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Anaerobic Digestion at Plant Chicago: Part 1

Following a collaboration with Loyola University Chicago (LUC) engineering students - Anna Zachary, Valentine Geze, and Brooke McDonald - Plant Chicago is taking steps to turn their capstone project into reality! Part one of this series - written by Zachary, Geze, and McDonald - lays a foundation for understanding anaerobic digestion and the origins of this particular undertaking at Plant Chicago.

In late summer 2020, Plant Chicago provided the three of us with a problem statement to design an anaerobic digester (AD) that would fit the size of their operations and provide usable outputs such that organic waste in their community can be diverted from landfills and used in a meaningful way.

Anaerobic digestion is a process where organic waste (e.g., fruits, vegetables, certain yard waste) is broken down via anaerobic microbes. Anaerobic microbes exist in environments which do not have oxygen. This process creates two outputs: digestate (liquid and solid) and biogas (a mixture of methane and carbon dioxide gasses). The liquid digestate output is abundant and can be diluted for use as fertilizer in soil or as hydroponic solution in Plant Chicago’s hydroponic systems. Biogas has a similar chemical makeup to natural gas; however, biogas can come directly from plants. This enables the next crop of vegetation to absorb the carbon created by combustion of the previous batch of biogas. Natural gas, in contrast, is pumped from gas sources deep underground. The removal of this underground carbon source by humans drastically increases the rate at which carbon is released into the atmosphere compared to that which would be released through natural events. In terms of biogas use at Plant Chicago, it could fuel a grill for use during community events.

Figure 1: The final AD design given to Plant Chicago

The AD that has been designed for Plant Chicago, seen in Figure 1, fits in a 275-gallon Intermediate Bulk Container (IBC) and can process one to nine gallons of organic waste per day. The AD will output the same amount of liquid digestate as the amount of organic waste put in. When the team began the project, Plant Chicago thought that using a pre-designed system was the

best option. Our team was to determine methods by which the pre-designed system would be useful and safe. However, through analysis of cost and risk surrounding this pre-made system, our team determined that the drawbacks outweigh the benefits for Plant Chicago. The capital cost and operations and maintenance costs were high, there would be more output of digestate and biogas than Plant Chicago would need, and fixing issues within this type of system is difficult due to ambiguities in the design. We ultimately discussed our concerns with Plant Chicago and determined that the best way to move forward was designing the entire system ourselves. This led to the DIY design delivered to Plant Chicago in May.

Our team’s DIY design is centered around a 275-gallon IBC tank. The tank has three holes drilled into it for a feedstock inlet, gas outlet, and digestate outlet. The digestate outlet and feedstock inlet holes are placed in opposite corners on the IBC tank to allow for maximum retention time of feedstock in the AD (IBC tank), which ensures complete anaerobic digestion and a maximum biogas yield. This design is low-cost (at approximately 2% of the cost of the pre-designed system) and easy and inexpensive to maintain due to the accompaniment of a user manual giving a comprehensive explanation of the system. In addition, the parts are off-the-shelf, making them easily replaceable if any issues occur. The design produces quantities of digestate and biogas specifically scaled to Plant Chicago, which aligns with their goal of redirecting waste from a landfill more closely than the pre-made AD would have.

After extensive research, multiple design iterations, and design verification testing, our team created a unique solution to Plant Chicago’s problem statement which upholds their mission and meets their desired criteria. The final AD design diverts landfill waste into a renewable energy source, produces useful amounts of biogas and digestate without creating surplus, and enriches Plant Chicago’s day-to-day operations. This award-winning* design exemplifies the most sustainable and efficient way for Plant Chicago to utilize biogas, negating energy losses and maintaining a circular economy mindset. It creates ample educational opportunities for the future, making it a steppingstone for greater composting and clean energy endeavors at Plant Chicago. Its do-it-yourself and off-the-shelf nature makes the design accessible to Plant Chicago and anyone in the community who wants to build one of their own. This design puts sustainability, education, circular economy, and community at its forefront. Combating climate change and reducing waste is a large feat. Reversing climate change’s effects starts at home and in our communities. By basing decisions on circular economy principles and educating our communities, we can reduce our individual footprints, create waste-free futures, and foster a sustainable environment.

*The LUC team presented this project at the Illinois Water Environment Association Student Design Competition in Spring 2021 and won first place.

AD Educational Graphic
Download PDF • 139KB

Final Report
Download PDF • 1.34MB

Plant Chicago Anaerobic Digester User Manual
Download PDF • 1.65MB

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