AWWA WQTC58862

Layer aeration is a depth-discrete artificial circulation technique. The method "redistributes" water temperature (heat) and oxygen at middle depths of the water column. A "layer" is created which is bounded above and below by thermoclines. Layer aeration has been used to reduce epilimnetic nutrient loading from the hypolimnion, to restore cool water habitats suitable for cold water fisheries and as zooplankton refuge, and for managing water supply sources. Relative thermal resistance to mixing (RTRM) profiles have demonstrated that stable stratification with multiple thermocline peaks. Layer aeration of a 523 acre, 70 ft deep, eutrophic water supply lake has been performed annually since 1987. An 18-year data record identifies very significant improvements in resource quality and raw water supply quality. The response to layer aeration occurred in several stages. Overall, summer Secchi disk transparency increased from <1.8 m (<6 ft) to >4.6 m (>15 ft), due to elimination of bluegreen algae blooms (Anabaena sp. and Aphanizomenon sp.). Deepening of the "compensation depth" into the hypolimnion contributed to restoration of 3300 acre-ft of aerobic cool water habitat. The habitat improvement resulted in the re-establishment of large-bodied Cladocera (Daphnia sp.) which increases the rate of algal biomass removal by grazing and further improved raw water supply quality. Several long-term water supply reservoir hypolimnetic and layer aeration cases are reviewed in this paper that are relative to limnological characteristics and water treatment relationships. Includes reference, figures.