Win-win math – Can anaerobic digestion make 2 + 2 = 10?

By Sonia A. Hall

Cows in a field
Dairy nutrient management, Tillamook, Oregon. Credit: NRCS Oregon under Creative Commons license CC BY-ND 2.0

I’m not very good at telling people what they should do (my sons excepted, as I’m sure they’ll be happy to tell you). However, I am by nature a problem-solver, so I can’t help but get excited about things—ideas, tools, practices, approaches—that have the potential to solve multiple problems at the same time. You know, the so-called win-win solutions, that in my mind can make 2 + 2 = 10. That is why I’ve enjoyed working on reports (to come out soon) and a webinar series (also on its way) on anaerobic digestion technologies.

Let’s start with a definition, for those of you who may not be familiar with AD. Anaerobic digestion is the microbial metabolic process that degrades organic matter in environments void of oxygen (see this fact sheet for details). Okay… so what does that do? Anaerobic digesters are used worldwide to produce bioenergy and to sustainably treat organic waste from municipal, industrial, and agricultural operations (from that same fact sheet). That is, you can feed organic wastes—food processing left-overs, manure, food waste from municipal waste streams—into an AD facility, where microbes basically eat it up and, because they don’t have oxygen, they “breathe out” more methane than carbon dioxide. That methane can be used to generate energy, replacing other fossil fuels. The remaining material—the microbes’ waste—has nitrogen, phosphorus, and other nutrients that can be used as fertilizer. It also has fiber, which can be used on its own or to produce biochar, in either case benefitting the soil if it is applied to fields.

So here’s my math:

2 = 2 Air: The need to control odors was identified in a recent survey (stay tuned, it is not yet published) as one of the top challenges for efforts to improve organics management in Washington state. Because AD facilities help prevent the production of smelly gases, clean them or use them, these facilities are much less odorous than facilities that treat wastes in other ways.

2 + 2 = 4 Water: Washington State currently generates 1.5 million tons of manure, that contain thousands of tons of nitrogen and phosphorus. If this manure —after some basic processing—is applied to agricultural fields in amounts that supply more nutrients than crops require, then the excess nutrients can run off into rivers and streams or leach into groundwater. This can contribute to water quality problems. An AD facility with nutrient recovery can recover these nutrients from the wastewater, reducing water quality impacts.

2 + 2 + 2 = 6 Nutrients: The nutrients captured to avoid impacts to water quality can be used as fertilizer. They are in a much more compact (and sometimes more useable) form than the original manure, so they can be shipped further afield, to acres that are in need of nutrients. When appropriately managed, this can provide nutrients to fulfill crop needs, while avoiding the issues associated with over-application.

2 + 2 + 2 + 2 = 8 Economic: All these benefits are well and good, but do the economic numbers pan out? That’s a big part of the questions that WSU researchers are evaluating, as part of an effort to design a biorefinery. That’s a facility that integrates a core biomass conversion process—in this case, anaerobic digestion—with different technologies to produce multiple, valuable co-products from organic wastes, in a way that both addresses emerging environmental concerns and is economically viable.

2 + 2 + 2 + 2 + 2 = 10 Climate: This is the clincher, as far as discussing AD on is concerned. In 2011, managing manure in Washington State generated greenhouse gases equivalent to 1.2 million metric tons of carbon dioxide (Adelsman 2014). That’s the amount produced by 252,600 passenger vehicles driven for a whole year (based on this Greenhouse Gas Equivalencies Calculator). WSU’s Climate Friendly Farming project estimated that a similar amount of carbon dioxide equivalents—1.103 million metric tons—could be mitigated in Washington by applying AD and nutrient recovery technologies, using both manure and organics from municipal wastes, under certain, very conservative conditions* (Kruger and Frear 2010). This is why we’ll be talking about AD in future posts: AD can help turn a dairy from a net source to a net sink of greenhouse gases.

Hilarides Dairy (California) truck at a CNG dispensing pump. Credit: OWS (formerly Phase 3 Renewables).
Hilarides Dairy (California) truck at a CNG dispensing pump. Credit: OWS (formerly Phase 3 Renewables).

Washington State University’s Center for Sustaining Agriculture and Natural Resources has a Waste 2 Fuels program that is striving to more rigorously quantify the air, water, nutrient, economic, and climate impacts of integrating emerging, next-generation technologies within anaerobic digestion systems on U.S. dairies. Their results will provide the “real” math. But hopefully my creative calculations give you a sense for why AD is worth looking into.

So join the conversation. Tell us what information you need and what questions you have, and we’ll ask the experts to give us answers.

* The very conservative conditions used in estimating AD’s ability to mitigate greenhouse gas emissions were: 40 AD facilities with nutrient recovery technology, and co-digesting manure from 70,000 cows mixed with 20% of municipal organic waste. The greenhouse gas savings are due to a whole package of climate-friendly technologies used with AD.

Sonia Hall is an Associate in Research with the Center for Sustaining Ag & Natural Resources at Washington State University.

Adelsman, H. 2014. Washington State Greenhouse Gas Emissions Inventory, 2010-2011. Publication No. 14-02-024. Washington State Department of Ecology. Olympia, Washington. Online Access.

Kruger, C.E., Frear, C. 2010. Lessons Learned About Anaerobic Digestion (Chapter 12). In Kruger, C., G. Yorgey, S. Chen, H. Collins, C. Feise, C. Frear, D. Granatstein, S. Higgins, D. Huggins, C. MacConnell, K. Painter, C. Stöckle. Climate Friendly Farming: Improving the Carbon Footprint of Agriculture in the Pacific Northwest. CSANR Research Report 2010-001. Washington State University: Climate Friendly Farming


2 comments on "Win-win math – Can anaerobic digestion make 2 + 2 = 10?"
  1. Dear Sonia
    Many thanks for your important article on the so many advantages of AD process.
    We can add also an other environmental advantage which is the very important COD reduction after AD process wich could be 90% for wastewater sludge
    Best Regards
    Hassan from Morocco

    1. Glad you found the article useful, Hassan. We will be discussing additional aspects of anaerobic digestion and nutrient recovery in the future as well – stay tuned!

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