Cultivation of Arthrospira Platensis Using Different Agro-Industrial Liquid Anaerobic Digestates Diluted with Geothermal Water: A Sustainable Culture Strategy

The anaerobic digestion sector is undergoing worldwide expansion, and alternatives of using digestate as substrate for microalgae cultivation in order to improve the profitability and environmental impact of biogas plants are important. This work investigates the feasibility of growing Arthrospira platensis (A. platensis) on diluted liquid anaerobic digestates using geothermal water (GW) as alternative source of water. To this purpose, digestates from four industrial biogas plants were collected and used to carry out A. platensis cultivation assays. In the first part of the study, a screening experiment was performed to determine the ideal dilution factor suitable for A. platensis growth using each individual anaerobic digestate diluted with demineralized water (DW). Depending on the type of digestate, the most appropriate dilution factors ranged from 2x to 60x corresponding to optimal initial ammonium and turbidity values of 40-95 mgN.L-1 and 10-70 NTU, respectively. These dilution factors were consequently used to investigate the opportunity to substitute DW by GW for the culture of A. platensis. No significant changes were observed in the growth profiles when DW was replaced by GW for the four digestates tested. Regarding the final concentration reached by the different digestates, values ranged from 0.58 to 0.77 gdry matter.L- 1 with DW as compared to values from 0.53 to 0.66 gdry matter.L- 1 with GW. Experimental findings also showed the strong nutrient bioremediation potential of A. platensis culture, with removal efficiencies ranging from 96 to 97 %, 78 to 100 %, 45 to 66 % for N-NH4+, P-PO43- and K+, respectively. These promising results indicate that geothermal water could be used as substitute to fresh water for diluting anaerobic digestates and cultivating A. platensis, in a more sustainable way compared to conventional culture approaches using fresh water.