IRRIGATION AND SEWAGE FARMS 135 



Gs found requisite ; thus, Jones^ recommends at least i acre of 

 good soil for 500 persons at 15 gallons per head. Professor 

 Robinson gives the average of English sewage farms as 149 

 people to each acre irrigated with 38 gallons of sewage per 

 head per day, and this is frequently too limited an amount of 

 land. 



Madras in igoi daily disposed of some 4,000,000 gallons 

 of its sewage on farms in various parts, and eventually 

 the whole 15,000,000 gallons are to be dealt with on a farm 

 with sandy soil near the sea, where indefinite extension is 

 possible. Nothing but successive crops of Harriali grass were 

 grown on the farms by the contractor, who paid an annual rent 

 of 50,000 rupees. In the transit for three to six miles a con- 

 siderable amount of nitrification occurs in the sewage. S^ydney 

 and Melbourne, and some other places in Australia, have large 

 sewage farms on areas of sandy so il. 



At Berlin, on a s and subso il, i to 2 J tons of *' waste lime " per 

 acre have been spread with benefit over fields previously 

 drenched with sewage. For clay, ashes from the town refuse 

 are dug or ploughed in. Deep steam-sloughing, and even sub- 

 soiling to turn in the sludge, are at intervals necessary, since 

 crude sewag e discharged direct on land rapidly coats it with a 

 felted layer of black decomposing matter, which hinders the 

 access of oxygen, chokes the plants, and soon creates a nuisance. 



Dr. Divers describes the cultivation of rice in Japan by 

 surface irrigation on clay land. The sewage as collected is 

 placed in large tanks in the fields, covered over with a loose 

 roof of straw, where it is allowed to ferment for some time, till 

 all the urea is converted into ammonia. The liquid passes over 

 terraces from field to field till it reaches the watercourse in the 

 valley. Dr. Kellner ascertained that the crops were good ; the 

 liquid flowing over the soil gave up much of the mineral con- 

 stituents, nitrates were formed and absorbed, and the effluent 

 was very satisfactory. 



In broad irrigation there is always a risk that a portion of 

 tlie raw sewage may escape wholly unpurified. On clayey 

 soils the liquid passes almost entirely over the surface, and this, 

 if a sufficient distance be given, has been found, as above, to 

 effect a great purification, with, however, generally a nuisance. 



I Mere deep trenching of heavy soils, laying pipe drains, and 

 filling up with ballast, etc., yields an almost unoxidized and 



^ " Manual of Hygiene," 1896, p. 484. 



