BACTERIA BEDS are largely an English out growth, since 1891, of the work on intermittent filtration. There have been various modifications of these beds, such as the use of coal, burnt clay, and coke, for filtering material; placing the beds in tiers, or in terraces; and aim ing to use the anaerobic and aerobic bacteria to gether, or the latter alone. But the essential fea ture of bacteria beds is the retention of the sewage a longer time in the beds than is possible with intermittent filtration, after which there is a resting period, similar to that in the older process, but shorter. The bacteria beds were evolved in England because of the scarcity of sandy land suitable for intermittent filtration. It being necessary to transport sand or some other filtering material, and make it up into wholly artificial beds, it was imperative that the more expensive beds should treat the sewage at a higher rate. This was found to be possible, but the purification not being sufficiently com plete for all conditions, a second, or even a third bed was added where necessary.
There are many claimants for the introduction of bacteria beds, but it appears that the first and most practical early work was that begun in 1892 at the Barking chemical precipitation plant of the London sewerage system by W. J. Dibdin,
chemist to the London County Council, aided by Cleorge Thutlichum, with a filter bed consisting of 3 feet in depth of coke, broken to small fragments.
In 1391 Sidney Loweock constructed a novel sewage purification plant for a private residence at Ashstead, England, in which he embodied, prob ably for the first time, the principle that the bac terial treatment of sewage involved two distinct stages: the breaking down of the solid organic matter, or liquefaction, followed by nitrification. For the first stage he employed a closed tank, filled with broken stone. Ma sewage rose up ward through this tank, then passed down through a series of nine perforated trays, each containing a thin bed of coke. The object of so many trays was to secure a more minute sub division of bacterial labor.
It is too early to say what rate of filtration will prove feasible with bacteria beds, but it seems doubtful whether the 500,000 to 1,000,000 gallons or more per acre, claimed in England, can he practical for a series of years without either poor results or large outlays for replacing clogged filtering material.