Sustainability assessment of water resource recovery facilities

In Denmark, several wastewater treatment operators aim to transform their wastewater treatment plants (WWTPs) into water resource recovery facilities (WRRFs). This transformation to WRRFs envisages obtaining climate- and resource-neutral plants beyond energy neutrality. However, circular systems do not guarantee climate-neutrality nor sustainability. Therefore, holistic quantification involving a broad environmental impact assessment is necessary to evaluate the transition to WRRFs. Wastewater operators do not yet have decision support methods to help them quantify impacts on new solutions beyond sole economic terms. As a result, the main aim of the PhD was to assess the sustainability of transitioning to novel resource recovery and pollution control technologies by applying well-established methodologies for quantitative sustainability assessments, such as carbon footprint (CFP), life cycle assessment (LCA), cost-benefit analysis (CBA) and multi-criteria decision analysis (MCDA), to facilitate wastewater operators’ decisionmaking process.

The first step of the thesis was to assess the current decision support tools used by wastewater operators to estimate the environmental impacts of their facilities. The thesis found that wastewater operators are acquainted with CFP evaluations. They reported ambitions for CO₂ neutrality to the Danish EPA, and many operators account for CO₂-eq emissions voluntarily. We assessed the internal greenhouse gas (GHG) accounting of three WRRFs and found several challenges in this quantification. Their accounting is inconsistent and difficult to compare across plants and years to track the WRRFs performance development in CO₂–eq emissions. Challenges in the plants’ CO₂ accounting include the lack of the definition of a functional unit, the lack of proper standards in system boundaries and emission factors especially for direct emissions of nitrous oxide and methane. Also, downstream emissions of sludge management are reported inconsistently. For example, some operators did not credit avoided impacts of fertiliser substitution although including the environmental burden of sludge composting.

The full environmental impact dimension is not identified by the climate impacts only. We applied LCA as a prominent decision support tool for evaluating WRRFs with a broad range of environmental categories. We performed an LCA of retrofitting an existing WWTP in Copenhagen to a future WRRF with complex technologies including sensors for nitrous oxide online monitoring, power-to-hydrogen combined with biological methanation, phosphorus recovery from sludge ashes, rotating belt filters for increased primary sludge and biogas production and the anammox technology for nitrogen removal from the reject dewatering. Our LCA results quantified reduced climate change impacts of the novel WRRF compared to the baseline plant; however, climate neutrality was not yet guaranteed as the operators should undertake further efforts to reduce direct N₂O emissions. Novel resource recovery technologies may further increase the need for renewable electricity consumption and chemicals increasing impacts of mineral resource depletion category. We also concluded that performance evaluations may change with holistic assessments expanding coverage to more and more impact categories. For instance, the phosphorus recovery technology increased climate change impacts but decreased mineral resource depletion impacts.

Assessing environmental impacts with LCA also has a few shortcomings. For instance, trade-offs between categories exist, making its application difficult during the decision-making process. Furthermore, LCA does not yet address emerging pollutants in wastewater, such as microplastics or emissions of some pharmaceutical ingredients. To overcome this shortcoming, we integrated LCA in an economic CBA for the case study of sludge pyrolysis with biochar and activated carbon production. We converted the potential non-market environmental benefits and costs into monetary values. Integrating LCA in a CBA allowed to provide a single economic score for each alternative, demonstrating that sludge pyrolysis is environmentally and economically beneficial compared to direct use on land. However, it is challenging and time-consuming to estimate monetary values of environmental externalities, where a market does not exist. This also applies to the monetisation of social impacts of novel resource recovery technologies.

A more easing approach might be selecting relevant and relatively simple operational criteria for WRRFs. By involving stakeholders from the wastewater sector, we established a multi-attribute framework that included criteria and indicators under four sustainability dimensions of environment, economy, society and technical aspects. MCDA provided single total sustainability scores for each alternative and eliminated trade-offs, based on eliciting stakeholders’ preferences over the criteria. The framework was applied to the retrofitting of the Avedøre treatment plant in Copenhagen to WRRF. Also from an MCDA perspective, in spite of the increased financial costs, it was found beneficial to retrofit Avedøre to a WRRF than continuing business-as-usual.

Overall, this PhD work combined well-established methodologies to provide unprecedented evaluations of a complex set of technologies in real-case WRRFs. Thereby, the thesis brings three novelties to the understanding of resource recovery from wastewater:

1) The thesis provides concrete recommendations on how to report, quantify and standardise carbon footprint accounting in the wastewater sector.

2) The thesis identifies and quantifies potential unwanted impacts and benefits of shifting towards more advanced and circular treatment plants.

3) Finally, the thesis proposes a multi-criteria framework paving the way to implement quantitative LCA, economic CBA and MCDA in the Danish wastewater sector.