CHAPTER VI 



IRRIGATION AND SEWAGE FARMS 



Broad irrigation — Soils — Application of lime or ashes — Trenching — 

 Organisms in soils — Suitable crops — Transpiration of water — 

 Statistics of sewage farms — Systems of distribution — Ridge and 

 furrow — Catchwater — Intermittent irrigation with under- 

 drainage — Merthyr Tydvil — Calculation of dilution by subsoil 

 water — Irrigation with previous treatment — Areas required — 

 General aspect of land treatment. 



EARTH-disposal is the main factor in the three forms of 

 irrigation : 



A. Broad Irrigation^ defined by the Royal Commission on 

 Metropolitan Sewage Discharge, 1884, as " the distribution of 

 sewage over a large surface of ordinary agricultural land, having 

 in view a maximum growth of vegetation (consistent with due 

 purification) for the amount of sewage supplied." 



B. Irrigation with Copious Under drainage, similarly defined 

 under the head of" Filtration " as "the concentration of sewage, 

 at short intervals, on an area of specially-chosen porous ground, 

 as small as will absorb and cleanse it, not excluding vegetation, 

 but making the produce of secondary importance. The inter- 

 mittency of application is a sine qua non even in suitably con- 

 stituted soils, wherever complete success is aimed at." 



c. Mixed Systems, including Previous Sedimentation or Chemical 

 Preparation. — These, which may be called shortly the broad, 

 the intermittent, and the mixed systems of irrigation, are 

 " sewage farm " schemes, and are jointly saddled with the 

 following difficulties : 



(i) The unsuitability of the only land often attainable. 



(2) Local opposition, and the very high prices generally 

 demanded for the area. 



(3) The failure, under these conditions, of making the sale of 

 the produce remunerative. 



A. Broad Irrigation, This, the oldest method, demands a very 

 large extent of land (approaching i acre per 100 of population), 



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