BY HARDOLPH WASTENEYS. Lon 
predjudice which exists against the use of alum in con- 
nection with the purification of a water supply, whether 
this predjudice is justified or not, I venture to assert that 
the comparison 1s all in favour of plain sand filtration, especi- 
ally when it is considered that the average results compared, 
are not the best results obtained from plain sand filtration, 
though unfortunately I have no better figures available 
from observations conducted throughout a similar period. 
(Appendix X.) 
The next method for consideration is Anderson’s Pro- 
cess. This process comes under my heading of Mechanical 
Filtration, though it has not the objectionable feature of 
the method just described, namely the use of alum as a 
coagulant. The process in brief is as follows :—Metallic 
iron, in the form of cast iron borings or steel punchings is 
placed in an iron cylinder, through which the water passes 
on its way to the sand filters. This cylinder is so arranged 
that by its slow rotation the iron is continually lifted and 
showered down through the water, which is being passed at a 
moderate speed through the cylinder. Air also is introduced. 
By means of this contrivance a small quantity of iron 
is dissolved in the water through the agency of the carbonic 
acid contained therein. This iron is dissolved in the form 
of ferrous carbonate, which on coming into contact with the 
air is oxidised and precipitated as ferric hydrate in particles 
more or less coarse, according to the nature of the water. 
This precipitate settles rapidly, carrying down with it and 
possibly oxidising the organic matter in the water, and at the 
same time removing the bacteria also. Subsequently the 
presence of this flocculent precipitate in the water permits 
of its rapid and efficient filtration through a simple sand 
filter. 
The use of this process for the purification of our lagoon 
was suggested to me by Mr. Parkinson, a well-known civil 
engineer of this city, who was kind enough to lend me a 
copy of the proceedings of the Institution of Civil Engineers 
containing a paper by Messrs. Chadwick and Blount, on the 
results of their experience with this method as a means of 
purifying the waters of lakes similar to ours in Mauritius 
and South Africa. The results described in that paper 
were so very good that it was determined to try the method 
en our lagoon water. 
I 
