216 ACTIVATED SLUDGE PROCESS. 



(2) A sludge of great fertilising value is obtained in place of 

 the highly offensive, objectionable and practically valueless sludges 

 obtained from all other processes, including the latest Inihoff 

 tank. 



(3) The land required is from one-fifth to one-tenth that of 

 any other complete process. 



(4) The first cost of the plant is considerably less than that 

 for any other complete process. 



(5) Taking into account interest on capital expended, the 

 annual running cost will be less. 



(6) In hot climates it is entirely free from the fly nuisance. 

 It has been known for many years that if sewage be exposed 



to the air for a sufficient period of time, the organic contents 

 are gradually oxidised and a deposit of humus is formed ; and, 

 also, if the process be sufficiently prolonged, the ammonium salts 

 and nitrogenous organic matter are largely oxidised to nitrates. 

 Numerous investigators have from time to time endeavoured to 

 utilise aeration methods. These efforts date back as far as 1884, 

 when the report of the Royal Commission on Metropolitan Sewage 

 Disposal was published. Since then many experiments have been 

 carried out in England and America, but without any great 

 success until 1913. In that year Dr. Fowler, who was then at 

 the Manchester University, and Dr. Ardern, Chief Chemist to 

 the Rivers Board Committee, together with Mr. W. T. Lockett 

 and Mr. Mumford, experimented on new lines with most successful 

 results. Their view was that the process of aeration was entirely 

 due to bacterial activity. A discovery was also made of a certain 

 organism, "M7," which is not found in sewage treated by any other 

 method. The "M7" is also found in colliery waters. The process of 

 purification was then known as the "M7" process. When the process 

 began to be applied practically the name was changed to "Acti- 

 vated Sludge" on account of the sludge, which is one mass of 

 intensified aerobic bacteria, and which is circulated through and 

 through the sewage continually. The whole of the purification 

 depends on the thorough mixing of an active sludge with the 

 sewage, and hence the name "Activated Sludge." 



Some people are under the impression that the process involves 

 the inoculation of the sewage with a given organism under pres- 

 sure of air. This is not so. There is no inoculation of any organism 

 whatsoever. In an ordinary sewage filter the sewage passes in 

 thin films in the presence of air over surfaces coated with an 

 active bacterial deposit. Costly surfaces are thus required, aeration 

 is by no means perfect, and surface action is limited. 



In the activated sludge tank the bacterial sludge, which is 

 built up in the sewage by processes similar in principle to the 

 formation of the active bacterial deposit in the filter medium, is 

 thoroughly intermixed with the sewage by means of air in a 

 fine state of division. Thus practically infinite surface is obtained, 

 and is continually renewed, and consequently the conditions of 

 bacterial oxidation approach theoretical perfection. This is the 

 principle which differentiates the activated sludge process from 

 all other processes of aeration which have previously been 



