Effluent water and biogas desulfurization
In both solid and liquid waste treatment plants (biomethanization plants) and wastewater treatment stations a gas is produced that is a mixture of methane, carbon dioxide and other impurities, called biogas. As the methane content of biogas is approximately 50-70%, aside from the fact it cannot be released into the atmosphere due to its high pollutant potential (it is one of the main greenhouse gases), its high calorific value means it can be harnessed for electric power production (cogeneration). Thus, the production and reuse of biogas enables this type of installation to be ever more self-sufficient in terms of energy.
Bioreactors enable the H2S to be eliminated at an extremely low operating cost
Nevertheless, one of the primary impediments to the use of biogas for electric power production is determined by the nature of the impurities that accompany the biogas. Hydrogen sulfide (H2S) is one of the substances that most often contaminate biogas, and in the greatest amounts. It is a corrosive compound that attacks both the civil works of the installations where it is produced and the equipment responsible for producing the electrical energy. Its concentration in the biogas can vary between 1,000 and 20,000 ppm (parts per million in volume), but in order to be used in electrical energy cogeneration systems the H2S concentrations must be below 400 or 500 ppm.
The desulfurization techniques used up to now are based on the chemical oxidation of the hydrogen sulfide in scrubbers, connected in series. In the first stage, it is neutralized with an acidic solution (H2SO4) and then, in the second stage, an alkaline solution of NaClO and NaOH is used to cause the chemical oxidation. This option entails high consumptions of reagents in addition to presenting technical difficulties due to the presence of other chemical species (carbonatation of the CO2).
The alternative to the traditional solution is the elimination of the H2S using a fully biological process. Trickling filters are used, where the surface of the filter filler material forms a biofilm comprising sulfide-oxidizing bacteria, in other words, microorganisms specialized in the oxidation of small sulfur compounds, the process from which the necessary energy for their growth is obtained. These bioreactors enable the H2S to be eliminated at an extremely low operating cost, without the use of chemical reagents (which is an economic, safety and environmental advantage), and offer a consistently high disposal efficacy. Although the process is biological, these systems have proven to be very stable operating for long periods of time and adapt to the variability of the pollutant load to be broken down. For the commissioning of the trickling biofilter, the simplest effective option consists of inoculating the mixture of the biological reactor with liquor in an urban wastewater treatment plant. In a relatively short period of time a selection of the microorganisms occurs in favor of the sulfide-oxidizing bacteria and a high elimination yield can be obtained the same week the inoculation takes place, always depending on the loads to be treated.
The investment costs of a biological desulfurization process are slightly lower than those for the chemical system. Nevertheless, where the difference is very noticeable is in the operation costs, as chemical reagents are not used and there is hardly any waste. This factor makes it economically viable to convert traditional chemical systems to biological.