Greening A Paradise
There is a little piece of paradise in the foothills of the upper Piedmont area of North Carolina, but most residents just call it Eden. Like many small towns, Eden is prized for its unique attractions—a vintage drive-in movie theater, annual bluegrass concert and rubber duck regatta, to name a few—as well as proximity to big-city conveniences.
Eden, with a population of 19,000, prides itself on embracing up-to-date technologies to ensure that basic services such as water and wastewater treatment are safe and efficient. In the past, the city’s primary wastewater treatment plant (WWTP) was safe, but it was not as efficient as Superintendent Melinda Ward would like due to the high cost of running the system’s grid-powered aerators.
She sought a solution that would reduce operating costs while complying with U.S. Environmental Protection Agency (EPA) requirements—or even improving on them—and found an answer in solar-powered circulation technology. By taking over some of the mixing and oxygenation previously supplied solely by high-horsepower electric aerators, the solar-powered circulation units cut electric use almost in half and provided an 11-month payback.
Activated Sludge System
Activated sludge WWTPs are fairly common throughout North Carolina. The main WWTP serving Eden, the Mebane Bridge WWTP, treats about 3.5 million gal of wastewater per day. The incoming wastewater is made up of 80% municipal wastewater, 10% industrial wastewater and 10% raw water.
In the activated sludge process, a mixture of sewage and activated sludge is agitated and aerated. The activated sludge (“activated” in that the particles are teeming with bacteria and protozoa) is separated from the treated sewage by settling in the clarifiers and is then returned to the reactor basin to re-seed the new sewage entering the tank.
At the Mebane Bridge WWTP, a mechanical bar screen removes larger inert material, followed by a grit removal system. Extensive aeration in the reactor basins reduces and removes biochemical oxygen demand (BOD). Most of the sludge or biosolids are returned to the reactor basins after settling in the clarifiers, while the rest gets transferred to the digester for further processing. Clear water eventually leaves the clarifiers for chlorine disinfection, followed by dechlorination. The treated effluent is returned to the Dan River, meeting all state permit discharge requirements.
The south basin of the Mebane Bridge WWTP, the main reactor basin where aeration is supplied for mixing, is an earthen impoundment measuring 1.74 surface acres with an operating depth of 13.5 ft and an average detention time of approximately four days. The South basin required 12 20-hp aerators for a total of 240-hp of aeration. This amount of aeration kept total suspended solids (TSS) in the 3,000 to 4,000 mg/L range, as required to meet EPA National Pollution Discharge Elimination System (NPDES) requirements.
“We ran brush aerators constantly to try and mix it all, but they could only mix and aerate the top half,” Ward said. “The results were a high, wasteful level of dissolved oxygen [DO] at the top of the basin and a mass of sludge at the bottom that wasn’t impacted by the treatment process.”
DO concentrations measured 8 to 10 mg/L, much higher than needed, yet the aeration could not be turned down because it was needed to suspend the solids.
Ward considered several mixing options, most of which would have added to operating costs. She learned of SolarBee at a regional tradeshow, and after consultation with company engineers, installed a SolarBee SB 10000v18 unit and deactivated the three aerators closest to the solar-powered unit. Over time, Ward deactivated three additional aerators. “We could actually see the solids moving,” she said.
An additional benefit was better sludge settling at the clarifier, leading to reduced effluent TSS.
SolarBee’s long-distance circulation technology creates a near-laminar flow pattern that completely mixes the water column. The unit installed at the Mebane Bridge South reactor basin consists of three floats that provide buoyancy for above-water, near-surface and underwater components. Solar panels, a low-voltage, gearless motor and control box are mounted on the above-water frame. A distribution dish, impeller and battery are suspended from the frame just below the surface. A 3-ft-diameter, flexible intake hose is attached to the frame at the base of the impeller.
The SolarBee unit transports about 10,000 gal per minute (gpm) of water to the surface. Approximately 3,000 gpm of direct flow ascends through the intake hose; another 7,000 gpm of induced flow ascends external to the hose. Water departs from the unit radially without turbulence, both above and below the distribution dish, mixing with other surface currents to redistribute water across the treatment area.
One Year Later
Ward and the Eden wastewater team monitored effluent water quality parameters and electrical use and cost for the first year of solar-powered mixing (June 2009 to May 2010). They compared the results with comparable data from the year immediately preceding (June 2008 through May 2009), when only aeration was used. The goal with the single SolarBee unit in one basin was to meet NPDES standards while reducing electricity costs.
The parameters measured were wastewater influent quantity and effluent TSS, BOD, acidity or alkalinity (pH), DO, ammonia nitrogen (AN), total nitrogen (TN), total phosphorous and fecal coliform.
NPDES limits were:
- TSS mean monthly: 30 mg/L, mean weekly: 45 mg/L;
- BOD mean monthly: 30 mg/L, mean weekly: 45 mg/L;
- pH daily: 6 to 9 su;
- AN mean weekly: 14.8 mg/L; and
- TN mean monthly: 12.7 mg/L.
During the first year of the study, water quality parameters were generally unchanged or improved (see Table 1). The effluent TSS concentration dropped significantly, while good DO levels were maintained.
The results from an efficiency point of view were much more dramatic. As shown in Table 2, according to Ward’s electrical bills, annual electricity use declined by 1,692,000 kWh, or 42%. Annual electricity cost decreased by $61,101, or 31%, as the cost of electricity rose. The cost savings on electricity resulted in a 10.7-month payback period, promising long-term cost savings for the citizens of Eden.
Because of the improvements in processing and operational efficiency provided by a single SolarBee solar-powered circulation unit, Eden now uses three units in the reactor basin, and an additional unit in the digester, to further optimize its system and savings.
“The city is very happy with the energy savings achieved through solar-powered circulation,” Ward said. In addition to energy savings, the city eliminated 1,607,400 lb of CO2 emissions to air. The fact that Eden is also reducing its carbon footprint will help minimize global climate change and make this paradise greener.