Red mud-based inorganic polymer spheres: Innovative and environmentally friendly anaerobic digestion enhancers

Unlocking anaerobic digestion potential via extracellular electron transfer by exogenous materials: Current status and perspectives

The efficiency of energy transfer among microorganisms presents a substantial hurdle for the widespread implementation of anaerobic digestion techniques. Nonetheless, recent studies have demonstrated that enhancing the extracellular electron transfer (EET) can markedly enhance this efficiency. This review highlights recent advancements in EET for anaerobic digestion and examines the contribution of external additives to fostering enhanced efficiency within this context. Diverse mechanisms through which additives are employed to improve EET in anaerobic environments are delineated. Furthermore, specific strategies for effectively regulating EET are proposed, aiming to augment methane production from anaerobic digestion. This review thus offers a perspective on future research directions aimed at optimizing waste resources, enhancing methane production efficiency, and improving process predictability in anaerobic digestion.

Evaluation of the Nature and Concentration of the Surfactant on the Properties of Red Mud/Metakaolin Porous Geopolymers Foamed with Aluminium

The chemical foaming technique is possibly the most common method of producing porous geopolymers. Despite this, to date, the role of the content and type of surfactant on the pore size distribution of porous geopolymers is not fully perceived, as constant surfactant dosages are usually employed. In addition, the comparison of literature studies is challenging since a distinct mixture of designs is employed. This investigation intends to provide additional insights on the topic, focusing on synthesizing red mud/metakaolin geopolymer foams and envisioning their use in thermal insulating applications. Various mixtures were prepared using three commercially available surfactants, namely Hostapur OSB, sodium dodecyl sulfate (SDS), and Triton X114. The content of the surfactant (0.025, 0.05, and 0.075 wt.%) and the amount of the foaming agent (aluminum powder, Al; 0.05, 0.075, and 0.10 wt.%) was modified, keeping the binder composition constant and the physical properties of the produced geopolymers were characterized. Results show that the combination between sodium dodecyl sulfate (0.025 wt.%) and aluminum (0.10 wt.%) leads to the strongest reduction in the foam density, the lowest value here reported being -400 kg/m3. On the other hand, samples produced with Hostapur OSB have much higher open porosity (up to 47.7%) and water absorption (up to 80.4%) values, showing that this surfactant leads to a pore network with higher connectivity. In addition, the microstructure of the foams, particularly pore morphology (size and shape) and connectivity between the produced pores are highly dependent on the type of surfactant, sodium dodecyl sulfate generating coarser pore size distribution with round, but mostly closed pores, while a narrower pore size distribution coupled with smaller size pores is seen with the Hostapur. These results suggest the feasibility of tuning the foams' properties (porosity and mechanical performance) according to the application by the proper combination of the type of surfactant and their concentration, enabling their use as thermal and acoustic insulators or as filters/membranes in wastewater treatment systems.

Anaerobic co-digestion of cheese whey and septage: Effect of substrate and inoculum on biogas production

Cheese whey is an industrial waste generated from the cheese processing unit of the dairy industry and requires treatment before its disposal. The present study investigated the possibilities of improving the digestibility from anaerobic digestion of lipid rich dairy by-product, cheese whey using septage as the co-substrate with different inoculum. Biochemical methane potential assays were conducted under mesophilic temperature conditions and results were validated using Modified Gompertz Model. Two sets of BMP tests were done; to assess the individual and combined digestion abilities of septage in anaerobic co-digestion of whey and to assess the ability of 3 inoculum sources (cattle manure, sewage sludge, and acclimatized anaerobic sludge) in the co-digestion process. The results indicated that septage is an excellent co-substrate that has better adaptability with cheese whey and the optimum mix ratio was found as 40:60 (S_CW: S_SP). BMP tests were also conducted with inoculum at S/I ratio of 1 and statistical analysis was performed to study the synergistic effect of both co-digestion and inoculum. The tests revealed that the cattle manure resulted in the highest biogas production (342.22mL/gVS) at 60% whey fraction. Modified Gompertz model fitted the experimental data well and identified an increase in lag phase times when whey fraction is increased. Comparatively higher lag phase times ranging from 1.98 to 4.35 days were obtained for sewage sludge inoculated samples. The maximum methane production (P_max) was obtained at 60% whey fraction (369.63 ± 4.05mL/gVS) at a very short lag time of 0.76 ± 0.17days for cattle manure inoculated mixture.

Batch mesophilic anaerobic co-digestion of spent goat batch mesophilic anaerobic co-digestion of spent goat straw bedding and goat cheese whey: Comparison with the mono-digestion of the two sole substrates

Spent livestock bedding is a valuable resource for the production of green energy (methane) in rural areas. Comparison and evaluation of batch anaerobic digestion and co-digestion of different mixtures of goat straw bedding (SGSB) and goat cheese whey were carried out. Biochemical methane potential (BMP) tests of the 100% SGSB, 95% SGSB-5% whey, 90% SGSB-10% whey, 85% SGSB-15% whey and 100% whey were found to be 423 ± 7, 354 ± 9, 371 ± 2, 293 ± 1, 274 ± 2 mL CH₄ g^-1 VS. Two different kinetic models were evaluated. The logistic model revealed a decrease in the maximum methane production rate (R_m) from 34.7 ± 1.5 to 14.1 ± 0.9 mL CH₄ g^-1 VS·d^-1 when the percentage of whey in the mixture increased from 0 to 15% as a consequence of the increased ammonia released during the co-digestion of increased concentrations of whey. The lowest value for the maximum methane production predicted by the model (P) was found for 100% whey (274 ± 10 mL CH₄ g^-1 VS). A two-substrate model was applied to describe the evident existence of rapid and slowly degradable material. Regarding the hydrolysis kinetic constants predicted by this model, considerable increases in the rapid biodegradation stage (k_rapid) were observed when comparing to the values found for the slow (k_slow) biodegradation stage in all the cases tested. The increases between both constants rose from 5 to 42% when the percentage of whey increased.

Role of waste-based geopolymer spheres addition for pH control and efficiency enhancement of anaerobic digestion process

In anaerobic digestion processes, pH has a vital role due to the direct impacts on the microbial community. An eco-friendly approach has been applied to control pH in anaerobic bioreactors, using waste-containing fly ash geopolymer spheres (GS) instead of powdered chemical compounds, to promote continuous alkalis leaching. The influence of GS porosity and concentration on the behavior of anaerobic sequential batch reactor treating cheese whey was evaluated. Results showed that the use of GS with the highest concentration and porosity promoted an increase in methane yield up to 30%, compared to the assay with powdered chemical compounds addition. In addition, GS boosted butyric acid production to the detriment of propionic acid, which favored methane production by a factor up to 1.2. This innovative approach indicates that GS addition can regulate pH in anaerobic digesters treating challenging wastewaters and, simultaneously, improve not only its efficiency but also the sustainability of the entire process.