Exhaust flue gases obtained from fossil fuels combustion or other industrial activities contain high amounts of SOx which must be removed before their emission to the atmosphere. Environmental pollution such as acid rain and photochemical smog is caused by flue gases containing sulfur dioxide and nitrogen oxides. SOx emissions also affect adversely human health, livestock and plants. Physical and chemical treatments of flue gases as dry sorption, wet scrubbing and catalytic oxidation have been extensively used to desulfurize them while recovering sulfur as a precipitate or as sulfuric acid. These technologies are efficient but highly expensive for those applications not related with petrochemical industry since requires chemicals and energy inputs. Therefore, biological recovery of sulfur contained in flue gases becomes an interesting alternative to physical and chemical treatments for being an environmentally friendly technology. Although biofiltration is a promising alternative to conventional physical and chemical air pollution control techniques, it has not been widely studied and applied for sulfur oxides treatment. Therefore, the main goal of this research line is the development, operation and optimization of a biological process which includes: 1. An absorption unit as the first step to remove Sox from exhaust flue gases and 2. A subsequent biological reduction-oxidation unit to recover elemental sulfur from dissolved sulfite contained in absorption effluent.
To this aim a three-step process has been initially proposed as an approach to recover elemental sulfur from SOx emissions. The process consists of a previous absorption-oxidation of SOx in water at slightly alkaline pH, the biological reduction of SO32- to S2-, using crude glycerol as the carbon source, and the partial oxidation of S2- to S0, performed under micro-aerophilic conditions. To optimize this process many variables must be studied as the residence times, both hydraulic and cellular, pH, temperature, carbon sources,… etc. The main disadvantage of this process is its performance in three different units, which increases the capital costs. In contrast, the key advantage is that the elemental sulfur obtained in the last reactor is really clean since has less than 6% of impurities and organic material due to the low biomass yield associated to sulfide oxidizing bacteria growth under micro-aerophilic conditions. Then, the elemental sulfur obtained from this recovery process could be used in several applications but especially in those industrial processes using sulfur as raw material.
A part from the valorization of flue gases, desulfurization of biogas can be also considered as a valorization process. As mentioned in the research line “Treatment of highly-loaded off-gases”, the valorization of biogas obtained from the anaerobic digestion of organic material performed in WWTP, solid waste treatment plants and other industrial facilities is carried out in our group through biofiltration in biotrickling filters.