Evaluation of the reuse of regenerated water from microalgae–related wastewater treatment processes in horticulture

Although the use of microalgae-based wastewater treatment (MWWT) processes is demonstrated to recover nutrients and produce clean water for agriculture, very scarce evidence of this use has been reported. This work aims to solve this gap and provide reliable data at this respect. For that, raceway reactor was operated in continuous mode for the treatment of municipal wastewater, the recovery of nutrients and water, and the production of biomass. After confirming the accomplishment of regulation for water discharge the treated water was evaluated for its use as irrigation water for lettuce, using both clean water and untreated wastewater as control. The microbiological quality of inlet and outlet water was analyzed by evaluating the total load and bacterial composition. Fertigation parameters, nutrient absorption and yield were measured. The wastewater treatment step confirms removal rates of 50 gCOD m-2·day-1, 1.7 gN m-2·day-1 and 0.8 gP m-2·day-1, in addition to a biomass production rate of 23 gbiomass·m-2·day-1, moreover the accomplishment of water discharge limits. Treatment with microalgae shows an improvement in the composition of bacterial taxa. The use of this water enhanced the root absorption of water and nitrate. The lower value of the drainage electrical conductivity when using water from MWWT indicates a higher metabolic activity in the roots, due to an increase in the absorption of nutrient ions concerning the volume of water absorbed. The treated water with microalgae was the treatment that showed the highest growth values (fresh and dry weight). Wastewater treatment process demonstrated to be efficient and releases up to 165,000 m3·ha-1·year-1 of clean water for irrigation. The use of MWWT processes has shown an increase in the beneficial bacterial load, the yield of the crops and to decrease the environmental pollution, therefore it is a clear example of an activity to be considered in the circular economy.