Life cycle assessment of nutrient removal technologies for the treatment of anaerobic digestion supernatant and its integration in a wastewater treatment plant

The supernatant resulting from the anaerobic digestion of sludge generated by wastewater treatment plants (WWTP) is an attractive flow for technologies such as partial nitritation–anammox (CANON), nitrite shortcut (NSC) and struvite crystallization processes (SCP). The high concentration of N and P and its low flow rate facilitate the removal of nutrients under more favorable conditions than in the main water line. Despite their operational and economic benefits, the environmental burdens of these technologies also need to be assessed to prove their feasibility under a more holistic perspective. The potential environmental implications of these technologies were assessed using life cycle assessment, first at pilot plant scale, later integrating them in a modeled full WWTP. Pilot plant results reported a much lower environmental impact for N removal technologies than SCP. Full-scale modeling, however, highlighted that the differences between technologies were not relevant once they are integrated in a WWTP. The impacts associated with the WWTP are slightly reduced in all categories except for eutrophication, where a substantial reduction was achieved using NSC, SCP, and especially when CANON and SCP were combined. This study emphasizes the need for assessing wastewater treatment technologies as part of a WWTP rather than as individual processes and the utility of modeling tools for doing so. •LCA was used to compare different side-stream treatment technologies.•At pilot plant scale, N removal technologies had the lowest environmental impact.•If part of a full WWTP, P-crystallization technologies presented the lowest impact.•Eutrophication impacts were reduced significantly using side-stream technologies.•Global warming, acidification, and toxicity impacts were only marginally reduced.