Effects of eukaryotic predation on nitrifying MABR biofilms

Area of Science:

• Environmental microbiology
• Biofilm engineering
• Wastewater treatment

Background:

• Nitrification is a key process in wastewater treatment using membrane-aerated biofilm reactors (MABRs).
• Previous studies indicated eukaryotic predation impacts heterotrophic MABR biofilms by creating voids and promoting sloughing.
• The effects of predation on nitrifying MABR biofilms, crucial for ammonia removal, remained uninvestigated.

Purpose of the Study:

• To investigate the impact of eukaryotic predation on the structural stability, bacterial community, and performance of nitrifying MABR biofilms.
• To quantify changes in biofilm properties such as void ratio, mechanical strength, and detachment under predation.
• To assess the influence of predation on nitrification flux and the relative abundance of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB).

Main Methods:

• Nitrifying biofilms were cultivated in flat-sheet MABRs.
• Optical coherence tomography (OCT) was used for biofilm imaging.
• Detachment tests were performed under increased shear flow, and a shear rheometer measured biofilm mechanical properties.
• Fluorescence in situ hybridization (FISH) and quantitative PCR (qPCR) analyzed the nitrifying community composition.

Main Results:

• Predation increased internal void ratios from 54% to 69% and significantly weakened biofilm mechanical properties (storage and loss moduli).
• Relative biofilm detachment increased from 4% to 18%, while average biofilm thickness decreased from 502 µm to 266 µm.
• Nitrification flux decreased from 1.00 to 0.61 g NH4+-N/m²day, and predation reduced NOB relative to AOB, indicating nitritation.

Conclusions:

• Eukaryotic predation substantially alters the structural integrity, bacterial community dynamics, and removal efficiency of nitrifying MABR biofilms.
• The observed effects of predation on nitrifying biofilms differ from those in heterotrophic systems, necessitating specific modeling considerations.
• Current biofilm models that lump predation effects into detachment or decay coefficients may inaccurately represent nitrifying MABR biofilm behavior.