AGRICULTURAL VALUE OF EFFLUENTS 333 



from being economical. The free ammonia present would be a 

 marketable article if it could be cheaply extracted. In 1882- 

 1883, Dupre suggested its separation by blowing air through 

 the liquid and absorbing the ammonia in acid, the aeration at 

 the same time improving the sewage and reducing the nuisance 

 at discharge. He states with reference to London, that the 

 sewage contains 3 to 4 grains of ammonia per gallon, equal to 

 31 tons of ammonia in 140 million gallons — one day's discharge, 

 giving 120 tons of sulphate of ammonia, worth from £14. to ;f 20 

 per ton, or a total value per annum of about ^400,000. " By 

 blowing air into the sewage much of the ammonia would be 

 expelled, and if only a fraction of it were recovered, the expense 

 of aeration would be covered." 



It does not seem that this idea has been attempted on a large 

 scale. It would include, in common with other artificial 

 methods that we have seen, a continuous mechanical expense, 

 hence natural nitrification is more economical. See also p. 194. 



Besides the plants mentioned in Chap. VI. as being grown 

 on sewage farms, the Sutton Urban District Council have 

 found peppermint a lucrative crop and one very suited to 

 irrigation with a bacterial effluent. The oil can be easily 

 distilled on the works with waste steam. The yield from the 

 2J acres cultivated at Sutton in 1898 was 61J lbs. sold as first- 

 class oil at 24s. 3d. per lb., realizing £yS' The amounts and 

 acreage since then are reported as : 1902, 8 acres, ;f 235 ; 1903, 

 6Ja., £179 ; 1905, 6a., ;f 145. Similar successes have been 

 recorded in connection with one or two other sewage works. 



Trade Effluents. 



Where land or precipitation is solely relied on, trade effluents 

 are a source of great difficulty. For instance, in a town in the 

 north " they had six times the ordinary flow of sewage, owing 

 to brewery refuse, complaints were very numerous, and they 

 had to reconstruct their sewers and get a farm of 500 acres, at 

 a cost of £250,000. Wherever they had a staple trade they 

 could not rigidly enforce the law, and they frequently had 

 water discharged into the sewers at 212° F." The Belgian 

 Government are experimentally treating trade wastes at a 

 special station at Verviers. 



The investigation of the sediment at the bottom of a stream 

 is of great importance. An instructive case is cited by Marsson^ 



^ Mitteilungen aus der Kimiglichen Prufungsanstalt, Berlin, Heft 2, pp. 28-31. 



