Abstract: The energy balance of a hypothetical microalgae-based wastewater treatment plant (WWTP) has been performed for thirteen geographic locations covering the whole range of latitudes, longitudes and climate conditions of the different Spanish regions. The proposed WWTP includes high rate algae ponds (HRAPs) for secondary treatment and nitrogen removal, anaerobic codigestion of primary sludge and the biomass grown in the HRAPs, and a combined heat and power unit for electricity and heat production. The operation of the HRAPs was optimized using the BIO_ALGAE model, which also predicted the biomass production of the HRAPs under the different climate conditions. Under the assumptions of this study, the electrical energy balance resulted neutral or even positive in all the locations during the whole year, in spite of the climatic conditions variations. However, the heat balance resulted closer to the neutral footprint. The most favorable locations (Almeria and Seville, south of Spain) were analyzed in detail, confirming the feasibility of a positive electrical energy balance, while the heat balance resulted slightly negative in the cold season. Along with the solar radiation, the air temperature and its variation during the year are determinant to predict the feasibility of the heat balance in the proposed WWTP scheme.

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