Posted April 22, 2016
Water. Cool, clear water.
Our daily existence depends on access to water – and we need it clean. For drinking. For cooking, bathing and recreation. For life itself.
Sometimes we take clean water for granted, but we really shouldn’t. Just ask folks who live in parts of the country that are arid 365 days a year or in places experiencing drought. Appreciation of our water resources prompts stewardship, environmental awareness and a basic commitment to clean up. So, to mark Earth Day 2016, let’s talk clean water.
Modern wastewater treatment facilities are the first responders in the transformation of used water – from urban and suburban runoff, agriculture and the daily needs of every man, woman and child – into water that’s usable again.
Enter energy. No one should be surprised that the process of wastewater treatment takes loads of energy. The Blue Plains Advanced Wastewater Treatment Plant that serves the District of Columbia and parts of Maryland and Northern Virginia, the third-largest plant of its kind in the world, is the largest single-source consumer of electricity in D.C., according to plant officials. As we’ve noted throughout this series, electricity generation is increasingly being fueled by clean-burning natural gas. The U.S. Energy Information Administration expects that this year, for the first time ever, natural gas will be the United States’ leading energy source for power generation. Bottom line: Energy is fundamental to clean water for all.
API colleague Jessica Pointer and I recently visited Blue Plains to see how a metropolitan wastewater treatment facility works. Blue Plains is situated on the Potomac River due south of downtown D.C., on the bank opposite Alexandria, Va. Here’s what the 153-acre complex looks like from the air:
Some facts about Blue Plains:
- The plant opened in 1937. Before that, wastewater flowed through the District in open sewers, emptying untreated into the nearest waterway – uh, not good for the Potomac and Anacostia rivers or the Chesapeake Bay.
- Today, Blue Plains averages 370 million treated gallons of water per day and more than 1 billion gallons per day at peak flow. The 370 million gallons is about what it would take to fill RFK Stadium, home of the D.C. United soccer team.
- Wastewater comes to the facility through 1,800 miles of sewers around the District and from the Potomac Interceptor, a large sewer line that begins at Dulles International Airport west of D.C., which gathers wastewater from Virginia and Maryland.
- The plant features primary, secondary and advanced treatment – including nitrification and denitrification, multimedia filtration and chlorination/dechlorination.
- It uses about 30 MW of electricity, one-third of which is generated through a process called thermal hydrolysis and anaerobic digestion – solids left over from wastewater treatment are pressure-cooked to produce combined heat and power.
Our tour guide, DC Water’s Bipin Pathak, explains that the treatment process begins when wastewater arrives at the plant. A series of screens removes objects and large particles. A grit chamber takes out rocks and other non-degradables. The two-story building where this happens is filled nearly floor to ceiling with a series of metal contraptions behind guardrails that pull out all kinds of things that are waste but not to be processed. This stuff is loaded into trucks and hauled to a landfill.
Primary treatment occurs in a cone-shaped sedimentation tank that lets solid particles settle and fall out the bottom while wastewater flows outward, over a set of weirs, low barriers that pool the water behind them. A metal arm on the surface skims fats, oils and grease off the top that’s then taken to a landfill. From above the round, water in the concrete tanks appears still, like a little pond, but the water actually overflows the sides, through metal teeth and then down into a moat below.
Secondary treatment uses microbes to treat organic material like fats, sugars and short-chain carbon molecules. Microbes need oxygen and food. The wastewater contains the food; oxygen is pumped into tanks, and the microbes get to work. The tanks are round and concrete like the primary tanks, but the surface is frothy and bubbly, red-brown in color – and oh my, the aroma! You wouldn’t want to drop your cellphone in there. Once the microbes are done, they settle out at the bottom.
Blue Plains officials say this is where many wastewater treatment plants stop. At Blue Plains, advanced treatment processes are introduced to oxydize nitrogen from ammonia to nitrate, converting the nitrate to nitrogen gas that safely releases into the air. It includes multimedia filtration – running the treated plant flow through sand and anthracite before it’s disinfected and residual chlorine is removed. The final plant effluent after processing that goes into the Potomac looks like drinking water. “You guys see the nasty things out front,” Pathak says. “Now it’s pretty clean.” He smiles, fetches two beakers filled with clear water and challenges our tour group to figure out which one is treated plant water and which one is tap water, ready to sip. No one volunteers for a taste test.
That might be more than you wanted to know, but it’s important. Because, again, all of those processes – the pumping, the machinery, the processing systems and more – require energy. Now, the plant gains back some of the energy it consumes in a phase called thermal hydrolysis. This uses heat and pressure to prepare collected solids from other processes for feeding to microbes in digesters. They produce methane gas to fuel electricity-generating turbines. “We are feeding one bacteria to another bacteria,” Pathak says.
It’s a big, complicated process, all resting on a foundation: Your water and mine, cleaned with a big helping hand from energy.
Some photos that API's Jessica shot at Blue Plains
ABOUT THE AUTHOR
Mark Green joined API after a career in newspaper journalism, including 16 years as national editorial writer for The Oklahoman in the paper’s Washington bureau. Mark also was a reporter, copy editor and sports editor. He earned his journalism degree from the University of Oklahoma and master’s in journalism and public affairs from American University. He and his wife Pamela live in Occoquan, Va., where they enjoy their four grandchildren.