Two decades of heavy metal fluctuations in wastewater sludge in Lithuania with evolving trends and implications for treatment efficiency

This study evaluates the temporal trends and seasonal variations in heavy metal concentrations within sewage sludge from the Kaunas wastewater treatment plant in Lithuania between 2000 and 2022. It integrates two decades of data with detailed seasonal analyses for 2021 and 2022, providing a comprehensive view of both long-term trends and short-term fluctuations. Monthly wastewater and sludge samples were collected, and concentrations of Cr, Zn, Hg, Cd, Ni, Cu, and Pb were analyzed using atomic absorption spectrometry. Statistical evaluations of temporal and seasonal data were conducted to identify trends and correlations between metal concentrations and external factors. Significant declines were observed in Cr, Pb, and Hg levels, highlighting the effectiveness of regulatory measures and treatment improvements. However, Zn and Cu concentrations remain elevated, posing challenges for sludge management and reuse. Seasonal spikes in Zn and Cr, particularly in May–July and October–December, were linked to industrial activities and stormwater runoff. The study also explores Lithuania's recent regulatory changes that permit higher Zn concentrations in sludge for agricultural use, reflecting evolving sludge management strategies.