A Swampy Situation

July 22, 2011
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Greenfield Lake, an idyllic cypress swamp surrounded by walking paths and bridges, is a popular recreation area in Wilmington, N.C. Residents and visitors enjoy picnicking, gardens, an amphitheater, boating and fishing.

But when blue-green algae took over the swamp, the water turned to pea soup and weeds grew thickly along the shore. The shallow, 100-acre reservoir is primarily fed by stormwater via several drainage pipes throughout the basin. Boaters preferred to go elsewhere.

To clean up the water and bring back boaters and visitors, city officials installed four SolarBee SB10000v12 units that provide long-distance, near-laminar flow. The solar-powered, floating circulators operate day and night with a low-voltage motor and on-board battery system for continuous, energy-efficient water quality improvement.

Improving Water Quality

“Pea soup” water is typical of stormwater ponds, reservoirs and lakes that are susceptible to unhealthy and harmful algae blooms (HABs). These large-celled blue-green algae can predominate in warm, stagnant, nutrient-rich water. An environmentally preferable alternative to chemical treatment, long-distance mixing creates enough horizontal and vertical water movement to sufficiently disrupt large-celled blue-green algae and instead allow the “good” small-celled algae to predominate.

The goal of a horizontal circulator is not to add oxygen to a reservoir. That is the work of green algae. The purpose of a horizontal mixer is to help the good algae survive. When the good algae survive, the levels of algae, zooplankton, fish, dissolved oxygen and pH will stay in balance.

At Greenfield Lake, SolarBee circulators draw in water from all directions, pushing it upward and sending it out across the surface in a thin layer. Good green algae and diatoms are pulled up from below, lifted to the top and sent out across the surface, where they receive sunlight and are available to zooplankton. Gentle mixing and surface renewal, which keep green algae suspended where they can receive sunlight, allowed Greenfield Lake to return to a healthy state.

Recreation Returns

Within two months of installing SolarBee circulators, Wilmington city officials reported positive comments from visitors: the lake looked better than it had in recent history. Fishers returned and canoe rentals doubled. Greenfield Lake is now a healthy reservoir that has won the battle against blue-green algae. Even alligators appear to appreciate the improvement, if not for the water quality then for a chance to soak up some rays on the SolarBee units.

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