To stem the continuing loss of clarity in Lake Tahoe, stringent numerical discharge requirements for storm water discharges in the Tahoe basin have been promulgated. In 2008, discharges to surface water must meet limits for turbidity (20 NTU), total phosphorus (0.1 mg/L), total nitrogen
(0.5 mg/L), total iron (0.5 mg/L), and oil and grease (2 mg/L). The California Department of Transportation (Caltrans) has undertaken a multi-year small-scale storm water treatment pilot project to identify, test, and evaluate potential treatment technologies to meet the effluent limits. Treatment
technologies investigated include simple “non-mechanized” sedimentation and granular media filtration systems (with and without chemicals), plus more complex “mechanized” systems involving combinations of coagulation, flocculation, sedimentation, filtration, and ion
exchange. During the first two years of operation covered in this paper, thirty different non-mechanized systems and eight mechanized systems were tested. In terms of turbidity and phosphorus removals, conventional storm water treatment systems (detention and filtration without chemicals)
were consistently unable to meet the discharge limits, whereas similar non-mechanized systems preceded by chemical coagulation performed much better, frequently meeting the limits. Sedimentation without chemicals followed by filtration through activated alumina or expanded shale adsorptive
media also performed well in removing turbidity and phosphorus, but resulted in elevated pH and dissolved aluminum concentrations. Most of the mechanized systems were successful in meeting the turbidity and phosphorus limits, though these systems are not well suited to the roadside environment.
The nitrogen limit was not consistently met by any of the systems tested.
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