Sunny Solution

Oct. 7, 2008

About the author: Chris Knud-Hansen, Ph.D., CLM, is senior limnologist for SolarBee, Inc. Knud-Hansen can be reached at 866.469.9606 or by e-mail at [email protected].

Energy is a major ongoing cost in the operation of aeration and mixing equipment (e.g., surface aerators and diffuser systems) and other mixing devices used to treat industrial and municipal wastewater lagoons, sludge storage ponds and water reuse storage ponds. As energy costs continue to rise, so will the operational costs.

Solar-Powered Water Circulators

One way to significantly reduce energy costs in treatment and storage ponds is to improve mixing efficiency while incorporating solar energy as the primary power supply. Since 1998, solar-powered water circulators have displaced aeration and mixing equipment in more than 150 wastewater treatment systems nationwide.

These machines, called SolarBees, are upflow pumps capable of moving up to 10,000 gal per minute, or 14.4 million gal per day (mgd), while consuming 36 W to power a ½-hp brushless, direct-drive, direct-current motor that is 90% or more efficient. There are three 80-W photovoltaic panels that charge an on-board battery, thus enabling the units to run day and night totally on solar power.

A specially designed distribution dish allows water pumped to the surface to flow long-distance horizontally away from the machine in a near-laminar flow. Water moves radially out to the pond’s margins before circulating downward to the depth of the intake hose and back to the machine. There is a bottom plate beneath the intake hose that sets the floor of the circulation pattern, so water moves back to the machine horizontally, not vertically.

These circulators have variable intake hose depths that can be adjusted to maximize specific application benefits. For water column mixing, the intake tube can be set at any depth depending on how deep the mixing needs to be for specific treatment objectives. For example, the intake can be as little as 2 ft for controlling noxious odor without releasing aerosols to the atmosphere.

Because of the energy-efficient near-laminar flow, each circulator can affect a large area of influence. Depending on the loading, one machine can be effective in ponds from 1 to 20 acres in surface area. One unit typically displaces 20 to 40 hp of grid-powered mixing energy, providing energy savings while maintaining or improving treatment capabilities through circulation. The unit requires 36 W but is capable of displacing about 25,000 W (30 hp of aeration) on average.

WWTP Energy Savings

The parts of a wastewater treatment pond system where solar-powered circulation has been beneficial are primary, partial mix ponds, total mix ponds, facultative ponds and sludge storage ponds. Partial and total mix ponds use intense aeration for both mixing and adding oxygen to the water column. Often, the energy required for mixing is several times beyond that needed for adding oxygen, and substantial economic and grid-based energy savings have been realized by using solar-powered mixing.

For example, in one California 0.8-mgd wastewater treatment plant (WTTP), energy savings totaled $37,000 per year and received a California Energy Commission Grant by removing 60 hp from peak load. These results are similar to those of many other WWTPs that have saved up to $30,000 or more per year per machine by switching to solar-powered circulation.

In facultative ponds with solar-powered circulation, the organic loading rate can increase three- to five-fold, thereby avoiding the need for aeration or the expense of building a new capital and operating an intensive treatment plant. Realized benefits in sludge storage ponds include odor control without the need for brush aerators. Payback for the circulators—based on economic savings from reduced grid energy use—has often been less than two years.

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About the Author

Chris Knud-Hansen

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