27.6 g/L 457.8 g/L 18.2 g/l 12.8 g/L 0.4 g/L 0.8 g/L 0.9 g/L
a Average of three parallel channels, with a detention time about 5 days.
Source: Kamber, D.M., Benefits and Implementation Potential of Wastewater Aquaculture, EPA Contract Report 68-016232, U.S. Environmental Protection Agency, Office of Water Regulations and Standards, Washington, D.C., 1982.
Excellent metal removals have been demonstrated in the type of constructed wetlands described in Chapter 6 and Chapter 7. Tests at pilot wetlands in southern California, with about 5.5 days' hydraulic residence time, indicated 99, 97, and 99% removal for copper, zinc, and cadmium, respectively (Gersberg et al., 1983); however, plant uptake by the vegetation accounted for less than 1% of the metals involved. The major mechanisms responsible for metal removal were precipitation and adsorption interactions with the organic benthic layer.
Removal of metals in land treatment systems can involve both uptake by any vegetation and adsorption, ion exchange, precipitation, and complexation in or on the soil. As explained in Chapter 9, zinc, copper, and nickel are toxic to vegetation long before they reach a concentration in the plant tissue that would represent a risk to human or animal food chains. Cadmium, however, can accumulate in many plants without toxic effects and may represent some health risk. As a result, cadmium is the major limiting factor for application of sludge on agricultural land.
The near-surface soil layer in land treatment systems is very effective for removal, and most retained metals are found in this zone. Investigations at a rapidinfiltration system that had operated for 33 years on Cape Cod, Massachusetts,
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