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Denver Metro District: Quantifying Energy and Greenhouse Gas Mitigation Opportunities for Biosolids

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Opportunities for wastewater utilities to reduce primary energy use and greenhouse gas emissions are discussed and quantified based on an evaluation that included the Metro Wastewater Reclamation District (Metro District) Robert W. Hite Treatment Facility. Targeted GHG reduction tips for wastewater treatment plant (WWTP) operators are detailed focusing on water reuse and efficiency, energy efficiency and recovery, nutrient cycling, and infrastructure design for various greenhouse gas (GHG) inventory boundaries (i.e., Scope 1, 2, and 3), with emphasis on the impacts of biosolids processing.

For WWTPs in general, biogas CHP, followed by co-firing digested solids, electricity efficiency, and avoided fertilizer represent the best ways for WWTPs to reduce their carbon footprint, respectively.

For the Metro District, digester gas CHP at the Metro District was shown to reduce facility Scope 1 and 2 GHG emissions by 40%. The inclusion of slightly controversial carbon sequestration credits for land applied biosolids heavily shifted results for comparisons of land application versus co-firing digested solids in coal-fired power plants. Without carbon sequestration credits, co-firing digested solids was found to have a superior GHG benefit relative to land application (−0.50 mtCO2e/MG vs. −0.14 mtCO2e/MG, respectively). If carbon sequestration credits are added to land application calculations (−0.14+ −1.13 = −1.27 mtCO2e/MG), avoided fertilizer is significantly more beneficial than digested solids co-firing. Because raw solids co-firing negates existing biogas CHP benefits, raw solids led to an increase in existing Metro District GHG emissions, despite having a higher Btu content relative to digested solids.

The Metro District's current Local Government Operations Protocol Scope 1 and 2 GHG emission factor is 1.29 mtCO2e/MG (2.24 mtCO2e/dmt biosolids) and its Scope 1, 2, and 3 GHG emission factor is 1.83 mtCO2e/MG (3.19 mtCO2e/dmt biosolids). When all GHG mitigation strategies presented in this study are fully utilized without carbon sequestration, the Metro District has the potential to reduce its current Scope 1 and 2 GHG inventory by over 75% to 0.318 mtCO2e/MG (Scope 1, 2, and 3: 0.791 mtCO2e/MG). When carbon sequestration is included in mitigation modeling scenarios, the Metro District can realize a net negative Scope 1, 2, and 3 GHG footprint, with (−0.034 mtCO2e/MG) or without (−0.523 mtCO2e/MG) potential avoided water savings.

Current GHG protocols do not allow for the inclusion of avoided carbon sequestration benefits. Thus, it is essential for WWTPs to monitor whether carbon sequestration credits are integrated into future protocols, as this can heavily shift a WWTP's decision to land apply biosolids or to co-fire digested solids at a coal-fired power plant. If carbon sequestration credits are allowed in future protocols, it is essential that WWTPs have operational control over land applied biosolids to receive this credit. At present, carbon sequestration credits are not allowed in existing protocols, and consequently, co-firing of digested solids is superior to land application of biosolids and it is essential that the WWTP retains the Renewable Energy Certificates to claim the offsets for the co-fired biosolids.
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Keywords: CHP; Life cycle assessment; anaerobic digestion; combined heat and power; energy; fertilizer; greenhouse gas; sustainability

Document Type: Research Article

Publication date: 2010-01-01

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