<p>A novel hybrid process combining a 3-stage rotating biological contactor (3-RBC) with a chitosan–PEG-coated polypropylene microfiltration (MF) module was developed and validated for treating leachate generated from a municipal solid waste composting facility in Behshahr (Mazandaran Province, Iran) with a BOD₅/COD ratio of 0.53. At the optimal hydraulic retention time (HRT) of 5 days, the standalone three-stage RBC achieved 71.8 ± 3.6% COD and 70.4 ± 1.7% ammonia–nitrogen (NH₃-N) removal. Integration of the MF module significantly enhanced overall performance, yielding 74.6 ± 3.1% COD removal, 93.9 ± 0.8% NH₃-N removal (effluent 3.03 ± 0.07 mg/L), and &gt; 99.9% TSS and turbidity reduction, while removing 96.3% Zn, 91.8% Ba, 84.6% Mn, 79.0% Fe, and 62.0% Ca without chemical dosing during operation. The chitosan–PEG coating reduced fiber diameter from 6.91 ± 2.50 µm to 5.16 ± 1.87 µm and maintained stable flux above 25 L/m²·h for over 120 days without chemical cleaning. Mature biofilms exhibited dense cauliflower-like morphology with abundant intracellular polyhydroxybutyrate (PHB) storage granules (127.0 ± 43.9 nm). Compared to conventional activated sludge, SBR, MBR, and coagulation-based systems, the hybrid RBC–MF process demonstrated superior ammonia and heavy metal removal, high effluent quality, low energy consumption (~ 0.4 kWh/m³), and operational simplicity. The effluent met Iranian standards for NH₃-N (&lt; 5 mg/L), TSS (&lt; 40 mg/L), and turbidity (&lt; 50 NTU), and EU requirements for NH₃-N (&lt; 10 mg/L) and TSS (&lt; 35 mg/L). However, residual COD (~ 1,500 mg/L) exceeded Iranian (COD &lt; 60 mg/L) and EU (COD &lt; 125 mg/L) discharge limits, indicating the need for tertiary polishing for direct discharge to surface waters. This biological–physical hybrid system offers a low-maintenance, cost-effective solution for sustainable composting leachate treatment, particularly suitable for water-scarce and environmentally sensitive regions.</p> Graphical abstract <p></p>