PROTECTION OF RIVER AND HARBOR WATERS 



19 



covered with deposits of glacial drift sand, ideal in character for sewage 

 purification. All that is necessary is to expose and level off areas of this 

 sand, to lay lines of underdrains a few feet below the surface to carry off 

 the effluent, and to install devices for discharging the sewage on the surface. 

 A bed may be dosed on one day out of three, or in smaller portions several 

 times a day. In winter the beds are furrowed so that an ice roof forms on the 

 top of the ridges while the sewage finds its way along the furrows between 

 and, although less efficient in winter than in summer, the microbes do their 

 work at all seasons well enough for practical purposes. The effluent from 

 an intermittent filter, properly built and carefully operated, is a clear 

 liquid, colorless or slightly yellowish in color, with no odor or only a slightly 



Double contact beds for purification of sewage. 

 can Museum 



Photograph of a model in the Ameri- 



inusty one, practically free from putrescible organic matter and low in 

 bacteria — a liquid that can be discharged with impunity into even the 

 smallest watercourse. 



The successful and economical use of the process of intermittent filtra- 

 tion is limited to those regions where ample areas of the right soil are easily 

 available. In clayey or chalky regions, sand beds must be artificially con- 

 structed with material brought from a distance, and this would make the 

 cost of the Massachusetts method almost prohibitive. In England where 

 the sewage problem pressed hardest for solution, sand is usually not avail- 

 able and it was almost essential that further improvements should be 

 made. 



