Full-scale Digester Micro-Aeration Studies to Improve Biogas Quality

ABSTRACT
Biogas generated from anaerobic digestion typically requires gas conditioning before it is used to generate energy. Elevated concentrations of hydrogen sulfide (H2S) in the biogas is often a limitation for downstream processing of biogas. Full-scale tests using micro-aeration technology was undertaken at several water resource recovery facilities and showed that this new process-integrated technology has potential using minimum plant upgrading while contributing to the sustainability and economic efficiency of the energy recovery process of waste sludge digestion at water resource recovery facilities (WRRFs). Keywords: anaerobic digestion, biogas, energy recovery, hydrogen sulfide, micro-aeration.

BACKGROUND
Biogas generated from anaerobic digestion of sludge typically requires gas conditioning before it is used to generate energy. The high concentration of H2S in biogas typically entails a limitation for downstream processing of the biogas as it creates maintenance related corrosion problems in combustion engines, and the release of SOx in ‚ue gases. Traditional physical/chemical end-of-the-pipe technologies (i.e. chemical scrubbing using caustic/hypochlorite or iron oxide-based adsorbents) are relatively expensive and can present relatively high environmental impacts. Biotechnologies (i.e. aerobic or anoxic biotrickling filtration) or physical-chemical combinations with biotechnology (i.e. caustic scrubbing followed by biological oxidation) have recently emerged as cost-effective and more environmentally friendly alternatives. Nevertheless, these biotechnologies are still limited by their high investment costs and their generated byproducts such as sulfuric acid and/or elemental sulfur slurry that need further processing or disposal. Micro-aeration, which involves the dosing of a small amount of air during the digestion process, is a process-integrated biochemical method of removing H2S from the biogas. Micro-aeration has been researched and used in industrial biogas plants, but rarely tested at municipal WRRFs. It has been applied in eastern-Europe in full-scale sludge digesters at WRRFs, with H2S removal efficiency of more than 90% in most cases. Moreover, micro-aeration improved the degradability of COD and volatile suspended solids in all cases. The studies discussed in this presentation are, as far as the authors know, the first full-scale tests in Spain and North-America that try to overcome the lack of full-scale experience with micro-aeration in digesters at WRRFs.

OBJECTIVE
The main objective of this presentation is to share information about micro-aeration and explain how it has potential as an effective digester biogas conditioning technology. The presentation will include research and full-scale testing data and information. These full-scale experiences may provide a stepping-stone for further development and wider implementation.

METHODOLOGY
Micro-aeration technology was tested over a period of several months in different full-scale digesters at different WRRFs. A small amount of air was injected in the sludge recirculation mixing pipe after the pump using fine bubble air injection nozzles or in the headspace of the digesters. The biogas H2S concentrations and sludge characteristics such as pH, alkalinity, total solids, volatile solids were measured daily, while the biogas production and sludge feed were measured continuously.

RESULTS
A comparison between the different types of digesters (i.e. digesters with mechanical mixing and digesters with internal biogas recirculation for mixing) are made to provide better understanding in how micro-aeration can best be applied. An example of the reduction of the H2S concentration in the biogas before and after introducing the micro-aeration process is illustrated in Figure 1. The H2S concentration in the produced biogas dropped by more than 80% from 5,000 ppmv to less than 1,000 ppmv when the micro-aeration was stopped. Directly after stopping the micro-aeration the H2S concentration increased back up to 5,000 ppmv. Figure 1: Reduction of H2S in the biogas during the initial phase 1 of one of micro-aeration tests. The presence of small amounts of oxygen in the anaerobic digestion process in the full-scale test did not significantly impact neither on organic matter removal nor on the biomethane productivity when the amount of air injected is limited. In fact, these full-scale tests (like other bench-scale studies) revealed that, besides efficiently desulfurizing biogas, micro-aeration can potentially improve organic matter degradation during municipal sludge digestion. The volatile solids content dropped in this full-scale test by a few percent from about 66.6% without to about 63.6% with the micro-aeration technology. This drop in volatile solids and a potential more stable digestion process was likely caused by the higher hydrolysis rates and/or alleviation of sulfide toxicity. The potential capital cost savings as well as operational cost savings that micro-aeration technology can provide is substantial. The cost saving on iron sponge media change-out are determined and will be presented. Some of the lessons learned from the full-scale tests include (1) the effectiveness of the air injection method using fine bubble air injection nozzles, (2) the identification of the locations where not to install the nozzles and (3) the quantification of the consequence of overdosing of the injected air amount. Multiple full-scale application of MA in digesters have been reported for different type of digesters (Table 1). The overview shows the different full-scale applications including digester size, organic feed material, biogas production, digester mixing method, biogas production and H2S reduction obtained in the biogas. In general, 80 to over 95% of H2S reduction have been achieved in multiple full-scale applications. Table 1: Example of Full-scale Digester Micro-Aeration Studies and Applications for Biogas Desulfurization.

CONCLUSION
From the full-scale tests and related research, it can be concluded that micro-aeration technology holds great potential for being an effective gas conditioning technology for different types of digesters along with requiring minimal capital investment and operating costs. Micro-aeration would also further contribute to the sustainability and economic efficiency of the energy recovery process during the waste sludge digestion at a WRRF.