The fouling rate of membranes has a significant effect on the cost, performance and efficiency of the membrane separation process. This fouling is closely related to the source water quality and the effectiveness of the membrane system’s pretreatment. The objective of membrane pretreatment systems is to economically remove as much of the debris, particulate, suspended solids, silt, microorganisms and any other solid material that may foul from the source water just prior to the membrane separation process, leaving behind only dissolved minerals. Any solid material that remains in the water after pretreatment will accumulate on the surface of the membrane and will reduce the membrane’s effectiveness over time.
The appropriate selection of a pretreatment technology should include a lifetime cost-benefit analysis. This analysis should closely compare the cost of the pretreatment technology under consideration with the savings benefit in the reduction of fouling the membrane. Experience has shown that automatic self-cleaning screen filtration offers one cost-effective solution to removing most of the particulate and suspended solids from the seawater source, thereby significantly reducing membrane fouling. Filtering is available from 10 micron and larger with self-cleaning screen filters.
Filtration ChoicesA number of self-cleaning screen filtration technologies are available today. They all incorporate a weave-wire or wedge-wire screen as the filtering element for the water. However, significant differences are found in how they drive the mechanism that cleans the screen. These differences greatly affect the filter’s efficiency in cleaning the screen - especially during high particulate load conditions in seawater - and the filter’s maintenance costs. A closer look is needed at how these technologies:
- Filter seawater.
- Clean the screen.
- Drive the cleaning mechanisms.
Cleaning the Screen. A pressure differential across the screen increases as suspended solids build on the screen. When this pressure differential reaches a preset threshold (for instance, 0.3 to 0.5 bar), the screen cleaning cycle begins. To start this cleaning cycle, a flush valve opens at the flush outlet, venting the flush chamber to atmosphere. This allows water to flow back through the suction nozzles, sucking the debris off the screen. The debris is sent into the flush chamber and out the flush outlet.
Moving the Suction Nozzles. Once the suction nozzles start sucking debris off the screen, the nozzles must traverse over the surface of the screen to clean the whole screen. Three types of mechanisms move these nozzles. They are:
- Hydraulic piston type, which utilizes a
hydraulic piston with velocity motor.
- Electric type, which utilizes a gearbox and screw
with electric motor.
- Hybrid type, which utilizes a screw with velocity motor.
Advantages of the hydraulic piston type include low cost and simple controls, and power is not required.
Disadvantages include the potential for inefficient screen cleaning. The rotation of the suction nozzles does not relate to the longitudinal movement of the purge tube against the piston. This means that suction nozzle overlap is not guaranteed. The higher the pressure, the faster the piston moves, and hence the more likely an un-cleaned spiral path remains on the screen after one pass, requiring multiple passes to clean.
In addition, the hydraulic piston type mechanism only cleans in one direction: the purge tube moves toward the piston. The flush valve must close so the piston can return the purge tube to its original position. This can result in more flush waste and more frequent flush cycles, which can cause high system pressure fluctuations.
The electric motor type also has been available for about 45 years. In this design, the flush (exhaust) valve opens and the suction nozzles suck the debris off the screen. An electric motor and gearbox are attached to the purge tube - or, as some manufacturers call it, the “scanner shaft.” The electric motor drives a gearbox that is attached to and rotates the purge tube. A PLC monitors a set of limit switches and reverses the rotation of the electric motor to return the purge tube to its original position.
Advantages of the electric motor type include efficient screen cleaning. The suction nozzles rotate on a screw, and the pitch on the screw is designed with the suction nozzle size such that 100 percent of the screen is cleaned in one pass.
Disadvantages of the electric motor type include the fact that a PLC is required for control. Many parts require maintenance such as the electric motor, gearboxes and limit switches. Limit switch failure results in damages to the gearbox and screw that can be expensive to repair. Also, this mechanism only cleans in one direction: while the purge tube moves toward the electric motor. This also can result in frequent cycling of the flush valve during high load conditions, which can cause high system pressure fluctuations.
The hybrid type was introduced recently and draws from the advantages of the piston and electric motor types (figure 1). In this design, the flush valve opens and the suction nozzles suck the debris off the screen. This flush water passes through a velocity drive, where the velocity of the flush water causes the purge tube to rotate. The purge tube rotates on a bidirectional screw like the level winder on fishing reels or ships’ anchors.
Advantages of the hybrid type include efficient screen cleaning. The controlled rotation of the suction nozzles on the bidirectional screw ensures 100 percent cleaning of the screen in one pass. The bi-directional screw cleans in both directions. Electric motors or pistons are not required to return the purge tube to its original position, and power is not required. The flush valve does not cycle during high load conditions, so there is less system pressure fluctuation. The design uses fewer parts, which means fewer parts need maintenance. Because the shaft does not exit the filter body, seals are not required, and the risk of leaks is eliminated. Finally, the hybrid type offers simple controls and less flush waste.
The primary disadvantage of the hybrid type is that it is more expensive than the other designs.
In conclusion, membrane fouling is greatly reduced with the use of pretreatment to remove suspended solids. Automatic self-cleaning screen filters offer an economically viable and efficient solution. The appropriate selection of an automatic self-cleaning screen filter should include simplicity, efficiency and expected maintenance costs for optimal performance of the membrane separation process.