Microbial changes in anaerobic digestate when air bubbles are introduced

Introducing air bubbles into anaerobic digestate—whether as micro-aeration, air mixing, or nanobubble technology—can induce significant microbial changes that impact both the structure and function of the microbial community.

Key microbial changes observed:

• Enhanced Hydrolysis and Substrate Breakdown: Air bubbles, particularly at controlled low intensities (micro-aeration), can accelerate the hydrolysis of complex organic matter, increasing the availability of substrates for downstream microbial processes681.

• Shift in Microbial Populations:

  • Facultative and Aerotolerant Bacteria: Micro-aeration tends to promote the growth of facultative and aerotolerant bacteria, such as Pseudomonas, Actinomyces, and Alcaligenes, which can thrive under low-oxygen conditions16.

  • Sulfide-Oxidizing and Syntrophic Bacteria: Enrichment of sulfide-oxidizing bacteria (Magnetospirillum) and syntrophic acetate-oxidizing bacteria (Synergistaceae) has been observed, supporting improved hydrolysis and syntrophic metabolism6.

• Effect on Methanogens:

  • Methanogen Sensitivity: Methanogens, especially strict anaerobes like Methanosaeta, are sensitive to oxygen. High or uncontrolled aeration can irreversibly inhibit their activity, reducing methane production1.

  • Optimized Micro-aeration: At carefully controlled, low micro-aeration intensities (e.g., 1–2% O₂ in the headspace), the abundance of methanogenic archaea can actually increase, particularly during acetate conversion, leading to higher methane yields16.

• Increased Microbial Diversity: Studies show that micro-aeration and air mixing can enhance overall microbial diversity in the digestate, potentially increasing process resilience and efficiency48.

• Reduced Sulfide and VFA Accumulation: The introduction of air or oxygen nanobubbles can stimulate facultative and sulfide-oxidizing bacteria, reducing the accumulation of toxic hydrogen sulfide and volatile fatty acids, which stabilizes the digestion process711.

Mechanisms and Implications:

• Micro-aeration (dosing small amounts of air) can be beneficial if precisely controlled, as it stimulates hydrolytic and facultative microbial activity without causing significant inhibition of strict anaerobes18.

• Nanobubbles of air or oxygen have unique properties (high surface area, long residence time) that further enhance substrate accessibility and microbial interactions, improving both hydrolysis and methanogenesis when appropriately applied171115.

• Over-aeration or uncontrolled introduction of air can lead to the inhibition of methanogens, reduced methane production, and possible process failure18.

Summary Table: Microbial Changes from Air Bubble Introduction

Microbial Group/Process	Change with Controlled Air Bubbles	Risk with Excessive Air
Facultative/aerotolerant bacteria	Increased abundance, enhanced hydrolysis	May dominate, outcompete methanogens
Methanogenic archaea	Can increase (with low O₂, esp. acetate-fed)	Inhibited, leading to lower methane
Syntrophic/acetate-oxidizing bacteria	Enriched, supporting VFA conversion	May be outcompeted by facultatives
Sulfide-oxidizing bacteria	Enriched, reducing H₂S toxicity	-
Microbial diversity	Enhanced, more resilient community	Potential loss of strict anaerobes

Conclusion:
Introducing air bubbles into anaerobic digestate, if carefully controlled (micro-aeration or nanobubbles), can enhance hydrolysis, increase microbial diversity, and even boost methane production by favoring certain facultative and syntrophic bacteria. However, excessive or poorly controlled aeration can inhibit methanogens and disrupt the anaerobic process, underscoring the importance of precise process management14687.

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC12125087/
2. https://www.sciencedirect.com/science/article/pii/S0043135425004610
3. https://en.wikipedia.org/wiki/Anaerobic_digestion
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC10535533/
5. https://dspace.lib.cranfield.ac.uk/bitstream/1826/4772/1/Anaerobic_digestion_foaming_causes-2009.pdf
6. https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4267008
7. https://pubmed.ncbi.nlm.nih.gov/33730622/
8. https://pubmed.ncbi.nlm.nih.gov/31923694/
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC4196933/
10. https://azuraassociates.com/why-is-there-foam-in-my-digester/
11. https://sci-hub.se/downloads/2021-05-13/f3/chuenchart2021.pdf
12. https://www.sciencedirect.com/science/article/pii/S2590123024005474
13. https://scispace.com/pdf/effect-of-aeration-by-micro-bubbles-on-methane-production-in-48mc51x811.pdf
14. https://www.mdpi.com/2076-2607/9/12/2581
15. https://www.sciencedirect.com/science/article/pii/S2589014X24001804
16. https://www.epa.gov/sites/default/files/2018-11/documents/multistage-anaerobic-digestion-factsheet.pdf
17. https://atrium.lib.uoguelph.ca/bitstream/10214/21172/8/Arias_Daniel_202009_MSc.pdf
18. https://pubmed.ncbi.nlm.nih.gov/36584718/
19. https://www.epa.gov/sites/default/files/2020-11/documents/agstar-operator-guidebook.pdf
20. https://backend.orbit.dtu.dk/ws/files/289348804/revised_manuscript_Clean.pdf