Evaluation of Rinse Water Recirculation Alternatives at Lackarebäck Water Treatment Plant

Abstract

Water treatment plants generate large volumes of residual streams during normal drinking water production. At Lackarebäck, one of two water treatment plants in Gothenburg, these streams currently amount to approximately 6,300 m3 per day and are discharged without any reuse. This thesis evaluates three predefined alternatives for recirculation using a cost-benefit analysis (CBA), considering economic performance, climate impact, operational risks, and qualitative benefits over a 50-year analysis period. The evaluation method monetizes changes in raw water abstraction, energy use, and wastewater discharge. Climate impacts are assessed through changes in energy consumption and greenhouse gas emissions and monetized by using a CO2 shadow price to produce a net-present value (NPV) that includes financial costs and benefits as well as climate effects. Operational risks are evaluated qualitatively using a risk matrix, and selected benefits that are difficult to monetize reliably are assessed qualitatively alongside the economic results. Sensitivity, scenario and threshold analysis are performed to assess how uncertainties affect the results. All three alternatives produce a negative NPV in the base case. Alternative A recirculates ultrafiltration backwash water and first filtrate, while Alternative B also includes carbon filter backwash water and clarified supernatant from sedimentation. Alternative C is the most extensive option with additional sedimentation before recirculation. Alternative B, which recirculates the second largest amount of water, stands out as the most balanced alternative based on the evaluation. It also shows a positive NPV when considered under optimistic assumptions. Alternative B is therefore recommended as a basis for further development but must be verified through sampling and more detailed cost and climate estimations.