https://www.sciencedirect.com/science/article/pii/S2589014X25003986?via%3Dihub
Nitreogen-Nanobubble water was applied to high-solids anaerobic digestion with percolate recirculation.
Methane yield increased by ∼43 %.
Improved biogas quality with reduced hydrogen sulfide and increased methane content.
Enhanced enrichment of syntrophic microbial communities
High-solids anaerobic digestion (HSAD) of source-separated organics
(SSO) is a key strategy for sustainable waste management and energy
recovery, but its intensification through advanced technologies is vital to
enhance energy recovery and process stability. This study investigated
the impact of nitrogen nanobubble water (N2-NBW) supplementation on
HSAD of SSO with percolate recirculation. Two bench-scale HSAD reactors
were operated under mesophilic conditions, one supplemented with N₂-NBW
in the percolate tank, while the control operated without NBW addition.
The N₂-NBW-amended reactor achieved ∼43 % higher cumulative methane
yield than the control along with improved methane content, and reduced
hydrogen sulfide (H2S) levels. Although total volatile fatty acids (VFAs) levels
were similar between systems by day 28, the N2-NBW reactor maintained a
relatively lower VFA-to-alkalinity (VFA/Alkalinity) ratio (0.33 vs. 0.40) and higher
alkalinity (22,540 vs. 18,824 mg CaCO3/L), indicating improved buffering capacity.
Microbial community analysis revealed an increased abundance of Methanosarcina
and vadinCA11, indicating the development of a more resilient microbial community.
These findings demonstrate that N2-NBW is a promising intensification strategy for
enhancing HSAD efficiency and resilience.
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To the best of our knowledge, this is the first study to incorporate N2-NBW in an HSAD system
with percolate recirculation, thereby filling a critical gap in the literature and expanding on earlier
efforts to incorporate NBW in conventional HSAD systems without percolate recirculation.
In addition to improving methane yields, the application of N2-NBW led to more resilient
methanogenic communities enriched with Methanosarcina species. In this study, NBW
was generated and used only during the reactor startup. A 2-h operation of the nanobubble
generator produced 15 L of NBW, of which only 1.5 L was used in the experiment.
The recirculation pump, being the primary energy-consuming component during NBW
operation, required ∼1.5 kWh during this period. Based on the average electricity cost
in Canada (∼CA$0.20/kWh; Source: energyhub.org, accessed on October 4, 2025),
the corresponding energy cost for NBW production was ∼CA$0.02/L. Considering
the observed 43 % increase in methane yield and this modest energy requirement
for NBW generation, integrating NBW into HSAD systems appears promising.
However, as this study was conducted at the laboratory scale, further pilot-scale
testing and comprehensive techno-economic analyses, including both capital
and operating costs, are necessary to assess its technical and economic feasibility fully.
This study demonstrates the potential of N2-NBW as a process intensification strategy
for HSAD with percolate recirculation. The test reactor achieved higher methane yields,
improved biogas quality, and lower H2S and FAN levels compared to the control.
Despite similar VFA concentrations, the N2-NBW system maintained a lower
VFA/alkalinity ratio and higher buffering capacity, indicating better process stability.
Microbial analysis revealed increased abundances of beneficial taxa such as
Methanosarcina, vadinCA11, SHA-98, and Cloacamonales, supporting enhanced
methanogenesis and substrate degradation. Based on these promising results,
further studies are recommended to assess long-term effects and broader
applicability across diverse waste streams.
