112 TYPICAL QUEENSLAND LAGOON 
the lake has been a constant trouble on account of vegetable 
growths is said to remain “clear and pure as crystal, and 
nearly as pure as distilled water; this statement is made 
after 30 years experience by Mr. C. Eliot, City Superintendent 
of the Spring Valley Waterworks, of San Francisco, in a letter 
to the Committee of the American Waterworks Association, 
on Animal and Vegetable Growths affecting water supplies, 
1890. When a reservoir was too large to permit of the con- 
struction of a roof over it, the same object is said to have been 
accomplished by the construction of a large raft which floated 
on the surface of the water, and from under the centre’ of 
which the supply was drawn. 
Of other methods of purifying lake waters in situ the 
most important is that devised by Dr. G. T. Moore, and which 
is reported to have been successfully used in several lakes in 
America. The method consists essentially in the addition 
of sulphate of copper to the water in the lake with 
the object of destroying the micro-organisms contained 
therein. The method will be dealt with later on in this paper. 
In nearly all other methods the purification of the water 
is effected outside the lagoon. 
These methods comprise—Settlement in covered basins, 
either by simple subsidence or subsidence aided by coagulants 
such as lime or alum, and with or without the use of screens, 
Sterilization by distillation, which is as a rule too costly 
to be used in conneetion with a city supply, and lastly Filtra- 
tion. ; 
Filtration is the most important of these methods and 
the most generally used. It may be divided into two great 
sections—Plain Sand Filtration and Mechanical Filtration. 
The first section includes all sand filters, both continuous 
and intermittent in action, which depend for their efficacy 
on the power of the sand alone to retain and remove suspended 
matter in the water, and on oxidation effected by the life 
processes of bacteria. These filters are generally operated | 
at slow rates, such as from 14 to 5 million gallons per acre per 
day. 
In the second section, I include all those methods of 
filtration in which sand filters are used in conjunction with 
coagulants and all filter presses of canvas or other porous 
