How to increase the recovery rate of an industrial ro membrane?

Recovery rate refers to the percentage of feed water (after pretreatment) that is converted into product water (permeate) in a reverse osmosis (RO) system.

1. System recovery rate Y (%) = (Product water flow rate / Feed water flow rate) × 100%

① Recovery rate of a single RO membrane element: This is determined by the manufacturer. For example, it is typically around 15% for brackish water membranes and 10% for seawater membranes. For a standard 40-inch RO membrane element (1 inch = 2.54 cm), the actual operating recovery rate is generally not recommended to exceed 18% (except for second-pass RO systems, where it is usually not more than 30%).

② Single-pass recovery rate of an RO system: This refers to the recovery rate of a single pass, i.e., product water from one pass divided by feed water of that pass. It is determined by the configuration and arrangement of membrane elements.

③ Overall system recovery rate Y of an RO system: This is the total recovery rate during actual operation. It is influenced by both the single-pass recovery rate and the concentrate (brine) reuse ratio, and it is the most commonly considered recovery parameter in system design and operation.

① Increasing the number of membrane stages to improve recovery

The concentrate discharged from the first membrane stage can be directly fed into the second stage for further treatment. Multi-stage configurations (such as three-stage or four-stage systems) can achieve higher recovery rates.

In a staged RO system, the upstream stage typically handles a larger flow than the downstream stage; therefore, the number of membrane elements in the first stage is usually greater than in subsequent stages.

As the number of RO stages increases, the concentration factor of the concentrate also rises. For example, at 50% system recovery, the concentration factor is approximately 2; at 75% recovery, it increases to about 4; and at 80% recovery, it reaches around 5.

Due to concentration polarization within the membrane system, the salinity near the membrane surface increases significantly. As a result, fouling may occur faster than expected because the feed water becomes progressively concentrated. In brackish water desalination applications, recovery is typically set in the range of 50–80%. However, the final recovery selection depends on operating conditions, raw water characteristics, and other design factors. Setting an excessively high recovery rate may increase the risk of scaling and fouling.

Note: Extending the membrane housing length can also increase the number of membrane elements in series. However, overly long pressure vessels may cause insufficient pressure at the last elements and reduced flux, negatively affecting overall system recovery. At the same time, longer housings require higher feed pressure, leading to increased energy consumption.

② Improving recovery through concentrate recirculation

Concentrate recirculation refers to returning a portion of the RO system's concentrate stream back to the inlet of the high-pressure pump (with or without additional pretreatment, which makes a significant difference). After mixing with the feed water, it re-enters the membrane system for further reverse osmosis treatment. This is an effective method to improve system recovery, especially suitable for systems with relatively low production capacity where the flow cannot fully utilize long membrane vessels (e.g., 12-meter pressure vessels).

Common recirculation strategies include:

• Partial recirculation of first-pass RO concentrate back to the raw water tank
• Full recirculation of second-pass RO concentrate back to the raw water tank
• EDI electrode rinse water recycled to the second-pass RO permeate tank or upstream of the second-pass RO system

However, concentrate recirculation increases the pollutant concentration at the feed inlet, which raises the risk of scaling and fouling in the RO system. Therefore, strict operational control and system management are essential. Based on raw water quality analysis, it is necessary to properly design pretreatment, recovery rates, operating temperature, and other system parameters.

It is strongly discouraged to increase system recovery simply by adjusting feed/concentrate valve openings and ratios without proper design consideration. Such improper operation can accelerate membrane fouling and lead to severe system damage.

Note: When no concentrate recirculation is applied, the relationship between membrane element recovery and system recovery generally follows standard design correlations.