Wastewater Meets Membranes

Membrane filtration offers a number of advantages over many other solids/liquids separation technologies in wastewater treatment applications

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A continuous process, resulting in automatic and uninterrupted operation.

Low energy use involving neither phase nor temperature changes.

A modular design ," no significant size limitations.

A minimum number of moving parts and low maintenance requirements.

No effect on the form or the chemistry of contaminants.

A discrete membrane barrier to ensure physical separation of contaminants.

No chemical addition requirements.

Before installing a membrane technology, the plant must conduct testing to determine:

Whether or not the particular polymer provides the desired separation.

What membrane device configuration is optimum in the application.

What processing conditions produce the best results (pressure, flow rates, temperature, etc.).

In general, every stream must be tested to determine design factors such as the specific membrane polymer, membrane element design, total membrane area, applied pressure, system recovery, flow conditions, membrane element array and pretreatment requirements.

Specific stream properties that influence these design factors include those listed in Table 3.

In an ideal system, all contaminants to be removed are separated by the membrane and exit in the concentrate stream. In reality, no membrane is perfect in that it rejects 100 percent of the solute on the feed side. This solute leakage is known as "passage." Expressed as "percent passage," the actual quantity of solute that passes through the membrane is a function of the concentration of solute on the feed side.

Under high-recovery conditions using reverse osmosis and nanofiltration technologies, the concentration of salts on the feed side is increased; therefore, the actual quantity of salts passing through the membrane also increases. Many applications demand that, in addition to a minimum concentrate volume, the permeate quality be high enough for reuse. This "Catch 22" predicament ," where permeate quality decreases as recovery increases ," can impose design limitations. Additionally, the osmotic pressure increases resulting from recovery increases also impose a design limit. Generally, pumping pressures in excess of 1,000 pounds per square inch (psi) (68 bar) are impractical for most applications.

Membrane testing options

Recovery, osmotic pressure, permeate quality, recycle and other factors serve to underscore the value of testing the specific waste stream as thoroughly as possible. Because analysis results on effluent streams often vary as a function of time, it is important that either a composite or a "worst case" sample is obtained for test purposes.

A cell test, an applications test and/or a pilot test should be used to evaluate membrane technology with a particular effluent stream.

Cell test.

This test uses small ," approximately 15-square-inch cut pieces ," of sheet membrane mounted in a "cell" that exposes the membrane to the test solution using the crossflow mechanism. This test is effective for quick evaluation of a number of different membrane polymers to the determine degree of separation.

On the plus side, the cell test is fast, involves inexpensive equipment and requires only small quantities of test solution. However, the test cannot determine the long-term chemical effect of a solution on the polymer and does not provide engineering scale-up data. In addition, it gives no indication of optimum membrane element configuration and does not provide data on the fouling effects of the test solution.

Applications test.

An applications test typically involves the evaluation of a 30-gallon [gal.] to 50-gal. sample of solution on a production-sized membrane element. The element is mounted in a test machine that has the engineering features of production systems. For a given element, the test can be completed within one to two hours.

The test is fast and provides scale-up data such as flow, element efficiency, osmotic pressure as a function of recovery and pressure requirements. It also can provide an indication of membrane stability. On the down side, however, it provides neither the long-term chemical effects nor sufficient data on the fouling effects of the test solution. The figure illustrates a typical applications test schematic.

Pilot test.

A pilot test usually involves placement of a test machine (such as that used for the applications test) in the process, operating on a "side-stream" for a minimum of 30 days. The test accomplishes all of the functions of the applications test. It also provides long-term membrane fouling and chemical stability data. It can be expensive to perform, however, in terms of monitoring and time requirements.

Typical Applications Test Schematic



Membrane technologies can be a viable option for a chemical plant's wastewater treatment. Their selection and use, however, involves some testing work upfront. When the proper membrane system is selected through testing, chemical plants will be able to reap the benefits associated with the technology.

Cartwright is president of Minneapolis-based Cartwright Consulting Co., which specializes in the application of innovative treatment technologies to water purification, wastewater treatment and chemical/food processing. Contact him at cartwrightconsul@cs.com, or visit the company's Web site at www.cartwright-consulting.com.

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