Environmental Services

Wastewater Treatment Plant Processes



The plant uses six wastewater treatment and solids handling systems:


Incoming wastewater from four interceptors is mechanically screened and degritted in one facility. Grit, scum and compacted screenings are hauled directly to a municipal solid waste incinerator.


After Preliminary treatment, the wastewater flows into Primary Settling tanks. The flow velocity is slowed to approximately one foot per minute,which allows the heavier solids to settle to the bottom of the tank and the grease and scum to collect on the surface. The heavier solids that settle to the bottom are called primary sludge. The primary sludge is pumped to the gravity thickener for more dewatering. The grease is concentrated and then blended with the grit and screenings from preliminary treatment.


The secondary treatment system consists of four parallel, 2.5 million gallon, four-pass aeration basins configured as a Biological Nutrient Removal (BNR) process. This system removes nitrogen to meet the new limits set by the state in the plant’s operating permit. The aeration tanks use fine bubble diffusers in a series of aerated zones followed by anoxic zones. Two additional BNR tanks are being added to provide the capacity needed to meet the permit.

activated sludge aeration tanksactivated sludge aeration tanks
Activated sludge aeration tanks

Six center-feed circular clarifiers follow this treatment. The waste sludge is withdrawn and is concentrated in a dissolved air flotation thickener before it is dewatered. Three more clarifiers are being constructed to handle the additional capacity anticipated during the next twenty years and to accommodate high flows during wet weather events.

secondary clarifiersecondary clarifier distribution chambers
Secondary ClarifierSecondary Clarifier Distribution Chambers
activated sludge blower systemactivated sludge blower system
Activated sludge blower system

Last Modified: October 22, 2013
2100 Clarendon Blvd. Arlington, VA 22201 Tel: 703-228-3000 TTY: 703-228-4611