A manual of hygiene and sanitation . the under-drains. Eecently it has been suggested that the filter-beds beconstructed directly over the storage reservoirs for thefiltered water, the beds being supported on suitable steelcolumns resting on concrete foundations in the bottom ofthe reservoirs. The bottom layer of the filter, composedof gravel or broken stone, Avould rest on steel tubes orbars several feet above the level of the water in the reser-voir, thus allowing the filtrate to be aerated as it fallsthrough the intervening space. Theoretically, it would 190 WATER, seem that the plan is a g
A manual of hygiene and sanitation . the under-drains. Eecently it has been suggested that the filter-beds beconstructed directly over the storage reservoirs for thefiltered water, the beds being supported on suitable steelcolumns resting on concrete foundations in the bottom ofthe reservoirs. The bottom layer of the filter, composedof gravel or broken stone, Avould rest on steel tubes orbars several feet above the level of the water in the reser-voir, thus allowing the filtrate to be aerated as it fallsthrough the intervening space. Theoretically, it would 190 WATER, seem that the plan is a good one, and actual results indi-cate that it practically is so. Some of the advantages arethe absence of under-drains and loss of the resistancefactor due to them, the aeration of the filtrate as indi-cated, and also the practically continuous aeration of thefilter-bed itself, thus enabling the saprophytic bacteria inthe upper layers to carry on their work of oxidizing andnitrifying the organic impurities of the water. One Fig. ?I I I ?! I I I I I I I J I I I I J i Io 15 ao 25 Feet Simplest form of regulating outflow from filter-beds. Stralau filters atBerlin. (Hazen.) serious objection to such a filter is that an accidental over-flow would contaminate at once all the filtered water inthe storage reservoir. Although it has been the custom to keep the depth ofwater upon the filter-beds in excess of the loss of head,this is not essential. On foreign filters the usual depth isfrom 36 to 52 inches, though less than this might sufficein many instances. The necessity of regulating the inflowand outflow and of maintaining a constant level must notbe overlooked if uniform results are desired. Summarizing the preceding statements, the loss of head BATE OF FILTRATION. 191 and rate of filtration will depend upon the depth of wateron the filters, the thickness of the sand layer, size ofsand-grains, resistance of under-drains, temperature, etc.,and all these will likewise affect both
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