Trap grease is a mixture of wastewater, solids, fats, oils and greases (FOG) generated in the sewer system. The handling and disposal of trap grease have been a long-standing issue for the US. Every year, several million tonnes of trap grease are landfilled, causing various environmental concerns. Recently, utilizing the FOG portion of the trap grease for biodiesel production has been recognized as a potently viable solution to minimize the pressure on landfill. While the technical feasibility of this solution is being tested and improved, understanding the energy consumption and greenhouse gas emission emission during the trap grease biodiesel production life cycle is an indispensable step toward evaluating its cost-effectiveness and sustainability.
This study presents, for the first time, a life cycle model that evaluates the energy consumption and greenhouse gas emission emission for producing biodiesel from trap grease. Regression equations (in supporting material) for material use and energy input/output are derived from a comprehensive review of the existing literature. For the nominal scenario (4% FOG in trap grease and solids used for anaerobic digestion), the total energy consumption is 31.39 MJ/gal biodiesel produced and the energy return on investment (EROI) is 1.7. The corresponding greenhouse gas emission emission is -55.5 g CO2-eq/gal, which is due to the significant carbon credit from avoiding the use of electricity from grid.
This study also identifies the uncertainties within trap grease composition and anaerobic digestion (AD) process, and applied a Monte Carlo simulation to quantify their effect on the modeling results. The energy consumption values range from 20 to 760 MJ/gal, with over 80% of the data points ≤ 100 MJ/gal. The greenhouse gas emission emission values range from -2,700 to 25,000 g CO2-eq/gal, with over 90% of the data points ≤ 10,000 g CO2-eq/gal. The sensitivity analysis shows that FOG concentration and methane generation rate (from AD) have the highest influence on the results.
The comparison with benchmark studies (biodiesel from other feedstocks) shows that trap grease could be a low energy consumption and low greenhouse gas emission emitting feedstock under ideal conditions (e.g. high fats, oils and greases concentration, low FFA concentration, high AD performance). The life cycle model presented in this study could be used by wastewater treatment plants to evaluate the potential of utilizing trap grease for biodiesel production by using site-specific data. This model may also be used to evaluate the trap grease-to-biodiesel fuel pathway against certain greenhouse gas emission-related programs (e.g. Renewable Fuel Standard).
Qingshi Tu1, Bryant E. McDonnell2 . Monte Carlo analysis of life cycle energy consumption and greenhouse gas emission for biodiesel production from trap grease. Journal of Cleaner Production, Volume 112, Part 4, 2016, Pages 2674–2683.Show Affiliations
- Department of Biomedical, Chemical and Environmental Engineering, University of Cincinnati, 2600 Clifton Avenue, Cincinnati, OH 45221, USA
- Water Resources Engineer, ARCADIS-U.S., Inc., 4335 Cornell Road, Cincinnati, OH 45241, USA
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