Assessing the Long-Term Efficiency of the MULESL System: A Sustainable Solution for Wastewater Treatment and Agricultural Water Reuse

Abstract

In light of escalating concerns surrounding contaminants of emerging concern (CECs) and the demand for sustainable water reuse, this study evaluates the long-term performance of the MULESL (MUch LEss SLudge) system in treating municipal wastewater for agricultural reuse.The system demonstrated robust and stable removal of conventional pollutants under variable hydraulic loads, achieving reductions of 94% and 90% for total and soluble COD, respectively, with residual BOD₅ of 9 mg/L. High removal efficiencies were also observed for suspended solids and nitrogen species (TSS >94%, VSS >93%, TN 83%, TKN 91%, NH₃ 92%), while pH and conductivity remained within regulatory limits. After a screening with a database including more than 40,000 pharmaceuticals, 46 were selected as the most abundant in influent wastewater and chosen for investigating their fate during the treatment. Metformin, caffeine, and 1,7-dimethylxanthine were the most prevalent. The MULESL system achieved an overall pharmaceutical removal of up to 85%. Microbiological quality was significantly enhanced through post-treatment: UV disinfection time of 20 s reduced E. coli to <10 CFU/100 mL, complying with stringent Italian and European reuse standards, while peracetic acid disinfection (2-3 mg/L) provided slightly lower but effective performance. The high treatment efficiency is attributed to the biofilm-granular structure, long solids retention time, and enhanced sorption and biodegradation processes. Overall, the MULESL system proves to be a compact, resilient, and effective technology for long-term removal of conventional pollutants and CECs, with strong potential for safe and sustainable agricultural water reuse following minor post-treatment optimization.