Solids Handling

Solids production at the Mandan Wastewater Treatment Plant is waste activated sludge (WAS) from the treatment process. The mixed liquor from the aeration basin flows to the final clarifier where gravity thickening occurs. Settled solids are either returned (RAS) to the anaerobic selector tank to aid in biological treatment or wasted (WAS) to the facultative sludge basin (FSB).

The Wastewater Treatment Plant currently produces about 2,800 pounds per day of waste activated sludge. The extended sludge age (20 to 40 days) of the aerated Biolac system provides stabilization of the biosolids. In the FSB through aerobic/anaerobic stabilization, biosolids are further treated. The FSB was designed to store 5.34 million gallons of biosolids.

In 2009, construction of a biosolids load-out facility and the purchase of semi-tankers, a field applicator, and a dredge have allowed the city to dispose of biosolids on an annual basis. Stabilized biosolids are dredged from the FSB, deposited into a 6,000-gallon semi-tanker, transported to land application sites and injected through the use of a tractor and a 6,500-gallon field applicator equipped with subsurface knife injectors and coulter wheels. Based on the facility design, Mandan will generally land apply about 500 dry tons of biosolids per year.

The city of Mandan operates in compliance with a permit issued by Region 8 of the United States Environmental Protection Agency (USEPA) that authorizes the disposal of sewage sludge by means of land application, landfill and surface disposal under the National Pollutant Discharge Elimination System (NPDES).