Up until 1956, Alexandria 's wastewater and untreated sewage went straight into the Potomac, its streams and tributaries, and eventually the Chesapeake Bay. But today, the Alexandria Sanitation Authority treats millions of gallons of wastewater 24/7. Here's how it works...

First, the Preliminary/Primary Treatment separates the liquid from the solids.

• When used water arrives at ASA's water resource facility, it is run through a series of screens. The first one removes the big stuff - from tennis balls to garden mulch.

• After more filtering, the wastewater is spun in a centrifuge and is sent to eight “settling” tanks. Gravity pulls the heavier solids to the bottom where they're removed. Lighter oils and grease rise to the top and are skimmed off.

The Biological Nutrient Removal system removes nitrogen.

• The wastewater then flows through a series of huge vessels, called Biological Reactor Basins. Each one holds about four million gallons - almost seven times the size of an Olympic swimming pool. It's here that ASA goes green. More than 99% of the nitrogen - the stuff that feeds algae blooms - is removed.

• Next: millions of microorganisms feast on the wastewater's nitrogen compounds and convert them into harmless and odorless nitrogen gas. Other gas byproducts even help power the plant's equipment.

The Tertiary Treatment and Disinfection/Post Aeration process removes remaining suspended solids and phosphorus.

• The water is then pumped into 12-foot deep tanks where the solids and microorganisms can settle to the bottom and another pollutant - phosphorus - can be removed.

• Finally, the water goes through special settling tanks and filters. Rather than adding harsh chlorine, ASA uses ultraviolet lamps to zap any harmful germs and bacteria. And as a last step, ASA boosts the water's oxygen levels to make it eco-friendly, and releases it into Hunting Creek and the Potomac.

Meanwhile, another process handles solids.

• Most of the solid materials collected throughout the treatment process are sent to a special ASA processing facility that turns them into a clean, fertilizer-like material that is spread on farmers' fields in a number of Virginia counties.

• ASA's sophisticated odor control system scrubs the air; most people in Alexandria have no idea a wastewater treatment facility is their neighbor.