762 



REPOET 1891. 



introduction of air to allow the tank to lie emptied, and as the oil is kept cool, and 

 always under pressure, it is impossible for a dangerous explosive mixture to 

 accumulate inside the tank. 



Evaporation, 



3. On the Hevolvinq Purifier for the Treatment of Water h/ Metallic Iron. 

 By W. Anderson, D.G.L., F.R.8.,'M.Inst.C.E. 



After pointing out the advantage of being able to purify in a satisfactory 

 manner the water of rivers available for the supply of towns, the paper proceeds 

 to relate some of the recent experiences of the system first introduced at Antwerp 

 some 7 years ago. Practical working in several places has shown that satisfactory 

 purification can be obtained after treatment with iron at a much greater rate of 

 filtration and through a thinner layer of snnd than in ordinary filtering arrange- 

 ments—a speed of as much as 100 gallons per square foot per twenty-four hours 

 being the usual rate of running at Dordrecht, for example. The complete installa- 

 tion is briefly described, and the marked efl'ect iii reducing organic contamination, 

 in the arrest of free ammonia, and in the destruction of microbes is attributed to 

 the formation of ferric oxide, which acts as a coagulant, depositing a very fine 

 filtering medium on the surface of the sand in the filter beds. The comparative 

 cost of the ordinary systems of sand filtration and the author's method are 

 contrasted, and a considerable economy, both in capital, outlay, and in working 

 expenses, is shown to exist. 



The installation at Agra, on the river Jumna, is next described, and the highly 

 satisfactory results obtained noted. The experimental apparatus on the Seine, 

 near Paris, is mentioned, and figures are quoted to sliow the large degree of purity 

 attained. The eflJ'ect of the iron treatment on waters containing very finely- 

 divided argillaceous matter, like the Nile, the Mississippi, and other rivers, is 

 described, and abundant evidence is given that these waters, which will not 

 subside clear in any reasonable time and cannot be filtered bright, yield imme- 

 diately to the iron treatment. Several instances of successful application in the 

 United States are given, and especially at Chicago, where a reduction of albumi- 

 noid ammonia from 3^08 to •122 parts in a million has been obtained. 



The paper goes on to describe some improvements connected with the intro- 

 duction of air and small doses of carbonic acid, which are found to be beneficial in 

 obstinate waters, such as those highly coloured by peaty matters ; and it is also 

 shown that the introduction of air is often beneficial, even with waters which 

 purify readily, by expediting the action and so increasing the efficiency of the 

 plant ; and it is suggested that the energetic action which often takes place with 

 very badly contaminated waters may be due to the carbonic acid generated by 

 putrefaction. 



The whole of the results dealt with have been obtained either from existing 

 installations or from experimental plant working on a large scale. 



