The City of Dubuque is widely considered to be a model “sustainable” community. Nowhere is this more apparent than with the City’s decisions related to wastewater biosolids digestion and follow-up projects and studies related to renewable energy, biogas conditioning and cogeneration, co-digestion, and food residuals management. We were hired to conduct long-range planning for the aging wastewater treatment plant, which was originally built in the 1960s and 1970s. The plant had historically incinerated wastewater sludge followed by landfilling of the ash, and one of the goals of this project was to determine whether a more sustainable approach for solids management could be implemented. Throughout the planning effort, the City’s sustainable goals were incorporated into the evaluations through carbon footprint and greenhouse gas emission analyses, energy evaluations, and related measures. In the end, anaerobic digestion of the City’s biosolids was recommended and ultimately implemented. That successful project was followed by the biogas reuse project, which included gas cleaning, cogeneration, and codigestion of high-strength waste feedstocks with sludge.

Currently, we are assisting the City in implementing a pipeline-quality gas project to take advantage of D3 renewable identification numbers (RINs) available for the gas, which has a much higher value than cogeneration of electricity and heat. Collectively, phased projects include the following components:

  • Class A anaerobic digestion (temperature phased anaerobic digestion)
  • Centrifuge dewatering and land application
  • Gas cleaning – Hydrogen sulfide, siloxanes, moisture
  • High-strength waste codigestion
  • Cogeneration – microturbines (600 kW expandable to 1.0 MW)
  • Heat recovery for digester and building heat
  • Pipeline quality gas – pressure swing adsorption system

In addition, we conducted a feasibility study to determine the potential of accepting solid food residuals at the treatment plant. This effort resulted in a preliminary design of a wet processing facility to accept approximately 20 tons/day of separated food scraps, grinding and slurrying processes, and pumping to discharge the material to the treatment plant’s anaerobic digesters.

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