Sweden’s pulp and paper sector accounts for a significant proportion of national energy usage, and generates wastewater that causes eutrophication of nearby sinks. In this paper, the possibility of optimizing a biological purification process at the Stora Enso Skoghall mill south of the city of Karlstad in central Sweden, with respect to electricity usage and the addition of nutrients, has been investigated. A computational model of the treatment process was developed, based on process data obtained from the mill, and nine different scenarios compared subsequently, with energy use, environmental impacts and operational expenses, as criteria. These nine scenarios were compared with the reference case, which is the status quo of the biological treatment at the mill. The results depicted that the most energy-efficient and cost-effective alternative was a combination of measures such as lowering the oxygen level in the MBBR from 3 mg l-1 to 2 mg l-1 and using Hyperclassic in the aerated lagoon; an arrangement that yielded a 48.5% reduction in operational expenses, and a 60% decrease in the energy use, vis-à-vis the reference case, without affecting the efficiency of the purification process. This also uncovered an opportunity to mitigate the global warming impact and the eutrophication potential, by approximately 100 tons of CO2-eq. year-1 and 140 kg PO43--eq. year-1. All attempts to optimize the use of resources and decrease the anthropogenic environmental footprint ought to be made to come closer to the targets set by the United Nations’ sustainable development goals (SDGs). The authors’ conclusion predicated on the results of the modelling and analysis done in this study is that the potential of seemingly small process modifications, such as lowering the oxygen level in the MBBR, and applying a more optimal dosage of nutrient salts, must not be overlooked.