Sunday, July 4, 2021

HL Mooney Advanced Waste Water Treatment Plant

 It has been years since I’ve been down to visit the HL Mooney Advanced Waste Water Treatment Plant. This past week I attended a Zoom virtual tour of the operation. Since I was there the HL Mooney AWWTP has completed an expansion and now is licensed to operate at around 24 million gallons a day. Expanding as Prince William Counties’ population grows. In addition, to the HL Mooney plant, Prince William owns a share of the capacity of the UOSA plant in Fairfax. Prince William’s share of UOSA is 19.8 million gallon a day. HL Mooney serves the easter population of Prince William County. The UOSA plant serves the western portion.

From PW Water Academy

Sanitary sewers carry wastewater from homes and businesses to the raw wastewater pumping stations around the eastern portion of the county. From the pump stations, the waste is pumped tot the treatment plant. The wastewater flows by gravity, once it reaches the plant.

Solids go to the landfill

Bar Screens with three eights of an inch holes let water pass, but not trash (such as rags, diapers, wood, tires and other junk.). The trash is collected and properly disposed of. The screened wastewater is pumped to the Grit Chambers that are the primary settling basins.

The Grit Chambers or settling basins slow down the flow to allow smaller particles like coffee grinds and dirt to settle from wastewater by gravity. After the waste water has been screened and allowed grit to settle out, the primary wastewater flows onto the next stage of treatment. The next step is the equalization basins which aerate and control the flow. In the primary clarifiers settling is chemically enhanced. Scrapers collect the solid matter that remains (called "primary sludge"). A surface skimmer collects scum or grease floating on top of the basins.

The next step is the Aeration Basins supply large amounts of air to the mixture of primary wastewater and helpful bacteria and the other microorganisms that consume the harmful organic matter. The growth of the helpful microorganisms is sped up by vigorous mixing of air (aeration) with the concentrated microorganisms (activated sludge) and the wastewater. Adequate oxygen is supplied to support the biological process at a very active level. The ratio of food (organic matter) to organisms to oxygen is continually monitored and adjusted to meet daily variations in the wastewater. There are five parallel basins.

The secondary clarifiers allow the clumps of biological mass (the microorganisms) to settle from the water by gravity. 90-95 % of this mixture, called "activated sludge," is returned to the aeration basins to help maintain the needed amount of microorganisms. The waste stream passes through a series of filters where methanol and a carbon source are used to remove nitrogen.

The final step in the treatment is ultraviolet disinfection (which replaced chlorine disinfection). After disinfection, the water passes through a step aerator which looks like a set of cement steps and is discharged to Neabsco Creek, a Potomac River tributary. Currently, the water released to the creek contains 1 mg/l Total Suspended Solids (TSS), is non detect for Biochemical oxygen demand (BOD), 0.1 mg/l phosphorus, and is currently 3 mg/l of nitrogen according to the permit. The water released meets the National Pollutant Discharge Elimination System (NPDES) criteria. The final effluent is monitored daily.

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