There are two microbial-based technologies that are successfully used to divert the organic fraction of municipal solid wastes (MSW) from landfill sites – composting and anaerobic digestion.
This diversion is relevant to climate change in that when organic matter decomposes in landfill sites methane is generated. This methane is potentially released into the atmosphere where it acts as a greenhouse gas.
Commercial composting operations use windrows, aerated static piles or in-vessel systems. The product is an agricultural compost. Alternatively, MSW can be anaerobically digested to produce methane that is burned in a gas engine to generate electricity. A solid or liquid digestate is also produced that can be used as an agricultural fertilizer.
Integrating composting with other technologies
I have a strong interest in the integration of microbial-based technologies. I was therefore very interested to see a document recently published by the Washington State University and the Washington State Department of Ecology called Advancing Organic Management in Washington State. The paper states that in 2014, 258 million tonnes of MSW were generated in the USA with only 34.6% recycled.
The paper looks at opportunities for producing not just low-value agricultural compost at composting facilities but also high-value products. The authors suggest that integrating the composting process with other technologies to construct different forms of biorefineries will accomplish this.
In particular they look at producing alternative jet fuel (AJF) which will have the effect of reducing the carbon footprint of its manufacture and lowering costs. They also look at integrating composting with anaerobic digestion and the production of biochar.
They evaluate the mass and energy balances of a number of technology combinations and assess their economic viability.
The composting baseline
The authors designed a hypothetical composting facility with the following characteristics. This was used as the basis for all the integration models studied:
- A capacity of 160,000 wet tonnes of MSW a year
- A daily throughput of 667 tonnes a day
- Wood chips used as a bulking agent
- Windrow composting
- A total of 8 weeks processing
Technologies for producing alternative jet fuel (AFJ)
Five technologies were considered:
The various scenarios for a biorefinery using these technologies are shown in the following figure.
Comparison of different biorefinery scenarios
- The minimum selling prices (MSP) for the products generated by each of the AFJ technologies used on their own were calculated and were found to be higher than current market prices.
- It was determined that cases using Virent’s BioForming, anaerobic digestion and fast hydrolysis as standalone technologies were not financially viable with a tipping fee of $60/tonne. However, they could be viable if the tipping fees, or site capacities, were increased.
- It was determined that cases using slow pyrolysis or LanzaTech’s ATJ as standalone technologies were financial viable with a tipping fee of $60/tonne.
- Most importantly, it was found that integrating these standalone AFJ technologies with a composting operation allowed a reduction of both capital and operating costs. This resulted in a reduction in the MSPs of 29-46% compared to standalone AFJ technologies.
- The conclusion of the study is that composting facilities can be used as a platform for the production of a variety of high-value products (fuel, methane, and biochar) when integrated with these other technologies.
This kind of work is supportive of the concept of multi-technology integration in the form of a biorefinery to reduce costs and increase the range of products made.