Water treatment as a rule-of-thumb art is a century old. As a scientific concept it is scarcely half that age. As an applied art, working to known ends by known means-engineering, chemical and biological-it falls almost within the present century. Although it is true that unnumbered centuries ago various crude forms of water treatment were in household use, ages passed before cities attempted to improve the quality of their water supplies. It was not until comparatively recent times that the aim was other than reduction of turbidity. The means to this end most commonly employed was sedimentation and filtration, the latter often being "natural filtration" -really wells or galleries for the development of underground supplies. Sometimes the primary aim was storage for a reserve supply which incidentally gave a trilogy of benefits: clarification, color removal and bacterial reduction; the first early recognized, the second appreciated later on; the third and most important unknown until within the last century.Folk days and folk ways. It took no genius to see that after water from a turbid source had stood in a household jar for a time it became clarified by simple deposition of its suspended matter. Chance, more likely than design, showed that precipitation of mud would be hastened in time and increased in amount by dropping alum into the water jar.By the various means indicated clarification without, and then with, coagulation became established, first and perhaps for centuries for household or temple use only; then, for city use. If the water was colored as well as turbid, its color might also be reduced by coagulation. Filtration, on a small scale, may have come into use just as naturally; sometimes being taken over from a household or industrial art or perhaps being suggested by noting the clearness of water drawn from a well or gallery in the sand. Boiling was introduced in early Grecian times, but seems to have been confined mostly to water carried by armies on the march in waterless areas.
Water quality in the lower Colorado and Gunnison Rivers in western Colorado, and many of their tributaries, is impaired by selenium, which originates from the local Mancos shale. Because of the diffuse and widespread nature of this source, there are limited opportunities to reduce selenium inputs. One option is to treat selenium-contaminated surface water at strategic locations, such as point-source discharges. Gravel extraction is common along these rivers, and treatment of discharges from pit dewatering presents an opportunity for reducing selenium loading. The pilot test goals were: 1) demonstrate that a passive BCR can accomplish high-efficiency selenium removal at the pilot scale 2) determine the relationship between selenium removal efficiency and detention time 3) assesses the influence of seasonal temperature fluctuations on treatment performance 4) determine design parameters for a full-scale system (i.e., one with a footprint on the scale of up to a few acres). A single 4,380 cubic foot pilot BCR was constructed to treat flows ranging from 2 to 24 gallons per minute. The vertical-flow reactor media contains cow manure, hay, sawdust, wood chips, and limestone. Influent was drawn from a dewatering trench in a gravel pit next to the Colorado River near Grand Junction, Colorado. The pilot operated, with varying detention times, over a thirteen-month period from September 2008 until October 2009. The pilot achieved maximum selenium removal rates of 98% with a hydraulic retention time of 2.4 days and a minimum effluent concentration of 0.0005 mg/L (0.5 µg/L). The highest mass removal rate achieved by the BCR was 73 mg/day/m 3 and the cumulative mass of total selenium removal was 600 grams. The BCR treatment process was effective throughout the cold winter months during which total selenium removal rates remained greater than 90%. The total capital cost for the pilot BCR was $39,200 or $0.50 per 1,000 gallons treated. The operations and maintenance costs for a passive BCR system are minimal. Based on the successful operation of the pilot, including high rates of selenium removal and consistent year-round operation, the BCR technology appears to be an effective, low-cost selenium treatment option. This project was funded by the US Bureau of Reclamation Science and Technology Program, Project #4414.
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