Ultrafiltration membranes for reverse osmosis pretreatment
Long life and efficient operation of reverse osmosis (RO) membranes largely depends on the quality of the feed water that a pretreatment system can provide. Whether RO membranes are desalting seawater, producing ultrapure water for industrial processes, or removing impurities from drinking water, pretreatment systems must provide a consistent supply of high quality feed water to ensure trouble-free RO operation.
Ineffective or unreliable pretreatment can adversely affect the RO system with problems such as high rates of membrane fouling, excessive cleaning requirements, lower recovery rates, high operating pressure, reduced membrane life and poor quality product water.
Each of these factors contributes to higher operational costs and lower RO plant productivity.
ZeeWeed immersed hollow fiber ultrafiltration (UF) membranes are increasingly being selected over conventional granular media systems for their ability to provide superior protection of valuable RO systems from particulate or biological fouling. UF membranes form a physical barrier that effectively blocks virtually all suspended particles from entering the feed water stream regardless of the turbidity of the raw water. This is particularly important for desalination plants where turbidity can vary greatly during sea storms. Conventional systems may not be able to consistently produce high quality water under such conditions.
UF membrane fibers are inherently insensitive to upsets caused by high turbidity or variable raw water quality. ZeeWeed systems can deliver RO feed water with a turbidity of less than 0.1 NTU and a low silt density index (SDI), typically less than 2.5, often less than 1.5 (See Fig. 1).
Compare and contrast
Most conventional pretreatment systems (Fig. 2) for seawater desalination consist of an open seawater intake, screens for coarse prefiltration, chemical additions (break-point chlorination, acid addition, in-line coagulation, addition of a flocculation aid) and single- or double-stage sand filtration.
The final step of pretreatment is a cartridge- or bag-type guard filter with a mesh size of five to 10 microns to protect the RO membranes, which suggests that relatively large particles may carry over from the pretreatment process to the RO membranes.
A conventional granular media pretreatment system may produce feed water of an acceptable quality when it is properly optimized and is receiving good quality raw seawater.
However, seasonal changes or storm events can cause changes in water quality that cannot be adequately handled by a conventional system.
As a result, the conventional pretreatment system may not always achieve SDI values that meet the requirements set by the RO membrane manufacturer. Overall, conventional granular media pretreatment systems have several major disadvantages when compared to ZeeWeed UF membranes that contribute to higher rates of RO membrane fouling and shorter RO membrane life including:
- Inability to provide a positive physical barrier to colloidal and suspended particles;
- Significant fluctuations of RO feed quality during varying or high turbidity raw water conditions—may not be able to achieve SDI below 3.0;
- Low removal efficiency of particles greater than 15 microns);
- Possibility of breakthrough during filter backwash and carryover of high concentration of colloidal particles immediately following a filter backwash; and
- Large footprint requirement as a result of slow filtration velocities.
ZeeWeed UF pretreatment systems (Fig. 3) occupy a significantly smaller process footprint than conventional systems because fewer process steps are required. With a nominal pore size of 0.02 microns, the membranes form a physical barrier against suspended particles, colloidal materials, algae and bacteria, resulting in excellent RO feed water quality even with high influent turbidity feed water (see Table 1).
As a result, chemical dosing for coagulation/flocculation can be greatly reduced or eliminated. Often, only a mechanical screen is required ahead of the membrane tanks to remove large debris that may damage the fibers.
The membrane fibers are loosely suspended in easy-to-handle membrane cassettes (Fig. 4) that are immersed directly into open process tanks (Fig. 5). The cassettes are connected to permeate collection headers and aeration hoses. Permeate pumps apply a slight suction to the end of each membrane fiber, drawing water into the membrane in an outside-in flow pattern. This not only requires less energy than pressure-driven membrane systems, but also simplifies membrane cleaning and inspection.
Fibers can be easily cleaned through a fully automated, clean-in-place backpulsing procedure that reverses the flow of permeate and pushes clean water from the inside to the outside of the membrane to dislodge particles from the pores and restore optimum permeability.
Coarse bubble aeration is also used to scour debris from the outside of the membrane surface. When necessary, in-tank chemical cleaning can be automatically performed if membrane fouling reduces permeability below a specified performance level.
The modular configuration of ZeeWeed membrane cassettes makes them ideal for retrofitting or expanding plants that use conventional granular filter media.
Membrane cassettes can be stacked directly in existing filter media tanks and can leverage a great deal of existing infrastructure. In many cases, such retrofits can double or even triple treatment capacity within the same plant footprint. By increasing capacity without expanding the plant footprint, plant operators can reduce capital expenditures and substantially increase the efficiency of their facility. UF membranes have several noteworthy advantages over conventional systems including:
- Positive barrier to particulates and pathogens;
- Significantly reduced RO membrane fouling and cleaning frequency;
- Extended RO membrane life;
- Reliable production of high quality water regardless of raw water turbidity;
- Higher RO membrane flux;
- Greater plant availability resulting from consistent production of excellent RO feed water and reduced cleaning requirements;
- Smaller footprints—lower land acquisition and system costs;
- Lower consumption of operational and cleaning chemicals; and
- Modular design to enable simple and efficient retrofits of granular media basins.
Reuse and desal
ZeeWeed UF membranes continue to gain acceptance as critical components for the successful pretreatment of source water prior to RO. The systems consistently outperform conventional pretreatment for RO systems in seawater desalination and ultrapure water production.
Scarce water resources in many parts of the world are compelling municipalities and industry to examine the best available technologies for water reuse and seawater desalination as a way to secure adequate water supplies for residents and businesses. For those countries that are leading the way in adopting UF pretreatment for RO, ZeeWeed membranes are critical components of many small- and large-scale facilities throughout the world including:
Singapore: A municipal wastewater treatment plant uses ZeeWeed membranes for the pretreatment of secondary effluent prior to RO. Membranes remove colloidal material and suspended solids from the treated effluent and consistently produce feed water with an SDI below three. The plant has a current capacity of 11.3 mgd and a total capacity of 31 mgd. Water produced by the plant is used for industrial influent, and a small amount is released back into local reservoirs for indirect use in potable reuse applications.
China: One of the nation’s largest independent power producers is turning to Zenon ZeeWeed immersed membrane technology for pretreatment of seawater for its RO desalination system. The desalinated water will be separated into two streams—one for use as boiler feed water at the plant, and the other for potable water. Any surplus potable water will be sold to the local community. The ZeeWeed system is one of the largest UF pretreatment installations of its kind, producing just over 20 mgd of feed water for the power plant.