Solar-powered aerator reduces energy drain on North Dakota wastewater lagoon

Aug. 14, 2002
A three year study completed in May shows that all 16,000 residents of Dickinson, ND, have good reason to smile about the Public Works Department's new solar powered pond circulators.

Aug. 14, 2002 -- A three year study completed in May shows that all 16,000 residents of Dickinson, ND, have good reason to smile about the Public Works Department's new solar powered pond circulators.

In 1998, the city was facing some tough issues regarding wastewater treatment. Its 133-acre lagoon system was the town's biggest energy consumer and required constant "hands-on" attention. Even with 500 horsepower of aeration and pumping equipment, it could barely keep up to the city's needs due to strict ammonia limits for discharging water to the Heart River.

Of the city's 1.4 million gallons per day of wastewater, only 12% could be cleaned up well enough to be discharged to the river. So most of the water was disposed of by irrigation. Since there was virtually no excess capacity, if an irrigation system went down the city had to scramble quickly to get it going again.

And at one point a key irrigation system started to erode a river bank, with the potential to cause serious new environmental problems.

Consequently, the city was forced to start looking at options, including construction of a new multi-million dollar mechanical treatment plant. Mechanical plants have much higher ongoing operating costs than lagoons, not only due to energy cost but also for fulltime employees, equipment maintenance, and chemicals. A mechanical plant would have meant large sewer rate increases for their customers.

By 2001 and after installation of some solar technology, the city's lagoon system met all water quality standards for discharging all of the water it needed to into the river. Irrigation was all but eliminated and the system energy requirement had dropped from 500 horsepower to only 130 horsepower, enough energy savings to supply about 240 homes with electricity year-round.

In addition, the lagoon system now requires very little management or maintenance effort, and has plenty of capacity to meet the city's needs far into the future.

"The sun produces about 5,000 horsepower of 'free' energy per surface acre," commented Joel Bleth, President of Pump Systems Inc., the manufacturer of the "SolarBee" solar-powered pond circulators installed by the city. "The SolarBee captures more of that energy to clean up the water than grid-powered aerators do."

According to Bleth, aerobic bacteria in the water need adequate oxygen just like fish and humans, as they consume the sewage and break it down. But while grid-powered aerators produce oxygen, they also produce turbulence as the high horsepower motors force air bubbles into the water and then mix them throughout the lagoon. The turbulence impedes many natural processes such as algae growth, photosynthesis and sludge digestion by anaerobic bacteria at the bottom of the lagoon.

In front-end lagoons, which receive the highest ammonia and other nutrients from the raw sewage, the SolarBees provide gentle but thorough mixing which allows algae to produce high volumes of pure dissolved oxygen for the bacteria to use.

The algae production also raises the pH, which allows ammonia to be gassed out of the water and causes other nutrients to be transformed into harmless chemical compounds that settle to the bottom of the lagoon.

Therefore low levels of nutrients are passed on to the next lagoon. When solar aeration systems are used in several lagoons in series, the result is that the water quality in the final lagoon is high enough to discharge to a river, having very little algae, ammonia, or other nutrients, and a normal pH.

The SolarBees used by the City of Dickinson are 16 ft. in diameter by 4 feet tall and are constructed primarily of stainless steel. They float mostly submerged in the lagoon, with concrete mooring blocks and stainless steel anchor chain to accommodate changing water depths.

Each machine has three 55 watt solar modules that convert sunlight to electricity, powering an electric motor that turns a large pump impeller that brings water up from low in the lagoon and spreads it across the surface.

The larger SolarBee models produce flow rates up to 10,000 gallons per minute. According to a May 2002, report called "Watergy" by the international organization Alliance to Save Energy (ase.org), co-chaired by ND Senator Byron Dorgan, 7% of total world energy and 3% of total US energy is used to process water and wastewater.

Since demand for both water and energy are expected to grow quickly in the years ahead, large energy savings are possible in this field. ASE coined the word "watergy" to demonstrate the linkage between these two large problems.

Bleth feels the SolarBee can be an important part of the solution, not only in wastewater but also in fresh water lakes where harmful algae blooms can cause fish kills and, in the case of drinking water reservoirs, problems of taste, odor, and excessive treatment costs.

So far most of the 250 SolarBees manufactured since 1998 have been placed in wastewater systems, but several of them have been placed in fresh water lakes and, so far, appear to have eliminated the harmful algae blooms. More tests on fresh water lakes are being conducted in 2002.

Conventional rooftop solar modules often have a long payback time, 20 years or more, But the SolarBee payback time is much shorter, often only 1-3 years. The City of Dickinson purchased sixteen SolarBees between 1998 and 2000, and will purchase four more in 2002 to eliminate the need for the last 130 hp of grid energy in their system.

The city will have less than $500,000 total invested in their SolarBees, but the entire investment has already paid for itself in energy costs, labor, and avoided costs.

The SolarBees have an expected 25-year life. For more information, including several charts with BOD figures, visit www.solarbee.com and click on "WW Case Study (PDF)" on the left side of the home page.

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