Comparison of leachate percolation and immersion using different inoculation strategies in thermophilic solid-state anaerobic digestion of pig urine and rice straw

Microbial community dynamics and functional potentials in the conversion of oil palm wastes into biomethane

Oil palm processing generates substantial waste materials rich in organic content, posing various environmental challenges. Anaerobic digestion (AD), particularly for palm oil mill effluent (POME), offers a sustainable solution, by converting waste into valuable biomethane for thermal energy or electricity generation. The synergistic activities of the AD microbiota directly affect the biomethane production, and the microbial community involved in biomethane production in POME anaerobic digestion has been reported. The composition of bacterial and archaeal communities varies under different substrate and physicochemical conditions. This review discusses the characteristics of POME, explores the microbial members engaged in each stage of AD, and elucidates the impacts of substrate and physicochemical conditions on the microbial community dynamics, with a specific focus on POME. Finally, the review outlines current research needs and provides future perspectives on optimizing the microbial communities for enhanced biomethane production from oil palm wastes.

The impact of immersed liquid circulation on anaerobic digestion of rice straw bale and methane generation improvement

Immersed liquid circulation is assumed to improve solid-state anaerobic digestion (SS-AD) with digestate flow convection on the surface of solid-state bed (SSB), which depends on SSB concentration and circulation rate (CR). In this study, the impact of CR on rice straw SS-AD was investigated within a 30 L pilot digester. Results showed that SSB threshold concentration for efficient biogas conversion was 10%-12% TS, achieving the methane yield of 185.3 mL/g VS. Within the threshold, methane production progress and VFAs release could be enhanced simultaneously by rational CR increasing, but no significant methane yield improvement was observed; above, the rapid and stable biogas generation could be acquired with a competitive methane yield of 174.7 mL/g VS (150% CR). No matter within or above the threshold, efficient lingo-cellulosic degradation was always accompanied by the moderate CR for effective methane generation. SSB was proposed to be above threshold for industrial application.

Strategies to improve solid state anaerobic bioconversion of lignocellulosic biomass: an overview

Solid state anaerobic digestion (SSAD) of lignocellulosic biomass may be attractive solution for its valorisation. Compared to liquid state anaerobic digestion (LSAD), SSAD can handle higher organic loading rates (OLR), requires a less water and smaller reactor volume. It may require lower energy demand for heating or mixing and has higher volumetric methane productivity. Besides numerous benefits of SSAD processes and progress in system design, there are still obstacles, which need to be overcome for its successful implementations. This review aims to compile the recent trends in enhancing the bioconversion of agricultural stubbles in SSAD. Several pretreatment procedures used to breaking lignin and cellulose complex, method to overcome carbon to nitrogen ratio imbalance, use of carbon-based conducting materials to enhance Volatile Fatty Acids (VFA) conversion and additives for achieving nutrient balance will be discussed in this review. Leachate recirculation and its impacts on SSAD of agricultural stubbles are also discussed.

Biochar improves heavy metal passivation during wet anaerobic digestion of pig manure

Anaerobic digestion (AD) is regarded as an effective treatment to stabilize organic materials and recycle the energy in pig manure. In this study, 0%, 3%, 5%, and 7% biochar (based on dry weight) were added to pig manure to investigate its influence on improving biogas production and reducing heavy metal bioavailability. The potential ecological risk of heavy metals (namely Mn, Zn, Cu, Ni, As, Cd, Pb, and Cr) in digestates was also assessed. Results show that the methane yield was significantly (P < 0.05) increased by 26.7%, 23.0%, and 26.4% following addition of 3%, 5%, and 7% biochar, respectively. Moreover, there was a significant change in the heavy metal speciation in amendment each group. The 5% biochar group showed the highest passivation rate of Ni, As, and Pb, while the highest passivation rate of Cd, Cr, Mn, and Zn was observed with 7% biochar. Although the anaerobic digestion process slightly increased the ecological risk of heavy metals, all tested digestates were still classified as a moderate risk. Results of this study can provide a reference for the treatment of heavy metal pollution in large- and medium-sized anaerobic digesters treating pig manure.

Dry anaerobic digestion of organic waste: A review of operational parameters and their impact on process performance

Dry digestion is a suitable technology for treating organic wastes with varying composition such as the organic fraction of municipal solids waste. Yet, there is a need for further research to overcome some of the disadvantages associated with the high total solids content of the process. Optimisation of inoculum to substrate ratio, feedstock composition and size, liquid recirculation, bed compaction and use of bulking agents are some of the parameters that need further investigation in batch dry anaerobic digestion, to limit localised inhibition effects and avoid process instability. In addition, further attention on the relation between feedstock composition, organic loading rate and mixing regimes is required for continuous dry anaerobic digestion systems. This paper highlights all the areas where knowledge is scarce and value can be added to increase dry anaerobic digestion performance and expansion.

Biogas production from palm oil mill effluent and empty fruit bunches by coupled liquid and solid-state anaerobic digestion

Biogas production of palm oil mill effluent (POME) and empty fruit bunches (EFB) was performed by coupled liquid (L-AD) and solid-state (SS-AD) anaerobic digestion processes. POME was fed to L-AD digester, while mixed of effluent from L-AD and EFB was fed to SS-AD digester. The maximum overall methane production of 60.9 m³-CH₄·ton^-1 waste was obtained at an optimal hydraulic retention time of 30 days and an organic loading rate of 1.66 gVS·L^-1-reactor·d^-1 for L-AD and 6.03 gVS·L^-1-reactor·d^-1 for SS-AD with L-AD effluent recycling rate of 16.7 mL·L^-1-reactor·d^-1. The bacterial community in the L-AD reactor was different from the SS-AD reactor, while the archaeal community was similar in both reactors. Synergistaceae, Caldicoprobacteraceae and Lachnospiraceae were increased in the SS-AD reactor. Coupling L-AD and SS-AD is able to increase energy production by 29% and 71% compared to the L-AD and SS-AD alone, respectively, with no outsource SS-AD inoculum required.

Sequencing batch dry anaerobic digestion of mixed feedstock regulating strategies for methane production: Multi-factor interactions among biotic and abiotic characteristics

This study investigated the synergistic effects and regulation strategy of multiple factors for improving methane production in sequencing batch dry anaerobic digestion (SBD-AD) using corn stalks (CS) and cow dung (CD). The regulation of the spray frequency (SF) and inoculum content (IC) significantly improved methane yield, which increased feedstock ratios (FRs) by 12.4-121.3%. Moreover, the relationship between SF and IC produced distinct interaction modes. An FR of 4:6 increased the SF to 2 h for the CD-rich condition, and an FR of 6:4 decreased the SF during a 6 h interval and increased the IC for the CS-rich condition, resulting in increases in methane yield and the conversion efficiency of volatile fatty acids (VFAs). Methanogenesis (Methanogens) played a key role in SBD-AD. The nutrient substrate (NH₄-N⁺) and key enzyme activities of methanogens were significantly affected such that the synergistic effect of the acetoclastic and hydrogenotrophic methanogenesis pathways was likely strengthened.