Microalgal-Biotechnology As a Platform for an Integral Biogas Upgrading and Nutrient Removal from Anaerobic Effluents Melanie Bahr, † Ignacio Díaz, ‡ Antonio Dominguez, ‡ Armando Gonza ́ lez Sa ́ nchez, § and Raul Muñ oz* ,† † Department of Chemical Engineering and Environmental Technology, University of Valladolid, C/Dr. Mergelina s/n, 47011 Valladolid, Spain ‡ BIOGAS FUEL CELL S.A., Parque Tecnoló gico de Gijó n, C\ Luis Moya 82, Edificio Pisa 1° izq, 33203 Gijó n, Spain § Instituto de Ingeniería, Universidad Nacional Autó noma de Me ́ xico, Circuito Escolar, Ciudad Universitaria, 04510 Mexico City, Mexico * S Supporting Information ABSTRACT: The potential of a pilot high rate algal pond (HRAP) interconnected via liquid recirculation with an external absorption column for the simultaneous removal of H 2 S and CO 2 from biogas using an alkaliphilic microalgal-bacterial consortium was evaluated. A bubble column was preferred as external absorption unit to a packed bed column based on its ease of operation, despite showing a comparable CO 2 mass transfer capacity. When the combined HRAP-bubble column system was operated under continuous mode with mineral salt medium at a biogas residence time of 30 min in the absorption column , the system removed 100% of the H 2 S (up to 5000 ppm v ) and 90% of the CO 2 supplied, with O 2 concentrations in the upgraded biogas below 0.2%. The use of diluted centrates as a free nutrient source resulted in a gradual decrease in CO 2 removal to steady values of 40%, while H 2 S removal remained at 100%. The anaerobic digestion of the algal-bacterial biomass produced during biogas upgrading resulted in a CH 4 yield of 0.21-0.27 L/gVS, which could satisfy up to 60% of the overall energy demand for biogas upgrading. This proof of concept study confirmed that algal-bacterial photobioreactors can support an integral upgrading without biogas contamination, with a net negative CO 2 footprint, energy production, and a reduction of the eutrophication potential of the residual anaerobic effluents. ■ INTRODUCTION Biogas from the anaerobic digestion of solid wastes constitutes a valuable bioenergy source with the potential to partially alleviate the world’s dependence on fossil fuels. According to the EurObservER 2012 report, 1 the production of electricity from biogas in the EU-27 increased by 18% from 2010 to 2011, while biogas heat sales to factories or heating networks increased by 16%. The primary biogas production in the European Union in 2011 accounted for 10.1 Mtoe, and there is still a huge potential for anaerobic digestion in the treatment of municipal and agricultural solid wastes in most EU countries. 1 In this context, a decrease in the biogas CO 2 content, which accounts for 25-50% of the biogas on volume basis, will result in lower transportation costs and an increase in the biogas energy content. Likewise, a reduction in the H 2 S content (0-2 %vol) is also crucial for biogas management since H 2 S is highly corrosive, toxic and malodorous. 2 A wide range of technologies based on physical-chemical and biological mechanisms have been applied for the removal of CO 2 or H 2 S from biogas, but there is a lack of technologies capable of simultaneously coping with both biogas contaminants. 3,4 Physical/chemical technologies such as membrane separation or chemical scrubbing can cope simultaneously with both biogas pollutants, but are often less environmentally friendly and exhibit prohibitive operating costs. 4 On the other hand, and to the best of our knowledge, there is no single biological technology capable of simultaneously removing H 2 S and CO 2 since aerobic or denitrifying biofiltration only removes H 2 S, while conventional microalgae photobioreactors are only efficient for CO 2 capture. 2,5,6 Algal-bacterial symbiosis in photobioreactors represents an opportunity to simultaneously remove both biogas pollutants at a low energy cost and environmental impact. In these systems, microalgae would use solar energy to fix the CO 2 from biogas via photosynthesis, with the concomitant production of O 2 . This in situ generated O 2 is subsequently used by sulfur oxidizing bacteria to oxidize H 2 S to sulfate. 7 The operation of these algal-bacterial processes at high pH values (by using alkaliphilic sulfur oxidizing bacteria and high pH-tolerant microalgae) would significantly enhance the mass transport of the acidic gases H 2 S and CO 2 from the biogas to the algal- bacterial cultivation broth, thus allowing for an integral biogas upgrading. 8,9 Another advantage of this novel biotechnology is Received: August 13, 2013 Revised: November 23, 2013 Accepted: December 3, 2013 Published: December 3, 2013 Article pubs.acs.org/est © 2013 American Chemical Society 573 dx.doi.org/10.1021/es403596m | Environ. Sci. Technol. 2014, 48, 573-581