Simultaneous dissolution and uptake of nutrients in microalgal treatment of the secondarily treated digestate

Anaerobic digestion processes result in large volumes of digestates which still require the development of elaborate and viable management options to improve the sustainability of these processes. Coupling of secondary anaerobic treatment with a microalgal nutrient removal process may serve benefits such as residual biogas and associated greenhouse gas capture from the digestate content, microalgal biomass build-up and its further valorization. This study investigated the applicability a microalgal nutrient removal process in a secondarily treated digestate (the digestate of a digestate) as a complementary unit. The results indicated that up to 100% dissolved reactive phosphorus (DRP), 97.8% total dissolved phosphorus (TDP) and 93.7% ammonium nitrogen (NH4+-N) removal could be attained in the treatment of the digestate of a digestate using microalgal cultures. Folding phosphorus concentrations via dissolution was found to compensate for dilution related phosphorus deficiency for microalgal growth. Microalgal biomass obtained by the end of operation could be concentrated from 6.4-15.5 mg/L to 164.2-502.6 mg/L (2100-7900%) by simple gravity settling which was correlated with the prevalence of agglomerated particles in the size range of 10-100 mu m. The results demonstrated not only the applicability of the microalgal process after secondary anaerobic treatment of the digestate, but also brought about the dissolution concept for nutrients during microalgal growth.