Aeration of Stratified Lakes: Theory and Practice

Abstract

ABSTRACT Layer aeration (U.S. Pat. 4,724,086) can be described as a depth-discrete artificial circulation technique. Like destratification, it uses both photosynthetic and diffusional sources of oxygen to overcome oxygen demand. Like hypolimnetic aeration, it maintains necessary vertical temperature structure for coldwater fish and zooplankton refugia. Layer aeration alters thermal structure, creating several functional thermoclines, while retaining stratification stability. Persistent metalimnetic anoxia and increased eddy diffusional transport across the thermocline, common problems with hypolimnetic aeration, are overcome by layer aeration. A 7-year study was conducted to demonstrate aeration efficiency, ecosystem response, and water supply benefits from layer aeration of a 212 ha (523 acre), 21 m (70 ft) deep, eutrophic water supply lake using two 30 HP compressors. Aerator oxygen input was a function of influent dissolved oxygen (DO) and temperature. Oxygen input efficiencies ranged from 2% to 13% m−1. Redistribution of available DO during layer aeration essentially doubled oxygen input, reaching a maximum of over 99% efficiency compared to the oxygen content of compressed air. Specially designed air-water separators increased oxygen input of both hypolimnetic and layer aerators (by 1–2% m−1). The ecosystem response to layer aeration occurred in several annual stages. Overall summer Secchi disk transparency increased from <1.8 m (<6 ft) to >4.6 m (>15 ft) due to elimination of Cyanobacteria blooms. Descent of compensation depth into the hypolimnion contributed to restoration of 2.5 × 106 m3 (2000 acre-ft) salmonid habitat and an additional 1.6 × 106 m3 (1300 acre-ft) zooplankton refugia. Large-bodied Cladocera (Daphnia sp.) returned as a dominant zooplankton population in 1990. Deep nitrification increased, and the ecosystem respiratory quotient (ERQ) decreased from 1.51 to 1.14, indicating a shift in respiratory processes from anaerobic to aerobic. Water supply benefits included elimination of prechlorination, extended granulated activated carbon (GAC) bed longevity, avoidance of taste and odor episodes, and reduced treatment cost. Aeration system sizing, the ammonia anomaly, and ecosystem energetics are discussed relative to ecosystem responses to aeration.