Although the reverse osmosis membrane technology was developed many years ago, additional applications at water treatment facilities are still being developed. While the source of the water to be treated does not influence the technology’s capabilities, its performance and design requirements are significantly affected by feed water characteristics and the intended use of the treated water.

In general, it is useful to divide the feed water into three categories:
• Raw water – water from a natural source, such as a well, river, lake, or ocean, or from a municipal drinking water treatment plant
• Wastewater – water that flows out of an industrial or commercial facility or a sewage treatment plant
• Process water – wastewater used in a manufacturing process

As water reclamation and reuse is continuously emphasized, it is important to understand the distinctions between these source waters and their impacts on membrane system designs.

Sources of raw water, including seawater, have a relatively narrow range of chemical characteristics and are well understood and present minimal design challenges. For example, to design a reverse osmosis treatment system with spiral-wound membrane elements, membrane manufacturers provide a computer program to design a water purification system, as long as they have some technical understanding of the materials. It is only necessary that this person know and understand the following:
• Feed water analysis
• Purified water quality requirements
• Total volume requirement (per day, per minute)
• Desired system recovery

A typical membrane process is illustrated in Figure 1.

Raw water
The primary goal for most water purification applications is to meet a specific treated water-quality requirement, while system recovery is usually a second requirement. System recovery is the percentage of feed water flow that passes through the membrane and becomes the permeate. Recovery is the purview of the system designer, and typically ranges from less than 50 percent for low flow applications to 85 percent for very large applications. One advantage of high recovery designs is that a relatively small quantity of concentrate (that percentage of the water that does not pass through the membrane and carries away the contaminants removed from the water supply) is discharged. Due to these lower flow rates, other advantages include a smaller feed pump and generally smaller pipe sizes.

The primary disadvantage of high recovery is that the concentration of contaminants in the concentrate stream dramatically increases as recovery is increased. The increased concentration can result in scaling and other fouling problems on the membrane surface.

Wastewater
A primary goal of reverse osmosis membrane processing is to decrease the concentrate stream so that it is as small as possible in order to facilitate further treatment or disposal. Therefore, recoveries are generally set very high, typically above 90 percent. Additionally, in the case of most industrial wastewater streams, the water characteristics are usually unique. Consequently, testing should be performed in order to develop system design data. This testing can take the form of cell testing, applications testing and/or pilot testing, but it is absolutely essential in industrial wastewater applications. None of the currently available computer programs are capable of designing an effective wastewater treatment system.

For reverse osmosis, the ionic concentration increases (resulting from the effects of high recovery) Since the mechanism involves a percent rejection of salts (typically above 90 percent), a percentage (albeit small) of the salts concentration at the membrane surface will pass through the membrane into the permeate stream (100 minus the percent rejection). As the concentration of salts increases due to higher recovery operations, an elevated quantity will pass through the membrane, thereby lowering the quality of permeate. This high recovery will also produce an elevated osmotic pressure in streams that have significant salts content. Osmotic pressure is basically the resistance of an ionic solution to being pumped through a reverse osmosis or nanofiltration membrane, and is a function of both the type of salts and their concentration.

It is important to remember that the concentration of contaminants seen by the membrane is roughly the arithmetic average of the feed and concentrate streams. Under high recovery conditions, the concentrate stream concentration is extremely high, which will increase the average concentration. Therefore, the TDS of the feed stream does not have to be particularly high to result in a high osmotic pressure condition. One of the outcomes of a test program is the determination of osmotic pressure as a function of recovery for a given waste stream.

An important parameter for wastewater applications is to decide the ultimate discharge of the concentrate stream. Depending upon the situation, options include evaporation to dryness, collection in an ion-exchange resin or other adsorptive medium, or simply the removal of this stream for disposal.

Process water
Of the three general applications for membrane technologies, chemical processing is the most complex and diverse. Membranes are used for applications where certain contaminants are separated from each other into discrete streams, and then one or more are possibly recovered for reuse.

