Reducing Costs With RO
Ultra-low-energy RO membranes help beverage facilities save energy and costs
Today, beverage production facilities make a wide range of products, including carbonated soft drinks, energy drinks, juices, teas and purified drinking water. The number one ingredient in all of these products is water. Naturally, the quality of the water, as well as the operation of the water treatment system, are key to the quality and profitability of the facility.
Reverse osmosis (RO) membrane systems have been replacing conventional water treatment systems, such as lime softening, for several years and are now often the technology of choice for the beverage industry. RO systems produce ingredient water with low total dissolved solids (TDS) content and provide a barrier to bacteria and trace organic materials that can affect the taste and quality of the final product. Lower-TDS ingredient water allows a facility to produce a wider range of products, such as energy drinks, that have more stringent requirements for TDS, sodium, alkalinity and other contaminants.
One of the main factors in the cost of operating an RO system is electricity consumption, because RO requires an electric-motor-driven high-pressure pump to generate the water pressure needed for the process. GE Power & Water has developed a new ultra-low-energy (ULE) RO membrane that uses significantly less pressure than other RO membranes and results in less electricity consumption, but produces the low-TDS ingredient water required for a variety of beverage products. The result is operating cost savings for the production facility as compared with alternative RO membrane systems.
Case Study Overview
A ULE RO system was commissioned for a beverage production facility in August 2011. Its previous water treatment scheme used a conventional lime treatment system to lower alkalinity to make carbonated soft drinks, but the system was aging and in need of replacement. The facility also needed to increase capacity.
It specified ultrafiltration and RO as replacement technology because it is more reliable, more automated and provides a better barrier to bacteria and organic material than the old system. GE proposed the new ULE RO membrane system, as it would meet the water quality targets the facility sought and would save it a significant amount in electricity costs.
The RO production rate of the new system is 400 gal per minute (gpm). It uses 90 ULE RO membrane elements with 365 sq ft per element.
At this facility, the water temperature varies between 38°F and 78°F, so all of the following operation data has been normalized to 77°F (25°C).
The case study results are:
- An average trans-membrane pressure of 52 psi (3.6 bar);
- A 98% average TDS rejection per element;
- A 99% average divalent ion rejection per element (hardness, sulfates and alkalinity); and
- A 92% average ionic silica rejection per element.
Average trans-membrane pressure is calculated by taking the average of thefeed pressures to the first RO element and the last RO element in the system and then subtracting any permeate backpressure. Ionic silica is more difficult to reject with RO membranes than other ions because it has a low charge density. A result of 92% ionic silica rejection is quite good given the low operating pressures of the ULE RO system. The system performance currently is exceeding the water quality requirements of the production facility and saving the customer operating expenses because of the lower pressure required.
Different applications may require different types of RO systems based on water quality requirements and energy saving desired. The following comparisons used test conditons with 77°F water temperature.
Conventional high-rejection brackish RO membranes test at 225 psi and achieve 99.5% sodium chloride rejection. These elements are used when dissolved mineral rejection is critical, such as in ultra-pure water treatment systems, like those used for power plant boiler makeup water, pharmaceutical plant USP-grade water, or electronics fabrication facility water to rinse semiconductor chips and other electronics in the manufacturing process.
Conventional low-energy RO (LERO) membranes test at 115 psi and achieve 99% sodium chloride rejection. LERO membrane systems are becoming more popular in general industrial applications, drinking water applications and beverage applications in which 99% sodium chloride rejection is adequate for the application and the 49% lower operating pressure is a benefit in energy savings.
ULE RO membrane elements test at 65 psi and achieve 98% sodium chloride rejection. These membranes save an additional 43% in operating pressure compared with LERO membranes, while giving up 1% sodium chloride rejection. The customer in the beverage facility case study was able to meet its quality requirements with the ULE membrane and save operating expenses with the lower operating pressure.
The first consideration in designing a ULE RO system is that the operating pressure is much lower than conventional RO systems. As a result, the size of the high-pressure pump is much smaller in terms of boosting pressure and will cost significantly less. The membrane housings can have a lower pressure rating and also cost less. In addition, the “high-pressure” piping in a ULE RO system can be PVC rather than stainless steel, which also saves costs.
The second design consideration is the arrangement of the membrane elements in the system. The conventional approach to RO system design is 12 to 18 8-in. membranes in series to achieve a recovery rate of 70% to 85%. This means placing a number of membranes in parallel operation to get the total volume of water needed.
As water flows through the channels of a membrane element, called cross flow, some pressure is lost. Over the entire set of membrane elements in series, this can be 50 to 80 psi of pressure. With conventional RO systems, the operating pressure is often 200 psi or more, so a pressure drop of 50 to 80 psi is manageable. ULE RO systems that operate at only 60 to 80 psi to start with, however, will not function properly if they lose nearly 100% of their pressure through normal pressure drop. The result would be that the first elements in the system would produce most of the permeate and the last elements in the system would produce little, if any, permeate.
One way to design a ULE RO system without high pressure drop is to use fewer elements in series. This approach can work in smaller-volume systems and/or at low water recovery rates. When the system is of large scale (greater than 200 gpm permeate), however, it is difficult to use this approach.
Another option is to use intermediate booster pumps to raise the operating pressure for the last membranes in the system so they operate at similar pressures and flow rates as the first elements. This approach costs more in capital expense, but is energy efficient.
A third design approach is to use permeate back pressure on the first elements to keep them from producing too much water. This option raises the overall operating pressure of the system, so the last few membranes have more pressure available and produce more permeate. The permeate back pressure approach is a lower-capital-cost alternative than intermediate pumping, but is less energy efficient. All of these options are viable, and the choice depends on the end user’s requirements for capital and operating costs.
ULE RO Applications
ULE RO membrane systems are best applied where RO technology is desired to reduce the dissolved mineral content, organic materials and bacteria from a water stream, but 99.5% or higher sodium chloride rejection is not required to meet the needs of the end user. Beverage producers often can reach the water quality targets for a variety of beverage products with only modest reductions in TDS, hardness and alkalinity, yet they desire a membrane technology, as it provides a barrier to bacteria and trace organic removal, which can affect final product quality.
Another application that would benefit from ULE RO membranes is municipal drinking water plants. Often, these plants desire RO membrane technology to reduce TDS, color, odor, sulfate, BOD, COD, arsenic, nitrates, bacteria or any combination of contaminants from the water supply. Typically, high-rejection RO membranes are not required to reach the goal.
The world’s water supplies continue to be depleted, and municipalities and industrial companies look to reuse their wastewater streams for utility water makeup, irrigation water and process water. In many of these applications, RO technology is desirable for modest TDS reduction, but with higher levels of color, TOC, BOD, COD and bacteria reduction. In these cases, ULE RO is a good fit because the larger organic compounds and bacteria in the water are rejected as well as with any high-rejection RO system, yet the system can operate at much lower pressures and still meet the finished water TDS goals in most cases.
The beverage facility case study represents a successful demonstration of the new ULE RO membrane in a full-scale 8-in. membrane element. After eight months of operation in the water treatment system, the performance is stable at less than 60 psi average trans-membrane pressure with greater than 98% average TDS rejection. Additional testing and certifications are required before full commercial release of the ULE RO element and RO systems using them, but thus far performance has been positive.
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