BACTERIAL PURIFICATION 2ig 



. 22, says that '* the destruction of soHd organic matter takes 

 lace most advantageously during the period of rest. The 

 ludge of a manufacturing town is more resistant than domestic 

 ludge." I pointed out in 1896 that as the organisms pro- 

 ucing nitrates require much oxygen (p. 121), and we know 

 hey do not thrive in a water-logged soil, their action is almost 

 ntirely confined to the periods of emptying and resting empty, 

 he prevalent fault of these fine or secondary beds on the 

 utton system is the deficiency of aeration, resulting in a 

 enerally low nitrification, shown also by the presence of 

 itrites. Thus at Exeter in i8g6 I found that the discharge 

 om the Dibdin filters contained ordinarily only 2*7 to 3 c.c. 

 of oxygen per litre, or less than half saturated, with about 

 Ml part per 100,000 of nitrogen as nitrates, whereas in a filter 

 wvhich had rested for some time the nitric nitrogen in the first 

 ^discharge was 2 parts per 100,000. The Leeds report, 1900, 

 mentions that large quantities of nitrates were produced in the 

 resting periods. In one instance where the filtrate had deterio- 

 rated and the nitric nitrogen was o*i6 part, after a rest of 

 eighteen days it advanced to 3*4 part per 100,000, and the 

 capacity at the same time increased 21 per cent, (see also 

 Chap. XL). It will be noticed, further, that the Sutton sewage 

 has already been broken down to a very considerable extent, as 

 shown by the 12*53 parts of free ammonia, and only i'i3 parts 

 of albuminoid. 



The total cost of the farm when formerly worked on the 

 chemical precipitation and broad irrigation system was for the 

 year ending March 31, 1895, £15 iis. iid. per 1,000,000 gallons 

 (taking into account the amount earned by the farm and sale of 

 I sludge, which was ;fii7 7s.) ; in 1899 it was ^^3 19s. with the 

 biological system. 



In June, 1899, I made an examination of the Sutton results 

 for the Local Government Board. The samples of screened 

 sewage, coarse and fine bed effluents were so collected as to 

 represent the working of one pair of beds on one day, the 

 average samples being obtained by taking equal volumes at 

 intervals of five minutes throughout the whole period of filling 

 or discharge. Gauging of coarse bed, 6,600 gallons ; fine bed, 

 4,369 gallons. The volume sampled was, therefore, approxi- 

 mately 19,800 gallons of screened sewage, of which 13,107 

 gallons were subsequently passed through the fine bed. The 

 flow through the coarse bed (33 X 55 feet = 20if square yards) 



