BACTERIAL PURIFICATION 



207 



mainly to the special organisms of disease, but gradually 

 [developing a knowledge of the larger class that are not patho- 

 [genic, but effect ordinary changes in organic matter. 



In November, 1887, the Massachusetts State Board of 



Health commenced their experiments on the purification of 



^ater and sewage by chemical precipitation and by filter-beds, 



ind in the two first volumes of their reports, extending to i8go, 



letails are worked out of different filtering media, size of grains, 



:hickness of strata, influence of time, temperature, and methods 



>f procedure, the results of about 4,000 analyses of raw sewages 



ind effluents being tabulated. At first they aimed at the 



fremoval or destruction of bacteria by straining and chemical 



, means, without practical success ; later they studied inter- 



jmittent filtration with the actual assistance of aerobic organisms. 



[All that was found necessary to completely destroy dead organic 



latter was to provide conditions favourable to the action of 



[bacteria, giving suitable material on which the organisms 



[would be retained, surrounding them at certain intervals with 



jair, and providing periods of rest. They selected as suitable, 



[sand from 4 to 5 feet in depth, under-drained, allowing the 



sewage to flow on the sand only six hours out of each twenty- 



tfour. 



On this principle at Worcester, Mass., sixteen filtration beds 

 |of about I acre each were constructed of coarse sand from which 

 ^all pockets of clay and quick-sand had been removed. They 

 .are separated by dikes, tamped with clay, and drained by lo-inch 

 >ipes with open joints, 6 feet deep and 50 feet apart. The feed 

 [is by split pipes laid on the beds. 



This intermittent sand filtration is capable of purifying per 

 [acre about 100,000 gallons of domestic sewage in each twenty- 

 four hours, so far as to remove any danger of subsequent 

 ; putrefaction ; with 20,000 to 30,000 gallons per day, the pro- 

 [duct is chemically and bacterially exceptional.^ 



On this aerobic plan, if a filter-bed were overworked, it 

 rapidly choked, and putrefaction occurred in the interior owing 

 to a deficiency of aeration, so that it was necessary to have 

 " very slow motion of very thin films of liquid over the surface 

 of particles having spaces between them sufficient to allow air 

 to be continually in contact," a condition, however, which did 

 not prevent the sand filters from becoming over-burdened, and 



^ "Purification of Sewage by Bacterial Methods," L. Kinnicut, September, 

 1900 {Journal of the New England Water-works Association, xv., 2). 



