Skip to main content

Can D.C. help flush away misconceptions about biosolids?

In an effort to move beyond the ick factor, District of Columbia water officials are among those reconsidering their approach to solid waste.

It’s been a part of my life, day in and day out. But once it was out of sight, it was out of mind. I hadn’t really spent any time thinking about it; but now, when I do, my heart races. I’m talking about poop.

For the more polite society, I’ll call it "biosolids."

My eyes were opened to the exciting world of biosolids — to which we all contribute — by an amazing talk given by Bill Brower, DC Water’s Biosolids Operations Program Manager.

He and his team envision the District of Columbia’s water utility not as a "wastewater treatment plant," but as a "water resource recovery facility."

That’s because Brower believes that the water that leaves our toilets, showers and storm sewers is much more than waste: it’s brown gold.

A humble history

While in some cultures, such as Japan and China, "nightsoil" was historically valued "as if it were gold," Europeans and Americans simply dumped their excrement into cesspits and waterways for centuries, assuming that the sheer volume of rivers and oceans would dilute the waste.

As populations grew, as people died of cholera and as we started studying the health of natural systems, we realized that this assumption was a big, smelly mistake.

In developed countries today, and in some cities in the developing world (PDF), human waste is flushed away and treated in wastewater treatment plants, creating effluent and biosolids. Effluent, mostly made up of water, is treated and put back into rivers and streams. The remaining biosolids are sent to landfills, burned or (as DC Water now does) reused.

Adding more stuff to our landfills just equates to a smellier, uglier, unhealthier chunk of land.

Where’s the problem? Sending biosolids to landfills (PDF) can lead to groundwater or surface water pollution as nitrate, metals, organics and/or pathogens leach out. As biosolids decompose in landfills, they also release methane, a powerful greenhouse gas.

Moreover, adding more stuff to our landfills just equates to a smellier, uglier, unhealthier chunk of land. One alternative — burning (or incinerating (PDF)) biosolids — requires consuming oil and/or natural gas for power. Ultimately, most of the ash generated from burning ends up in landfills anyway, ending up as waste.

Brown gold

Reusing biosolids from city sewer pipes makes all the sense in the world. Not only does it avoid adding to already-overloaded landfills, it gives us a valuable product (PDF) at the end. Treated biosolids can be used on agricultural lands, forests, rangelands and land areas with such bad soil quality that they need a little boost. Biosolids are full of organic matter and nutrients, such as nitrogen and potassium. They improve the texture of the soil and help it retain more nutrients. They even can replace expensive chemical fertilizers — all while being less likely to leach nutrients into water systems.

While most of DC’s biosolids make their way to farms, there’s a need for them right in the city as well. Even the White House lawns have benefited from the city’s excrement throughout the decades. As Brower will tell you as he jumps into his DC Water truck to deliver free biosolids to over 50 D.C. community gardens and tree plantings, Washington's regular soil can be anything but inviting to seedlings.

Becoming biosolids

Consider the after-flush: DC Water collects wastewater from 2.2 million people in the District of Columbia region (including parts of Maryland and Virginia) and is designed to treat an average of 370 million gallons per day. Once trash and grit have been filtered out, the solids and liquids of the waste are roughly separated, creating goopy sludge.

At DC Water, biosolids are created through a process called thermal hydrolysis (breaking up the sludge using very high temperature and pressure), followed by anaerobic digestion (PDF) (letting "good" bacteria eat up the sludge, breaking it down into gases and water). Although there are other ways to process biosolids for reuse, this process has huge advantages:

  • The bacteria reduce the amount of biosolids that have to be handled and shipped, because they turn about half of the waste into water and gases (methane, carbon dioxide and ammonia). With the newly adopted process, DC Water hauled an average of 444 wet tons per day (PDF) of biosolids in May, compared to 2012’s average of 1,103 wet tons per day (PDF).
  • The methane gas produced by these bacteria is burned to generate about one-third of the energy needed to power the entire water treatment facility. That’s 13 megawatts less needed from the power grid.
  • Instead of letting biosolids break down in landfills and emit greenhouse gases, the controlled digestion of biosolids recovers these gases and dramatically reduces greenhouse gas emissions.
  • Plus, we get the benefit of a useful, great-quality product that safely can make its way back to the earth.

Before it’s shipped off, the water content of the biosolids is adjusted. The less water there is, the less it weighs (and costs) to truck away. The end result? A dark, earthy soil that you can’t help but run your fingers through.

Safety concerns

The U.S. Environmental Protection Agency has defined guidelines on the way that biosolids should be handled, and the National Academy of Sciences has concluded that "the use of these materials in the production of crops for human consumption, when practiced in accordance with existing federal guidelines and regulations, presents negligible risk to the consumer, to crop production and to the environment."

That’s because the steps used to process biosolids are designed to kill pathogens (germs and worms that make us sick) and to monitor (PDF) for contaminants (chemicals that can, at certain levels, harm our health).

DC Water tested its biosolids and found that the lead, triclosan (anti-microbial) and PBDE (flame retardant) levels were below common agricultural and household levels. PBDE levels, for example, were at 200 parts per billion ("The equivalent of a pinch of salt in 10 tons of potato chips," Brower said), compared to the 500,000 parts per billion found in household dust.

The future

If Victor Hugo is right and "the history of men is reflected in the history of sewers," I’m optimistic about the future. Our sewers are leading us to great things.

DC Water’s leadership is clear, as the utility prioritizes research to inform the value and future of biosolids. Through a rebate in its biosolids contract, DC Water invests $2 per ton of biosolids to funding research through nearby universities and the U.S. Department of Agriculture.

The utility also is preparing to expand its resource recovery, hoping one day to add food waste to its biosolids processing. As the value of biosolids becomes widely appreciated, the business model of selling this resource will become self-sustaining.

DC Water is being watched by wastewater treatment plants across the nation. If the utility’s resource recovery plan continues to succeed, many utilities may follow in its wake.

So, if you live in the District of Columbia area, take a second to consider the value of the brown gold you’re sending on its way the next time you flush. And if you don’t live in D.C., ask questions, tour wastewater facilities and learn all you can about whether your city is using your generous restroom contribution wisely. Don’t let your waste go to waste.

This story first appeared on:

Worldwatch Institute

More on this topic

More by This Author