There are many industrial processes that produce gaseous effluents with high concentrations of pollutants. Treatment of highly-loaded off-gases has been traditionally performed through physical and chemical processes, mainly adsorption with activated carbons or zeolites, absorption with organic solvents or alkali and acid solutions or catalytic or thermal oxidations. The application of biological technologies has been frequently considered only for odorous gaseous emissions or low-loaded off-gases. Nevertheless, the use of microorganisms to treat waste streams has been lately extended to treat a wide range of pollutants and concentrations contained in gaseous effluents since is technically suitable, environmentally friendly and a cost-effective alternative to physical-chemical processes. Moreover, three different configurations of reactors have been proposed, applied and studied under different conditions in order to treat adequately waste gases with different characteristics: biofilters, biotrickling filters and bioscrubbers. However, the optimization of these biological technologies and the application limits of these systems have not been extensively investigated. Then, this research line aims at filling this lack of information for the treatment of off-gases with high concentrations of two different compounds: ammonia and hydrogen sulfide. The main cases under study are the biogas and other energy-rich gases desulfurization, which is the biological removal of H2S for biogas upgrading, and the treatment of composting off-gases containing large loads of NH3.
1. Removal of H2S for biogas upgrading
Biogas is an energy rich effluent which can be burned in co-generation engines providing an environmentally friendly and low cost solution for nowadays growing energy demand. However, H2S contained in biogas must be removed to avoid engines damaging due to acidic corrosion. Biological removal of H2S has been efficiently performed using biofiltration processes, as biotrickling filters. However, the main disadvantage of this process is the accumulation of elemental sulfur in the trickling bed when high loads of H2S are treated. This situation is caused by an inefficient oxygen transfer that does not allow the full biological oxidation of H2S to sulfate. Hence, the aim of this research is to optimize the oxygen supply as well as to improve its transport to the liquid phase or biofilm. The first step towards achieving this aim is to calibrate and validate an accurate mathematical model that describes properly biogas desulfurization in a BTF at different conditions (transfer phenomena, kinetics, …). Then, the model obtained allows simulating different control strategies for optimizing biogas desulfurization. In our research group this study is particularly focused on the modification of trickling liquid velocity to reach this aim since is one of the most important parameters that influence the stability of the operation of this type of bioreactors.
2. Treatment of composting off-gases
There are many industrial processes that produce gaseous effluents with high concentrations of nitrogen compounds, mainly ammonia (NH3) and may contain traces of organic compounds which require treatment before release into the atmosphere. This is the case for example of the gases produced in facilities for waste composting on farms or cattle farming. In these cases, the ammonia is not only a compound that must be removed for health reasons, but also for technical reasons (to allow biological treatment facilities to operate satisfactorily) and environmental reasons (combustion generated oxides nitrogen). In these facilities, the concentrations of ammonia can easily rise up to 0.01 and 1% (from 100 to 10,000 ppmv) and taking into account the air flow ventilation installations, loads can exceed 100-150 g NH3/ m3h. Consequently, the treatment of these gaseous streams is now essential before being emitted into the atmosphere since the nitrification process has associated many substrate and product type inhibitions. Then our target falls on finding out where the limits of the system are to optimize the process in order to be implemented as a treatment alternative to physical-chemical processes.
It must be mentioned that in this research line, as well as in “Treatment of low-off gases” research line, many experimental and mathematical tools have been used in order to obtain the maximum knowledge about these processes:
- Respirometric and titrimetric techniques. These techniques have been used in order to perform the stoichiometric and kinetic characterization of the microbial cultures withdrawn from the packing material of the biofiltration units. Kinetic models describing the biological processes have been obtained and used in general models describing biofiltration in biotrickling filters.
- FIA and CFA monitoring techniques. Analysis of dissolved sulfide and sulfide and ammonia concentrations in the gas phases of the biofiltration units have been performed through hand-made Flow Injection Analysis (dissolved sulfide) and Continuous Flow Analysis (NH3 and H2S) systems. The continuous monitoring of different species through FIA and CFA during operation provides relevant information about the processes in general.
- Modeling and Control techniques. The formulation of theoretical models describing biofiltration processes serve to predict the behavior of bioreactors under different situations as well as to serve as a powerful tool for design, optimization and process control. The establishment of control protocols and strategies permit a robust, stable and secure operation under optimal conditions.
- FISH, DGGE and Pyrosequencing analysis. Molecular biology techniques used to study the microbial diversity found in biofiltration processes helps to understand better the biological reactions, mechanisms and kinetics. Moreover, the comparison of microbial populations under different conditions allows obtaining optimum results since, depending on the pH, temperature and pollutant loads set the diversity and the activity of bacteria is completely different.