62 A Solution of the Sewage Problem. [January, 



was invented by David Forbes, F.R.S., when compared 

 with those with which we have been dealing, appears a very 

 admirable one. It is, however, from a technological point 

 of view, somewhat deficient in economy. For the ingredient 

 added to the sewage is expensive, too expensive with regard 

 to the return in practical good obtained from its use in agri- 

 culture ; and unfortunately those plants which require most 

 phosphoric acid bear irrigation least. In a theoretical point 

 of view, if we overlook the infringement of the rule of 

 economy, the process is attended with a high degree of con- 

 sistency. By no means is the preceding statement true of 

 all precipitation processes. The lime process, as an instance, 

 produces a precipitate containing a large proportion of lime, 

 possessing but feeble or no manurial power, and readily 

 putrefying ; while the effluent water, instead of being pure or 

 even suited to the purposes of irrigation, contains introduced 

 foreign matter inimical to the land and the life of plants. 



The process of precipitating by sulphate of alumina 

 the valuable constituents of sewage, and utilising at the 

 same time the purifying power of charcoal and clay, is that 

 to which we decidedly give the preference, as by this means 

 the water is practically purified fit to be discharged into a 

 running stream, and the deposit is retained in a form entirely 

 inoffensive and capable of being turned into a dry and 

 portable manure. This process has been before the world 

 for some years as the ABC process, worked by a company 

 called the Native Guano Company, and the claims it set up 

 three years ago to have solved the great social problem we 

 may now pronounce to be fully justified by facts ; its prin- 

 ciples were correct, the mechanical arrangements for con- 

 ducting them being alone defective. The name of the 

 process has been derived from the initial letters of the 

 principal constituents of the precipitant : Alum, Blood, Clay, 

 and Charcoal. We will consider the action of these sub- 

 stances upon sewage, taking them in order. 



The alum was for a considerable time a source of 

 expense, it being added to the sewage in the form of 

 ammonia-alum. Ammonia-alum has the further disad- 

 vantage that the ammonia remains in the effluent water. 

 A much more economical, and as effective a substitute has 

 been found in a crude sulphate of alumina manufactured at 

 one-fourth the cost. 



The action of the sulphate of alumina may be briefly 

 described. 



In contact with sewage, — a slightly alkaline liquid 

 charged with nitrogenous organic matter, — the alumina is 



