As regulations tighten and the costs of membrane systems come down, low-pressure membranes are becoming the premier water filtration technology for drinking water. The benefits are apparent: high water quality, ease of operation and low operating and maintenance costs.
As the market grows, competition has increased, with new competitors entering the market regularly. While this benefits end-users by reducing systems’ prices, it also forces users to look closely at each competitive offering because not all membrane systems are equal; there are hidden costs and potential risks that can arise from using systems that use inferior polymeric membranes and/or integrity test methods that might lead to an unacceptable amount of breakage and potential drinking water contamination.
Membranes in general can be categorized based on the amount of pressure required to pass the raw water through the membrane. Low-pressure membranes (microfiltration or ultrafiltration) typically require less than 45 psi; high-pressure membranes (nanofiltration and reverse osmosis) operate at levels as great as 1,000 psi.
Low-pressure membranes can be subdivided into submerged or pressure varieties, cross-flow or dead-end mode and materials of construction. One of the most prevalent low-pressure membranes for almost all drinking water applications are pressure systems operating in dead-end mode using polyvinylidene-flouride (PVDF) fibers. These systems have been proven to be economical and reliable solutions for most water and wastewater treatment applications, including desalination.
The Importance of Reliability
Reliability in membrane systems is a critical consideration due to the ever-present need for drinking water in the communities in which they are operating. It is critical that these systems treat raw water consistently in terms of capacity and filtrate quality, and one key component of that is the strength of the media (polymeric membranes) and the testing and repair method employed.
The importance of these fibers cannot be understated. They serve as the barrier from pathogens that adversely affect public health, including Cryptosporidium and Giardia, which are not removed with conventional disinfection—a key driver in the use of membranes for drinking water applications.
While an integral membrane system removes up to 6-log of Cryptosporidium and Giardia cysts and oocysts, there are significant health concerns if fibers break and the pathogens are distributed into the water supply. Therefore, direct integrity tests that are capable of detecting even a 3-micron breach are needed to guarantee safe drinking water. All Pall Aria systems—from 25-gal-per-minute packaged plants to 100-million-gal-per-day (mgd) custom systems—utilize a pressure decay integrity method that can detect such breaches. Additionally, Pall employs a unique and effective method of isolating the breached fiber that takes only 15 minutes of an operator’s time and ensures safe drinking water for the public.
Fibers in Action
The 15-mgd Pall drinking water plant (coagulated/settled surface water source) in Westminster, Colo., began operating in 2001 and has experienced zero fiber breaks—that is, zero broken fibers out of more than 4 million fibers online. The system is comprised of eight racks of 82 modules each. Modules consist of more than 6,000 fibers.
The system has been in operation for nine years, and the membranes have provided a robust and reliable barrier against harmful contaminants and pathogens. Furthermore, the system utilizes the pressure decay test method daily to confirm the integrity of the system.
Another plant that benefits from fiber testing methodology is the 10.8-mgd drinking water plant in San Patricio, Texas. The system went online in 2000 and has experienced less than 20 fiber breaks in the lifetime of the original modules.
“When a fiber does break, the repair takes 15 minutes in the worst case, and as little as five minutes if the operator is good at it,” said John Herrerra, head operator at San Patricio.
Based on those times, operators at the San Patricio plant have spent, at worst, about 2.5 hours repairing modules in 10 years. This is significant because there are many systems being relied on today that have daily integrity issues that take hours to resolve.
Pall’s length experience compares very favorably to competing systems when evaluating planned systems for two reasons:
1) The consistency of filtrate quality produced by the membrane plant; and
2) The labor costs associated with maintaining the integrity of the plant.
On the first point, with more than 300 installations globally, Pall has gained a reputation for membrane integrity due to the high-crystalline PVDF membranes at the heart of Aria systems. While this is hard to quantify, it is a critical qualification to consider because it impacts public safety.
For example, in large plants, more than 100 fiber breaks per quarter is becoming the norm instead of the exception. These events, while repairable, lead to health concerns because of the pathogens that are not filtered during the events. To mitigate such risks, some specifications take measures that demand robust membranes. This includes warranties that specify a maximum number of broken fibers—perhaps five—in any module before the system manufacturer is required to replace them. These specifications also may state liquidated damages for any broken fibers that must be repaired during the life of the membrane modules (typically 10 years).
Quantitatively, labor costs can be calculated for the time operators are needed to repair broken fibers to ensure the system is integral (e.g., 2.5 hours at the Pall system at San Patricio). As previously mentioned, hundreds of fiber breaches per quarter are considered routine for inferior membranes, and the method of module repair is cumbersome and can take longer than one hour. Submerged membranes, for example, cannot be repaired in situ like their pressurized counterparts. These systems require special tools and procedures to repair even a single broken fiber if the integrity test system signals a breach to the operator. This can lead to lost production and the need for additional operators to maintain the system, which adds significantly to the lifetime cost of ownership of the plant.
Membrane strength is of paramount importance in the selection of a membrane system, primarily for public health reasons. Moreover, integrity issues add to labor costs because of the time operators are required to repair breached membranes.