Transactions of the American Society of Civil Engineers, vol. LXXII, June, 1911 / Water Purification Plant, Washington, D. C. Results of Operation.

Description of the Filtration Plant. —The Washington filtration plant was completed and put in operation in October, 1905. It consists of a pumping station for raising the water from the McMillan Park Reservoir to the filter beds; 29 filters of the slow sand type, having an effective area of 1 acre each; the filtered-water reservoir, having a capacity of about 15,000,000 gal.; and the necessary piping and valves for carrying water, controlling rates of filtration, etc.
In the pumping station, there are three centrifugal pumps, which are directly connected to tandem compound engines; two sand‑washer pumps; three small electric generating sets for furnishing electric light; and four 200‑h.p., water-tube boilers.
Each of the centrifugal pumps has a nominal capacity of 40,000,000 gal. per day when pumping against a head of 21 ft., and each sand‑washer pump has a capacity of 2,500,000 gal. when pumping against a head of 250 ft. The electric light engines and generators supply the current for lighting the pumping station, the office and laboratory and other buildings, and also the courts and interior of the filter beds, and for operating a machine‑shop.
The filters and filtered‑water reservoir are built entirely of concrete masonry. The floors are of inverted groined arches on which rest the piers for supporting the groined arch vaulting. All this concrete work is similar to that in the Albany, Philadelphia, and Pittsburg filters.
The filters contain, on an average, 40 in. of filter sand and 12 in. of filter gravel. The gravel is graded from coarse to fine; the lower and coarser part acts as part of the under‑drain system, and the upper and finest layer supports the filter sand. The raw water from the pumps is carried to the filters through riveted steel rising mains which have 20‑in. cast‑iron branches for supplying the individual filters. The filtered water is collected in the under‑drainage system of the several filter beds, and is carried through 20‑in., cast‑iron pipes to the regulator‑houses. These regulator‑houses contain the necessary valves, registering apparatus, etc., for regulating the rate of filtration, showing the loss of head, shutting down a filter, filling a filter with filtered water from the under‑drains, and for turning the water back into the raw‑water reservoir, or wasting it into the sewer. From the regulator‑houses, the filtered water flows directly to thefiltered‑water reservoir. Generally, five filters are controlled from one house, but there are two cases where the regulator‑houses are smaller, and only two filters are controlled from each.