218 y.CTIVATED SLUDGE PROCESS. 



are fitted to each distributing pipe so that the air supply may be 

 regulated and equally distributed. The air compressor is of the 

 Ingersoll Rand horizontal type, and is belt-driven by electric 

 motor (d.c.) capable of developing 40 b.h.p. When running at 

 235 r.p.m.j it has a piston displacement of 615 cubic feet per 

 minute and actual delivery of 562 cubic feet. 



The total capacity of the settlement tanks is sufficient to allow 

 a two hours' detention when working at the above rate, the sew- 

 age taking eight hours to pass through the tank. 



The Worcester sewage cannot be termed a strong sewage as 

 the flow is equal to approximately 40 gallons per head, and although 

 the City Engineer states that a variety of trade effluents are dis- 

 charged into the sewers, there appears to be none of an inhibitory 

 character present in the sewage. Apart from stoppages due to 

 structural alterations to the settlement tanks, the tanks have been 

 in continuous operation since May, 1916. 



During this period the rate of flow has varied from 60,000 

 gallons to over 1,000,000 gallons per day. The average daily flow 

 dealt with may be taken at about 750,000 gallons. 



In the following table are given the average results of 

 analyses made by the Worcester City Analyst of screened sewage 

 (nine samples), and final effluent (twelve samples), from June, 

 1916, to May, 1917. 



Results in Parts per 100,000. 



Screened Final 

 Sewage. Effluent. Purification. 

 Four hours' oxygen absorption ... 3.70 0.56 85% 



Albuminoid ammonia 0.76 0.14 82% 



Suspended solids 14.3 0.72 — 



The final effluent was perfectly clear and colourless and con- 

 tained very little suspended matter. These results indicate that, 

 a high degree of purification is obtained with the production of 

 an entirely satisfactory effluent. 



Under normal conditions the compressor speed is 160 r.p.m., 

 which gives a free air delivery of 384 cubic feet per minute and 

 about 20 b.h.p. The working air pressure is nine pounds per 

 square inch. This volume of air employed is equal to only seven 

 cubic feet per square foot tank area per hour, and is inclusive of 

 the air used by the airlifts for returning the sludge to the aeration 

 chamber. Working at a rate of 750,000 gallons per day the air 

 consumption is equal to 0.7 cubic feet free air (actual delivery) 

 per gallon of sewage treated. The electric motor driving the com- 

 pressor absorbs 16 units per hour, so that at three farthings per 

 unit, the cost at Worcester is equal to thirty-two shillings per 

 million gallons of sewage treated when working at the above 

 rate. 



Some little time ago sand drainage beds were constructed to 

 receive the surplus sludge, but the results of their operations I 

 have not been able to ascertain, so it is impossible to say with 

 any certainty what volume of sludge is produced per million 

 gallons dealt with. Experiments carried out by Dr. Ardern, of 



