Enhancing methane production and organic loading capacity from high solid-content wastewater in modified granular activated carbon (GAC)-amended up-flow anaerobic sludge blanket (UASB)

An integrated review on the role of different biocatalysts, process parameters, bioreactor technologies and data-driven predictive models for upgrading biogas

As energy consumption and waste generation from human activities continue to rise, the technology of anaerobic digestion (AD), which converts waste into bioenergy, has gained popularity. Biogas produced from AD commonly contains 60 % CH4, 40 % CO2 and a minor fraction of impurities. Currently, several anaerobic reactors have been designed to upgrade the biogas with biomethane content above 90 %. This review summarizes the current trends in the biological upgradation of biogas from a bio-circular economy perspective to achieve sustainable energy goals. Examples of applications reporting the latest advancements in treating industrial effluents using high-rate anaerobic reactors have been mentioned. The integrated anaerobic-aerobic hybrid reactor offers a solution to the limitations of traditional methods in treating diverse effluents. A special focus on biological upgradation techniques such as in-situ, ex-situ, and hybrid mechanisms have been briefed. The key advantage of hybrid upgradation is its ability to address the pH rise during in-situ process. Additionally, the applications of artificial neural networks and optimization to upgrade biogas production have been discussed. The review concludes with future research directives with emphasis on the economic viability of the approaches.

In-depth exploration of microbial electrolysis cell coupled with anaerobic digestion (MEC-AD) for methanogenesis in treating protein wastewater at high organic loading rates

High concentrations of protein wastewater often reduce treatment efficiency due to ammonia inhibition and acid accumulation caused by its low carbon-to-nitrogen ratio (C/N) after digestion, as well as its complex structure. This study investigates the performance of a microbial electrolysis cell (MEC) driving a protein digestion system with gradually increasing organic loading rates (OLR) of bovine serum albumin, elucidating microbial changes and methanogenic metabolic pathways on bioelectrodes under high OLR "inhibited steady-state" (ISS) conditions. The results showed that the accumulation of ammonia nitrogen (AN) from protein hydrolysis under high OLR conditions disrupted microbial growth and caused cell death on the electrode surface, hindering the electron transfer rate. Toxic AN reduced protein hydrolysis, led to propionate accumulation, inhibiting the acetoclastic methanogenesis process and favoring the hydrogenotrophic pathway. As OLR increased from 6 to 11 gCOD/L, cumulative methane production increased significantly from 450.24 mL to 738.72 mL, while average methane yield and production rate decreased by 10.51% and 50.28%, from 375.20 mL/gCOD and 75.04 mL/(gCOD·d) to 335.78 mL/gCOD and 37.31 mL/(gCOD·d), respectively. Despite these declines, the system maintained an ISS. Moderate OLR increases can achieve an ISS, boosting protein waste treatment capacity, methane production, and net energy output (NEO), with an OLR of 6 gCOD/L being optimal for maximizing NEO per unit substrate. These findings provide theoretical insights into the methanogenesis pathway of high OLR proteins in MEC-AD systems and offer an effective method for treating high OLR protein wastewater in future practical applications.

Enhancing anammox process with granular activated carbon: A study on Microbial Extracellular Secretions (MESs)

Granular activated carbon (GAC), a porous carbon-based material, provides increased attachment space for functional microorganisms and enhances nitrogen removal by facilitating extracellular electron transfer in the anammox process. This study investigates the effects of GAC on the biosynthesis of microbial extracellular secretions (MESs) and explores the roles of these secretions in anammox activities. Four lab-scale reactors were operated: two downstream UASB reactors (D1 and D2) receiving effluents from the upstream UASB reactors (U1: no-GAC, U2: yes-GAC). Our results indicate that MESs were enhanced with the addition of GAC. The effluent from U2 exhibited a 59.62 % higher amino acid content than that from U1. These secretions contributed to an increase in the nitrogen loading rate (NLR) in the downstream reactors. Specifically, NLR in D1 increased from 130.5 to 142.7 g N/m3/day, and in D2, it escalated from 137.5 to 202.8 g N/m3/day, likely through acting as cross-feeding substrates or vital nutrients. D2 also showed increased anammox bacterial activity, enriched Ca. Brocadia population and hao gene abundance. Furthermore, this study revealed that D2 sludge has significantly higher extracellular polymeric substances (EPS) (48.71 mg/g VSS) and a larger average granule size (1.201 ± 0.119 mm) compared to D1 sludge. Overall, GAC-stimulated MESs may have contributed to the enhanced performance of the anammox process.

A new substrate equalization method for optimizing the influent conditions and fluid flow patterns of a multifed upflow anaerobic sludge blanket reactor with mature anammox granules

To improve nitrogen removal efficiency (NRE) and achieve homogenous distribution of anammox sludge and substrate, a new substrate equalization theory and a cumulative overload index was proposed for multifed upflow anaerobic sludge bed (MUASB) reactors with mature anammox granules. The performance and flow patterns of MUASB reactors were investigated under various influent conditions. The results showed that the nitrogen removal performance and stability of MUASB reactors could be optimized by minimizing the cumulative load. The NRE gradually increased from 83.3 ± 2.2 %, 86.8 ± 4.2 % to 89.3 ± 4.1 % and 89.7 ± 1.6 % in feeding flow tests and feeding port tests, respectively. Furthermore, the flow patterns were compared based on residence time distribution and computational fluid dynamics, indicating that a better equilibrium distribution of microorganisms and substrates could be achieved in the MUASB reactors under the lowest cumulative load. Therefore, substrate equalization theory can be used to optimize the nitrogen removal performance of MUASB reactors with low-carbon footprints.