3i8 SEWAGE AND ITS PURIFICATION 



have not obtained any nitrification in mixtures of sea-water 

 with one per cent, of sewage, although there was a rapid 

 breaking down of organic matter with formation of ammonia 

 and odorous compounds. By trials with the marine salts 

 separately and together they found that they had an inhibitive 

 effect on oxidizing changes. Their conclusions are strongly 

 against sewage being discharged untreated. 



The principal objections to sewage outfalls that appear in 

 most official enquiries are: — (i) Smells; (2) Unsightliness and 

 inconvenience through turbidity of the water or floating debris, 

 formation of mud banks, and unpleasant deposits on the shores ; 



(3) Destruction of fish, and sometimes of aquatic vegetation ; 



(4) Contamination of water supplies drawn from rivers and 

 lakes ; and in special cases, (5) Pollution of shell-fish or of 

 watercress beds. 



As a result of the enquiries it has been frequently found that 

 the faults were not entirely due to crude sewage, and only 

 exceptionally to purified effluents. Taking the above points in 

 detail : — 



(i) Smells. Many sewage- and water-bacteria effect lique- 

 faction without creating any offence. On the other hand we 

 have described at p. 81 a number of natural causes of objec- 

 tionable odours unconnected with sewage. 



(2) Faults above mentioned that are occasioned by suspended 

 solids. Screening and roughing filters prevent the more glaring 

 evils, and in many cases this is all that is done, but it is rightly 

 made an essential of satisfactory treatment that the effluent 

 should contain very little suspended matter. As an example of 

 pollution, 26 miles of the river Severn were examined for the 

 Royal Commission on Sewage.^ This stretch receives the 

 untreated sewage of Shrewsbury (pop. 28,396, dry weather flow 

 about 900,000 gallons, mixing with about 100 volumes of river 

 water), besides less amounts from other sources. Chemically 

 the river shows a marked recovery from its Shrewsbury pollu- 

 tion within some 20 miles from the town, and it still further 

 recovers as it proceeds, in spite of some additional contamina- 

 tions.2 Hence it is fair to infer that with a good effluent there 

 would not have been any injury. But the results of discharging 

 sewage entirely untreated were found to be : 



{a) The bed of the river consists of putrefying sludge, and 

 the velocity of the stream is always tending to spread the 



1 Reports, vol. ii., 1902, pp. 93-133. 2 Uyid,^ p, 132. 



