Wastewater Nutrient Removal and Algal Biomass Production – An Energy Evaluation

The potential environmental benefits of algal biodiesel production have recently been outlined in several life cycle analyses evaluating the energetic balance of the process. These virtual production analyses require a wide range of assumptions, including the addition of fertilizers and carbon dioxide to achieve high algal yields in open ponds. This paper presents an energy balance based on the microalgal production in four pilot-scale reactors (2,500 gallons each) fed with wastewater effluent from a conventional activated sludge process for six months. The pilot data was used to estimate an energy balance for processing the total average 12 million gallons per day processed by the wastewater treatment plant. The algal dewatering step consumes a significant amount of energy and therefore, several thickening and dewatering processes were compared. An energy offset was applied for removing nutrients with the algal reactors rather than the biological nutrient removal processes typically utilized in municipal wastewater treatment. The results show that biofuel production is energetically favorable for open pond reactors utilizing wastewater as a nutrient source even without an energy credit for nutrient removal. The direct combustion of algal biomass was also considered as an alternative to lipid extraction and biodiesel production as it may be a more viable energy source than biofuel production, especially when the lipid content of dry biomass (10% in this field experiment) is lower than the high values reported in lab-scale reactors (50–60%).