Selection of microalgae in artificial digestate: Strategies towards an effective phycoremediation.

Digestate is a complex by-product of anaerobic digestion and its composition depends on the digestor inputs. It can be exploited as a sustainable source of nutrients for microalgae cultivation but its unbalanced composition and toxic elements make the use challenging. Screening algae in a simplified synthetic digestate which mimics the main nutrient constraints of a real digestate is proposed as a reproducible and effective method to select suitable species for real digestate valorisation and remediation. Growth performance, nutrient removal and biomass composition of eight microalgae exposed to high amounts of NH, PO and organic-C were assessed. Using a score matrix, A. protothecoides, T. obliquus, C. reinhardtii, and E. gracilis were identified as the most promising species. Thus, three strategies were applied to improve outcomes: i) establishment of an algal consortium to improve biomass production, ii) K addition to the medium to promote K uptake over NH and to reduce potential NH toxicity, iii) P starvation as pretreatment for enhanced P removal by luxury uptake. The consortium was able to implement a short-term response displaying higher biomass production than single species (3.77 and 1.03-1.89 mg mL respectively) in synthetic digestate while maintaining similar nutrient remediation, furthermore, its growth rate was 1.6 times higher than in the control condition. However, the strategies aiming to reduce NH toxicity and higher P removal were not successful except for single cases. The proposed algal screening and the resulting designed consortium were respectively a reliable method and a powerful tool towards sustainable real digestate remediation.