BACTERIAL PURIFICATION 279 



The plant and working at Manchester still admit of improve- 

 ment. The sewage is a difBcult one to deal with, but is not 

 worse than that of other centres which have met with better 

 success. The resolution of solids in the septic tank, estimated 

 as from 25 to 30%, is lower than at other places owing, 

 as is implied in the reports/ to the irregularity and frequent 

 disturbance in operating. The mode of distribution on the 

 beds by radiating troughs on the surface is not a good one, and 

 as to the unsuccessful trials with sprinklers, the report of 1903 

 itself says (p. 29) " it is difficult to draw trustworthy conclu- 

 sions from comparative experiments made in this manner." 



At Birmingham, since the abandonment of the lime process 

 {ante, p. 155) in 1900, the plant has been used for a modified 

 septic treatment. In 1902 out of 24 J million gals, of sewage 

 per day only 3 million gals, were domestic, the remainder being 

 chiefly trade effluents, into which sulphuric, hydrochloric and 

 nitric acids and salts of heavy metals had passed. It was 

 noticed that the nitrates disappeared in the sewage in hot 

 weather. The flow is received into three large tanks, each 

 divided into three parts by submerged walls rising to within 

 2 ft. of the surface. The first chamber retains the heavier 

 detritus, the other two arrest the bulk of the residual suspended 

 matter, and are used alternately and cleaned out once a fortnight. 

 The sewage passes on to 16 smaller septic tanks, where it stays 

 8 hours, and finally is distributed over land at an average rate 

 of 15,000 gallons per acre per day. It is reported that " an 

 enormous quantity of gas escapes from the septic tank, result- 

 ing in a loss of about 8 per cent, of the solid matter which 

 enters these tanks," that the suspended solids are very much 

 altered in quality, and that the sulphides developed precipitate 

 most of the heavy metals in solution. The effluent *' is not 

 merely harmless, but has considerable manurial properties ; for 

 in grass land irrigated by it the path of the water may be traced 

 by the increased vigour of the herbage."^ An important point 

 is that "the enormous fall in liquid sludge" — from 281,000 

 cub. yds. for 20,000,000 gallons daily dry-weather flow in 1896, 

 to 128,000 cub. yds. for 21,500,000 gallons in 1901 — '* is largely 

 due to the cessation of the use of lime and adoption of septic 

 treatment." With regard to organic improvement the analyses 

 given show that the arrangement of the tanks should be different. 



^ Manchester Rivers Department, 1903, p. 20 ; 1904, p. 17. 

 ^ J. Soc. Cheni. Industry, May 31. 1902. 



