Tracking nonregulated micropollutants in sewage sludge: antimicrobials, OH-PAHs, and microplastics — environmental risks, fertilizer implications and energy considerations

Abstract

This study analysed the content of selected antimicrobials agents (AAs), microplastics (MP), hydroxyl derivatives of polycyclic aromatic hydrocarbons (OH-PAHs) in stabilized sewage sludge and fertilizers produced from them. Eighteen AAs were identified and quantified in both sewage sludge and fertilizer samples using the LC-MS/MS method. The highest concentrations, exceeding 3,000 µg kg⁻¹, were found for sulfasalazine, clindamycin, ketoconazole and its deacetylated form, azithromycin, and desmethylated azithromycin. While the fertilizer production process successfully reduced the number of AAs present, 20 compounds persisted, with five exceeding 1,000 µg kg⁻¹, posing potential environmental concerns. The FTIR method revealed an average MP content of 2,429 ± 758 fractions in stabilized sewage sludge. Both black and colored microplastic fragments were detected, with an average of 1,070 and 665 particles, respectively. These findings suggest that microplastic contamination remains an issue even after sewage sludge stabilization. GC-MS/MS analysis identified six OH-PAHs in sewage sludge and fertilizer samples. In stabilized sewage sludge, concentrations ranged from 53 µg kg⁻¹ (2-HydroxyFluorene) to 587 µg kg⁻¹ (1-HydroxyNaphthalene), while in fertilizers, values ranged from 4.7 µg kg⁻¹ (2-HydroxyNaphthalene) to 31 µg kg⁻¹ (1-HydroxyPyrene). The fertilizer production process effectively removed from 46% to 88% of OH-PAHs, with 3-OH-BaP levels falling below detection limits. Despite the effectiveness of the fertilizer production process in reducing several contaminants (e.g., sulfamethoxazole, metronidazole, trimethoprim, pyrazinamide, sulfadiazine, delamanid, and piperacillin), certain pollutants, including clindamycin and ketoconazole, persisted. Additionally, the economic analysis of the annual profitability of processing sewage sludge into a fertilizer product was performed. The estimated costs and profits were taken into account. This analysis indicates that the total annual income from the operation of the installation will amount to USD 233,300. However, further research is needed to fully investigate and develop this method in reference of Circular Economy management.