Feasibility Study and Conceptual Design for Using Coagulants to Treat Runoff in the Tahoe Basin

by P. A. M. Bachand, (corresponding author), Ph.D., President; Bachand & Associates, 2023 Regis Dr., Davis, CA 95618,
A. C. Heyvaert, Ph.D., Assistant Research Professor; Desert Research Institute, 2215 Raggio Pkwy., Reno, NV 89512; and, Researcher, Tahoe Research Group, UC Davis, Davis, CA 95616.,
S. E. Prentice, Scientist; Bachand & Associates, 2023 Regis Dr., Davis, CA 95618.,
T. Delaney, Scientist; Bachand & Associates, 2023 Regis Dr., Davis, CA 95618.,

Serial Information: Issue 11, Pg. 1218-1230

Document Type: Journal Paper

Abstract: Fine particles entrained in storm-water runoff are likely to pass through storm-water treatment basins because of their slow settling velocities and the natural biotic and abiotic mixing processes common to ponds and basins. In Lake Tahoe, targeting fine particles <20 μm in diameter is critical to abating turbidity and phosphorus inputs disproportionately responsible for reducing the lake clarity and impacting regional water quality goals. Iron- and aluminum-based coagulant dosing has been commonly used in water and wastewater treatment plants for removal of fines, turbidity, and dissolved organic carbon. However, application of these coagulants for treating storm water is not common. This study used settling columns to show the feasibility of coagulant dosing to target fine particle removal from storm water in shallow treatment basins and wetlands. Coagulation reduced mean turbidity and phosphorus by 85–95% within 10 h of dosing, compared to 20 and 55% reductions in turbidity and phosphorus, respectively, for nontreated storm water over the same amount of time. To achieve equivalent treatment levels, an order of magnitude increase in time was required for the nontreated storm water. These results have important implications on approaches to treat storm water in the Tahoe Basin. First, these findings suggest that whereas most treatment basins and wetlands will not effectively remove fines and total phosphorus within a 24-h hydraulic residence time, those which utilize coagulant dosing should effectively remove fines and total phosphorus. Second, coagulant dosing relies on mechanical equipment such as pumps and flow meters. These equipments cannot accommodate normal variations in storm-water flow which can range over four orders of magnitude. Thus, to fully leverage the investment of this technology, modifications in hydrologic designs are necessary. We suggest equalization basins upstream of treatment basins to shift treatment from storm water entering a treatment complex to that leaving the equalization basin. This configuration buffers flows at the coagulant dosing location and increases the storage capacity of the storm-water treatment complex. Finally, given the paucity of available acreage in the Tahoe Basin and its high cost, coagulant dosing systems could be retrofitted to existing treatment basins and wetlands, enabling these treatment areas to be more effective in targeting phosphorus and fines, service drainage areas two or three times greater than currently, and reduce land area needed for treating storm water. We present a conceptual layout, a process and instrumentation diagram, and cost estimates to implement this technology at a larger scale. We believe that this technology should receive serious consideration for its application at a field or pilot scale where other potential issues can be further investigated and addressed.

Subject Headings: Stormwater management | Feasibility studies | Retention basins | Phosphorus | Conceptual design | Basins | Runoff | Settling basins | Wetlands (fresh water) | Turbidity | Lakes |

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