I 



CHEMICAL CHANGES 115 



Most of the sulphur, however, enters into combination with 

 the iron present in the sewage, forming insoluble ferrous sul- 

 phide, and giving a black colour to the suspended matter. 

 When the black matter is treated with acids, sulphuretted 

 hydrogen is evolved and the substance becomes brownish, just 

 as when strong acid effluents from factories are discharged into . 

 ditches or on to the black mud-banks of neglected rivers a j 

 liberation of HgS occurs. In the tank, however, the ferrous i 

 sulphide is protected by the ammonia ; on reaching the oxida- 

 tion stage it is converted into a basic ferric sulphate, forming 

 an ochreous coating on the materials, which considerably assists 

 in the transfer of oxygen and absorption of organic matter. 



At Melbourne a large proportion of sulphuretted hydrogen is 

 produced during the passage of the sewage through the 2 J miles 

 of pipes from the pumping-station. Being protected from light 

 and air during transit, a septic action is set up, and much local 

 complaint arose as to escaping smell. Sulphur has been burnt 

 and the SO2 forced into the sewage, with the result that com- 

 plaints ceased ; but directly the process is stopped the smell 

 returns. A remedy has been found in ventilation. ^ 



A proportion of the bacteria escape from the septic tank or 

 other anaerobic chamber, but a large number remain entangled 

 in a zooglcea mass either at the top or bottom of an unobstructed 

 tank, or as a layer on the surface of the flints or other filling 

 material. 



With the exception of not requiring extraneous heat, the 

 first stage of anaerobic resolution of organic substances is 

 analogous to the decomposition of coal in gas retorts, the chief 

 products, free hydrogen and methane (CH^) being identical; in 

 fact, the latter has been called " marsh gas," from its being pro- 

 duced in stagnant pools, where hydrolytic changes occur beneath 

 the surface. As disengaged from closed tanks, the gas is found 

 H^o burn with a blue flame, like that of an ordinary atmospheric 

 ■burner, giving great heat, which can either be utilized under 

 ■boilers or, by means of incandescent mantles, be applied to the 

 lighting of the works. At Exeter a gas-lamp of the usual street 

 pattern is fed from the gases of the septic tank (see Fig. 26). 



Even from open tanks gas can be collected. At Manchester 

 a small gasometer, about 4 feet in diameter, was floated on the 

 surface of one of the tanks, and from the spherical head of this 



1 Thvvaite's Report to Melbourne Board of Works, 1901. p. 55. 



8—2 



