346 SEWAGE AND ITS PURIFICATION 



is carried out of the tank in the effluent ; (3) the action of 

 a septic tank removes a large amount of iron from an acid iron 

 sewage ; (4) the reduction of organic matter, 26 per cent., is 

 much less than occurs with an alkaline domestic sewage ; 

 (5) the quantity of gas given off depends greatly on the amount 

 and character of the sludge at the bottom of the tanks, and on 

 the temperature, and is less in winter than in summer ; (6) the 

 average volume of gas, 2*2 gallons per 100 gallons of sewage, is 

 probably less than would be obtained where the sludge contains 

 more organic matter : (7) its composition is CH4, N, CO2, with 

 little, if any, hydrogen ; (8) the amount of sludge changed into 

 soluble or gaseous substances, 28 per cent., is probably less 

 than usual on account of the presence of so much ferrous 

 sulphide ; (9) the top crust or scum here contains much more 

 organic matter than the sludge ; (10) the formation, non- 

 formation, and disappearance of the crust appears to be an 

 incident, rather than a result, of bacterial action.^ Lime is 

 now added to this sewage. 



Effluents from oil, wool, and dye works at Trowbridge, 

 Wilts (Dibdin's report of Sept., 1900), interfered little with 

 bacterial treatment, and not at all when diluted, with the 

 exception of bichromate liquors. None of the fourteen effluents 

 tried, when added singly or in mixture, in the proportion of 

 one per cent, to sewage 15 minutes before cultivation, had any 

 antiseptic effect, while with seven per cent, of the mixture the 

 influence was only slight. 



Fibrous matters, such as those from wool manufacture, and 

 from horse-dung and wood-pavements, as mentioned in Dr. 

 Clowes' L.C.C. Report of 1899, seriously interfere with the 

 action of the bacterial filters, but are easily dealt with by 

 hydrolysis in a septic tank. 



In England,- at present, the relations between local authori- 

 ties and manufacturers in regard to the disposal of manufac- 

 turing effluents are contained in Section 21 of the Public Health 

 Act, 1875, which reads : — 



" The owner or occupier of any premises within the district 

 of a local authority shall be entitled to cause his drains to empty 

 into the sewers of that authority, on condition of his giving such 

 notice as may be required by such authority." 



And in Section 7 of the Rivers Pollution Prevention Act, 1876: — 



^ Third Annual Report of the Connecticut Sewerage Commission, 1902. 

 2 Roy. Com., 3rd Report, 1903. 



