ACTIVATED SLUDGE PROCESS. 219 



the Withington Sewage Works, are just to hand. Dried sludge 

 powdered 1,342 lbs. per 1,000,000 gallons sewage treated. 



Up to the present the settlement tanks appear to be the 

 limiting factor in regard to the volume of sewage with which the 

 installation is capable of dealing. 



The outstanding feature of this installation is the low air 

 consumption without the assistance of a pulsating air supply. It 

 may be stated that it is not more than one-half the volume 

 employed in the American installations, and it is very consider- 

 ably less than the estimate based on early investigations. 



Dr. Ardern, Chief Chemist to the Manchester Corporation, 

 informed the writer that at present six cubic feet (free air) per 

 square foot tank area per hour is about the minimum air supply 

 they have employed at their Withington Sewage Works. 



Dr. Fowler states that, broadly speaking, the activated sludge 

 process consists of three operations : — 



(1) A clotting or clarifying action; 



(2) A rapid carbon oxidation process; 



(3) A final nitrification. 



Working with a domestic sewage it is not necessary to push 

 purification to the point of nitrification in order continuously to 

 obtain staple effluents. It necessarily follows that, if ultimate 

 nitrification be not aimed at, the aeration period will be diminished 

 and considerable economy effected. 



Whether or no any proportion of the nitrogen in the sludge 

 is due to the presence of nitrogen fixing organisms remains to be 

 demonstrated. At the early stages of the activated sludge process 

 there was considerable discussion as to the best methods of aerating 

 the sewage, and many experiments were tried. If perforated 

 pipes are used there is a considerable waste of air by the creation 

 of large bubbles which do not ensure satisfactory mixing of the 

 sewage and the sludge. The experience gained so far with large 

 scale plants indicates that, despite certain disadvantages, the use 

 of diffusers offers a practical solution of the problem. Owing to 

 the risk of choking the fine pores with oily matter, dust, etc., they 

 were originally regarded with suspicion, but I understand that 

 later and more perfect forms of diffusers have reduced this possi- 

 bility to a minimum. Trouble was found during some of the earlier 

 experiments at Davyhulme, Manchester, in oil choking the under- 

 side of the diffusers. This was due chiefly to the fact that air 

 was drawn from high pressure sources and the oil used had become 

 carbonised, but if air is compressed to not more than about 10 lbs. 

 per square inch, the temperature of compression is not sufficient 

 to alter the nature of the oil, and thus the liability to choke is 

 small, provided the air compressed is clean. 



The porous tile diffuser, as compared with any other system 

 of aeration, has the following advantages which fully justify its 

 adoption : — 



(1) There are no moving parts in the sewage. 



(2) The air is evenly distributed. 



(3) The air used for aeration provides also for agitation and 

 circulation. 



