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 adjacent waterway. Currently, the water released  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.

Implications of Complying with the TMDL for Virginia

The Chesapeake Bay Commission was created in 1980 to coordinate Bay-related policy across state lines and to develop shared solutions. The six Bay watershed states are Virginia, Maryland, Pennsylvania, West Virginia, New York, and Delaware and the District of Columbia and are all parties to the Chesapeake Bay Agreement. Back in May 2010 the EPA announced that it will mandate that the six states and the District of Columbia who are parties to the Chesapeake Bay Agreement limit their nitrogen, phosphorus and sediment flow into the bay in compliance with an overall daily maximum, TMDL, allocated by the EPA. The six Bay watershed states (Virginia, Maryland, Pennsylvania, West Virginia, New York, and Delaware) and the District of Columbia have all developed Watershed Implementation Plans, WIPs, to meet these targets.

The EPA found that none of the states met the reasonable assurance standard they set, but that Virginia’s WIP had a “moderate need” for federal backstops (the WIPs from Maryland and Washington, DC were found to need only minor backstops). Virginia’s WIP was found to have serious deficiencies. It did not meet allocations for nitrogen (6 percent over) and phosphorus (7 percent over), but did meet allocations for sediment (12 percent under). EPA will invoke federal authority where they can to ensure the mandated reduction in nutrient pollution to the Chesapeake Bay. Though EPA states that they prefer the individual states develop their own approach to develop a final WIP with an acceptable level of assurance, so that EPA will not have to invoke federal actions. The states have more flexibility and can achieve reductions from a wider range of sources than EPA can. Dozens of small measures will allow the state to achieve the TMDL goals. The problem is creating the regulatory network to control the non-point source nutrient and sediment contamination.

If the states’ WIPs do not meet the EPA standard of reasonable assurance, the EPA’s only possible response is to further control the point source contamination it can regulate. This means that more stringent waste load allocations will be applied to waste water treatment plants (regulated via federal programs). The waste stream from wastewater treatment plants was assigned to be 4 mg/L total nitrogen and 0.3 mg/L total phosphorus. For municipal separate storm sewer systems, MS4s, the federal government has imposed the requirement that 50% of urban MS4 lands meet aggressive performance standard through retrofit/ redevelopment; 50% of unregulated land will be treated as regulated, so that 25% of unregulated land meets aggressive performance standard.

At the public hearing held in Annandale, Virginia, Anthony Moore, Assistant Secretary for Chesapeake Bay Restoration in Virginia stated that the reduced levels of discharge of total nitrogen would not be set so low as to cause algae blooms in the Occoquan Reservoir. This statement left me confused since it is the nitrogen and phosphorus contamination which cases the algae blooms. I went down to the HL Mooney Waste Water Treatment Plant in Woodbridge, VA to talk about the TMDL and algae blooms with Glenn B. Harvey, the plant Process Engineer. He provided me with additional insight into the very complicated balancing act necessary to restore the Chesapeake Bay.

HL Mooney is an AWWTP operating 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. HL Mooney serves around 130,000-150,000 people a day which represents about 40% of the population of Prince William County. Mr. Harvey explained to me that the sediment of the Occoquan contains amongst other contaminants nitrogen and phosphorus. He stated that when the nitrogen in the Occoquan waters falls too low, anaerobic nutrients are released from the sediment. According to Mr. Harvey, the nitrogen keeps it anoxic and not anaerobic. Unless, the entire Occoquan were dredged, which would make the water undrinkable for an extended period of time, release of nutrients from the sediment would have to be considered in any point sourc reduction.

HL Mooney is only a fraction (about 7%) of the size of Blue Plaines AWTTP. The monthly discharge averages are less than 0.1 for phosphorus, TSS 1 mg/l, BOD non- detect and nitrogen is currently 5 mg/l but according to Mr. Harvey will be 3 mg/l when the expansion is completed. He pointed out that the average monthly TMDL is about 50% more lenient a measure than the weekly measure that EPA is mandating. Mr. Harvey worried that the cost of the technology to achieve the weekly TMDL limit might be prohibitive. The more I examine the issure the more complicated it becomes. Weighing raising sewage and water rates on public systems to pay for the technology to meet the TMDL goal for the state and implementation and enforcement costs to have the Commonwealth regulate the non-point sources of contamination.