H2s free biogas. From anaerobic to microaerobic

Resum

Limited oxygen supply (or air) to anaerobic sludge digesters, thus creating microaerobic conditions, to remove hydrogen sulphide from the biogas was studied in pilot-plant scale digesters. Digesters with 200L working volume in mesophilic conditions fed with real sludge from a wastewater treatment plant were employed (WWTP). Microaerobic conditions successfully removed hydrogen sulphide from the biogas with an efficiency above 98% while the organic matter removal and methane yields were maintained. The process was found to be robust, recovering quickly from failures in oxygen supply or variations of the sulphur load to the digester and showed that the relation Oxygen rate / Biogas production was an appropriate parameter to control the oxygen dose. Both air an oxygen showed similar removal efficiencies for similar oxygen rates supplied to the digester. However, the utilization of air resulted in methane dilution in the biogas as a consequences of nitrogen presence; methane concentration in this conditions was reduced between 2% and 4% in the biogas. The main product of the oxidation was elemental sulphur, partly accumulated in the headspace of the digester. Molecular biology studies showed that sulphide-oxidising microorganisms developed in this area over the liquid surface. Sulphur accumulation was lower when biogas recirculation was employed to mix the digesters as a result of sulphur dragging from the headspace. Furthermore, this mixing method was also useful to remove dissolved sulphide from the liquid with an efficiency higher than 90%. Mixing conditions in the digester did not significantly alter the removal, neither than the dosing point. Nevertheless, oxygen supply to the headspace was found the optimal dosing point to limit oxygen utilization in other processes different from sulphide oxidation. The employment of air as oxidant reactive showed the better balance in economic terms when applied a full-scale WWTP. Implementation of the microaerobic removal can be achieved with payback periods of 3.7 years and operational costs were 4.7 lower than the utilization of ferric chloride. Finally, batch-tests to explore the kinetics of the degradation of cellulose showed that microaerobic assays presented a shorter lag-phase time in biogas production when compared to the anaerobic conditions, then accelerating initial degradation complex organic matter.