Thursday, May 5, 2011

Biosolids: an intersection of public health, clean water, and sustainable ag

Wastewater treatment is designed to protect human and environmental health

Puget Sound and the Seattle city skyline.
Each day ~175 million gallons of wastewater flows to King County (Seattle) treatment plants. That’s 63 billion 875 million gallons of raw wastewater getting treated every year (!!!). To be clear, that’s anything flushed or washed down the drain. We are so privileged, for without sewers and proper sanitation water born diseases like cholera, typhoid, and dysentery would run rampant.

From left: raw sewage, treatment plant effluent, reclaimed water.
To protect both human and environmental health, raw wastewater is treated. In my opinion this feat is arguably the greatest invention of modern society! By incredible acts of engineering, raw wastewater is piped to treatment plants where solids are removed via extensive biological processes and then water is disinfected. The clean water can then be safely discharged to surface water bodies like rivers, oceans and Puget Sound, or it can be used for irrigation.
Demonstration garden irrigated with reclaimed water and amended with biosolids compost.
Now what about those solids? Prior to treatment, solids are mostly feces, t.p., and food bits from garbage disposals. To treat the raw solids, or sewage sludge, most treatment plants use anaerobic digestion technology wherein solids are pumped into large digester tanks where microorganisms chow down until they die, then other microorganisms chow down on those dead bodies and so on and on until the product is no longer feces, t.p., and bits of food but rather just the chewed-up bodies of the solids-chowing microorganisms. Voila! Now we have “biosolids” which is the industry term for treated sewage sludge that can safely be used to amend soil. Microbes are so cool!

Pure biosolids cake, straight from the digesters.
A handful of biosolids composted with sawdust to make GroCo
Sustainability of biosolids
Similar to animal manure, biosolids are rich in organic matter and nutrients. Farmers clamor for this material because the growth response is incredible. See the photograph below where the biosolids amended plots far outgrew the synthetic fertilizer plots.

Dryland winter wheat. From left: no fertilizer control, synthetic fertilizer at 50 lbs N/acre, biosolids at 50 lbs N/acre, biosolids at 100 lbs N/acre
With amazing results like these, there aren’t enough biosolids to satisfy eastern WA wheat farmers. Obviously, there are enough nutrients in biosolids to bolster growth, but the true magic is the organic matter in biosolids, which unlike synthetic fertilizer, actually helps build fertile soil over time.

Photo credit: USDA NRCS. This is a photo of the WA state soil, "Tokul". Yes I am a geek for including a picture of a soil profile, but bear with me, I'm a soil scientist. The dark color in the upper horizons is a result of organic matter accumulation. Tokul soils are mostly found in western WA, and under conifer forest canopy.
Virgin soils like those found in grasslands, forests, and marshes are full of organic matter, however, human activities like construction and tillage degrade this natural resource. In the natural world, all organic matter gets recycled in the soil, including fallen leaves, excrement, and carcasses. Organic matter, like that found in biosolids, is full of nutrients. As microbes eat the organic matter, nutrients are slowly released to plants, and they excrete sticky substances called exudates (I love that word). The sticky exudates bind soil particles together, which helps prevent erosion and preserve topsoil. All this organic matter also helps loosen up compacted soil and retain water for thirsty plants. People like me will argue that organic matter might be our nation’s greatest resource since it’s organic matter that helps to protect our arable land. But enough with the dirt on dirt, the take home message is: organic matter is good for soil.


Volunteers haul biosolids compost to garden beds at Alleycat Acres' Beacon Hill site.
Some folks balk at the idea of biosolids. I used to, so I understand this reaction quite well. No doubt, there is a serious ick factor associated with poop. From childhood we are taught waste is dirty and a cause of disease (remember this is why we have wastewater treatment systems). But then I actually saw, touched, and smelled biosolids composts (pretty innocuous), and I started to think about the sustainability factor. It became clear to me biosolids aren’t icky, but actually a wonderful, renewable, natural resource worthy of reverence. Biosolids are full of nutrients derived from our food, which are nutrients that originated from farm soils. To close the nutrient loop, we need to return these nutrients to the soil, otherwise we need to mine or manufacture more fertilizer. The decision here is easy since one choice is sustainable and the other is not. The other important question to ask is: if we don’t use biosolids as a soil conditioner and fertilizer replacement, what do we do with them?

Andy Bary, WSU soil scientist, collects GPS data points at a long-term dryland wheat study site.
Me sampling a Yakima valley hops field, with a long history of biosolids application.
Skeptics claim biosolids are unsafe, unregulated, and unstudied. This is simply untrue. No other soil amendment or fertilizer has been so thoroughly studied or regulated. University and government scientists throughout the U.S. have studied biosolids for over 35 years and have repeatedly demonstrated their safety. Scientists continue to study biosolids for safety, efficacy, and larger sustainability issues, such as carbon sequestration. Are there pollutants in biosolids? In small concentrations – yes. We live in a dirty, chemical-ridden world, so of course there are pollutants. They are everywhere! While testing shows some level of pollutants exist in all soil amendments, we should know and appreciate that given the original mission of wastewater treatment - to protect human and environmental health - biosolids are the most regulated and monitored soil amendment available. Conspiracy theorists and skeptics abound when it comes to biosolids, but before we get wrapped up in the hysteria over ickyness, I ask that we all take step back and look at the larger sustainability picture.

UW undergraduate intern and I collect soil samples from a restoration site on Vashon Island, WA, where biosolids composts were used to restore the soil.
A year later wildlife and trees are starting to come back to the Vashon Island site.
Biosolids application to soil is proven as a safe renewable way to return nutrients and organic matter back to the earth. Biosolids are a product of wastewater treatment, which is designed to protect human and environmental health.

Pretty purple Scabiosa growing in biosolids amended soil.
It’s International Compost Awareness Week (May 1-7, 2011), and it’s time to get gardening! How about trying some GroCo, Tagro, or Soundgro for yourself! These amazing soil amendments are created locally with our very own class A biosolids products.
Since I live in an urban area, I would like to send out a big thank you to the wastewater treatment operators and program managers who work in the sewers to keep me, my neighbors, and Puget Sound healthy. Thanks folks! Keep up the good work!


  1. Excellent article! It explains a lot.

  2. This comment has been removed by the author.

  3. Thank you for your refreshing, informed take on biosoilds compost/ing. I mean, there's no better way of ridding our environment of all the poop we produce than to compost and reuse it. It only makes sense to reapply this nutrient rich material to our soil, improving soil quality so the cycle may continue. Thanks for spreading the word, Kate!

  4. WOW~ someone finally gets it. Great article

  5. Great post, thanks! Do you think you could provide some references for the photo's used? Or to some studies that have shown improved yield as the result of biosolids application?

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  10. Thank you for your refreshing, informed take on biosoilds compost/ing. I mean, there's no better way of ridding our environment of all the poop we produce than to compost and reuse it. It only makes sense to reapply this nutrient rich material to our soil, improving soil quality so the cycle may continue. Thanks for spreading the word, Kate!

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