220 ACTIVATED SLUDGE PROCESS. 



(4) Less air is used. 



To secure the utmost possible economy in air consumption 

 Messrs. Jones and Attwood have patented a mechanical gear 

 whereby a pulsating air supply is automatically obtained. This 

 has reduced the air consumption considerably and given very good 

 results. For example, at Davyhulme, Manchester, a pulsating 

 gear is in operation. The design of the cams at present employed 

 gives a ratio of the time of admission of air to the diffuser to 

 the period of rest of 1 to 2; the actual time of these periods can 

 be varied by regulating the speed of the cam shaft. As a general 

 rule each set of diffusers receives air for five seconds at intervals 

 of ten seconds. The ratio can be varied by substituting cams of 

 a different design. 



The next matter to consider is the sludge. Activated sludge 

 differs from ordinary sludge in its physical characteristics and 

 in its ready drainability. It is not gaseous like either Emsher 

 tank or septic tank sludge. In properly aerated sewage the sludge 

 settles to the bottom and the clarified liquor comes to the surface 

 quickly. The liquid separates from the sludge so quickly and 

 effectually that there need be little difficulty in drawing it off 

 from a sludge tank and de-watering from the surface as well as 

 from the bottom. Down to a 90 per cent, water content it drains 

 fairly easily, but, owing to its gelatinous nature, further natural 

 drying is a very tedious matter. The drying of the sludge I will 

 deal with later. 



The proportion of activated sludge to the sewage depends on 

 a variety of factors — tank capacity, air supply, character of sewage, 

 degree of purification required, etc. From the various experiments 

 tried in England and America, I think it may be accepted that the 

 volume of sludge should be maintained as low as possible, con- 

 sistently with adequate clarification. If a high nitrification is 

 desired a greater proportion of sludge will be necessary. The 

 average requirements are from 20 to 25 per cent, of sludge. 



One of the most interesting features of the whole process is 

 the conservation in the sludge of much nitrogen which had hitherto 

 been lost. There was, in fact, reason to hope that the value of 

 the nitrogen thus saved in the sludge would pay for a great part, 

 if not the whole, of the process of sewage purification. This great 

 saving of valuable nitrogenous manure is a point of great economic 

 importance. The percentage of nitrogen saved varies from four 

 to six per cent., according to the character of the sewage. 



The following average chemical analysis of the activated sludge 

 obtained from the Withington Sewage Works at Manchester will 

 be of interest: — 



Chemical Analysis. 



Loss on ignition 70.4 per cent. 



Mineral matter 29.6 per cent. 



Total nitrogen (as N) 6.0 per cent. 



Phosphate 4.2 per cent. 



Greasy matter, etc 7.3 per cent. 



