246 



SEWAGE AND ITS PURIFICATION 



top for ventilation of the broad beds. The latter have no wall, 

 but the bank of cinders is simply sloped, leaving free air- 

 way to the open sides of the raised tiled floor. There is an 

 18-inch supply to a pair of bays, 62 ft. width. '* We are using a 

 slightly coarser grade of * medium,' say, J to i J ins. diameter, 

 by 7 ft. depth, and find this prevents the pooling on the 

 surface, which is the only working defect of our first beds, 

 made with * medium ' from f\ to | in. In all cases with 

 some coaser stuff over the tile floor." It is proposed to com- 

 plete the Roughing Filters by cleansing apparatus including 

 upward air-blowing. 



In 1893 Mr. Wallis Stoddart pubhshed some experiments^ on 

 small model filters of coarse chalk, with arrangements for con- 

 tinuous dropping and trickling. He seeded the bed with 

 liquids containing ammonifying, nitrosifying, and nitrifying 

 organisms successively, and obtained different results according 

 to the rate of flow. The organisms were too much mixed in 

 the same area : he secured, however, ** a very constant forma- 

 tion of nitrate of lime," and with a polluted well water he 

 records the following purification : 



With a sewage percolating continuously through 5 feet of 

 coarse chalk with an upward current of air, the results were : 



