Nov 08, 2005

Advantages of Pressure MF Systems Over Vacuum Systems

Microfiltration is no longer the future of water filtration—it is the present

Introduced more than 20 years ago, early microfiltration water treatment systems were expensive and inefficient to run. Ten years later, a decade of significant membrane manufacturing improvements and other factors had made microfiltration a viable cost-effective alternative to conventional water treatment systems.
Today, the value of the U.S. membrane market is more than $2 billion and growing at a rate of 8% per year. Microfiltration is no longer the future of water filtration—it is the present.
Two types of membrane systems are available today, “pressure systems” and submerged systems, also known as “vacuum systems.”
These types of membranes are commonly manufactured in a hollow-fiber configuration. In a pressure system, feed water is forced through the membrane pores by a pressure pump. In a submerged system, the membranes are not placed in a housing or vessel, but placed in a basin or tank that is often open to atmosphere. In this type of system, filtrate is drawn through the membrane via a vacuum pump.
Though both are capable of delivering the benefits of membrane technology, pressure systems offer some inherent advantages over vacuum systems. These advantages are highlighted in this article.

Range of pressure

Pressure systems operate within a greater range of operating pressure, resulting in some advantages over vacuum systems.
A limitation of vacuum systems is that they can operate only within the range of atmospheric pressure, typically operating at pressures approaching 70% of the maximum pressure differential available (10-11 psid).
Pressure systems, on the other hand, rely on a greater reserve of pressure should upset conditions occur in the system, operating at 40 to 50% of the maximum transmembrane pressure (16-20 psig) when tested on the same waters. This builds an intrinsic safety factor into a design that allows the pressure system to better cope with process upset, changes in influent water quality or any other changes in plant operation.
Pressure systems are also immune to the impact of temperature changes that affect vacuum systems.
During winter, for example, feed water temperatures drop, which increases viscosity and the pressure required to force water through a membrane pore. Because absolute vacuum is -14.7 psig, the operating pressure of vacuum systems can never exceed 14.7 psi. In most cases, the maximum available pressure is -12 psig, which may be even less at higher elevations. This means vacuum systems must be designed at much lower fluxes than pressure systems. By their nature of operation, pressure systems operate in a much wider differential pressure range (0-40 psid), thus allowing them to provide consistent water flows regardless of feed water temperature.

Improved cleaning

Pressure systems offer safer, more reliable cleaning operations than submerged systems. In pressure systems, cleaning chemicals are entirely contained within the membrane modules, or chemical tanks, virtually eliminating the risk of exposure to both the chemicals and the fumes they may generate.
In contrast, fumes from the open tanks of submerged systems often require complex and expensive ventilation systems, for both the safety of operators and the longevity of the facility housing the membrane system, as the fumes can corrode metal buildings or structures within the treatment plant. Cleaning chemicals and any fumes they may generate, are more easily controlled in the closed loop system of the pressure membranes.
An additional benefit of pressure systems is that their closed design makes them resistant to tampering or outside contamination, either accidental or deliberate. Process water is completely insulated through the membrane filtration process in pressure systems, unlike vacuum systems where process water is filtered from large open tanks. It is very difficult for accidental spills to contaminate finished water in pressure membrane filtration. By contrast, in vacuum systems, any contaminant dissolved or smaller than the membrane pores that is spilled into the open tanks can contaminate the filtered or treated water.
In the end, the closed design leverages these inherent advantages to deliver higher levels of safety and security than the submerged system.

More reliability

Integrity testing is critical to ensuring product quality, and pressure systems have several advantages over vacuum systems when it comes to integrity testing membrane fibers. Pressure systems use the highly automated and widely accepted “Pressure Decay Test,” which examines the filter during the diffusive flow portion of the flow curve. Membrane fibers can be pressurized at more than two-and-a-half times those in a vacuum system, enabling small breaches of membrane integrity to be detected much more easily and rapidly than in vacuum systems.
These points regarding integrity testing become even more critical in light of the U.S. EPA’s proposed Long-Term 2 Enhanced Surface Water Treatment Rule, released for public comments in 2003, which requires membrane systems to undergo direct integrity testing and continuous integrity monitoring for compliance. Among the performance criteria specified are that testing frequency shall be no less than once per day, and that direct integrity testing be able to detect a breach equal to or less than 3µ.
In a pressure system, each module is equipped with a clear coupling on the filtrate side of the module, allowing the plant operator to quickly identify the module with the broken fiber. In the rare instance where a membrane integrity breach occurs, fiber repair can be done onsite without removing the module from the rack, typically taking less than 15 minutes.
As an illustration of fiber reliability, a Pall installation in California has experienced only four fiber breaks in two years with a flow rate of 20 mgd.
In vacuum systems, tracing back to the individual module where a membrane integrity breach occurred is more difficult and time consuming. In the event of a broken fiber, a rack of modules has to be lifted out of the basin, and the damaged module has to be removed for repair.
Piping leaks in pressure systems, being under positive pressure, are easily detected and addressed. Because vacuum systems are under negative pressure, they will leak from out to in, making leak detection much more difficult. This problem is further compounded by the fact that there is a continuous concentration of contaminants over the course of a day in the entire plant, as well as from the first train to the last. Any leak in the membrane means extremely dirty water will contaminate the filtrate.

Lower costs

Pressure systems deliver lower total costs than vacuum systems when users take into account operations, construction and equipment. A major reason for this is that pressure systems typically require a much smaller footprint than comparable vacuum systems. Recent bids have shown that pressure membrane systems, with their compact, skid-mounted and modular design, offer the smallest system footprint among all the major membrane systems in the industry.
The larger operating pressure range that pressure systems offer makes it possible to design them at much higher fluxes. This translates to less membrane surface area, fewer membrane modules, and consequently, lower costs. Continuing process optimization and advancements in treating different types of waters have enabled pressure systems to deliver lower operating and maintenance costs than submerged systems.
Because pressure membrane systems are designed as modular units, there’s no need for costly permanent hoisting equipment. Membrane modules are manually installed. In instances where a module needs to be removed, the specific module rack is isolated, and a single operator without the need of any module removal equipment can quickly remove the module.
Vacuum systems, on the other hand, require a permanent hoisting mechanism to remove or put in place membrane cassettes, adding size, cost and complexity to the system design. Vacuum systems also require expensive basins that demand either major excavation work or a taller building than pressure systems, which require only a concrete slab.

Final analysis

Membrane pressure MF/UF systems are commercially viable and competitive with vacuum systems in water processing plants, offering many advantages over vacuum systems that are not always incorporated into the economic evaluations.
However, these advantages can improve plant operation, facilitate equipment maintenance and ensure that effluent water quality meets requirements.
Pall has a continuous process improvement program to advance the capabilities of pressure systems. These improvements leverage the available pressure of pressure systems and have enabled higher operating flux rates, which in turn enable smaller and more cost-effective systems. Operating protocols are constantly being upgraded for enhanced flux maintenance, which can further reduce operating costs.

About the author

Jesus Martinez is a project director in the Pall Water Processing division. He can be reached by e-mail at jesus- [email protected].
Pete O’Connell is vice president, applications engineering in the Pall Water Processing division. He can be reached by e-mail at [email protected].