CFD simulation and performance evaluation of gas mixing during high solids anaerobic digestion of food waste

Role of biogas stirring in alleviating acidification and promoting methanogenesis during the anaerobic digestion of food waste: Macroscale and microscale perspectives

Anaerobic digestion (AD) represents an important solution for reducing the annual increase in global food waste, by facilitating the recycling of food resources. In the present study, we introduce a biogas stirring reactor (RB) to address the over-acidification in AD systems, enhancing methanogenic efficiency. Compared with the mechanical stirring reactor (RM), the RB exhibited higher methane yields (219 ± 17 mL CH4/g volatile solids (VS)added/d) and stable pH values (7.0 ± 0.2) during the steady-state phase. Computational fluid dynamics analysis highlighted the superior vertical mixing and flow velocity in RB, which reduced the sludge particle size (359 μm in RM vs. 100 μm in RB) and extracellular polymeric substance content (283 mg/g VS in RM vs. 126 mg/g VS in RB), thereby enhancing microorganism interactions and AD efficiency. Notably, biogas stirring enriched hydrogenotrophic methanogens, particularly, Methanobacterium, rebalancing the AD process. Microbial community analysis confirmed the positive impact of biogas stirring on hydrogen utilization by microorganisms. Our findings highlight the effectiveness of biogas stirring in enhancing the methane yield and present a viable approach for managing over-acidification in AD systems.

CFD modelling and simulation of anaerobic digestion reactors for energy generation from organic wastes: A comprehensive review

Anaerobic digestion (AD) has been recognized as one of the most viable options for the treatment of a wide range of waste materials. Complex structure of wastes is safely broken down to destroy pollutants and pathogens. Biogas is produced as a by-product of this process which is considered as a clean energy resource. However, provision of controlled environment for microbial activities is critical to ensure the required process efficiency. This can only be achieved with the efficient design of controlled vessels used for anaerobic digestion, termed as AD reactors. AD functions such as mixing, hydrodynamics, multiphase interaction, heat transfer, temperature distribution and bio kinetics are significantly affected by the reactor shape, design and configurations, hence making it essential to optimize the reactor design before installation at large scale. Mostly, such optimization is carried out with the help of lab scale experimentations and testing protocols which result in high costs for repeating several design experiments. Computational fluid dynamics (CFD) is an applied mathematical tool which helps to understand and predict the fluid dynamics, heat flow as well as species transport in different domains. This approach contributes to minimize the experimental costs while optimizing the reactor configurations in less time. The current review is presented to summarize and discuss the core characteristics of AD process followed by concerned CFD attributes. Research gaps and critical challenges are identified in different aspects such as reactor design, and configuration, mixing, multiphase flow, heat transfer, biokinetics as well as machine learning approaches.

Mass Transfer Enhancement in High-Solids Anaerobic Digestion of Organic Fraction of Municipal Solid Wastes: A Review

The increasing global population and urbanization have led to a pressing need for effective solutions to manage the organic fraction of municipal solid waste (OFMSW). High-solids anaerobic digestion (HS-AD) has garnered attention as a sustainable technology that offers reduced water demand and energy consumption, and an increased biogas production rate. However, challenges such as rheology complexities and slow mass transfer hinder its widespread application. To address these limitations, this review emphasizes the importance of process optimization and the mass transfer enhancement of HS-AD, and summarizes various strategies for enhancing mass transfer in the field of HS-AD for the OFMSW, including substrate pretreatments, mixing strategies, and the addition of biochar. Additionally, the incorporation of innovative reactor designs, substrate pretreatment, the use of advanced modeling and simulation techniques, and the novel conductive materials need to be investigated in future studies to promote a better coupling between mass transfer and methane production. This review provides support and guidance to promote HS-AD technology as a more viable solution for sustainable waste management and resource recovery.

Effect of optimized intermittent mixing during high-solids anaerobic co-digestion of food waste and sewage sludge: Simulation, performance, and mechanisms

Inadequate mixing has been proven to be a major cause of anaerobic digester failure. This study revealed the mechanism of mixing intervals on high-solids anaerobic co-digestion (HS-AcoD) of food waste (FW) and sewage sludge (SS). Optimized intermittent mixing time (15 min/h) was determined through computational fluid dynamics (CFD) simulation. Experimental results indicated that the simulated intermittent mixing could shorten digestion time and increase cumulative methane output (366.8 mL/gVS) compared with continuous mixing and unmixing. Mixing could considerably accelerate substrate solubilization and hydrolysis. Maximum rates of acidogenesis (53.35 %) and methanogenesis (49.41 %) were observed with an optimized intermittent mixing (15 min/h). Vigorous mixing induced apoptosis and disrupted syntrophic metabolism, whereas intermittent mixing promoted the syntrophic metabolism between Syntrophomonas and Methanobacterium, and led to an enrichment of genes involved in acidogenic and methanogenic pathways. These findings have important implications for the development of an optimized intermittent mixing strategy for maximizing HS-AcoD efficiency of FW and SS.

Insights into high-solids anaerobic digestion of food waste enhanced by activated carbon via promoting direct interspecies electron transfer

High-solids anaerobic digestion (HS-AD) of food waste frequently confronted the acidification and failure under high organic loading rates (OLRs). Results indicated powdered activated carbon (PAC) addition significantly enhanced methane production and process stability than granular activated carbon, and columnar activated carbon at higher OLRs via accelerating the propionate consumption. Potential direct interspecies electron transfer (DIET) partners, including various syntrophic oxidation bacteria and methanogens, were enriched with the activated carbon (AC) addition. Furthermore, DIET contribution to methane production was 35% by PAC, predicated by the modified Anaerobic Digestion Model No.1 (ADM1). This study deeply elucidated the DIET mechanism and offered the potential foundations for the selection and applications of AC-based materials in HS-AD of food waste.