Sewage Sludge Land Application in Romania and Central and Eastern Europe (2000–2024): Trends, Valorisation Rates, and Circular Economy Implications

The transition towards a circular economy in the wastewater sector necessitates the sustainable management and valorisation of sewage sludge. This study provides a comprehensive longitudinal analysis of sewage sludge land application in Romania (2006–2024), contextualized within the broader Central and Eastern European (CEE) region (Poland, Czechia, Hungary, Bulgaria, and Slovakia; 2000–2024). Utilizing harmonized datasets from the Romanian National Institute of Statistics (INSSE TEMPO) and Eurostat, this research applies non-parametric Mann–Kendall trend tests, Sen’s slope estimator, Modified Trend-Free Pre-Whitening Mann–Kendall (TFPW-MK) tests to address serial autocorrelation, ordinary least squares regression with comprehensive residual diagnostics, exploratory detrended correlation matrices, and subperiod sensitivity analysis. Results indicate that Romanian municipal sludge applied to agricultural land increased from 0.38 thousand tonnes (kt) of dry matter (DM) in 2006 to a peak of 63.08 kt in 2022 and 53.97 kt in 2024 (+53.59 kt DM absolute increase; ~142-fold relative expansion), with the agricultural valorisation rate rising from 0.17% to a record 31.29% in 2024 (TFPW-MK Z = +5.000, p < 0.001). Municipal sludge production followed a non-linear trajectory, contracting to a historical minimum of 65.80 kt DM in 2012 before expanding to 283.34 kt in 2017 and reaching an infrastructural plateau around 172.49 kt in 2024 (2013–2024 subperiod τ = +0.030, p = 0.945). Conversely, reported industrial wastewater volumes experienced a substantial decline (TFPW-MK Z = −4.545, p < 0.001), reflecting industrial restructuring and a 2012/2013 reporting reclassification. Comparative CEE analysis reveals marked heterogeneity: Czechia (6.50 kg DM/capita) and Poland (4.07 kg DM/capita) lead in per capita agricultural recycling, while Romania reached 2.83 kg DM/capita (4.00 kg DM/ha Utilized Agricultural Area). An agronomic mass-balance assessment indicates that applying 53.97 kt DM under statutory limits requires a theoretical national land footprint of 10,794–17,990 ha (<0.133% of Romania’s agricultural area and <0.220% of conventional arable land), demonstrating that macro-scale physical land competition with certified organic farming does not exist. Furthermore, exploratory detrended and first-difference correlation analyses demonstrate that raw correlations between sludge and mineral fertilizers reflect parallel multi-decadal growth rather than guaranteed farm-level substitution. These empirical findings provide robust benchmarks for national circular sludge strategies under the recast EU Urban Waste Water Treatment Directive (Directive (EU) 2024/3019).