Wastewater Management

Annual report on collecting, treating and recycling wastewater and biosolids, July 2015 - June 2016 (printed version) (expanded on-line version)  

What are microbeads and how can they affect water quality in rivers, lakes, etc.?

WUNC-TV's two-part series on water and wastewater featuring OWASA (May 2015)

OWASA operates a system of pipes and pumping stations to collect the community's wastewater (domestic sewage) for treatment at the Mason Farm Wastewater Treatment Plant, which is near the Finley Golf Course in Chapel Hill. We operate about 324 miles of wastewater collection piping that has been constructed with a downhill slope to allow the wastewater to flow by gravity. Where collection pipes encounter a hill or become very deep, a pumping station is needed to pump the wastewater uphill to a point where it can flow again by gravity. We operate 21 pumping stations  and we have 14 miles of pressurized sewer pipes that carry wastewater from the pump stations.

The collected wastewater enters the Mason Farm Wastewater Treatment Plant, which uses physical, chemical and biological processes to treat the wastewater to a very high level before releasing it to Morgan Creek or for use as Reclaimed Water for non-drinking purposes. The wastewater treatment plant can treat a peak month average flow of 14.5 million gallons of wastewater per day (MGD). The  flow to the wastewater treatment plant averages about 8 MGD.

Wastewater Management

Wastewater enters the plant at our headworks facility, where large debris and grit (sand) are removed. The wastewater then enters the primary clarification process where heavier solids fall to the bottom of a settling tank and the cleaner wastewater leaves at the top of the tank.

The next step in the treatment process is called secondary treatment where the wastewater flows into large aeration basins where it is mixed with air to support the growth of microorganisms that naturally live in wastewater. The microorganisms clean the wastewater by consuming the suspended and dissolved organic waste and other contaminants thereby reducing the amount of pollutants and nutrients (nitrogen and phosphorous). Chemicals are added to optimize this treatment process. The wastewater remains in the aeration basins for about three to four hours to allow time for the microorganisms to do their job. This is the most important step in the process to make the dirty water clean.

The wastewater then enters the secondary clarification process where the microorganisms clump together and settle to the bottom of the tank and the treated wastewater leaves at the top of the tank. The microorganisms removed from the bottom of the tank are recycled back to the aeration basins to begin their work all over again to treat the incoming wastewater.

Operator checking aeration basins

The treated wastewater leaving the secondary clarification process then flows through sand filters where most of the remaining suspended particles are removed. These filters can also remove additional nitrogen from the wastewater. The filtered water is disinfected with ultraviolet light prior to being released to Morgan Creek, a tributary of Jordan Lake. A portion of the highly treated wastewater is recycled as Reclaimed Water (RCW).

The solids removed from the primary and secondary clarification processes are thickened and pumped to large anaerobic digesters in which microorganisms break down biodegradable material in the absence of free oxygen under high temperatures for at least 30 days. The treated solids are referred to as biosolids which are beneficially reused as a low grade fertilizer and soil amendment. Please see our information about our Biosolids Recycling Program.

Please see information about our Odor Elimination Program. To report an odor event from our Mason Farm Wastewater Treatment Plant, please call 919-537-4376 at any time.

For information about our wastewater and biosolids programs in the fiscal year from July 2015 through June 2016, please see our Annual Report including the results from laboratory testing of our treated wastewater and biosolids. For a more detailed version of the report including data from testing the quality of our treated wastewater, please click here.