BACTERIAL PURIFICATION 247 



The flow on the model filter exhibited at the British Medical 

 Association at Bristol, in 1894, was 200 gallons per square 

 yard per day. The first successful working filter on this plan 

 was erected at Horfield, near Bristol, in September, i8gg, and 

 has since been running continuously night and day. ** The 

 composition of the filtering material is immaterial, so long as 

 it is insoluble and not too friable ; the size should be i^ to 

 I inches, in no case less than J inch, and the depth 6 feet." A 

 chief feature is the distributor (Chapter XII.). 



The nitrogen as nitrates and nitrites in samples of the Horfield 

 filtrates is given as 2*14, 2*57, and i*8i parts per 100,000. A 

 recent analysis of the filtrate shows that the oxidation is not 

 complete when the flow is increased: Saline ammonia, I'go ; 

 albuminoid, o'i2 ; oxidized nitrogen, 2*80; oxygen absorbed, 

 0*88; chlorine, 6*14; parts per 100,000. With this filter using 

 the distributor devised by him and with a precipitated sewage 

 or hydrolysed effluent of weak character, the rate of continuous 

 flow can be increased up to 1,000 or 1,200 gallons per square 

 yard per day and still yield a final filtrate which is non- 

 putrescible, with an appreciable amount of nitrates and dis- 

 solved free oxygen. Stronger sewages require a considerably 

 lower rate, but the fact remains that continuous filters of this 

 type and that used by Moncrieff can deal with sewage at a 

 much greater rate than intermittent ones, when a well-devised 

 distributor is used for insuring that the whole of the filter-bed 

 is utilized. 



Other advantages of continuous filters are : (i) The coarse- 

 grain material used in them does not require so deep a con- 

 struction of the filter as the fine-grain of intermittent filters ; 

 (2) it is cheaper than the latter ; (3) it does not need washing 

 so often and does not waste so much in washing ; (4) expensive 

 thick walls are not required ; (5) from an agricultural point of 

 view, a " contact-bed " wastes about half the nitrogen by dis- 

 persal mainly as nitrogen gas, a continuous filter preserves 

 very nearly all of it in fertilizing and innocuous forms (see 

 Chapter IX., p. 222, and Chapter XIV.). 



Mr. Stoddart points out that *' the improved filter does not 

 constitute a complete system of sewage disposal, as it is not 

 intended to deal with crude sewage." 



Repeating the three stages of natural sewage purification — 

 (i) Anaerobic — hydrolytic solution and ammoniacal change; 



(2) Partially aerobic — nitrites and simplified bodies ; 



(3) Complete oxidation and nitrification ; 



