Integrated fixed film activated sludge and algae system for advanced membrane-based wastewater treatment: A pilot plant experiment with real wastewater

Upgrading existing wastewater treatment plants to achieve higher efficiency requires compact process-intensification strategies, particularly where the construction of additional treatment units is constrained by limited land availability. This study evaluated a compact pilot-scale integrated fixed-film activated sludge (IFAS) membrane photobioreactor treating real municipal wastewater under low carbon-to-nitrogen (C/N) conditions. The system combined suspended activated sludge, attached biofilm, microalgae ( Chlorella sp. ) and submerged membrane filtration. Two operating periods were investigated by increasing the algae:sludge mass ratio from 1:5–1:3. The reactor achieved high carbon removal, with total chemical oxygen demand (COD) removal improving from 91.7 ± 4.3% to 95.9 ± 1.6% despite the higher influent organic load in the second period (1.5 ± 0.8 kg COD/m3 d). Ammonium removal also increased from 82.2 ± 26.4% to 96.5 ± 5.5%, indicating enhanced nitrification capacity. In contrast, total nitrogen (TN) removal remained moderate, decreasing from 49.5 ± 23.6% to 39.8 ± 21.4% as carbon availability declined (C/N in period 2: 1.2 g soluble COD/TN), confirming carbon limitation as the main bottleneck for complete nitrogen removal. Phosphate removal was poor overall, although a modest improvement was observed at a higher algal fraction. Biomass characterisation showed greater biofilm accumulation and algal proliferation in the second period, with attached biomass doubling and chlorophyll-a increasing from 2.7 ± 0.2–4.3 ± 1.5 mg/L. Overall, the results demonstrate the feasibility of this IFAS membrane photobioreactor as an advanced proof of concept for low-COD/N wastewater treatment, while highlighting the need for further optimisation to improve denitrification and phosphorus removal.