Treatment of Sewage with Basic Per- Salts of Iron. 149 



works is ten times as great as that of the experimental 

 tanks, the former being 80 feet wide to take 10,000,000 

 gallons per day, and the latter 8 feet wide to take 100,000 

 gallons. 



The result of this rapidity of flow is shown by the 

 percentage of purification in the filtered and unfiltered 

 effluents in Table 4 compared with those in Table 1. The 

 less perfect subsidence affects the effluent as it runs from 

 the tanks, but does not cause any material difference in the 

 percentage of purification after filtration, which arrests any 

 flocculent matters carried away by the too rapid flow. 



A third condition makes its appearance as a factor in the 

 practical application of the precipitant, and this is the 

 continuously varying nature of the sewage of an industrial 

 town. In the smaller experiments it was possible to cope 

 with, to some extent, the almost hourly variations of the 

 nature of the sewage, by attention to the supply of precipi- 

 tant, but on the large scale this becomes much more 

 impracticable, and whilst such considerable variations as 

 are found between the day sewage and the night sewage 

 may be fairly easily met, the only way to secure a uniformly 

 good effluent during the day is to add a sufficient quantity 

 of precipitant to purify the sewage at its worst. This is 

 especially the case if the attempt is being made to work 

 without filters or land to receive the tank effluent. 



With regard to the filtration of the effluents from the 

 tanks, it may be remarked that this was through about 

 three feet of sand and gravel, or through the same depth of 

 ordinary cinders and clinkers, the result being much the 

 same in either case. It is improbable that the filters 

 effected anything more than a straining from the flocculent 

 particles, as the rate of flow was too rapid for any appreciable 

 oxidation to take place. 



There is much to be said in favour of the use of some 

 straining medium after precipitation and subsidence as a 



