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Recent research has demonstrated that using biosolids as a soil amendment or fertilizer replacement helps to sequester carbon in the soil and offset the emissions associated with the use of synthetic fertilizer. The research has shown that fertilizing with carbon-rich biosolids offsets emissions in three main ways:

  1. Biosolids increase plant growth, which helps remove carbon dioxide from the atmosphere and deposits more plant residues in the soil.
  2. Using biosolids as a fertilizer replacement means that the carbon emissions associated with manufacturing synthetic fertilizer are offset.
  3. Biosolids are rich in organic matter and some of that will stay in the soil.

Carbon benefits diagram   

By sequestering carbon and avoiding synthetic fertilizer use, King County offset over 42,000 tons of CO2 equivalents in 2013. That's like taking 8,000 cars off the road!

To put soil carbon in perspective, it is important to understand where carbon is stored around our planet and what happens when this pattern changes. As shown in the chart, there are five major places, called ‘pools’, where carbon resides on Earth: the oceanic pool, the geologic pool (where fossil fuels come from), the soil pool, the atmospheric pool, and the biotic pool (the living tissues of plants and animals).

Increased atmospheric carbon in the form of greenhouse gases like carbon dioxide and methane is responsible for climate change, the most important carbon-related problem we face. After centuries of agriculture and deforestation, soil carbon worldwide has been depleted by an average of 30% compared to original or undisturbed amounts (Post & Kwon, 2000). This loss undermines soil stability and productivity and contributes to climate change, but it can be reversed by adding carbon back to the soil. Use of biosolids is an effective way to do this while simultaneously enhancing plant growth.

The increased plant growth from biosolids fertilization also helps climate: as plants grow faster and larger, they remove more carbon dioxide from the air via photosynthesis. Plants store that carbon in their biomass and ultimately return some of it to the soil when they shed their leaves and branches, storing yet more carbon in the soil rather than in the atmosphere.

Using biosolids instead of synthetic fertilizers also reduces the climate impacts of fertilizer use. Because synthetic fertilizers require large amounts of fossil fuel energy to produce, choosing biosolids instead allows us to avoid the greenhouse gas emissions associated with producing and burning those fuels.

Even the biosolids production process provides a chance to reduce our climate impacts: by capturing the methane produced during the digestion process, we can harness a renewable fuel source and further reduce our dependence on fossil fuels. Taken together, these benefits have allowed King County WTD to become one of the nation's first carbon neutral wastewater facilities.