Micro-Aerobic Digestion For Advanced Process Stability and Biogas Desulfurization

INTRODUCTION Many anaerobic digestors have not been fully driven to its stable performance potential. Despite the many merits, the anaerobic digestion (AD) process has several drawbacks such as low hydrolysis rates, unstable at high organic loading rate, and release of toxic/corrosive hydrogen sulfide (H2S). Suboptimal AD performance may mean under-utilised biogas production, foaming events, or poorly dewatered cake. Traditional AD processes will benefit from a facelift that maximizes biogas production, enhancing process stability and broadening substrate treatability without major infrastructure modification. Moreover, elevated concentrations of H2S in the biogas is often a limitation for downstream processing of biogas that typically requires expensive gas conditioning before it is used to generate energy. This facelift could be given by implementing micro-aeration, a process-integrated technology that is defined as a system with zero concentration of oxygen present in the sludge and limited (trace) consumption of oxygen. Bench-scale and full-scale tests of organic waste digestion processes using micro-aeration technology was undertaken. The tests confirmed that this process-integrated technology has potential to use minimum plant upgrading while contributing to the sustainability and economic efficiency of the energy recovery process as operated for example at water resource recovery facilities (WRRFs). Pre-treatment processes such as THP is installed progressively to improve the bioavailability of the organic substrate in the digester and bust the methane production. Despite the advantages of a high-thermal pre-treatment processes, microbes in the feedstock have been killed-off resulting in a different, and potentially a reduced diverse microbial community in the digesters, which could make the AD process vulnerable. Biogas generated from anaerobic digestion of sludge typically requires gas conditioning before it is used to generate energy as it can create maintenance related corrosion problems in combustion engines, and the release of SOx in flue gases. Micro-aeration has been researched and used in industrial biogas plants with H2S removal efficiency of more than 90% in most cases (Jenicek et al., 2017, Diaz et al., 2017, Kraakman et al., 2019). Moreover, micro-aeration has shown to improve the degradability of COD and volatile suspended solids in some cases and enrich the diversity of the microbial community potentially improving process stability. OBJECTIVE The main objective of this presentation is to share information about micro-aeration and explain how it has potential to enhance the digestion process and improve the biogas quality. The presentation will include bench-scale research and full-scale testing data as well as several lessons learned. METHODOLOGY Three main full-scale case studies at different WRRFs are used to illustrate the potential and the challenges when implementing micro-aeration technology. For these case studies micro-aeration was tested over a period of several months and up to one year. In all cases, air was injected in the sludge recirculation pipeline after the recirculation pump using fine bubble air injection nozzles. The biogas H2S concentrations and sludge characteristics such as pH, alkalinity, total solids, volatile solids were measured daily using industry standard methods, while the biogas production and sludge feed were measured continuously. Separately, bench-scale testing focused on investigating benefits from micro-aeration on digestion stability and performance. 20-L bench-scale digestion testing used Napier grass (5 months old, a typical energy crop) with and without micro-aeration with cattle manure-derived inoculum were performed at OLR of 5.0 g VS/L/day. Microbial community structure and function were investigated by the analysis of 16S rRNA gene and meta-genomics sequencing. RESULTS Bench-scale research at an organic loading rate of 5 g volatile solids (VS)/L/day showed increased digester stability and performance with micro-aeration, operating at residual acetic acid concentrations of 3.0 g/L, compared to instable control at 9.2 g/L without aeration introduction as shown in Figure 1 (Nguyen et all., 2019). The test digester with micro-aeration also showed an increase in methane yield. 16S rRNA gene sequencing analysis shows increase in relative abundance of facultative bacteria with micro-aeration, while the dominance of hydrogenotrophic methanogens remained similar with and without micro-aeration (Wu et all., 2021). Full-scale micro-aeration test were performed mainly as a means to reduce H2S concentration in the biogas. An example of the reduction of the H2S concentration in the biogas before and after introducing the micro-aeration process is that the H2S concentration in the produced biogas dropped by more than 80% from 5,000 ppmv to less than 1,000 ppmv in the first case study; the second case study showed a decreased H2S concentrations from 1,000 ppmv to 200 ppmv (see Figure 2). The presence of small amounts of oxygen in the anaerobic digestion process in the full-scale test did not impact 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. However, not all attempts of implementing microaeration have resulted in consistent H2S removal; the third case study showed that the overall average was only 40% suspected to be due to limitations in oxygen transfer. The potential capital cost savings as well as operational cost savings that micro-aeration technology can provide is substantial (see Table 1). Lessons learned to be shared as part of this manuscript and presentation are related to: - method and locations for micro-air injection - full-scale implementation design including safety measures - potential risk of elemental sulfur accumulation in downstream equipment - process model development to support process understanding and optimization CONCLUSION From the full-scale tests and related bench-scale research, it can be concluded that micro-aeration technology holds great potential for being an effective gas conditioning technology and enhance the digestion process, while requiring minimal capital investment and operating costs. Micro-aeration would further contribute to the sustainability and economic efficiency of the energy recovery process of organic waste digestion as operated for example at WRRFs.