152 



SCIENTIFIC NEWS. 



[Sept. 1st, 1887. 



DOMESTIC SANITATION. 



No. 5. — Sewage Treatment. 



THE aim in sewage precipitation is to add to the foul 

 liquid some substance by which its impurities are 

 deodorised, rendered insoluble, and caused to subside to the 

 bottom of the tank, leaving above a layer of clear water, 

 which may safely be let flow into any river. 



All this sounds exceedingly simple, but in practice it is 

 beset with not a few difficulties. This we shall at once 

 admit if we consider the multitude of processes which have 

 been projected and patented, many of them utterly absurd, 

 and many more, to say the least, quite impracticable. It 

 must be kept in view that the impurities in sewage and in 

 the waste waters of manufacturing establishments resolve 

 themselves into two main classes — suspended matters, and 

 such as are present in the form of solution. To the former 

 class belong all solid bodies, filaments derived from washing 

 garments, the excrements of animals, and fragments of 

 meat. The presence of the latter is shown by the eager- 

 ness with which crows, sea-gulls, etc., hover over sewage- 

 tanks and open sewers, watching to snatch up any floating 

 morsel. 



The dissolved, or, at least, soluble impurities include 

 soapsuds, urine, blood, etc. Now whether the solid or the 

 liquid pollution is the more dangerous to public health 

 might be debated with great learning and little profit. But 

 it must be admitted that the solids, or suspended impuri- 

 ties, are the more easily got rid of. Ordinary filtration 

 through sand or gravel removes them, whilst the liquid or 

 dissolved matter passes through the filter-bed with little 

 change. 



The processes which have been devised for treating 

 sewage by precipitation are essentially of two kinds. There 

 is what may be called precipitation simple, which was first 

 tried when the sewage question became pressing ; it con- 

 sists in adding to the foul water some soluble earthy or 

 metallic compound. Thereupon impurities seem to coagu- 

 late into denser masses, and gradually subside to the bottom. 

 The first agent employed for the purpose, and which yet 

 finds advocates by reason of its cheapness, was lime. Lime 

 certainly renders the water clear, but it has many dis- 

 advantages. It leaves the water in what chemists call an 

 " alkaline " condition (known as such by its power of turn- 

 ing certain red vegetable colouring matters blue). The out- 

 flowing liquid is deadly to fish — a fact of which poachers 

 have been aware for ages. If the lime is used freely, a 

 part of the suspended impurities are rendered soluble, and 

 the water, if analysed, may seem to have been actually 

 rendered worse. Lastly, the deposit or sediment produced 

 by adding lime to water has a most disagreeable smell, 

 often perceptible at distances exceeding half-a-mile. 



Certain other substances, especially solutions of alumina 

 and iron, such as alum and copperas, are more effective. 

 But their higher price is an objection in this age when 

 all other considerations are subordinated to momentary 

 cheapness. 



But to all these simple precipitation processes there is 

 urged an objection which can by no means be at once dis- 

 missed. It is contended that " precipitation merely clarifies 

 but does not purifj'," or in other words, that it removes 

 merely the suspended impurities, leaving those of a liquid 

 nature untouched. It has even been said that no metallic 

 precipitant is capable of removing dissolved organic matter 

 from solution ! 



This exaggerated utterance must be due to some one who, 

 if a theoretical chemist, had but a moderate acquaintance 

 with industrial chemistry. Otherwise he might have known 

 that no small part of the arts of dyeing and colour-making 



turns on the fact that dissolved organic bodies are precipi- 

 tated by means of metallic solutions, which the dyer calls 

 mordants. 



On the other hand it may be contended that the solutions 

 of organic matter which have to be encountered in treating 

 sewage are far weaker — and hence less easily precipitated 

 — than the solutions of organic colouring matters with which 

 the dyer has to deal. 



We may, however, the better afford to dismiss this 

 question as idle, since it does not in the least affect com- 

 pound precipitation, or those processes to which we referred 

 on p. 128 as being in principle inverted irrigation. 



This method of dealing with impure waters (other than 

 sewage) was known to the Chinese probably thousands of 

 years ago. Their great rivers, which serve in extensive 

 regions of the empire as the main water-supply are very 

 turbid, and in this state unfit to drink. But the Chinese 

 add to a tub of water a pinch of alum, and stir the whole 

 well up together. On standing the mud subsides, and as 

 it has previously absorbed the organic impurities and 

 putrid matters, the clear water above is not only free from 

 any offensive smell or taste, but perfectly wholesome. 



It has even been found that by the joint action of an 

 absorbent, like earth or clay, and a metallic salt, such as alum, 

 any disease-germs present may be effectually removed from 

 water. 



This has been most strikingly proved in Annam and 

 Cochin China. When the French first attempted to colonise 

 tliese regions, their troops, suffered cruelly from endemic 

 dysentery. But after they adopted the Chinese method, 

 and the soldiers were regularly supplied with water which 

 had been treated with a little alum, and allowed to settle, 

 the clear alone being poured off for use, the disease almost 

 disappeared. 



The Chinese principle was introduced into England for 

 the treatment of sewage about the year 1868, and is now, 

 with trifling variations, embodied in a number of processes, 

 patented or otherwise. Clay or arable soil, with or with- 

 out the accompaniment of sea-weed charcoal, waste carbon 

 from the manufacture of prussiate of potash, and various 

 mineral carbons is introduced into the sewage and mixed 

 up with it mechanically, when it is remarked that the ofien- 

 sive odour at once disappears, the particles of earth and 

 carbon having absorbed and become saturated with the 

 putrescent matter. The metallic salt, which may be sul- 

 phate or chloride of aluminium, sulphate or chloride of iron, 

 or aluminate of soda, etc., is then added, when the mixture 

 of clay and carbon subsides, carrying down in its pores 

 and entangled among its particles the organic matter, 

 disease-germs, etc., whilst above it floats a clear, inodorous 

 water, which may be safely admitted into any river. Into 

 the details of separating the water from the sediment, we 

 cannot here enter, especially as they cannot well be ex- 

 plained without the aid of diagrams. 



We may, however, say that the deposit when dried suffi- 

 ciently to be portable, contains nitrogen equal to 3 per cent, 

 of ammonia, and phosphoric acid equal to 5 per cent, of 

 tribasic phosphate of lime, being consequently much richer 

 than any farmyard manure. 



The advantage of such methods of precipitation is that 

 whilst they are- thoroughly efficient in a sanitary point of 

 view, and whilst they recover the greater part of the 

 manurial matters, they require but little space and can be 

 adopted, where from the peculiarities of the soil and the 

 climate, the high price of land, the populous character of 

 the neighbourhood, etc., it is not practicable to lay out an 

 irrigation-farm. 



In many places, where the sewage is rich and where 

 high farming prevails in the neighbourhood, the sale of the 



