Di Costanzo, Nicola Harnessing magnetism and biogas micro-oxygenation to enhance the biomethane and fertilizing potential of sewage sludge. [Tesi di dottorato]

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Abstract

Sewage sludge, a byproduct of wastewater treatment, presents a significant challenge due to its volume, composition, and environmental impact. It is a complex mixture of organic and inorganic matter, its characteristics varying depending on the wastewater source and the treatment process employed. Over the years, the growing population and urbanization have fueled the expansion of wastewater treatment plants, leading to a concomitant surge in sewage sludge production. This trend has been further exacerbated by stricter effluent quality requirements, which necessitate more intensive treatment processes and, consequently, higher sludge generation. Anaerobic digestion (AD) is a promising alternative to traditional sewage sludge disposal methods. AD is a biological process that converts sewage sludge into methane gas, a valuable renewable energy source. The generation of biogas comes along with the production of anaerobic digestate with potential fertilizer properties due to the significant content of both macro- and micro-nutrients. It also helps to reduce the volume and weight of sewage sludge, making it easier to transport and dispose of. However, AD is not without its challenges. The complex organic matter (OM) composition of sewage sludge can make it difficult to digest anaerobically. Digested sludge tends to concentrate potential contaminants, thereby limiting its reuse in agriculture to fall within the limits set by the European directive 86/278/EEC. To overcome these challenges, several studies have implemented processes that can break down OM components increasing biodegradability, dewaterability of sewage sludge, and, consequently, biogas production or that can act positively on the agronomical and hygienic properties of sewage sludge. Among these, the application of static magnetic field (SMF) has been scarcely evaluated and recently has gained increasing interest. Also, the effects of SMF application on the agronomic properties of digestate have not yet been addressed. Besides, inhibitory substances in the OM can also interfere with the anaerobic digestion process. Hydrogen sulfide (H2S) is a common byproduct in biogas, produced through anaerobic digestion of sulfur-rich organics and the activity of sulfate-reducing bacteria (SRB). H2S levels in biogas, which can range from hundreds to thousands of ppm, are influenced by the feedstock's sulfur content and the microbial competition for organic substrates. Elevated H2S levels not only harm biogas processing equipment but also lower its economic and reuse value. During combustion, H2S gets converted to sulfur oxides (SOx) and sulfuric acid (H2SO4), causing severe corrosion in infrastructure. Additionally, H2S and SOx can deactivate oxidation catalysts and inhibit methanogenic bacteria at concentrations above 50 mg/L, further stressing the need for careful management of H2S in biogas systems. Microaeration has been reported as an efficient and cost-effective method for in-situ H2S removal from biogas, but it leads to the dilution of the CH4 produced during the process mainly due to the presence of nitrogen gas. Injecting small amounts of pure oxygen (micro-oxygenation) instead of air in the digester headspace to prevent nitrogen gas contamination of biogas, can turn in an important challenge if the target is biogas upgrading to biomethane. Several studies have demonstrated the efficacy and cost-effectiveness of microaeration in removing H2S directly within biogas digesters. However, this approach suffers from methane dilution due to the presence of nitrogen gas introduced with air. To overcome this limitation and achieve efficient biogas upgrading to biomethane, micro-oxygenation - the controlled injection of pure oxygen into the digester headspace - emerges as a promising alternative. This method eliminates nitrogen gas contamination, thereby minimizing methane dilution and increasing the methane content of the upgraded biogas. The primary objective of this doctoral thesis was to devise a comprehensive strategy for enhancing the AD of sewage sludge, particularly in terms of increased biomethane yield, nutrient recovery, and inhibitory compound removal. Towards this goal, the impact of SMF on AD processes and the biological biogas desulfurization via in-situ micro-oxygenation in real-scale anaerobic digesters were investigated. The impact of SMF on AD of sewage sludge was evaluated in various settings: a) as post-treatment to improve the agricultural value of sewage sludge digestate by promoting the precipitation of valuable fertilizers (i.e., struvite); b) as pre-treatment to enhance the biomethane potential; c) during anaerobic digestion in a continuous side-stream recirculation system, ensuring uninterrupted sludge exposure to the SMF. Low-intensity SMF was found to be extremely effective in both batch and continuous configurations, as it produced an improvement in biomethane production up to 12.7% and 48.3%, respectively. In contrast, high-intensity SMF (1.5 T) pre-treatment exhibited detrimental effects on methane generation, resulting in a 15.1% decline, indicating an inverse relationship between SMF intensity and biogas production. Nevertheless, high-intensity SMF post-treatment proved valuable in promoting the chemical precipitation of beneficial compounds with fertilizer properties (e.g., struvite) in anaerobically digested sludge, thereby enhancing its fertilizer potential. This thesis work focused also on the biological removal of sulfur compounds from biogas, followed by biomethane production, using in-situ micro-oxygenation in large-scale anaerobic digesters that process high-solid AD under thermophilic conditions. A stable hydrogen sulfide (H2S) removal efficiency of 98.2(±1.3) % was achieved with an oxygen (O2) dose of 0.96 NL/Nm3 biogas. At the lowest O2 dose tested, removal efficiencies of 67.4(±0.7) % were observed. The response time of the biological desulfurization system to transient oxygen conditions was also evaluated through intermittent O2 injection, proving that full recovery of desulfurization performance was within 10 hours. Moreover, by carefully introducing small amounts of O2 into the anaerobic digester, the digestion performance was not negatively impacted and enabled the achievement of biogas quality that complies with the specifications for biomethane in terms of both O2 and H2S contents. Using a Next Generation Sequencing (NGS), targeting bacterial 16S rRNA gene, the main microbial families responsible for biological H2S oxidation developing on the digester surface and internal walls were identified (Lentimicrobiaceae, Caldicoprobacteraceae, DTU014, Syntrophomonadaceae, and Rhodobacteraceae). Therefore, by embracing the innovative treatments proposed in this thesis work, it seems possible to increase the environmental and economic benefits of sewage sludge management, turning it into a driving force for sustainable practices and resource conservation.

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