i873»] A Solution of the Sewage Problem. 63 



separated in flocks, and, by virtue of its remarkable affinity 

 for dissolved organic matter, each particle seizes hold of, 

 and drags down with it, a corresponding particle of nitro- 

 genous impurity. The blood here comes into play ; this 

 is essentially a liquid highly charged with albumen ; al- 

 bumen is instantly coagulated in the presence of alum ; 

 and in the same way as this ready coagulability of albumen 

 is utilisedjn fining wine and coffee, so it is made use of in 

 this process by joining with the alumina in its precipitation, 

 uniting it in a net-work of fibres, and giving it, as it were, 

 arms wherewith to seize upon and drag out of solution still 

 more putrescible constituents. 



But the precipitated hydrate of alumina is light in 

 character, and although it would ultimately settle, leaving 

 a clear liquid above it, the slightest agitation causes it to 

 float up, and thus renders it difficult, on the large scale, to 

 drain off the mud. At Paris sulphate of alumina has lately 

 been employed for clarifying several hundred thousand 

 gallons of sewage ; and among the many defects of this 

 process, that of imperfect settlement was by no means the 

 least. Here the action of the clay is apparent. This sub- 

 stance has a curious physical property ; when finely 

 ground up with water it forms a creamy emulsion, which 

 takes many days to settle ; many rivers, in time of flood, 

 owe their turbidity to this cause : the Seine at the present 

 time is a striking example, its water being in colour, 

 although not in actual impurity, as bad as the Thames be- 

 low London Bridge. But when this creamy liquid meets 

 with sulphate of alumina, the clay coagulates like albumen, 

 and settles down in heavy granular flakes. Now in the 

 ABC process these three precipitations — that of the alumina, 

 that of the albumen, and that of the clay — take place simul- 

 taneously, and in each other's presence ; they become 

 closely locked together in a triple alliance ; the heavy 

 character of the clay particles gives density to the mass, 

 and causes it to settle rapidly, and remain in a compact 

 form at the bottom of the tank. 



Were the object merely to produce an easily dried pre- 

 cipitate and a clear effluent, nothing more would be required ; 

 for not only has this precipitate carried down all the sus- 

 pended matter, but much of the dissolved nitrogenous and 

 albumenoid impurities have fixed themselves on to the 

 alumina, whilst the clay has also performed its part in ab- 

 sorbing and carrying down a good proportion of the ammonia* 

 But there still remains the probability, if not the certainty, 

 of foul gases being present, whilst the water, though clear, 



