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 <euro>/kg or 7.4 $/kg) and substantial savings for liquid digestate bioremediation (3.4 <euro>/m3 or 3.7 $/m3) and N removal (15.1 <euro>/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.