Treatment of Sewage with Basic Per- Salts of Iron. 147 



It was therefore arranged to try the effect of a larger 

 quantity of the Iron Salt on the 19th, 20th, and 21st of June. 



During this experiment the period of trial was 56 hours. 

 During this period 33 tons of basic persulphate of iron and 

 q\ tons of lime were used. The quantity of sewage passed 

 through the tanks was about 23 million gallons during the 

 56 hours. Per million gallons of sewage, therefore, was 

 used 28f cwts. of iron salt, and 8 cwt. of quick lime. The 

 iron salt is reckoned as a solution of basic persulphate of 

 1*5 sp. gr., the quick lime contained about 78 per cent, of 

 calcium oxide (CaO.). 



The effect of the increase of precipitant was most 

 marked. The effluent passing over the sill of the last tank, 

 instead of being turbid was exceedingly clear, although 

 often containing some particles of flocculent matter carried 

 over by the too rapid flow of the sewage through the tanks. 

 This effect is seen by comparing the percentage of soluble 

 impurities removed as shown in Tables 3 and 4. 



Table III. 



Result of 'treatment of ' Salford sewage, about 70,000,000 gallons per 

 day, with Basic Persulphate of Iron, at the rate of 29 cwt 

 per million gallons, together with 8 cwt. of lime per million 

 gallons. 



1893. 



June 19 

 ,, 20 

 ,, 21 



Crude Sewage. 



Effluent unfiltered. 



Effluent filtered (sand). 



Shaken 

 Alb. Amm. 

 per 100,000. 



Settled 

 Alb. Amm. 

 per 100,000. 



Alb. Amm. 

 per 100,000. 



% of purifi- 

 cation. 



Alb. Amm. 

 per 100,000. 



% of purifi- 

 cation. 



i-5 



i-6 



1 "4 



o-8 

 o-8 

 0*65 



0- 3 8 



o'35 

 o.35 



52 

 56 

 46 



0*24 

 0'28 

 0-24 



70 



68 

 65 



The method of sampling for the test was as follows : — 

 A sample of the sewage and of the effluent was taken every 

 hour, day and night. The samples were at the end of the 



