Digester Microaeration: A Comprehensive Full-Scale Case Study

Digester Microaeration: A Comprehensive Full-Scale Case Study 1. Introduction In-situ digester microaeration is an emerging solution for biogas hydrogen sulfide (H2S) control that offers several advantages including reduced chemical inputs and no need for additional treatment vessels. In this process, small quantities of oxygen are introduced directly into an anaerobic digester to facilitate conversion of hydrogen sulfide to elemental sulfur via sulfur oxidizing bacteria. The use of microaeration for hydrogen sulfide control has been described at bench scale (Azizi et al., 2023) and has been implemented to a limited extent in full-scale applications (Muller et al., 2022). However, very little information exists on the viability of using microaeration for hydrogen sulfide control in anaerobic digesters at municipal wastewater treatment facilities (Kraakman et al., 2023). 2. Objectives This study had several goals. A primary objective was to determine the optimal oxygen additional method and dose to facilitate in-situ biogas desulfurization via microaeration. This study was also designed to understand the system impacts of switching from ferric chloride addition to microaeration. This included looking at digestion impacts such as biogas quality and production, volatile solids destruction, and changes in sludge chemistry. Further, this study looked at other 'downstream' impacts including impacts to digested sludge thickening and filtrate processes, biosolids quality, biogas treatment, and corrosion concerns for the digester structure and appurtenances. 3. Study Setup This study involved microaerating a 1 million gallon mesophilic anaerobic digester treating municipal sewage sludges at the Nine Springs Wastewater Treatment Plant in Madison, WI. Microaeration was accomplished by addition of either compressed air or concentrated oxygen (~90% O2) using an oxygen concentrator. Oxygen was injected into the heated sludge recirculation line for the digester, along with ferric chloride. Oxygen and ferric chloride doses were adjusted over time to maintain biogas H2S concentrations similar to other digesters at the facility. The trial was conducted for 11 months. 4. Findings 4.1 Biogas Desulfurization and Biogas Quality Overall, results in Figures 1 and 2 show that microaeration is an effective method for reducing biogas H2S concentration. With air, biogas H2S was kept below 300 ppm while employing a ferric chloride dose 65% less than typical. However, with air, up to 330 cfh was needed to maintain H2S below 300 ppm. This resulted in a headspace oxygen content of 1.6-1.8% O2 and caused methane content to dip to 55% due to oxygen and nitrogen dilution. In contrast, when switching to concentrated oxygen, a gas flow of 55 cfh O2 was needed to maintain H2S below 200 ppm. At this dose the O2 content in the headspace was 0.6% or less and methane content was similar to the non-microaerated digesters (60% CH4). Overall, concentrated oxygen appears to be a more effective means of facilitating microaeration as desired results were achieved with lower oxygen dose, lower headspace oxygen content, and negligible dilution impact to the biogas. 4.2 Digestion Performance Microaeration did not result in identifiable negative impacts to digestion performance, and appears to have resulted in a slight improvement in volatile solids destruction. Digester pH and alkalinity increased slightly, ostensibly due to reducing in ferric chloride dose. Additionally, intermediate vs partial alkalinity ratio with microaeration remained similar to the ratio prior to microaeration. This demonstrates that the addition of oxygen directly to the sludge matrix did not have an inhibitory effect on methanogenesis. Figure 3 shows that volatile solids destruction in the test digester improved over time with microaeration. Prior to the study, the test digester had a lower volatile solids destruction that the average of the remaining digesters at the facility, whereas with microaeration the test digester showed a volatile solids destruction up to 4% higher than the remaining digesters. Biogas and methane yield also did not significantly change, further supporting the conclusion that injecting oxygen directly into the sludge matrix did not negatively impact methanogenesis. 4.3 Other Impacts of Interest One particular concern at the test facility is the formation of nuisance struvite due to high background magnesium and use of biological phosphorus removal. Dosing ferric chloride to the digesters is done to mitigate H2S, but this also helps bind phosphorus to reduce struvite formation potential in the digester. Over time the ortho-phosphate concentration in the sludge increased roughly 30%. However, there was no clear evidence of nuisance struvite formation in the digester as evidenced by lack of struvite accumulation in the sludge heat exchanger and associated piping as well as no discernable difference in material accumulation inside the digester during cleaning operations compared to other reactors. This observation, however, implies a greater side-stream phosphorus load back to the head of the facility. This load either needs to be dealt with in the main liquids treatment process, or could improve the viability of a phosphorus recovery process. 4.4 Microbial Corrosion Perhaps of greatest interest were findings during digester cleaning after 11 months of microaeration. As expected, the entire headspace of the reactor was coated in a yellow/white crust composed primarily of elemental sulfur. However, chemical analysis revealed that this crust also contained elevated concentrations of calcium, aluminum, iron, and magnesium, along with elevated sulfate. Presence of these constituents points to concrete degradation due to sulfuric acid production. Further, physical inspection of the concrete revealed surface softening, confirming that microbial corrosion of the concrete had occurred. While chemical analysis of the crust suggests that sulfide is primarily being converted to elemental sulfur, the level of concrete deterioration observed suggests long-term operation would be unwise without first treating the concrete to prevent corrosion. 5.0 Conclusions Results indicate that in-situ microaeration is an effective method to control biogas H2S. Use of concentrated oxygen appears to be more effective in terms of oxygen dose required and avoiding biogas dilution with nitrogen. Reduction in ferric chloride addition resulted in increased digestate pH and ortho-phosphate concentrations, but evidence of nuisance struvite formation was not identified. Evidence of sulfuric acid production and concrete deterioration were detected in the digester headspace, indicating that the use of microaeration is appropriate for applications where headspace construction is achieved with materials resistant to sulfuric acid attack.