Sewage sludge (SS) derived from Wastewater Treatment Plants (WWTPs) represents a valuable resource rich in organic matter and nutrients, including nitrogen (N) and phosphorus (P). These elements are major components of mineral fertilizers, i.e., essential macronutrients used in agri-food production. In addition, P is recognized as a non-renewable resource, mostly as a component of P-based rocks distributed in specific global regions. Therefore, recycling and recovering N and P from SS could generate value-added products such as (organo)mineral fertilizers, fuel, energy, etc., contributing to sustainable waste management practices and environmental protection. In this study, we studied, for the first time, long-term monitoring data spanning 18 years (2007–2024), focusing on the monthly dynamics of N and P concentrations in SS generated at the WWTP Zagreb, Croatia. It was confirmed that the long-term monitoring data provide valuable insights into the dynamics of N and P in SS, emphasizing the importance of continuous improvement in waste management practices to enhance nutrient recovery and environmental sustainability. Specifically, an analysis of variance revealed significant concentration heterogeneity (p<0.001) for both elements over the studied years: N concentrations ranged from 0.07% to 9.57%, with a mean value of 2.59% followed by a positive linear trend (y = 0.003x + 2.557), while P concentrations varied even more, from 0.03% to 20.3%, with a mean value of 2.01% and a negative linear trend (y = -0.081x + 2.784). The significant heterogeneity and opposite trends of N and P concentrations in municipal sludge highlighted that the recovery and recycling processes of those elements need to be optimized. Technological upgrades of the current secondary and tertiary phases of wastewater treatment technology could additionally enhance SS management, maximizing the recovery of valuable nutrients, strengthening water resource conservation (e.g., preventing water eutrophication), and reducing reliance on non-renewable P.