i 9 o SCIENCE PROGRESS 



published reports of work carried out by Jordan and others, 

 which went to prove that intermittent downward filtration, 

 through prepared artificial filters of suitable material, will 

 achieve all that is done by the land treatment, and this on a 

 much smaller area ; and, furthermore, that this purification was 

 the work of contained bacteria. In 1892 Scott-MoncriefT sug- 

 gested a system whereby the sewage was purified by a series 

 of stages. His filter-beds were filled with coke, flint, or gravel, 

 and the crude sewage passed into the bottom of the bed, rose 

 through it, and escaped at the top. The solid matter was kept 

 back at the bottom while it underwent solution by means of the 

 anaerobic organisms. The effluent escaping from the top was 

 found to contain very little suspended matter. Then it was led 

 along a series of channels, where it was freely exposed to the 

 air and thereby oxygenated, and hence nitrification ensues. 

 More recently Mr. Scott-Moncrieff has introduced other im- 

 provements into his method. 



From 1 891 to 1895 Mr. Dibdin carried out a number of 

 experiments with the metropolitan sewage which led him to 

 advocate what is sometimes called the " Sutton system of bio- 

 logical purification." Stated very shortly, this system consists 

 in first removing from the sewage, by means of fine metal 

 strainers, the large particles of matter, then exposing it to 

 ■downward filtration through beds of coarse material. These 

 beds are about four feet deep, and made of burnt ballast of such 

 a size that pieces will pass through a two-inch ring and be 

 rejected by a half-inch ring. The first bed deals with the 

 larger and coarser particles of sewage without fear of clogging. 

 Then the liquefied sewage passes on to a second bed of similar 

 size, but finer filtering material, |-inch to T Vinch ring. The 

 sewage is allowed to fill these beds almost to the top surface, 

 and then to rest for a period of two hours (hence the name 

 " contact beds "). Then the beds are slowly emptied and allowed 

 to remain empty for a period of several hours. Each bed is 

 charged twice daily, and by this intermittent application of the 

 sewage a certain amount of aeration of the bed is ensured. 



The septic tank method was devised by Mr. Cameron, of 

 Exeter, about this time also. This method provides a well- 

 defined line of demarcation between the stages of liquefaction 

 and subsequent purification. The process is carried out under 

 strictly anaerobic conditions by constructing a large under- 



