NA TURE 



543 



THURSDAY, JUNE 22, 1911. 



PROBLEMS OF POTABLE WATER. 



Modern Methods of Water Purification. By J. Don 

 and J. Chisholm. Pp. xvi + 368. (London : 

 Edward Arnold, 191 1.) Price 155. net. 



WHEN Mr. Don read his paper on "The Filtra- 

 tion and Purification of Water for Public 

 Supply " before, the Institute of Mechanical Engineers 

 in January, 1909, it was generally thought that a very 

 useful addition had been made to the literature on 

 this subject, and in following this up with the volume 

 under review, which is in effect an amplification of 

 the former publication, the authors have done a great 

 service to those whose duty it is to provide a pure 

 and efficient supply of water for domestic purposes. 



Following a general introduction, the first chapter 

 deals with the sources of supply. The possible pollu- 

 tions that may be expected are fully discussed, and 

 some most useful information is given of the best 

 means of detecting, and, as far as may be, prevent- 

 ing them. 



Obviously, all water destined for domestic purposes 

 cannot be taken from sources unimpeachable, both 

 as to purity and constancy of supply, and the next 

 two chapters in the book deal with the first steps 

 towards remedying these evils under the general 

 heading of " Storage." 



The main inference that is to be drawn from this 

 section is that, in addition to ensuring constant and 

 adequate supply from sources which depend largely 

 on the rainfall over a limited catchment area, storage 

 undoubtedly has a very marked effect in purifying 

 and rendering harmless an otherwise polluted and 

 dangerous water. The work of numerous investi- 

 gators is quoted in support of this contention, parti- 

 cularly the valuable researches of Dr. Houston on 

 the storage of water from the rivers Thames and 

 Lea, published in his reports as Director of Water 

 Examination to the Metropolitan Water Board. 



The result of these researches are very lucidly 

 summed up by the authors in the following sen- 

 tences : — 



" I. The microbes of disease, and those which are 

 indicative of sewage {B. coli) perish rapidly in stored 

 water. In about three weeks, generally speaking, 

 the safety change is complete, and the dangers 

 imminent from sewage pollution minimised. 



"2. After being impounded for two or three weeks 

 the water is in a better state from a chemical point 

 of view, seeing that there is a well marked decrease 

 of ordinary ammonia, oxygen consumed, oxidised 

 nitrogen, lime salts, and occasionally of albuminoid 

 nitrogen. 



"3. Storage deprives the raw water of nearly all 

 its sediment, and therefore serves to prolong the life 

 of the filter beds." 



There is one danger, however, occurring sometimes 

 after prolonged storage, which should not be lost 

 sight of; that is the abnormal development in the 

 reservoir of algae and other minute vegetable growths 

 at certain seasons of the year. These growths not 

 NO. 2173, VOL. 86] 



only cause great inconvenience by rapidly choking 

 the filter beds, but certain species give rise to exceed- 

 ingly unpleasant odours and tastes in the water, which 

 cannot usually be got rid of by filtration. A remedy 

 for these evils, however, is suggested in the book, 

 namely, the addition of exceedingly small doses of 

 copper sulphate, which, it is stated, not only will 

 cause the death of myriads of these organisms, but 

 if added in anticipation of a rapid development will 

 prevent the growth taking place. The use of hypo- 

 chlorites is also recommended. 



Following on this exhaustive and extremely valu- 

 able discussion of storage, the authors devote a large 

 section to the question of filtration. There are three 

 chapters devoted to this subject, entitled respectively, 

 "Sand Filtration," "The Management of Sand 

 Filters," and " Mechanical Filters." 



Under the first heading elaborate and exceedingly 

 interesting and instructive explanations are given of the 

 theory of sand filtration, which bring out very con- 

 clusively the precautions which are necessary to ensure 

 efficient purification. The action of a sand filter 

 appears to be threefold ; first, the mechanical strain- 

 ing of the grosser suspended matter, this causes a 

 film or skin composed of silt, algee, and bacteria to 

 form on the surface of the sand; secondly, the living 

 algce in this skin seem to have a power of consuming 

 the bacteria which come within their orbit, thus re- 

 taining many of the minute organisms which would 

 otherwise pass through the interstices between the 

 sand grains. The third action is brought about by 

 the slimy or gelatinous film which forms round the 

 grains of sand in the lower layers of the bed ; this 

 has the power not only of retaining most of those 

 microbes and minute particles of matter which escape 

 the filmy skin, but also by a process of adsorption, of 

 acting on the organic matter dissolved in the water, 

 " mineralising " it, and converting it into innocuous 

 nitrates, sulphates, and carbonic acid. 



It is pointed out that to ensure all these actions 

 taking place properly it is necessary that the rate of 

 filtration should be slow (about 4 inches per hour) and 

 uniform, as an irregular flow causes disturbance to 

 the microbes adhering to the slimy coating of the 

 sand grains. 



All the operations in connection with the manage- 

 ment of filters are dealt with under this heading, and 

 details are given of their construction. Descriptions 

 are given of many of the elaborate processes em- 

 ployed in different parts of the world, and the chapter 

 concludes with a discussion on the use of coagulants 

 for the rapid sedimentation of matter in suspension, 

 and also for the formation of an artificial filtering 

 skin. This leads up to an important chapter on 

 mechanical filters, which are rapidly gaining favour 

 in various parts of the world. Numerous appliances 

 are described, differing chiefly in matters of detail. 

 The general features are the use of a much greater 

 head of water or of artificial pressure, and the use 

 of an artificial skin produced by the addition of alum, 

 and, if necessary, lime or some other alkali, if the 

 water has not sufficient natural alkalinity to precipi- 

 tate the alumina. The filtering medium employed 



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