Start-up of an intermittent-aeration integrated fixed-film activated sludge (IFAS) - Oxic settling anaerobic (OSA) biological system coupling shortcut simultaneous nitrification-denitrification to near-zero sludge generation

The growing need to upgrade wastewater treatment plants to meet environmental standards is compounded by the rising production of sludge. This situation demands the development of innovative, easily applicable, and low-cost biosystems. The present study describes the start-up of an integrated fixed-film activated sludge (IFAS) system coupled with an oxic-settling-anaerobic (OSA) cycle, designed to achieve shortcut simultaneous nitrification-denitrification (SND) while minimizing sludge production. Shortcut SND was established already during the cultivation phase via effective inhibition of nitrite-oxidizing bacteria (NOB). Effluent COD concentrations remained consistently below 30 mg/L, with total nitrogen (TN) removal reaching up to 75 %. The observed sludge yield (Yobs) was 0.166 ± 0.049 gTSS/gCOD, significantly lower than conventional activated sludge systems (∼ 0.5 gTSS/gCOD). Up to 63 % P-PO43- removal was achieved at an HRT of 1 day, DO range of 0.2–2.3, and COD/TN of 12.2. These findings show a promising potential of the IFAS-OSA system for combined C, N, and P removal coupled to near-zero sludge production, providing a new compact solution to enhance the sustainability of wastewater treatment. Each stage of the IFAS-OSA start-up process is described accurately in terms of reactor performances, operational challenges, and possible remedial actions. Guidelines and recommendations are provided to support researchers and WWTP operators through the implementation and upscaling of the integrated system.