Material Matters

Polyethersulfone (PES) chemistry helps give greater fiber strength and efficiency to TARGA II HF treatment units from Koch Membrane Systems.
Material Matters
In the production mode, feedwater enters one end of the cartridge and flows under pressure through the length of the fiber.

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Membrane systems have gained wide favor for their ability to produce high-quality drinking water while providing a physical barrier to pathogens, including parasitic cysts.

Among the more recent introductions in this category is the TARGA II hollow fiber water treatment system from Koch Membrane Systems. These units are designed to provide compact, cost-effective treatment in municipal drinking water systems and in other applications including desalination pretreatment and industrial water treatment.

The membrane fibers themselves use polyethersulfone (PES) chemistry in place of the polysulfone used in the product’s previous generation. This ultrafiltration technology uses an “inside-to-outside” flow path. Kevin Phillips, product manager, described the treatment units in an interview with Water System Operator.

wso: What accounts for the compact size of these systems?

Phillips: The cartridges have a high membrane area. For example, in our 10-inch cartridge, we have 871 square feet of membrane area. We’ve also configured the rack and skid to create a footprint more than 30 percent smaller than previously. It’s not simply the cartridge size that’s responsible. We actually devised a way to hang the cartridges in a row instead of offset from each other.

wso: What makes this system especially effective for drinking water treatment?

Phillips: The membranes have a very tight pore size distribution of approximately 0.02 micron. As a result we have very good rejection of viruses as well as Giardia, Cryptosporidium and other microbiological contaminants.

The membranes have gone through a battery of tests for the California Department of Public Health in which fibers were deliberately broken and punctured as a test of filter integrity. Even with a partially compromised membrane, we achieved greater than 4-log (99.99 percent) removal of viruses.

wso: How would you characterize the advantages of PES membrane chemistry?

Phillips: For one thing, we designed the fiber to have a spongy structure so that it’s stronger and less susceptible to breaks. That means there is less operator involvement in repairing the membrane fibers. The cartridges are designed to last up to 10 years.

The membranes also have a low contact angle of 45 to 50 degrees as opposed to greater than 80 degrees previously. Contact angle is a measure of how easily water will flow into the membrane pores and also a measure of how hydrophilic the membrane is. The lower the contact angle, the greater the affinity for water, the lower the transmembrane pressure (TMP), and the lower the potential for fouling.

The membrane also can be cleaned at high pH. So if the water contains organic foulants that aren’t easily removed with chlorine cleaning alone, we can clean with sodium hydroxide at pH 12 to 12.5. That works well with total organic carbon (TOC) fouling.

wso: Please describe the basic functioning of this system.

Phillips: The flow of feedwater is from the inside of the fiber out through the fiber wall. Feedwater enters one end of the cartridge and flows under pressure through the length of the fiber. It becomes clean as it passes through the fiber wall and enters a center collection tube as permeate. The permeate exits the top of the cartridge. An optional retentate stream carries away suspended solids.

wso: How are these membranes cleaned?

Phillips: Debris particles are deposited on interior walls of the fibers, which are very smooth, so that the particles are not trapped within the membrane pores. Simple backflushing allows the debris to be easily lifted off the surfaces.

Every water is a little bit different. We can add chemicals to the backflush if necessary. For example, if there is iron or manganese fouling, we can add acid to dissolve that. We can also do a periodic maintenance clean, which is a short cleaning cycle similar to a clean-in-place (CIP), although it uses a lower chemical concentration. It’s very effective. The machine would go down according to a schedule for 40 or 45 minutes for automated cleaning with caustic and chlorine together, with just caustic, or with a mineral acid like sulfuric acid.

wso: What does a typical operating cycle for these filters look like?

Phillips:: The filters are designed to be fully automatic. The basic operation is that you would be in production making water for 20 to 45 minutes. Then the filter would go down for automatic backflush and back into production.

Automated maintenance clean can be triggered by time – such as an interval of four days — or by TMP. For example, suppose the source water is subject to upsets and increased turbidity. The TMP would rise to a given setpoint, say 10 psi, at which point a maintenance clean would be initiated. That would lower the TMP again, so you wouldn’t be using excessive pumping energy to force the water through the skid.

The other feature we have is an automatic integrity test. That is generally done once a day. We can customize that to meet state requirements.

wso: How are these units configured for specific waters and specific sites?

Phillips: We configure to the smallest footprint possible. The design also looks at the feedwater quality and the volume requirements. Based on our experience, we select the number of cartridges necessary, the backflush interval, the maintenance clean interval, and the necessary chemicals for backflushing. We design systems conservatively to run reliably. When we’re required to do a pilot test, we can pick the initial design parameters and then optimize them in the field for the exact water quality at that site.


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