The Orange Water and Sewage Authority: Energy Efficient Aeration and Mixing for Mason Farm Wastewater Treatment Plant


The Orange Water and Sewage Authority (OWASA) is a publicly-held water treatment agency serving the Carrboro – Chapel Hill area in North Carolina. The OWASA Mason Farm Wastewater Treatment Plant in Chapel Hill is nearly 86,000 square feet and services about 83,000 residents. The plant produces reclaimed water for the University of North Carolina at Chapel Hill and contributes to cooling tower make-up water, toilet flushing, and athletic field irrigation.


The Mason Farm facility is an activated sludge wastewater treatment facility that includes biological nutrient removal, deep-bed filtration, ultra-violet light disinfection, and anaerobic digestion. Solids are treated to Class A (safe for direct contact) via anaerobic digestion. The facility is equipped to use the biogas created by the anaerobic digestion process as fuel for their boilers that provide process heat for the digesters. OWASA’s biosolids are recycled beneficially either to local farmland in liquid form, or in dewatered form for composting at a privately-owned facility. Mason Farm produced about 1,150 dry tons of biosolids in FY 2015.

Over the course of 2013 and 2014, OWASA implemented a major renovation of the aeration and mixing systems at the Mason Farm plant, which accounted for an estimated 30 percent of the site's electricity consumption. Many of OWASA’s treatment systems (aeration and mixing) had been in place since the late 1970s. The agency had difficulty maintaining required dissolved oxygen (DO) levels, was experiencing higher energy consumption, and had odor control issues. Working on a tight timeline to meet a December 31, 2014 odor control requirement deadline, OWASA upgraded the aeration, mixing, and odor control systems using an innovative funding mechanism that included a combination of zero-interest loans and custom utility incentives. The project was completed in November 2014, and yielded strong energy savings.



To implement the project successfully and minimize any service disruptions, OWASA upgraded a few cells at a time and moved sequentially until the entire aeration system was replaced. The improvements to the mixing and aeration system include:

  • Four single-stage, integrally-geared 250-horsepower blowers, which can supply air at higher pressures, while using less energy. These replaced five existing blowers that were at the end of their useful lives.
  • Installation of 24 hyperboloid, platform-mounted mixers, replacing the existing maintenance-intensive jet mixers.
  • A fine-bubble diffused aeration system with stainless steel air header pipe, enabling improved oxygen transfer and treatment efficiency.
  • Incorporation of advanced sensors into the agency’s Supervisory Control and Automated Data Acquisition (SCADA) system to provide real-time energy consumption information.
  • Odor control improvements, including covers on the 10 aeration cells and new odor scrubbers.

The project reduced energy intensity from 4.89 kWh per 1000 gallons to 2.79 kWh per 1000 gallons, a 43 percent reduction. This translated into annual energy savings of 5.5 million kWh. With energy cost savings of $342,000, the project yielded a 23-year simple payback. To reduce the cost burden on customers, OWASA received a 20-year no-interest, $6.56 million loan from the North Carolina Clean Water State Revolving Fund. Duke Energy also provided $168,000 in energy efficiency incentives, which served to mitigate the project’s costs.

In addition, the new sensors in the SCADA system enabled better control of the plant’s energy consumption and process efficiency. The information provided by the sensors is used to alert plant operators of energy consumption spikes so they can shed loads during on-peak periods and avoid demand charges.



In addition to saving energy and reducing demand, the newly-configured aeration and mixing systems enable improved wastewater treatment process performance and greater operational control over the biological treatment process. Because of the odor control improvements, OWASA was able to meet the permit requirements from the nearby community. Also, the plant was able to maintain appropriate DO levels in the aeration basin, which prevents excess energy consumption to oxygenate the aeration process.


Annual Energy Use

Baseline (2012)
47,426 MMBTU
Expected (2015)
28,506 MMBTU

Energy Savings:


Annual Energy Cost

Baseline (2012)
Expected (2015)

Cost Savings:


Sector Type



Carrboro, North Carolina

Project Size

85,810 Square Feet

Financial Overview


Mason Farm Wastewater Plant
Mason Farm Wastewater Plant