Assessing the long-term efficiency of the MULESL system: a sustainable solution for wastewater treatment and agricultural water reuse

Abstract

This study evaluates the long-term performance of the MULESL (MUch LEss SLudge) system for treating municipal wastewater intended for agricultural reuse, with particular focus on contaminants of emerging concern (CECs). The system showed stable removal of conventional pollutants under variable hydraulic loads, achieving 94% and 90% reductions in total and soluble COD, respectively, with residual BOD₅ of 9 mg L⁻¹. High removal efficiencies were also observed for TSS (>94%), VSS (>93%), TN (83%), TKN (91%), and NH₃ (92%), while pH and conductivity remained within regulatory limits. From a database of over 40 000 compounds, 46 CECs were selected as the most abundant in influent wastewater; metformin, caffeine, and 1,7-dimethylxanthine were the most prevalent. The MULESL system achieved up to 85% overall CEC removal. Post-treatment significantly improved microbiological quality: UV disinfection (20 s) reduced E. coli to <10 CFU/100 mL, meeting stringent European reuse standards, while peracetic acid (2–3 mg L⁻¹) provided slightly lower but effective results. The high efficiency is attributed to the biofilm–granular structure, long solids retention time, and enhanced sorption and biodegradation processes, highlighting the MULESL system as a compact and resilient solution for safe wastewater reuse.