Designing a customized reverse osmosis system provides the greatest amount of control over permeate water quality. This is especially true for RO systems providing water treatment in specialized industries, such as pharmaceuticals. When water purification standards are particularly stringent, a custom-designed system allows the freedom to target specific permeate parameters.
How can a layman make the best decisions about a custom system? Learning more about the design process is a good start. These 10 steps offer a look at the process of designing a membrane system:
#1: Determine Water Quality Baselines
Much of the design process hinges on the feed water source and the necessary quality of the end permeate water. Before getting started, be sure to take samples for analysis of the feed water, and find out what the target parameters are for the permeate water.
#2: Choose the Flow Configuration
The flow configuration offers control over the circulation, which affects system operation and efficiency. Some common choices include:
- Plug flow is the most common flow configuration for RO systems, and with this configuration, the feed volume passes through the system once.
- Concentrate recirculation is an alternative for systems that have too few elements to sustain the right system recovery rate, and is ideal for waste water processing.
- Pyramid or concentrate staging moves the feed into a decreasing number of pressure vessels in each stage in a tapering configuration, and helps maintain consistent flow rates for system longevity.
#3: Select the Membrane Element Type
Free reign over the membrane element type allows for targeted performance, whether the user is looking to accomplish a high rejection rate or reduce energy consumption with an ultra-efficient membrane model.
#4: Designate the Membrane Flux
The average flux, or flow rate, should typically be chosen as a high flux value in systems that have relatively good quality feed water. Low flux systems, on the other hand, perform best with low quality feed waters that require more intensive purification and a slower process.
#5: Find the Number of Elements Needed
This is a simple calculation once you have the permeate flow rate (which was chosen in step 1) and the flux, which was chosen in step 4.
Where QP=permeate flow rate, and f=flux, and SE=membrane surface area, the equation is:
#6: Choose the Number of Pressure Vessels
This is another simple calculation once you have the number of elements.
Where NE=number of elements, NEPV= number of elements per pressure vessel, and NV is the total number of pressure vessels, the equation is:
Most large systems have pressure vessels that fit up to 6 elements each, though some vessels can contain 8 elements.
#7: Designate the Number of Stages
The number of stages helps determine the system recovery. In general, systems that are intended to process poor quality feed water should have a larger number of stages for a longer, more intensive filtration process. One-stage and two-stage systems are most common, but three-stage systems can be designed.
#8: Choose the Staging Ratio
The staging ratio is the relationship between the number of pressure vessels and the stages. Ideally, every stage in the system should operate at the same fraction of system recovery.
If R=ratio, Nv=number of vessels, and i=number of stages, the equation is:
#9: Balance the Permeate Flow Rate
In a well-designed system, the permeate flow rate is lower than the lead element flow rate. After setting the rest of the specifications, the flow rate should be tested thoroughly, and adjustments made accordingly.
If the ratio is too high, then it can be balanced by boosting the feed pressure, applying permeate back pressure, or using membranes with low permeability in the first positions of the system.
#10: Test & Refine the System
The final step is to have the reverse osmosis system tested and any refinements or changes made before finalizing the design. A simulation program can provide valuable data and system performance projections, such as feed pressure and permeate quality, as well as operational data.
This 10-step guide provides a look at the most important considerations when designing an RO system from scratch. Every specification needs equal consideration, and all choices should be made with permeate quality in mind. When designing a system, it’s best for most users to partner with an expert team of water purification engineers to achieve the best performance possible.
Considering a custom-designed reverse osmosis system? Contact AXEON Water Technologies today to discuss the process and specifications with our team of water treatment experts!
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