Testing requirements
With regard to wastewater and process water applications, it is imperative that every stream be tested to identify the following design factors:
• Optimum membrane element
configuration
• Total membrane area
• Specific membrane polymer
• Optimal pressure
• Maximum system recovery
• Flow conditions
• Membrane element array
• Pretreatment requirements

Specific properties of feed streams, which influence these design factors, include:
Stream chemistry
• Total solids content
• Suspended (TSS)
• Dissolved organic (TOC, MBAS, COD and BOD)
• Dissolved inorganic (TDS)

Chemicals of concern
• Oxidizing chemicals
• Organic solvents (particularly aromatic hydrocarbons)
• Saturated solutes

pH, operating temperature, osmotic pressure as a function of system recovery, and variation in chemistry as a function of time

To generate necessary design data, several testing options are available:
Cell Testing – Cell test devices are available for purchase or a qualified consulting engineering firm can provide testing. Such testing will evaluate small sheets of membrane candidates on the stream to be processed. Typically, the sheet is placed between two stainless steel plates, and the test stream is pumped across the membrane surface at a selected pressure and flow rate. The permeate is collected and analyzed for its degree of solute separation.

Cell testing only requires small volumes of the test solution. Several membranes can be evaluated in a short period of time. The cell test approach is useful as an initial step, primarily to select one or more membrane candidates for further evaluation. However, actual final design cannot be developed from this test.

Applications Testing – Applications testing utilize a full-sized membrane element in a test unit that is capable of simulating a production unit. Since the data from this testing will be used to scale up the design to full size, it is essential that the membrane element manufacturer supplies an element that is capable of such scale up.

The applications test equipment should be designed so that very high recoveries can be achieved without compromising the flow rates required to produce turbulent flow. This requires a pump that is capable of not only producing the desired pressure, but also a flow rate to create the minimum cross flow velocity across the membrane surface as well.

Materials of construction should be considered prior to testing; 316L stainless steel is essential for most applications requiring pressures in excess of 60 psi; below that, schedule 80 PVC is usually sufficient. For high chloride streams, special alloys may be required.

Applications testing can generate complete design data for the full-sized system. The test can be conducted with as little as 50 gallons (200 L) of the test stream, and, after setup, can be completed in one hour or less for each membrane element tested.

A typical applications test is conducted as follows:
1. To establish control conditions, high quality water (tap water or water treated with reverse osmosis or a deionizer) is used in the system at low recovery. Record data for each condition.

2. Feed water is then initially tested in the unit at a low recovery. After the system stabilizes, (usually in less than five minutes), the following data are recorded: feed pressure, pump pressure (pump discharge), system pressure (at the exit of the membrane upstream of the concentrative valve), recycle pressure, flow rates (usually feed, recycle and permeate) and recycle stream temperature. The system recovery is then increased incrementally while adjusting the recycle to ensure that the correct cross flow velocity is maintained.

At each recovery, in addition to the collection of flow and pressure data, analytical samples may be taken for performance evaluation. Of course, the choice of chemical parameters to be measured depends upon the separation goals of the test. It is unusual for system recoveries to exceed 95 percent; however, recoveries also depend upon the goals of the testing, and it is possible to run a well-designed test unit up to 99 percent recovery.

Once the optimum conditions have been established, such as operating pressure and maximum system recovery, the normalized performance data will enable the test engineer to determine the total membrane area required for the full-sized system.

Pilot Test – This test involves the operation of a prototype system that can be scaled up as needed in the final design.

This usually involves the placement of a test machine in the process, which operates continuously on a side-stream for a minimum of 30 days. The optimum run conditions (recovery, pressure, flow rates, etc.) are maintained during the testing period, and pretreatment requirements can usually be obtained during this testing.

Table 1 summarizes the important characteristics of these testing options.

Conclusion
Although water purification applications currently dominate the reverse osmosis membrane technology market, the potential for membrane separation technologies in wastewater and processing applications is basically untapped. To realize this potential, it is imperative that any candidate stream be properly tested. Such testing requires knowledgeable, experienced personnel to run and interpret test results on well-designed testing equipment. PE