142 SEWAGE AND ITS PURIFICATION 



In places where the soil is sufficiently porous the land is laid 

 out in a different way, the sewage being fed along the furrows 

 with the vegetation on the ridges, and the underdrains between, 

 so that the liquid reaches the roots from underneath, the excess 

 passing laterally to the drains. This method seems to be recog- 

 nised as the best for avoiding water-logging, " sewage sickening," 

 and other evils of sewage-farming by broad irrigation. It must 

 be remembered that the reliance is here on the filtering qualities 

 of the soil, the plants playing a subordinate part in utilizing 

 the nitrogen of the soil afterwards (Fig. 19). 



At Paris a portion of the city sewage is treated in this way 

 at Gennevilliers and Acheres. At the former the soil is sand 



Fig. 19. — Section showing Underdrains in Irrigation. 



mixed with clay, and the crops are various, but chiefly vege- 

 tables, with also fruit-trees, flowers, and some meadow land. 

 The irrigation is managed by flooding at intervals, the vege- 

 tables growing on ridges as described above. Part is worked 

 by private lessees and part by the State, and the results seem 

 to have been satisfactory until lately, when, owing to the 

 increase of population and greater volume of sewage, com- 

 plaints have been made to the municipality of flooding and 

 nuisance. For a number of years experiments have been con- 

 ducted at Gennevilliers and Acheres to ascertain the amount 

 of sewage that may be applied to land without injuring the 

 crops. It is stated that 144,000 cubic metres per hectare 

 (13,000,000 gallons per acre) annually may be turned on a field 

 of lucerne and 170,000 (15,000,000 per acre) on meadow land. 

 These figures are far in excess of anything hitherto accomplished 

 in regular daily work. 



Bechmann states that the experiments at the model garden 

 of Genevilliers show that from 80,000 to 130,000 cubic metres 

 of sewage per hectare (7,000,000 to 11,500,000 gallons per acre) 

 annually can be applied without prejudice to the success of the 



