Full-scale demonstration of micro-aeration for hydrogen sulfide control at the Chambers Creek Regional Wastewater Treatment Plant

Wastewater utilities are increasingly engaged in projects that beneficially utilize biogas in renewable energy applications. Although the degree to which the biogas must be cleaned or upgraded will depend on its end use, the removal of hydrogen sulfide (H2S) to varying degrees will always be required. Historically, hydrogen sulfide has been treated using chemical scrubbers or adsorptive processes, but these tend to be costly and maintenance intensive. The addition of small amounts of oxygen, whether by air or high purity oxygen systems, to anaerobic digesters has been reported to be an effective and economical mitigation measure for hydrogen sulfide. The Pierce County Department of Public Works and Utilities (County) uses digester gas at their Chambers Creek Wastewater Regional Wastewater Treatment Plant to power boilers which were installed in 2016. Although the boilers are relatively new, the County has had to replace fouled boiler tubes approximately every six months of operation due to the levels of hydrogen sulfide in the biogas (2200 ppm, on average). (Figure 1) Consequently, the County requested that Brown and Caldwell (BC) evaluate different technologies to reduce hydrogen sulfide concentrations to levels closer to boiler manufacturer recommendations (10 parts per million by volume [ppmv]), which would in turn reduce maintenance and costly tube replacement. Although the use of sacrificial media is a feasible alternative and one that the County may implement in coming years, the level of treatment required for the current biogas hydrogen sulfide concentrations would require frequent and costly media replacement. BC recommended that the County pilot microaeration, which would not reduce H2S concentrations to the levels required but rather extend the life of any adsorptive/sacrificial media and reduce associated operating costs. The pilot design was informed by a literature review conducted prior to implementation in order to ascertain appropriate design and control parameters and to review the particular importance of biogas residence time in gaseous hydrogen sulfide removal efficiency and air requirements for effective sulfide reduction. Residence times of approximately five hours showed a relatively consistent removal of 90%; the results of the literature review on air quantity required was mixed. Various sources indicated that, in pilot systems, approximately two to three times the theoretical air requirement was needed to affect the anticipated hydrogen sulfide reduction, while in full scale systems, about four times the theoretical air requirements are needed, potentially due to mixing and oxygen transfer as well as different surface area-to-volume ratios. A linear relationship between mass of air supplied and mass of H2S removed was reported to be approximately 2.05 kg O2/kg H2S removed, which is again four times the theoretical value based on oxidation to sulfur. Finally, oxygen concentrations in the digesters subjected to microaeration were found to be on the order of 1-2% The Chambers Creek microaeration pilot was designed in two phases. Data on two digesters receiving the same feed loads (total and volatile solids) were collected as part of phase one, prior to the beginning of aeration, in order to baseline existing conditions and variations in digester operation (see Table 1). Data were collected for approximately five months between August and December of 2021. Phase two began in January of 2022; one digester was chose as the treatment digester, and one as the control. 90 scfh of plant air was introduced into the sludge feed line upstream of the treated digester through an air injection assembly, with data continuing to be collected on both digesters for comparison. The initial aeration dose was chosen based on the County's desire to maintain the minimum reported oxygen level of 0.2% in the digester so as to minimize disruption to the digestion process. At biogas sulfide loads of approximately 16kg/day, the air flow rate of 90scfh corresponds to a ratio of 1.03 kg O2/kg H2S and a molar ratio of 1.09 mol O2/mol H2S. At these ratios, approximately 12% removal in hydrogen sulfide (mass basis) in the aerated digester was realized after three weeks of operation. Both the mass and molar ratios are generally lower than those identified in the literature review, data that, when coupled with the significantly lower biogas residence time, indicate that lower aeration rates may still positively impact sulfide removal. The full-scale demonstration of micro-aeration at Pierce County, resulted in an approximate 15% reduction in the biogas hydrogen sulfide levels; a reduction less that that reported possible in the literature. An evaluation of the data, digester microbial population data and digester gas system inspection provided valuable lessons learned surrounding the implementation of microaeration at scale. These include: - The addition of small quantities of oxygen do not negatively impact the anaerobic community. - Surface area is critical for microaeration to successfully remove H2S - Digester gas management systems in systems with microaeration should be designed such that biofilm and sulfur management are possible (Figure 2) - Microbial community analysis indicates that primary H2S metabolism is likely to be in the biofilm and not the bulk digestion phase. - Online monitoring is critical to process success and control, with feedback control (Figure 3) - Digestion processes that have relatively small headspaces (ex. egg-shaped digesters, submerged fixed covers) may want to consider ex-situ treatment of H2S. This paper will discuss the results of this pilot as well as the lessons learned and how they may be applied to future installations to mitigate the challenges of this emerging process.