STATE HORTICULTURAL SOCIETY 179 



By alternating the flow between several plats, and thus leaving 

 them to the operation of sun and air at intervals, it has been found 

 practicable to discharge the sewage of 1,000 people on each acre of 

 porous soil. Irrigation — " broad irrigation " as we may now call it — 

 requires an acre to each 100 people. The Royal Commission of Eng- 

 land in 18S2-84: recommended the " intermittent filtration plan " for 

 London. 



3 PRECIPITATION. 



There are many contrivances under this head all having for their 

 object the acceleration of subsidence of suspended matters. When 

 quantities become large, subsidence is slow, and decomposition sets 

 in To quicken subsidence various chemical substances are mingled 

 with the sewage in the settling tanks. Lime in the shape of milk 

 of lime at the rate of one ton of lime to one million gallons of sew- 

 age has come into commonest use. Other cheiuicals employed are 

 tar and chloride of magnesium or of lime, sulphate of alumina, pro- 

 tosulphate of iron, and a mixture of clay (alumina), blood and car- 

 bon. The system employing alumina, blood and carbon is called for 

 shortness the A, B, C process. 



After precipitation by whatever process, the efHuent water still 

 holding organic matter in solution may be variously disposed.of. It 

 may be discharged into the sea or a stream It may be used for irri- 

 gation or filtered through soil. The city of Birmingham, of 600,000 

 people, precipitates by the milk of lime process, and irrigates 1,200 

 acres of land with the waste water. 



The disposal of the sludge still remains. It is of little value ior 

 manure, and has a gelatinous consistency which makes it very diffi- 

 cult to handle. On the seaboard it can be puumped into barges and 

 carried out to sea. I meet nowhere with any satisfactory suggestions 

 for handling the sewage sludge of inland cities. It can be made into 

 bricks and has been made into a cement, but not at a cost to warrant 

 such manufacture. 



An experiment at Aylesbury, a town of 29,000 people about forty 

 miles northwest of London, offers a probable solution of the problem 

 for small cities. The A, B, C process of precipitation is used. The 

 sludge is dried, ground with some sulphate of magnesia and sold under 

 the name of native guano at $17.50 a ton. The material as it issues 

 from the filter press resembles oil cake, and is quite inoffensive. Ten 

 hundred weight of it doubles a crop. It is reported to be better than 

 stable manure or Peruvian guano. 



