Long-term outdoor operation of microalgae-based digestate treatment: impact of external drivers on process performances and techno-economic assessment

A pilot-scale algal pond for liquid digestate treatment (5.8 m2, 0.75–1.83 m3) was operated outdoor for 1145 cumulative days. Key performance indicators were correlated with relevant external drivers including influent characteristics, weather conditions, and operational parameters, gaining information for management and optimisation. A techno-economic assessment allowed to calculate and validate process costs for a scaled-up plant (2-ha). Despite external factors, thanks to consistent nitrification, ammonium removal was high and stable during all trials (85 ± 8%). Environmental and operational conditions strongly influenced phosphate removal (51 ± 21%) and biomass productivity (10.4 ± 6.6 g TSS/m2/d). High biomass productivities were mostly associated to global radiation, while phosphate removal was influenced by digestate characteristics. The relatively low biomass production costs (6.9 €/kg or 7.4 $/kg) and substantial savings for liquid digestate bioremediation (3.4 €/m3 or 3.7 $/m3) and N removal (15.1 €/kg N or 16.2 $/kg N) confirmed the high potential of microalgae-based technologies as a cost-effective and sustainable alternative to conventional approaches for wastewater bioremediation and resource recovery.