How light and aggregate size shape oxygen profiles in photogranules

Area of Science:

• Environmental Microbiology
• Biogeochemistry
• Wastewater Engineering

Background:

• The redox structure of photogranules is crucial for wastewater treatment, balancing oxygen production and microbial respiration.
• Spatial coupling of these processes is poorly understood, limiting optimization of photogranule bioreactors.

Purpose of the Study:

• To investigate how light intensity, organic carbon supply, and granule size affect oxygen gradients within photogranules.
• To quantify the spatial distribution of oxygen production and consumption in relation to granule characteristics.

Main Methods:

• Oxygen microprofiles were measured in photogranules of varying sizes under different light intensities and acetate concentrations.
• Light penetration was quantified, and a one-dimensional reaction-diffusion model was used to estimate oxygen dynamics.
• Granules from acetate- and methane-fed bioreactors were analyzed to assess legacy effects.

Main Results:

• Light penetration was limited to <700 µm, independent of granule size.
• Anoxic cores formed in granules at low light (<50 µmol PAR·m⁻²·s⁻¹) and moderate acetate (30 mg COD·L⁻¹).
• Model simulations indicated oxygen production at the surface and deeper heterotrophic respiration, with significant differences based on prior cultivation.

Conclusions:

• Light intensity and granule size are key parameters for controlling redox zonation in photogranules.
• Managing illumination and granule size distribution can enable concurrent aerobic and anaerobic processes.
• This approach can enhance the efficiency and functional flexibility of wastewater treatment systems.