Membrane system boosts water reuse

Kevin Phillips, product development manager at Koch Membrane Systems, reveals how new hollow fibre ultrafiltration membrane technology is proving key to successful water reuse.

The flow through the membrane cartridge.

Governments around the world are shifting focus to water reuse strategies as population growth and global development places a strain on existing fresh water supplies. Simultaneously, wastewater reuse in both the industrial and municipal sectors has become more prevalent in recent years as the cost and regulations for waste disposal increase.

Treated wastewater effluent can be used for agricultural and landscape/golf course irrigation, industrial cooling processes and indirect potable reuse. Water reuse strategies are taking hold around the U.S., with more systems coming online every day.

In New York State watersheds, many wastewater plants use UF membrane filtration before discharging wastewater effluent to aquifers. In more arid environments that are experiencing water scarcity, like those in California, Arizona and Nevada, membrane plants are common. In some cases reclaimed water is distributed around a municipality for a variety of purposes through a completely separate water main, leading to increased overall water use efficiency.

As some sources of usable fresh water dry up or otherwise become unavailable, other alternatives are becoming moreattractive, including difficult-to-treat sources such as brackish water, pond water or seawater. Ultrafiltration productsare being developed to process the vast number of feed streams that are not clarified enough to be processed by drinking water membranes currently in place.

Total organic carbon

In the case of surface waters, like reservoirs, lakes and rivers, these alternative water sources can be highly variable in terms of suspended solids or turbidity. High solids and increased levels of total organic carbon (TOC), tend to wreak havoc on conventional water treatment equipment. A storm rolling across a shallow lake can churn up sediment causing a peak turbidity event.

In a lake with an average turbidity of 5-10 NTU, a storm could cause a spike in turbidity levels to 100-200 NTU, or even as high as 1000 NTU. In the case of rivers, two inches of rain can cause excessive run-off from roads to flow into the river. When this flow of this murky water enters a treatment plant using sand filter or clarifier technology, the plant often cannot meet set turbidity output standards.

Filtration challenges are also increased by other factors such as seasonal change and variations in water temperature. Variability in suspended solids or turbidity is often introduced to water sources as seasons change. Filtration is more difficult in colder water, as increased viscosity makes it more difficult to push water through a membrane filter. Membranes can achieve higher recoveries for industrial water, work well for seawater pre-treatment and are even being used for potable water treatment.

In response to the need for consistent permeate under highly variable conditions, KMS developed the PURON MP ultrafiltration membrane. This product was specifically designed for high-solids water and wastewater applications, including surface water treatment, high TOC water treatment, RO pretreatment, and tertiary wastewater treatment. A combination of robust membrane fibres and a unique cartridge design allows the Puron MP to withstand the stress of high turbidity events with effective flux recovery.

The fibres, the cartridge and the complete PURON MP skid.

Development process

A four-year research and development process began with an early prototype placed at a reservoir test site with moderate to high TOC levels. This pilot, located near the KMS manufacturing facility in Wilmington, Massachusetts, United States, ran for 24 months as researchers and engineers evaluated operating modes, cleaning sequences and cartridge designs to come up with an optimum configuration. Tests were conducted on air scouring, backflushing and chemical cleaning and the results used to fine-tune the cleaning sequences.

Following extremely positive test results, engineers developed and tested additional prototypes with different packing densities to arrive at the optimum number of fibres to place in each cartridge. The optimised system has been tested on different applications at more than 10 surface water or municipal wastewater pilot sites in the United States, Australia, China, Brazil, Italy and Spain.

The tests have been used to optimise operating parameters and demonstrate long-term performance on a wide variety of feed water sources. Full scale systems are currently being installed at industrial facilities in China and Singapore. Pilot test findings show the membranes can tolerate an extremely high level of solids even after significant storm events, producing consistent permeate no matter what type of raw water is received.

At one pilot site, the feed water was a river with periods of extremely high solids due to rains and run-off. Even with measured turbidity peaks up to 8000 NTU, while the pilot would see an increase in transmembrane pressure (TMP), no unusual cleaning was necessary to return the cartridge to baseline TMP after the turbidity spike cleared. In general, even with these very high upsets, the permeate turbidity did not increase above 0.1 NTU.

A comparison between the two approaches.

Lagoon wastewater

At another site, feed water consisted of high fouling lagoon wastewater plant effluent with high TOC and incomplete nitrification. The change from fall to winter brought a snowfall that caused the lagoon to ‘flip’, causing TOC readings to jump from 12-15 ppm to more than 20 ppm. Even so, the membrane cartridge consistently produced permeate with very low turbidity and silt density index (SDI) values, demonstrating excellent water reuse potential regardless of feed water turbidity and TOC.

Operators back flushed the pilot with a 20 minute cycle during the lagoon flip event, allowing the facility to stay clean without high TMP. The pilot operated well through the winter with very low temperatures and in high feed TOC conditions at 25 gallons per square foot of membrane per day (GFD) with a clean in place (CIP) interval of 30 days.

In addition, the unit also demonstrated very high coagulant dosing tolerance. Tests with an on-line permeate UV254 instrument showed that coagulant dosage can be optimised for more efficient chemical consumption, rather than dosing excessive, expensive concentrations to ensure worst case conditions are managed.

The design of the Puron MP modules includes features that make them more forgiving for use in difficult-to-treat waters and better able to handle high solids without fibre sludging. Because the cartridge utilises a strong reinforced hollow fibre, the membranes are able to be bound or “potted” at the top end only.

Air scouring

This single-potting design permits the membrane fibres to move freely within the cartridge, allowing air scouring to penetrate the fibre bundle more completely and release accumulated solids to the bottom, where they are easily drained away. Also, backflush discharge is in the centre of the cartridge, the zone most likely to experience sludging.

By contrast, other hollow fibre membrane cartridges utilise traditional designs with potting at both ends. Thecomtightly packed dual-header configuration in these traditional modules restricts fibre movement, creating dead zones where solids can accumulate. This fibre sludging reduces membrane surface area, system output and energy efficiency.

Another advantage of the Puron MP system set-up is that being able to rely on automated cleaning sequences to keep the membrane clean during difficult turbidity events eliminates the need to take down the system for intensive chemical cleaning. Air scouring, backflushing and draining keeps TMPs lower so operators do not have to come in on an emergency basis to deal with these types of events.

The cartridge uses a virtually unbreakable braided reinforced fibre which minimises the type of damage that could shut a system down or contaminate the permeate. This results in improved uptime, ensuring the system can operate at full capacity so there are no damaging effects on downstream processes. The PURON MP system’s high flux and solids tolerance properties eliminate the need for costly clarifiers and2chemical pretreatments in many tough applications. This reduces the footprint required, limiting initial capital expenditure as well as operating costs associated with chemical use. Advanced membrane chemistry, tight pore size and distribution and effective air scouring can deliver more stable performance without the need for extensive chemical cleans.

The new pressurised hollow fibre UF cartridge technology by KMS produces high quality permeate at low fouling rates with difficult feed water. The cartridge design incorporates a single header design with free floating, reinforced fibres that allow for solids removal during cleaning. Simplifying operation, the new membrane eliminates clarifier pretreatment in many applications, minimises downtime, and reduces chemical usage. These benefits add up to a lower total cost of ownership.