Long-term improvement of urban wastewater treatment efficiency following biological modernization in a Baltic Sea region

Abstract The improvement of urban wastewater treatment systems remains a key priority across Europe in order to achieve the long-term objectives of the EU Urban Waste Water Treatment Directive 91/271/EEC and reduce nutrient loading to the Baltic Sea basin. This study evaluates the temporal development of wastewater treatment performance in Kaunas City (Lithuania) over a 25-year period (2000–2025), based on continuous laboratory monitoring and technological modernization of the treatment system. Five core parameters were analysed—biochemical oxygen demand (BOD7), chemical oxygen demand (COD), suspended solids, total nitrogen and total phosphorus—to quantify removal efficiencies and identify long-term performance trends. The results demonstrate substantial improvement in treatment effectiveness, with BOD7 removal reaching up to 98%, nitrogen up to 90%, and phosphorus 90–95%, while COD and suspended solids removal consistently exceeded 85%. Performance in most years met or exceeded EU regulatory thresholds. Since treated effluents are discharged into the Nemunas River—one of the largest contributors to the Baltic Sea catchment—enhanced nutrient removal contributes to broader international efforts aimed at reducing eutrophication pressure in this ecologically sensitive marine basin. This study highlights Kaunas as a relevant example of long-term transition toward advanced biological nutrient removal in Central and Eastern Europe. The findings provide evidence-based guidance for wastewater utilities and policymakers aiming to strengthen nitrogen removal stability and address future challenges such as emerging pollutants, pharmaceuticals and microplastics.