Treatment of urban runoff at Lake Tahoe: Low intensity chemical dosing

A systematic investigation of the impact of coagulant type and dose as well as temperature, mixing, and water quality on subsequent charge neutralization and removal of phosphorus and fine particles from urban/stormwater runoff entering Lake Tahoe was conducted. Dosing based on streaming current values resulted in turbidities of less than 10.9 +/- 0.35 NTU and filtrable and total phosphorous concentrations of less than 9.83 +/- 0.54 and 25.6 +/- 5.71 ppb, 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 pre-hydrolyzed forms of aluminum, high intensity rapid mix was counterproductive. Several classes of coagulants responded robustly to water quality and temperature changes. However, poly-aluminum 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.