IRRIGATION AND SEWAGE FARMS 147 



Later a bacterial scheme was introduced, and in 1900 

 [Mr. Mawbey issued his report on the experiments. These 

 [were of hmited scope and open to some objections, but under 

 local conditions it was considered that the best results were 

 [obtained by passing the raw sewage through (i) a " closed 

 letritus tank," (2) " clarifying bacteria beds, single contact, and 

 [hree fillings a day," followed by one application to old pasture. 



'he system has been adopted by the Corporation.^ 



It is obvious that the solids are an integral part of sewage, 

 md that their removal, entirely or in part, by any system 

 |f straining, settlement, precipitation, or filtration, results in 

 (he production of a "sludge," which has to be separately 

 treated; also that precipitation, if the effluent is afterwards 

 [o be applied to crops, must not involve such use of chemicals 

 LS may be injurious to the vegetation. Iron and aluminium 

 jalts, such as alumino-ferric, followed by lime, give a much- 



mrified effluent which has proved to be innocent in agriculture, 

 ilthough both this and simple sedimentation or filtration 

 remove from the liquid some of the constituents which, when 



)roperly fermented, are capable of assimilation by plants, and 

 ilso, along with the suspended solids, many of the bacteria 



^hich affect these changes. 



The areas that have been officially demanded in England 

 for the purification of sewage according to the process adopted 

 u*e as follows ; a much less amount has been often used suc- 

 :essfully with proper management and care, but local conditions 



lay even demand larger quantities : 



Population per Acre of Land. 



1. Irrigation without precipitation — 

 Stiff clay ... ... i acre for every 25 persons 



Loamy gravel ... ... ,, ,, 100 persons 



2. Intermittent filtration without precipitation — 

 Sandy gravel ... i acre for every too to 300 persons 



3. Irrigation and precipitation — 

 Clay ... ... ... I acre for every 200 persons 



Loamy gravel ... ... ,, „ 400 persons 



4. Intermittent filtration and precipitation — 

 Sandy gravel ... I acre for every 500 to 600 persons 



^ See further Mr. Mavvbey's paper, Congress of the Royal Institute of Public 

 [ealth, Exeter, 1902. 



TO — 2 



