Learn more about the Water Pollution Control Plant's Solids Master Plan.
Arlington County, the State Leader in Limiting Nitrogen Discharge Into County Waters
We've been doing amazing things to help protect and preserve your environment. And much of the work has occurred since we instituted Master Plan 2001. This comprehensive plan was our attempt to protect existing high-quality waters and reduce external bypasses (partially treated water that is discharged into County waters), with an overarching goal of supporting the Chesapeake Bay Preservation Ordinance, a statewide program to help restore the bay to health, first adopted by the County in 1992.
Why Limiting Nitrogen and Phosphorous Is Important
Excess nutrients like nitrogen and phosphorous allow algae blooms to occur. When the algae dies, it causes oxygen depletion from streams, rivers and the bay, resulting in dead zones, reduced life, and a reduction in the economic and recreational value of the bay and surrounding areas.
What We Did
With the Master Plan 2001, the County Board authorized up to $568 million to help protect, restore and safeguard state waters. In 2003, we pursued limit-of-technology standards concerning nutrient removal set by the Virginia Department of Environmental Quality (DEQ). And Arlington County has been more than up to the challenge for years now.
For 2012, the County registered an average of .79 milligrams per liter of nitrogen discharge into area waters. How impressive is this number? Well, current parameters set forth by DEQ call for a limit of 3 milligrams per liter of nitrogen discharge. In fact, in December 2012, Arlington achieved an incredible .25 milligrams nitrogen discharge rating. How low is that? No other state wastewater treatment plant has recorded anything close to that finding. Yes, you might’ve guessed that Arlington is ranked first in the state for milligram-per-liter nitrogen discharge. And the Master Plan 2001? It was completed under budget and approximately one and a half years ahead of schedule.
Your tax dollars are at work helping to save the mighty Chesapeake Bay: Residents should be proud of this fact.
Wastewater Treatment Plant Processes
The Water Pollution Control Plant uses the five wastewater treatment and solids handling systems listed below.
1. Preliminary Treatment
Wastewater reaches the plant via four interceptors. Bar screens and grit cyclones remove large objects and coarse solids from the water, including rags, plastics, coffee grounds, eggshells, grit, rocks, etc. These materials, along with grease and scum, are hauled directly to a municipal solid waste incinerator.
2. Primary Treatment
Next, the wastewater flows into primary settling tanks at approximately 1-foot-per-second, a slow speed that allows heavier solids to settle to the bottom of the tank while grease and scum collect on the surface. Solids that settle on the bottom, called primary sludge, are pumped into a gravity thickener for a dewatering process. Grease and scum are blended with grit and screenings from preliminary treatment.
To properly manage daily flow fluctuations and wet weather flow surges, the plant temporarily stores wastewater in equalization tanks, where it’s kept until it can be reintroduced into the wastewater treatment system.
3. Secondary Treatment
This is a two-step process. Chemicals (ferric chloride) are added to partially remove phosphorous. Wastewater is then combined with return activated sludge — the “bugs” — and travels through up to six parallel, 2.5 million gallon, four-pass aeration tanks, configured for biological nutrient removal.
The next step consists of processing the wastewater through circular secondary clarifiers, where the “bugs," which are slightly heavier than water, settle as activated sludge. Much of the activated sludge is returned to the aeration tanks while the excess is sent to a dissolved air flotation thickener, and then dewatered.
4. Tertiary Treatment/Chemical Addition
From the secondary clarifiers, wastewater goes through a process designed to remove nitrogen and the remaining phosphorus. Methanol is added to the water, which then passes through a sand filter where “bugs” grow. The " bugs" then convert the remaining nitrogen oxides to nitrogen gas.
A 5 percent sodium hypochlorite solution disinfects the water, then another chemical, sodium bisulfite, neutralizes the residual chlorine. Finally, air is diffused into the water to add oxygen so that fish and other life forms can survive in it. This makes the water safe for release through the plant’s outfall into Four Mile Run.
5. Treatment of Solids
Throughout the treatment process, solids that are removed from wastewater must be disposed of in a safe and environmentally friendly manner. The plant ships screenings and scum from the primary treatment process to a solid waste incinerator. The final treatment for sludge includes collecting, thickening and dewatering before disposal. Dewatered sludge then processes through lime stabilization, to reduce pathogens and odors.
The facility produces about 100 wet tons of biosolids each day. Arlington’s biosolids are land-applied on permitted sites throughout rural Virginia. This is one of the largest recycling programs in the County and minimizes the generation of greenhouse gases. The plant’s extensive chemical wet scrubber odor control system removes the bulk of odors that the various primary, dewatering, lime stabilization and truck-loading processes generate.