Anaerobic Biogas Digester: From Bench Build to Statewide Model
The idea
Methane is a high-leverage near-term climate target: tens of times more potent than CO₂ over its roughly twelve-year atmospheric lifetime. A large, overlooked source is compostable waste rotting in landfills. But that same methane, captured instead of vented, is renewable energy. My Honors thesis at Portland State asked a practical, three-part question: how much methane does Oregon’s landfilled compostable waste actually produce, how well can anaerobic digestion mitigate it, and could a digester affordable enough for a household backyard make a real dent?
The built artifact
Rather than only model the idea, I designed, built, and instrumented a working digester from accessible materials — HMWPE 55-gallon drums and PVC — for roughly $800, about half the cost of a comparable commercial unit. It integrated six subsystems, documented across six AutoCAD schematics, including:
- A central digestion tank with separated bioslurry and biogas headspace.
- A water-displacement methane collection tank with an autonomous over-pressure relief design.
- A dual-motor agitation system on a programmed timer — the novel element, which raised measured biogas yield by roughly 35% over passive digestion.
- A multi-stage chemical scrubbing train (sequential H₂S, CO₂, and water removal) that achieved 88–94% methane purity.
- Compression and storage for appliance use, plus a fire-safety analysis for its real backyard installation.
What the pilot showed
Over a 90-day pilot run, I tracked feedstock mass, slurry pH, temperature, pressure, and power draw weekly, deriving captured biogas mass via the ideal gas law. The run processed about 102 kg of household compostable waste and sequestered methane convertible to usable thermal, electrical, and mechanical energy. It also taught a real lesson in physical constraints: production stayed sluggish until an unusually cold Oregon spring finally warmed past about 60 °F, a reminder that bench results live at the mercy of the environment.
Scaling the question with GIS
A working backyard unit is a proof of concept; the interesting question is what happens at scale. I built a waste-to-methane-to-renewable-gas model across three demographic scales — statewide, Oregon’s ten largest cities, and nine federally recognized tribes — using EPA generation rates, Oregon DEQ waste data, and climate-adjusted emission factors. I classified communities by NLCD land cover and Köppen-Geiger climate, applying climate-specific methane factors and producing thematic maps and zonal statistics.
The model suggested that broad adoption could divert on the order of 967,000 tons of waste annually, generate roughly 30 million MWh of renewable natural gas, and curtail around 82,000 tons of methane per year — enough to meaningfully shift Oregon’s renewable energy share. The GIS analysis also surfaced a stark equity gap: urban communities recovered only about 4% of their compostables against roughly 72% for rural ones.
Why it matters
This is the project that crystallized why I do this work. It runs the whole arc I care about — designing hardware, measuring honestly, modeling impact, and mapping who’s left out — in service of turning a pollution problem into an energy solution. It earned recognition from PSU’s Honors College, but more than that, it’s the clearest statement of the stewardship that drives everything else on this site.