Anaerobic Digestion of Yard and Foodwaste to Export Nutrients?

There is a curious optimism that anaerobic digestion will resolve our urban and agricultural organic waste issues in British Columbia. There are a few important questions. Is anaerobic digestion economically viable or does it have to rely on government grants and ongoing subsidies? Does anaerobic digestion have any ability to move nutrients and organic matter out of a region of high concentration? Is this realistic optimism?

Metro Vancouver recently released a Recycling Market study, prepared by EBA Engineering Consultants and Cascadia Consulting Group (https://metrovancouver.sharefile.com/d/s39ad9b4eb9544b7b). The case for anaerobic digestion is linked with the possibility of creating fertilizers to export nutrients.

“A few private companies have determined that a business case for anaerobic digestion exists,  especially given current available capital funding. Given the overabundance of nutrients in the Lower Mainland, from municipal solid waste sources, biosolids and agricultural waste, there is significant interest in processing material to benefit from energy production, then composting or producing dry pellets for fertilizer or fuel with excess nutrients. The two types of anaerobic digestion are wet and dry. For wet digestion,moisture is added to the waste stream, which allows for separation of plastics, so it is beneficial for waste streams with higher plastics contamination. However, more energy is needed and nutrients are lost (Kelleher, 2007). The waste inputs for dry digestion typically have lower moisture content and do not require the addition of liquid for processing. Dry digestion has lower energy requirements than wet digestion, retains more nutrients, and generates less odour (Schafer et al., 2006). At the provincial level, an Integrated Resource Recovery approach is being explored as a way to manage community infrastructure to maximize the recovery of value from waste resources, including how to handle the overabundance of organics in the Lower Mainland (BC Ministry of Community Development, 2011; Paul, B., 2011).”

We in BC have a curious optimism that anaerobic digestion in general will be an economically viable solution in the future, so we are willing to provide extensive taxpayer money to move this idea forward. Further investigation reveals that the anaerobic digestion industry itself is divided on the economics of the process and whether or not dry or wet digestion is better and more economically viable. There are claims made for both – wet digestion gives a better gas yield and therefore is more economically viable – blending food waste and yard waste in a dry digestion process creates a better blend that can be further composted, but there are huge annual fluctuations in yardwaste volume and characteristics – anaerobic digestion is a integrated two part microbial process that is very prone to upset from dietary changes…. We will wait and see – some significant process and economic questions.

The BC Bioenergy Strategy (http://www.energyplan.gov.bc.ca/bioenergy/PDF/BioEnergy_Plan_005_0130_web0000.pdf) cites that 3% of British Columbia’s biomass is in the form of municipal solid waste and 10% is in the form of agricultural waste. This means that from potential future energy supply for BC, both municipal solid waste and agriculture are not that significant.

What about the bioenergy strategy to solve the excess nutrient challenge in south coastal BC? Wet digestion requires that all organic matter and nutrients are in a solution that is greater than 90% moisture. We already know from our sewage treatment plants that it is very costly to extract nutrients from water. “Dry” digestion is not quite dry – it requires a significant amount of “percolate water” that will also contain high amounts of nutrients, which is also costly to extract.

Given that we have an excess of both agricultural and urban organic waste in a small area, I believe that we need to find a cost effective way to stabilize the organic matter and package the nutrients so that they can be used elsewhere – in our own communities where they traditionally have not been used, and beyond our borders to provide organic fertilizers to other areas. I have not seen any good arguments as to how anaerobic digestion can achieve this. There is actually more evidence to suggest that nutrient export can be better achieved with a composting process, whereby the potential energy in the organic waste can be used to enhance the value of the recycled nutrients.

 

 

 

 

 

 

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