ACTIVATED SLUDGE PROCESS. 221 



Gelatine counts have shown a bacterial content of at least 

 thirty million organisms per cubic centimetre. In addition the 

 sludge, by reason of its nitrifying power, must of necessity contain 

 a large number of nitrifying organisms. Dealing with the quantity 

 of sludge per million gallons of sewage one must remember that 

 there is less sludge from a bacterial process than a chemical process. 

 The activated sludge process is a bacterial process, and the resultant 

 sludge per million gallons of sewage treated is about one to one 

 and a half tons dry, but, of course, that depends on the class of 

 sewage and the amount of moisture. 



The methods to be adopted for the extraction of moisture, 

 the pressing, drying and conversion of the sludge from what has 

 formerly been regarded as a troublesome and costly nuisance to 

 a welcome and valuable fertiliser, will depend upon the quantity 

 to be treated and the conditions under which it is dried. 



The problem of drying or de-watering of the sludge is one 

 where there is large scope for improvements. The writer had the 

 privilege of watching some experiments for drying the sludge 

 carried out by Mr. R. A. Sturgeon, of the Sturgeon Centrifugal 

 Company. The method adopted by Mr. Sturgeon for de-watering 

 is centrifugal force applied without using heat of any description. 

 If heat is applied in the drying process the value of the sludge 

 is immediately diminished, owing to lower percentage of nitrogen. 

 Here, in South Africa, we have the advantage of a good dry climate 

 and bright sunny days, which I think could be taken advantage 

 of and used for drying the sludge. If, for example, the liquid 

 sludge could be run on to an open sand filter, I believe it could 

 be dried very simply and cheap. The cost of drying the sludge 

 in England is thirty-six shillings per ton. I think, with the 

 more favourable atmospheric conditions prevailing in South Africa 

 this cost could be considerably reduced. So far the testing of the 

 fertilising value of the sludge has been conducted on a small scale 

 only, owing to the limited quantity of material available, but 

 when the installations now under way are in full working order 

 trials on a commercial scale will be possible, and it is satisfactory 

 to know that the Board of Agriculture in England is taking an 

 active interest, in the matter. To show the availability of the 

 nitrogen in activated sludge, two equal plots, A and B, were 

 prepared near Manchester, A with activated sludge in powder 

 form, B with farmyard manure. Calculating the amount of 

 nitrogen in farmyard manure to be five per cent., activated sludge 

 was added to plot A in quantities containing as nearly as possible 

 the equivalent amount of nitrogen, and in both were set an equal 

 number of seed potatoes. Plot A yielded 150 lbs., and plot B 

 62 lbs. of potatoes. These experiments were carried out by Mr. 

 Ernest Gaid, M.Sc, of the Manchester University. The writer 

 had the pleasure of seeing the results of many experiments on 

 horticultural plants carried out by Mr. Walter Jones at " The 

 Uplands," Stourbridge, and has obtained a photograph of a pair 

 of azalea plants, one treated with activated sludge and the other 

 untreated. In the one case the soil was top dressed on January 



