PROCEEDINGS OF SECTION I. 665 



the deposited matter; (2) a series of eight covered tanks, each 119ft. 

 long by 23ft. wide, of an average depth of 5ft. 6in. (inlet end 6ft. lOin. 

 in centre, 5ft. at sides ; outlet end, 5ft. 4in. in centre and 3ft. lOin. 

 at sides), and of an approximate capacity of 94,000galls. ; and (3) an 

 area of about eight acres of sand, 5ft. to 6ft. deep, subdivided by low 

 dykes into eight filter beds. The beds are structurally continuous 

 {i.e., there are no partition walls between them) and are emptied by a 

 series of effluent drains. 



The population served is nearly 30,000, and the daily average 

 flow is about 750,000galls. in dry weather. 



Stormwater is dealt with as follows : — Until the flow becomes 

 about three times the maximum normal daily flow, it is all run through 

 the tanks. Then, when quantities above this amount come down, an 

 overflow from the main carrier operates automatically, and the dilute 

 sewage is conducted on to one of the filter beds, which is kept almost 

 entirely for use as a stormwater filter. In the event of these arrange- 

 ments proving insufficient the excess runs over the end of the carrier 

 into a large concrete stormwater channel provided to carry ofi the 

 stormwater from Willoughby Falls Creek. Exact figures are not 

 available, but it is obvious that the sewage must be enormously diluted 

 when on rare occasions it reaches the harbor untreated. 



Provision is made for screening the sewage by means of grids at 

 the inlet chamber. The grids are constructed with iron bars lin. 

 apart. These are not used in ordinary dry weather flows, but are 

 placed in position so as to arrest floating matter brought down by any 

 rise in the flow, and thus assist in preventing buoyant solids from 

 reaching the harbor in times of heavy rain. 



If the whole of the tanks were in operation at one time there would 

 be sufficient tank space available to permit of the dry weather flow 

 remaining therein for 24 hours, but if a deduction be made for an 

 approximate average of 2ft. of scum over all tanks, this time would be 

 reduced to 15 or 16 hours. As a matter of fact, however, for the last 

 four months only three tanks have been in operation, so that the actual 

 time the sewage is in the tank is less than six hours. 



The scum formation in these tanks is, and has been, of particular 

 interest. Before the tanks were covered the scum was of such thick- 

 ness and strength in parts that persons could walk on it. The surface 

 became dry and hard, and supported a luxuriant growth of self-sown 

 grasses and vegetables, and undoubtedly caused such obstruction in 

 the tanks as to interfere seriously with their working. It was thought 

 that by covering the tanks the scum would be prevented from drying 

 and that liquefaction would be favored. The scum was, as it is now, 

 by far the thickest at the inlet ends of the tanks. 



