reduced to 1 foot in thickness.f 
* ' Water Supply of Cities and Towns.' By W. Hnmber. Crosby Lockwood 
and Co., 1876. 
t ' Waterworks for the supply of Towns.' By Hughes. Weale's Series, 1859. 
' Water Supply of Cities and Towns.' By W. Humber. 
E 2 
