From Flush to Four Mile Run: Inside Arlington’s Water Pollution Control Plant

Text and images by Laura Gilmore, except as noted.

Early each morning, millions of gallons of wastewater begin an unseen journey beneath Arlington County. Every sink drain, shower, and toilet output of household wastewater eventually converges at one place: the Arlington Water Pollution Control Plant (WPCP). During a recent tour, ARMN members had the opportunity to see how this 24/7 facility quietly protects our waterways, and why our everyday choices at home matter more than some people might realize.

Located in south Arlington near Four Mile Run, the plant treats wastewater from homes and businesses across Arlington and neighboring communities. On an average day, it processes 21-23 million gallons of wastewater, serving approximately 250,000 people in Arlington while nearly 20% of the plant’s flow comes from the waterways in neighboring localities of Alexandria, Fairfax and Falls Church. 

After treatment, the water is discharged into Four Mile Run, which flows into the Potomac River and ultimately the Chesapeake Bay watershed. Protecting these downstream ecosystems is the plant’s core mission. The tour highlighted how the plant combines engineering, biology, and chemistry to transform wastewater into water that can safely be returned to the environment.

Wastewater, or influent, is either pumped or flows by gravity to the WPCP. Large bar screens and grit removal systems capture items that should never have entered the sewer system in the first place, such as rags, plastics, wipes, coffee grounds, eggshells, and small rocks. These materials are removed before they can damage equipment or clog pipes.

Next, wastewater moves slowly through primary settling tanks. Here, heavy solids sink to the bottom, forming primary sludge while oils, grease, and scum float to the surface. These materials are removed for further processing through a gravity thickener for dewatering while the partially clarified water continues through the system. This step alone removes a large portion of the suspended solids in wastewater.

In aeration tanks, the real stars of the process take over: microorganisms. Wastewater is combined with activated sludge, a community of bacteria and other microbes that feed on organic pollution. Air is pumped into the tanks to support these microbes as they consume organic material and nutrients.

The mixture then moves into circular secondary clarifiers where the microbes settle out. Some of this microbial sludge is recycled back into the system to keep the treatment process going.

Because excess nutrients can harm aquatic ecosystems, the plant removes nitrogen and phosphorus using additional biological and chemical steps. Methanol is added to encourage specialized bacteria to convert nitrogen compounds into nitrogen gas, which safely returns to the atmosphere. This nutrient removal is especially important for protecting the Chesapeake Bay, where excess nitrogen contributes to harmful algal blooms and low-oxygen dead zones. Arlington’s facility has historically achieved nitrogen discharge levels well below regulatory limits, demonstrating how effective modern wastewater treatment can be.

Before discharge, the treated water passes through filtration and final polishing steps to remove any remaining particles or nutrients. A 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 as effluent through the plant’s outfall into Four Mile Run. 

Wastewater treatment does not just produce clean water; it also generates biosolids from the material removed during treatment. The WPCP produces about 100 wet tons of “Class B” biosolids each day, which are land-applied on permitted sites throughout rural Virginia. Arlington County is currently planning upgrades through its Re-Gen program, which aims to modernize solids treatment and potentially produce higher grade  “Class A” biosolids, a safer product that can be reused as a nutrient-rich soil amendment used for agriculture, gardens, forests, and landscaping. This shift reflects a broader trend in wastewater treatment: recycling waste into valuable resources.

However, even the most advanced treatment plant works best when the community supports it.

Here are a few simple ways residents can help:

• Don’t flush wipes since “flushable” wipes do not break down like toilet paper and are one of the most common causes of clogs and equipment problems in wastewater systems (including the recent Potomac Interceptor spill).
• Keep fats, oils, and grease out of drains as grease hardens in pipes and contributes to sewer blockages.
• Dispose of chemicals properly since household chemicals, medications, and hazardous waste should never go down the drain.
• Reduce nutrient pollution by using fertilizers responsibly and managing yard runoff to protect waterways downstream.

These small actions reduce strain on treatment systems and protect the streams and rivers that many of us work to conserve.

For many residents, wastewater treatment remains largely invisible as out of sight is often out of mind. The Arlington Water Pollution Control Plant, however, is a critical link connecting our daily lives with the health of the Potomac River and Chesapeake Bay.

The tour helped remind us that watershed stewardship does not start at the stream; it starts at the sink.

This educational opportunity helped the Arlington Regional Master Naturalist Chapter better understand our Wonderful Water resources in Virginia (the 2026 service theme for Virginia Master Naturalists). #WOW