Online Detection of Compromised Hollow Fiber Modules Using Membrane Integrity Sensor

Membrane Integrity Sensor

One of the key issues with operating membrane treatment plants is ensuring that the membranes maintain their integrity throughout the process. Most methods of detecting defects (Pressure Decay Test, Bubble Tests) can only be conducted offline. The membrane integrity sensor is an online instrument for detecting adverse changes in membrane integrity by accurately monitoring the permeate stream. This reduces the cost of operating UF membrane systems, improves reliability and allows for higher efficiency in the plant.

A novel approach for characterizing the particulate load and fouling propensity of water has been developed in the Temasek Professor programme at NTU. This process has been patented and initially designed as an ‘Integrity Sensor' (IS)[1]. The current arrangement (Figure 1) for the IS uses two membrane discs (M1 and M2) connected in series with pressure transducers measuring TMP1 (P₂ – P₁) and TMP2 (P₃ – P₂). Sample is pumped in crossflow across M1 and a steady permeate stream is pulled through M1 and M2. For a ‘clean' sample stream the ratio TMP1/TMP2 remains constant, whereas a solids-laden stream causes TMP1/TMP2 to steadily rise, signifying a loss of integrity down stream.

A tweak in the design of the original integrity sensor has increased the versatility and the potential lifespan of the sensor.

Figure 1 Membrane Based Sensor – Old Design (left) and New Design (right)

There have been two major changes to the design. The first is replacing the second membrane with a needle valve which mimics the resistance of the second membrane. The second is the installation of a bypass line on the sample line. The function of the second membrane is to provide a reference resistance for the fouled first membrane. A needle valve provides the required resistance with some advantages. One of the advantages is that a needle valve is much less prone to fouling than a membrane due to its much larger orifice. This increases the time required before the first membrane must be replaced. Without a second membrane, it is also much easier to design a backwash regime to keep the first membrane clean. Most importantly, a needle valve allows the operator to easily change the “reference” resistance. The utility of this feature will be explained in the next section. The bypass line allows the operator to change the driving pressure in for the sensor, up to a maximum of the line pressure. One way the sensor loses sensitivity is when the flux through the membrane drops to such a low value that only tiny amounts of foulants are deposited onto the surface of the membrane. By starting with a low pressure, and cranking up the pressure when the flux drops, the first membrane does not need to be replaced as often.

Trial runs of the sensor have been conducted at the Bedok NEWater Factory. Figure 2 shows a successful test of the integrity sensor in the plant. Using feed from the plant, it was possible to detect a hollow fiber UF module that had 0.07% of its fibers broken. This is evidenced by an increase in the sensor reading, π.

Figure 2 Detection of Compromised hollow fiber modules

In the full paper, we will discuss other results obtained from the plant, including long term tests (>1 week), the correlation of the sensor reading to SDI as well as the detecting breakages in hollow fiber modules. The implications of using the sensor for improving the reliability of the entire plant will be explored.

References

J. Phattaranawik, A.G. Fane, F.S. Wong, Detection apparatus and method utilizing membranes and ratio of transmembrane pressures, PCT/SG2007/000130, WO/2007/129994, Filing date 10 May 2007

---

[1] J. Phattaranawik, A.G. Fane, F.S. Wong, Detection apparatus and method utilizing membranes and ratio of transmembrane pressures, PCT/SG2007/000130, WO/2007/129994, Filing date 10 May 2007