Environmental Services
Wastewater Treatment Plant Processes
Tertiary/Chemical Addition
Phosphorus Removal: This process has two stages. Each has three 2.2 million gallon reaction clarifiers and can be arranged either in series or in parallel. The current and most often used mode of operation is in series. Ferric Chloride (at a 34 percent concentration) has been used since 1981 to achieve chemical precipitation of phosphorus. Polymer addition also is available to enhance precipitation and settling, but is not regularly used.
Gravity Filtration: Eight rectangular basins with beds of media consisting of gravel, sand, and anthracite coal granules follow the phosphorus removal process. The water filters down through the bed of media by gravity. Additional solids and phosphorus removal is achieved through filtration.
Granular Activated Carbon Adsorption: Eight rectangular basins are currently used in a filtration mode with reduced carbon bed depths to supplement the filtration by the gravity filters. The process is designed so that the carbon units can be used either in series or in parallel with the gravity filters. Plans call for the basins to be upgraded significantly in the future.
See a quicktime movie of the carbon filters being demolished (requires Quicktime software), as part of the upgrade going on at the plant!
Disinfection: A five percent Sodium Hypochlorite solution is used for disinfection. Chlorine residuals are currently maintained at 0.50 mg/l.
Dechlorination: This process was added in 1991. Sodium Bisulfite is used to neutralize any potential chlorine residual left in the wastewater so no chlorine is discharged to Four Mile Run.
Dewatering Processes
A dewatering building, including three new high-solids centrifuges, was built and put in service in early 1998. All dewatered sludge is now used for land application or sent to a landfill instead of being incinerated.
A gravity thickener is used to increase the percentage of solids in the primary sludge and a flotation thickener processes the secondary waste activated sludge. The chemical sludges from tertiary treatment are blended with the primary sludges. The combined thickened sludge is then pumped into one of two storage tanks.
Polymer is mixed with the thickened sludge which is pumped to three high-solids centrifuges. Thickened sludge fed to the centrifuges is dewatered to achieve 23 to 28 percent solids. The dewatered sludge is then held in four storage bins that can hold roughly 60 wet tons each.
Stored dewatered sludge is processed by lime stabilization, using granular hydrated lime to reduce pathogens and odors. Virginia health regulations require a 2 hour minimum biosolids pH of 12.0 and a minimum 22 hour pH minimum of 11.5 for lime stabilized biosolids that are applied to the land. An estimated 100 to 150 wet tons of biosolids are produced each day. Arlington County employees monitor the biosolids to insure pH control is consistent. The biosolids dewatering operation is supported by an extensive chemical wet scrubber odor control system.
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Land Application Process
Arlington County's biosolids are land applied on permitted sites throughout rural Virginia and Maryland. Biosolids are transported and applied at approved farm sites by a contracted service. Arlington County's biosolids have been designated "Class B" which means that the biosolids contain small amounts of trace metals and organic compounds. Class B biosolids are very beneficial to land reclamation and provide valuable nutrients for secondary feed crops. Class B biosolids are not distributed for direct public use.
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| Laboratory Staff |
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| Wastewater Microorganisms |
