The Relationship Between Permeate Volume and Water Temperature in Reverse Osmosis (RO) Operation

Inlet water temperature is one of the most significant and direct operating parameters affecting the permeate volume of an Ro System. With a constant inlet water pressure, the higher the inlet water temperature, the greater the permeate volume of the RO system; conversely, the lower the inlet water temperature, the significantly lower the permeate volume. This is not a simple linear increase, but its impact is very significant. The underlying scientific principles mainly involve the following two points:

1. Changes in Water Viscosity

This is the primary reason. Water molecules are more tightly bound together at low temperatures, resulting in poor flowability and increased viscosity. High-viscosity cold water needs to overcome greater resistance when passing through the extremely small pores (approximately 0.0001 micrometers) of the Ro Membrane, naturally slowing down the flow rate and reducing the permeate volume per unit time.

Conversely, as the temperature rises, water molecules become more active, their viscosity decreases, and they become "slipperier," allowing them to pass through the membrane pores more easily and quickly, thus increasing the permeate volume.

2. Changes in Membrane Material Properties

The RO membrane itself (usually a polyamide composite material) has a certain degree of temperature sensitivity. Increased temperature causes a slight expansion of the pore structure of the polymer membrane, causing the pores to open slightly. This reduces water resistance to some extent, further increasing the permeate flow rate.

In RO system design and operation, a crucial concept is the Temperature Correction Factor (TCF).

The rated permeate flow rate of an RO system is typically based on a standard feed water temperature, most commonly 25°C. The relationship between actual permeate flow rate and standard permeate flow rate can be calculated using an empirical formula:

Actual permeate flow rate = Standard permeate flow rate × TCF

TCF is usually calculated using an exponential formula, commonly in the form:

TCF = exp[K × (1/(273+T) - 1/298)]

Where T is the actual feed water temperature (°C), K is a membrane material-related constant (typically between 2000-3000), and 298 is the Kelvin temperature (273+25) corresponding to the standard temperature of 25°C.

Generally, using 25°C as a baseline, for every 1°C decrease in temperature, the permeate flow rate decreases by approximately 2% to 3%. Conversely, for every 1°C increase in temperature, the permeate flow rate increases by approximately 2% to 3%.

Impact of Water Temperature on System Operation and Countermeasures

Low Water Flow Rate in Winter: This is the most common problem. Low water temperatures in winter result in severely insufficient permeate flow, potentially failing to meet water demand. Solutions include:

Installing a Feed Water Preheater: Heating the feed water to above 15-20°C using electricity or steam can significantly alleviate this problem.

Increasing the Number of Membrane Elements: Designing the number of membranes based on the minimum expected water temperature during the initial system design phase, but this increases initial investment.

Increasing Operating Pressure: Temporarily increasing the pressure of the high-pressure pump to "force" high-viscosity cold water through the membrane, but this increases energy consumption and may exacerbate membrane fouling.

High Water Flow Issues in Summer: While high water temperature and high water production seem like a good thing, they also present potential risks:

Excessive Recovery Rate: An excessively high system recovery rate (product water/feed water) can lead to a sharp increase in salt concentration on the concentrate side, easily causing scaling and membrane fouling.

Slightly Decreased Membrane Desalination Rate: At excessively high water temperatures, water molecules move too vigorously, potentially carrying more salt ions through the membrane, resulting in a slight decrease in product water quality (conductivity). RO membranes typically have a maximum temperature limit (generally 45°C).