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No. Even with a two-pass RO unit, you will not consistently produce USP grade water. While it may be possible to produce USP grade water immediately after startup, the water quality will likely not meet the USP requirements as the membranes age. The use of Deionization bottles or Electro-deionization will be required after the RO to meet USP requirements.

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It is difficult to predict the permeate quality of an RO. RO product quality relies heavily on the quality of the feed water and the types of membranes used. The most effective tool that helps determine the permeate quality is an accurate water analysis. MECO can evaluate the water analysis and run RO projections to help determine the permeate quality of any RO design.

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Hot water sanitization and chemical sanitization are both used to sanitize RO units. Hot water sanitization has a much higher initial cost, as it requires more instrumentation, a heat exchanger, controls, and special membranes. The high heat (180F) can also reduce the life of the membrane if frequent sanitizatons are required.

Chemical sanitization has a much lower initial cost. Chemically sanitized units do not require stainless steel feed lines, special membranes, heat exchangers, controls, and added instrumentation. A CIP skid is required, but if a customer already has one for an existing piece of equipment, it can easily be adapted for use on the RO, as well. The drawback is that sanitization chemicals have to be purchased, handled, and disposed of every time the unit is sanitized.

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RO units need a good bit of attention. The RO unit should be monitored on a daily basis and records should be kept on pressures, flowrates, and water quality. This helps determine when the RO unit is experiencing problems with scaling, biofouling, membrane degradation, etc. Since problems with RO units are usually not associated with catastrophic failures, having the ability to trend data makes diagnosing a problem much easier.

Even with proper care, RO membranes will have to be replaced. The average life of RO membranes is about 3 years. Replacement will require a full shutdown of the RO unit. Depending on the number of membranes in the system, this can be an expensive maintenance cost that should not be overlooked.

Membrane cleaning is a task that will have to be performed on a regular basis. Cleaning of the membranes is required for removal of scale and biofouling. Depending on the size of the RO unit, it should be noted that cleaning may be a very time consuming task and the RO unit must be shutdown.

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The first consideration you should make is what type of materials of construction do you need. PVC is an inexpensive material to make an RO out of. The capital cost of the unit will be greatly reduced at the expense of a more sanitary design. If a sanitary design is a major concern, then a stainless steel O.D. tube permeate is required at a greater cost.

Membrane selection is another important area of concern. Different membranes have different rejection rates. Typically, the highest rejection rates are associated with the highest operational pressures. The higher the operational pressure, the more pump horsepower is required to boost the pressure. A larger horsepower pump will incur a higher electrical utilities cost for daily operation.

Membrane flux is the amount of water that can be passed through a square foot of membrane. The unit of measure for flux is referred to as GFD (gallons per square foot per day). The higher the flux rate, the higher potential for scaling problems. It is best to consider a conservative flux rate when designing an RO system. Running the membranes close to their maximum flux will increase the potential for scaling and will shorten the life of the membranes.