BACTERIAL PURIFICATION 275 



It became evident that the main difficulties with contact beds 

 were due to their being used chiefly as simple strainers, and 

 that a previous preparation of the sewage was necessary. 

 Except with the septic tank, this had taken the form of screen- 

 ing, sedimentation, or precipitation. 



The enquiry included a comparison between a closed and an 

 open septic tank. For the latter one of the large precipitation 

 tanks at Davyhulme was used, raw sewage being allow^ed to flow 

 over the end sill in a very thin stream, passing through the tank 

 almost continuously at a rate of 1,700,000 gallons per 24 hours. 

 Similar phenomena to those at Exeter are described, the liquid 

 becoming covered with a scum which excluded the air, while 

 " up to the present time the only notable quantity of sludge 

 which can be perceived. ... is immediately beneath the inlet 

 penstocks. . . . An enormous quantity of the sludge which 

 would otherwise accumulate has been destroyed in this way." 

 The effluents from both closed and open tanks are shown to 

 be closely similar.^ 



With reference to the question of closed or open septic tanks, 

 I may remark that some of the advantages of the former are 

 that the gases can be utilized and all smell avoided, that the 

 temperature is more even, and interference from frost or wind 

 is prevented.'^ At the same time it was found at Manchester 

 that the outfall sewage was 10° F. warmer than the temperature 

 of the air. 



The deposit from this open septic tank in April, 1900, con- 

 tained organic matter 5*24, inorganic 678, water 87*98 per cent. ; 

 or in the solids, 57 per cent, inorganic matter and 43 per cent, 

 organic. The sludge of the chemical precipitating tank was 

 much more prone to putrefactive decomposition than that of 

 the septic tank. 



It was agreed that in the management of contact beds : — 



^ In an enquiry at Yeovil, in March, 1900, the balance of opinion was that in 

 the case of a strong sewage a closed tank is necessary. Open septic tanks are 

 now often called "scum tanks." Drs. Kenwood and Butler state (Sanitary 

 Institute, April. 1901) that in the Willesden and Finchley scum tank the sludge 

 maintains a fairly uniform bulk, and certainly does not accumulate sufficiently to 

 require removal for many months, and that at Acton after a year's working the 

 deposit averages only a few inches with a spongy scum 8 to 10 inches deep. The 

 albuminoid ammonia is reduced 40 per cent., the sludge contains 78 per cent, of 

 non-volatile matter, while the scum contains 39 per cent. The deposited sludge 

 can be removed with little offence, and at Finchley has been spread over small 

 areas of land, with no offence, even in the immediate neighbourhood. 



- Mr. Barbour states that at Saratoga, in the winter of 1903-4, the scum froze 

 to a depth of 4 in. under the concrete groined roof covered with 18 in. of soil ; 

 " what would have happened if the tanks had been uncovered is a question worth 

 considering." — Trans. Am. Sec. C.E., LIV. , part E, 1905. 



18—2 



