Submerged Osmotic Processes: Design and Operation to Mitigate Mass Transfer Limitations

Area of Science:

• Membrane Science and Technology
• Biochemical Engineering
• Environmental Engineering

Background:

• Submerged forward osmosis (FO) is crucial for bioreactors (e.g., osmotic membrane bioreactors, microalgae photobioreactors) and concentration processes where high viscosity or fragile components limit conventional pressurized systems.
• Existing FO research predominantly focuses on cross-flow configurations, leaving submerged systems with limited understanding of mass transfer limitations.

Purpose of the Study:

• To investigate mass transfer limitations in submerged plate and frame FO modules.
• To provide recommendations for optimizing the design and operation of submerged FO systems.
• To compare different configurations and mitigation techniques for enhanced performance.

Main Methods:

• Operation of a submerged plate and frame FO module under vacuum-assisted osmosis.
• Evaluation of different draw channel designs (U-shape vs. straight).
• Assessment of external concentration polarization (ECP) mitigation techniques, including air scouring and feed solution circulation.

Main Results:

• Vacuum-assisted osmosis significantly enhances permeation flux in submerged FO.
• Straight draw channel designs are superior to U-shape designs due to reduced pressure drops and dead zones.
• Air scouring is more effective than feed solution circulation for mitigating ECP in submerged FO, though application-specific adaptations are necessary.

Conclusions:

• Submerged FO systems require specific operational modes like vacuum-assisted osmosis for optimal performance.
• Module design, particularly the draw channel geometry, critically impacts efficiency.
• Effective ECP mitigation strategies, like air scouring, are essential for submerged FO, with further research needed on synergistic effects with fouling control.