Evaluation of Permeate Quality in Pilot Scale Membrane Distillation Systems

Area of Science:

• Water treatment technologies
• Membrane science
• Chemical engineering

Background:

• Membrane distillation (MD) is a promising technology for water desalination.
• Understanding permeate quality in pilot-scale MD systems is crucial for practical application.
• Feed leaks and membrane integrity are key factors affecting permeate quality.

Purpose of the Study:

• To analyze the salinity of permeate produced by pilot-scale membrane distillation systems.
• To investigate the impact of operating conditions and feed salinity on permeate quality.
• To explore the mechanisms behind feed leaks and their effect on permeate quality.

Main Methods:

• Experiments were conducted using three different spiral-wound commercial membrane modules (Solar Spring 10 m², Aquastill 7.2 m², Aquastill 24 m²).
• Permeate conductivity was measured during intermittent operation to assess salinity.
• The effect of varying feed salinity (up to 1.8 M) and vacuum-enhanced air-gap MD (V-AGMD) configuration was studied.

Main Results:

• Permeate conductivity initially decreased from a high value to a constant range of 3-143 µS·cm⁻¹ for seawater feed.
• The final permeate quality achieved was independent of operating conditions, only the time to reach it varied.
• Increased feed leak was observed in the V-AGMD configuration and with increasing feed salinity, though a conclusive explanation for the latter remains elusive.
• Pore wetting due to crystallization was ruled out as permeate quality was restored after washing.

Conclusions:

• Pilot-scale MD systems can achieve high permeate quality, with transient high conductivity at the start of operation.
• Permeate flux appears to dilute minimal feed leaks, contributing to the final stable permeate quality.
• Further research is needed at higher salinities and at the membrane level to fully understand feed leak behavior.