212 SEWAGE AND ITS PURIFICATION 



From 10 p.m. on Saturday till 6 a.m. on Monday the filter 

 rested empty, making a period of thirty-two hours each week. 

 This weekly rest involves the storage of the crude effluent in 

 reservoirs for the corresponding period — a practice which has 

 many objections. At Exeter, where the flow through the 

 septic tank is continuous, and no reservoirs are employed, the 

 cycles are continued by means of the automatic gear throughout 

 the entire week ; but if a filter shows signs of exhaustion, which 

 occurs at long intervals, or rarely through accident, it is thrown 

 out of use for one or two weeks till recuperated. 



The I -acre filter is still in use. After five years' working it 

 was reported to be free from clogging, and not impaired in 

 working capacity. It will be noticed that the filtering material 

 is only 3 feet deep, and that it is used for treating an effluent 

 from precipitation by Hme and copperas. In 1897-98 new coke- 

 beds were constructed at the northern and southern outfalls 

 for dealing with raw screened sewage, and were made of greater 

 depths — 6, 9, 4, 12, and 13 feet. In a report of the London 

 County Council giving the results of the working of these 

 beds up to August 9, 1898, Dr. Clowes and Dr. Houston show- 

 that they have been continuing the experiments on the lines 

 of Mr. Dibdin, with special reference to the following 

 points : 



" {a) The effect of using the coke in fragments about the size 

 of a walnut. 



" {h) The effect of increasing the depth of the layer of coke 

 beyond the usual limit. 



" (c) The extent to which the raw sewage underwent purifi- 

 cation by the treatment. 



" {d) The practicability of maintaining the constant " (mean- 

 ing, clearly, regular intermittent, not continuous) *' passage of 

 raw sewage through the same coke-bed without deterioration, 

 either in the bed or in the efiluent. 



** {e) The amount of sewage which could be treated daily by 

 a superficial unit of the coke-bed. 



''(/) The extent to which the effluent underwent further 

 improvement by its passage through a second similar coke-bed. 



'' ig) The suitability of the effluent for maintaining the life 

 of fish. 



** {h) The effect of the treatment on the number and nature 

 of the bacteria which were present in the raw sewage." 



The report shows that the size of coke is of importance : 



