Troubleshooting Declining Salt Rejection And Permeate Flow in RO And NF Systems

A decline in salt rejection or permeate flow is among the most common performance issues in reverse osmosis (RO) and nanofiltration (NF) systems. When system performance drops, the first step toward resolving the issue is to identify where the problem is occurring and determine the underlying cause. This process typically starts with a review of system operating data or readings from online monitoring instruments.

If operational data alone cannot pinpoint the problem, one or more membrane elements must be removed for analysis to locate the leakage point or damaged section.

Step 1: Identify the Distribution Pattern

To trace the problem effectively, each pressure vessel on the permeate side should be equipped with a sampling port to measure individual TDS, conductivity, or other relevant water quality indicators.

During sampling, care must be taken to prevent mixing of permeate from adjacent pressure vessels, which could distort the test results. Once proper samples are collected, measure the TDS concentration in each sample.

In nanofiltration systems, it is also necessary to analyze for sulfate ions or other specific components related to system performance.

The test results from all vessels within the same stage should fall within a consistent range. It’s normal, however, for the average permeate TDS or conductivity to increase from the first stage to the second, since the second stage feedwater is the concentrate from the previous one.

To determine solute leakage across each stage, the feed concentration for every stage must also be measured. The salt passage rate—the ratio of permeate concentration to feed concentration expressed as a percentage—can then be calculated. Elevated salt passage may occur in specific vessels or across an entire stage.

Step 2: Inspect Individual Membrane Elements

If one pressure vessel shows noticeably higher permeate TDS than others in the same stage, its membrane elements should be investigated in detail.

A common method is to insert a ¼-inch (6 mm) plastic sampling tube into the permeate tube running through the center of the membrane elements. This requires disconnecting the vessel’s permeate line from the main manifold or removing the permeate end cap on the opposite side.

If the vessel remains connected to the common permeate header, make sure that permeate from other vessels cannot interfere with the measurement.

With the system operating under normal conditions, the initial water sample from the sampling tube is not representative. Allow several minutes for flushing and stabilization before measuring the permeate TDS with a handheld conductivity meter. The recorded values will reflect the membrane’s local performance.

Pull the sampling tube back in 8-inch (200 mm) intervals, measuring the permeate conductivity at each position. The first reading is typically taken about 6 inches from the deepest point near the end adapter (commonly known as the “grenade”), followed by measurements every 8 inches until the tube is fully withdrawn.

This procedure provides a conductivity distribution profile along the pressure vessel, allowing the operator to evaluate each membrane element as well as all interconnectors and adapter O-rings. It is useful to mark the sampling tube at the measurement intervals for accuracy and consistency.

Under normal conditions, the conductivity of the permeate should increase gradually from the feed end toward the concentrate end of the vessel. Any abrupt deviation from this trend indicates an abnormality.

A sudden spike in conductivity at a connection point often suggests a damaged O-ring or leak at an interconnector. In contrast, a general increase in conductivity across an element indicates that the membrane sheet itself is compromised.