AWWA ACE63256

In what is believed to be the most intensively studied application of hypolimnetic aeration in a water supply reservoir, significantly lower concentrations of iron, manganese, phosphorus, sulfide, chlorine demand, and ammonia have been observed since the installation of hypolimnetic aeration systems in two water supply reservoirs in Virginia. Substantial reductions in sulfide and iron in the lake bottom waters were observed immediately after aerator installation even though dissolved oxygen concentrations were not maintained in the bottom waters throughout the period of thermal stratification. No reduction and even increases in total manganese concentrations were observed in the bottom waters of the lakes in the first several years following aerator installation. This trend was likely related to the increased mixing that occurred due to aerator operation and an increase in hypolimnetic (bottom water) temperature. Following the expansion of aerator capacity (and the amount of oxygen transferred to the lake bottom waters) dissolved oxygen concentrations improved in the hypolimnion and manganese concentrations were observed to decrease over a period of several years. Water quality improvements associated with hypolimnetic aeration allowed discontinuation of prechlorination at Norfolk's water treatment plants to remove iron and manganese and the reduction in chlorine demand of the lake water contributed to lower total chlorine usage. Due to elimination of prechlorination and lower chlorine doses for primary disinfection, THM concentrations in compliance samples were also observed to decrease following aeration. Increases in filter run times and concurrent lower filter backwash water losses were also observed leading to an increase in system capacity. Includes 6 references, tables, figures.