SOFTENER OR REVERSE OSMOSIS – WHICH ONE IS BEST FOR YOUR BOILER?

Traditionally, there are two main methods for boiler feed water treatment. Those methods are: water softener and/or reverse osmosis. The degree to which water needs to be treated depends on several variables, such as the specific type of raw water used and the operating pressure of the boiler itself. In boilers, it is a common problem to experience calcium carbonate scale formation. Using less than pure water even with miniscule levels of contaminants can cause deposits and corrosion.

Knowing which is the right option for your boiler comes down to understanding what each of these water treatment systems do and how they can benefit your boiler. Here’s a quick breakdown to help you differentiate the two.

SOFTENER – HOW IT WORKS

Hard water is caused by high levels of calcium and magnesium in the water. Water softeners treat the water by using salt and ion-exchange resins to remove these minerals. When the water comes in contact with the resins, the calcium and magnesium ions transfer out of the water and onto the active sites on the resin. Then they are replaced in the solution by the sodium ions.

Is water softener alone sufficient for treating boiler feed water?

Water softening alone, at times, can be sufficient for low pressure boilers. Check with the manufacturer for their recommendation. In addition to scale depositing calcium and magnesium minerals, boiler feed water often contains other deposits and minerals that are not effectively removed by the ion exchange process of a water softener. Adding a RO filtration system will reduce any chemicals needed and will supply the boiler with consistent high-quality water.

How long does it take for a water softener to regenerate?

The length of time it takes for a water softener to regenerate depends on a number of factors, namely the brand and size of water softener used. Most water softeners require regeneration after a set number of gallons of water passes through the system. This monitor can determine exactly when your softener needs to regenerate, allowing you to achieve a savings in salt and water required for the regeneration process. Savings can be estimated at the time the unit is sized.

REVERSE OSMOSIS – HOW IT WORKS

Reverse osmosis is the process used to purify water used for drinking and cooking. The water in our RO system goes through three stages. In the prefilter stage, large particles like silt and rust are removed by the filter. Next the water goes through the membrane. Hydrogen and oxygen pass through the pores while dissolved solids are caught and removed. Finally, the water goes to the holding tank after going through one last stage of filtration to remove any leftover contaminants.

What is the difference between reverse osmosis (RO) and nanofiltration (NF or Nano)?

Reverse osmosis and Nanofiltration are nearly identical in terms of appearance and functionality. Both systems force water through a membrane filter to remove unwanted contaminants.

The primary difference between reverse osmosis and nanofiltration is that nanofiltration does NOT reject dissolved minerals. Nanofiltration allows between 10% and 20% of dissolved minerals to remain in the water while reverse osmosis rejects 90% to 99% of these same minerals. Nanofiltration is a newer technology that was developed specifically for purifying drinking water. Since the same minerals that are beneficial to human health can wreak havoc on a boiler system or cooling tower, reverse osmosis remains the filtration method of choice for industrial use.

Can a reverse osmosis (RO Filtration) system achieve 100% recovery or 100% separation of particulates from process water?

Reverse osmosis systems CAN reject particles down to the size of .0001 microns. A good reverse osmosis system will reject 95%-99% of most feed water contaminants. Industrial reverse osmosis systems typically recover anywhere from 50% to 75% of filtered feed water. So, for every 100 gallons filtered, about 50-75 gallons is recovered as product water.

WHICH ONE IS BEST FOR YOUR BOILER?

In order to gain a better, practical understanding of using RO for boiler pretreatment, it is helpful to compare a typical soft water pretreatment program to that of a RO pretreatment program.

Advantages of RO over a softener boiler pretreatment program include:

• 90 to 99.9% removal of dissolved solids to allow boiler to operator at much higher cycles of concentration (up to 100 cycles).
• Removes more than just hardness (calcium & magnesium).
• Lower boiler makeup and blowdown rates.
• Lower fuel usage.
• Less internal boiler treatment chemicals required.
• Less alkalinity resulting in less neutralizing amine demand.
• Cleaner boilers.
• Reject may be of good enough quality for cooling tower makeup or wash water.
• If using softeners with RO, the regeneration costs (including salt and water) may decrease depending upon the current condensate return and blowdown rate.
• No regenerant chemicals.
• For the right waters, lower manpower requirements may be realized.

Disadvantages of RO as compared to a softener boiler pretreatment program include:

• Increased electrical costs.
• If acid pretreatment is used, acid handling will be required.
• Product water flow rate is pretty much fixed while ion exchange can manage variable flows.
• An antiscalant may be necessary upstream of the RO.
• With polyamide (PA) membranes, chlorine must be removed upstream of the RO.
• Increased monitoring responsibilities including RO normalized permeate flow, percent salt passage, pressure drops, SDI’s, conductivity, temperatures, etc.
• Commonly 25% of water into an RO comes out as reject. If this water has no other uses, it is sent down the drain.
• Membranes must be cleaned and replaced periodically.
• Microbiological fouling can be an issue.
• Flow is temperature sensitive.
• May need additional pretreatment equipment.
• In some waters, higher manpower requirements are seen.

Conclusion

The big picture for a properly applied RO pretreatment system in an ideal situation is: lower water costs, lower sewer costs, lower fuel costs, lower treatment chemical costs, lower or elimination of regenerant chemical costs, higher boiler reliability, and higher steam purity. As discussed, exceptions to this big picture do exist, and how an RO boiler pretreatment system fits into each unique situation must be carefully considered.