Teaching life cycle assessment in environmental engineering: a disinfection case study for students

Abstract

To help educators increase the extent and effectiveness of integrating sustainability into undergraduate education, a case study in life cycle assessment (LCA) is developed and presented using the context of disinfection of wastewater. Design and operating parameters are presented for three alternatives: chlorine/sulfur dioxide, ultraviolet (UV) light, and sodium hypochlorite/sodium sulfite. The case study includes student learning objectives, design assumptions, system boundaries, supporting calculation files, descriptions of LCA simulation scenarios, expected simulation results, and interpretation. LCA simulations, using the ISO methodology approach, are performed with varying assumptions about design flows, study duration, electricity fuel mixes, an alternative LCIA methodology, and weighting scenarios. Results are presented primarily at the midpoint level, and the effects of weighting are illustrated using a ternary plot. Life cycle costing is performed by calculating net present worth cost of construction materials and selected ongoing operation and maintenance costs. After interpreting simulation results, students should be able understand and apply several LCA skills including identifying significant impact categories, describing tradeoffs between different life stages, identifying “hot-spots” in the life cycles, illustrating the impacts and limitations of weighting, and observing differences across LCIA methodologies. Using the assumptions herein, chlorine disinfection results in larger initial impacts due to the larger basin required for hydraulic retention time (HRT), but operating impacts associated with electricity consumption cause the UV impacts to overtake those of the chlorine alternative. The results are sensitive to the LCIA method, the electricity grid’s fuel mix, and the electricity consumed per unit of wastewater disinfected. Finally, consideration of non-environmental and non-cost factors (risk, safety) provide students with an opportunity to reflect on broader societal impacts. Adaptable for various audiences and to provide differing levels of technical rigor, the case study should aid students in understanding and becoming proficient in performing LCA to facilitate life cycle thinking. It is the author’s hope that by providing a transparent, comprehensive LCA case study comparing engineering alternatives, educators can better integrate life cycle thinking and systems thinking into engineering curricula.