Treatment of urban runoff at Lake Tahoe: low-intensity chemical dosing.

A systematic investigation of the effect of coagulant type and dose and temperature, mixing, and water quality on subsequent charge neutralization and removal of phosphorus and fine particles from urban and/or stormwater runoff entering Lake Tahoe (Sierra Nevada mountains, western United States) was conducted. Dosing based on streaming current values resulted in turbidities of less than 10.9 +/- 0.35 NTU and filterable and total phosphorus concentrations of less than 9.83 +/- 0.54 and 25.6 +/- 5.71 microg/L, respectively. Inadequate slow mixing could be partially compensated for by increased settling time; however, such quiescent conditions are difficult to obtain in natural systems. For prehydrolyzed forms of aluminum, high intensity rapid mixing was counterproductive. Several classes of coagulants responded robustly to water quality and temperature changes. However, polyaluminum chlorides modified with silica or sulfate, with low to medium basicity, were consistently the best performers in these tests, in terms of simultaneously removing phosphorus and fine particles under a wide range of operating conditions with low doses.