Monday, October 25, 2010

HL Mooney Advanced Waste Water Treatment Plant


I went down to the HL Mooney Advanced Waste Water Treatment Plant, AWWTP, in Woodbridge, VA to talk with Glenn B. Harvey, the plant Process Engineer. HL Mooney AWWTP operates at around 13 million gallons of waste water a day. The plant was built originally in 1979, expanded in the 1990’s and is being expanded to 23 million gallons a day today. Expanding as Prince William Counties’ population grows. The plant capacity is being expanded from 18 million gallons of waste water a day to 24 million gallons of waste water a day capacity, in addition, the effectiveness of the nitrogen removal is going from 8 parts per million to 3 parts per million in this plant upgrade. The cost of this project is $120 million dollars paid for by the Service Authority water and sewage fees. HL Mooney serves around 130,000-150,000 people a day which represents about 40% of the current population of Prince William County.

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.

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 ont to 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 and is discharged to the 1,700-acre Occoquan Reservoir, which serves as the boundary between Fairfax and Prince William counties. Currently, the water released to the Occoquan contains 1 mg/l Total Suspended Solids (TSS), is non detect for Biochemical oxygen demand (BOD), 0.1 mg/l phosphorus, and is currently 5 mg/l of nitrogen, but will be 3 mg/l for nitrogen after the construction project is complete. The water released meets the National Pollutant Discharge Elimination System (NPDES) criteria. The final effluent is monitored daily.


After talking to Mr. Harvey, I was amazed by how similar the operation of my little alternative on-site sewage system is to the basic steps in the AWWTP operation.

1 comment:

  1. I like this blog it’s very informative and attractive also. Thanks to author for this post it’s very easy to understand. Sewage treatment plant and equipment

    ReplyDelete