Role of hydraulic retention time in integration of microalgae and activated sludge process for nutrient recycle from diluted dairy liquid digestate

The integration of microalgae and activated sludge (MAS) processes presents a promising method for recycling nutrients from animal digestates. However, the effects of operational factors such as hydraulic retention time (HRT) on this process remain unclear. Therefore, this study utilized semi-continuous photobioreactor systems with a working volume of 800 mL to investigate the impact of three different HRTs (4 d, 6 d, and 8 d) on the performance of MAS processes for nutrient recycling from diluted dairy liquid digestate (DLD). Results indicated that the 8 d HRT yielded the highest biomass concentration, ranging from 2.20 to 2.52 g/L. However, extending the HRT beyond 6 d decreased biomass productivity. The growth of MAS biomass favoured nutrient removal, with C. vulgaris the primary contributor. Longer HRTs improved nutrient removal, with the 8 d HRT achieved the highest total nitrogen (TN) removal of 87.68 ± 4.57 % and complete phosphorus elimination. Complete total ammonia nitrogen (TAN) removal occurred when HRT ≥ 6 d. All systems exhibited poor COD removal due to the poor biodegradability of DLD and the algal organic matter (AOM) produced by microalgae, with the highest COD removal of 16.43 ± 3.65 % (8 d HRT). Metagenomic analysis revealed that HRTs affected bacterial communities significantly, with the highest richness and diversity occurring at a 6 d HRT. Dominant phyla in all three systems were Proteobacteria and Bacteroidetes. Longer HRTs enhanced lipid accumulation and reduced carbohydrate contents but had minimal effect on higher heating value (HHV), which ranged from 20.63 to 21.72 MJ kg−1. These findings have significant implications for developing efficient and sustainable MAS-based treatment systems.