Winter Low-Temperature Impact on RO System and Permeate Flow Reduction Improvement Measures
In reverse osmosis (RO) systems, a significant drop in permeate production during winter is a normal physical phenomenon caused by reduced feedwater temperature, but excessive decline indicates that system design or operation needs optimization. Water viscosity increases at low temperature, membrane permeability decreases, and overall mass transfer efficiency is reduced.
1. Effect of Low Temperature on Membrane Permeability
RO membrane flux is highly temperature-dependent. When water temperature drops, water viscosity increases and diffusion rate through the membrane decreases, resulting in lower permeate flow at the same operating pressure.
Key characteristics:
Permeate flow decreases approximately 2–3% per °C drop
Higher operating pressure is required to maintain same output
Salt rejection may slightly improve due to reduced diffusion rate
This is a physical limitation of membrane transport, not a system failure.
2. Insufficient Feedwater Temperature Compensation
If no temperature compensation strategy is used, winter performance decline becomes more severe.
Common issues include:
No temperature correction in design recovery rate
Fixed pressure operation without adjustment
Lack of preheating in feedwater line
Outdoor piping exposed to cold air causing further cooling
Without compensation, system output can drop 30–50% in cold conditions.
3. Pretreatment System Temperature Loss
Pretreatment units may amplify temperature-related performance loss.
Typical factors:
Open sand filters exposed to ambient cold air
Long pipelines causing heat dissipation
Storage tanks without insulation
Cold shock entering membrane directly
Cold feedwater entering RO membranes directly reduces flux sharply.
4. Increased Viscosity and Pump Load Effect
Low temperature increases fluid viscosity, which affects hydraulic performance:
Higher resistance in filters and pipelines
Increased pressure drop across system
Feed pump operating closer to limit conditions
This indirectly reduces effective net driving pressure across the membrane.
5. Membrane Fouling Sensitivity in Low Temperature
Although fouling is slower at low temperatures, the system becomes more sensitive to existing deposits.
Effects include:
Minor fouling causes larger flow reduction
Spacer channel resistance becomes more significant
Cleaning frequency impact becomes more noticeable
Even slight scaling or biofilm can amplify winter performance decline.
6. Improvement and Optimization Measures
To reduce winter production loss, the following engineering improvements are recommended:
Install feedwater heating or heat exchange system to maintain stable inlet temperature
Insulate raw water tanks, pipelines, and pretreatment units
Increase operating pressure within membrane design limits during winter
Adjust recovery rate downward slightly to maintain stable flux
Implement temperature-compensated flow control logic in PLC system
Ensure proper circulation in storage tanks to avoid stratification
Reduce heat loss by minimizing outdoor pipeline exposure
These measures help maintain stable driving force across the membrane.
7. Operational Strategy Adjustment
In winter operation, system control should be adapted rather than forcing summer parameters:
Gradually increase pressure instead of abrupt adjustment
Monitor permeate conductivity and flux trend together
Avoid overloading pump beyond cavitation margin
Schedule preventive cleaning if flux decline exceeds expected temperature effect
Proper winter operation is a balance between pressure compensation and membrane protection.
Conclusion
RO system production decline in winter is primarily caused by increased water viscosity and reduced membrane permeability at low temperatures. However, excessive reduction is usually related to insufficient thermal management, lack of system compensation, and poor pipeline insulation. By combining temperature control, hydraulic optimization, and intelligent operating adjustments, stable winter production can be effectively maintained.
References
U.S. Environmental Protection Agency (EPA), Membrane Filtration Guidance Manual
American Water Works Association (AWWA), Reverse Osmosis and Nanofiltration Manual of Practice
World Health Organization (WHO), Desalination and Water Treatment Guidelines
Dow / DuPont Water Solutions, Reverse Osmosis Membrane System Design Guide
Water Research Foundation (WRF), Temperature Effects on Membrane Filtration Performance
