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Operating Electrodeionization in a High-Silica Feedwater Environment

High silica feedwaters pose two challenges for ultrapure water membrane systems. On the front end, silica can scale the RO membranes, limiting use of some feedwaters and high water recovery. On the back end, silica can scale the IX resins in the electrodeionization (EDI). It is also a challenge to attain the very low ppb levels of silica in the ultrapure product water required by power and electronics applications.  The use of a premium silica antiscalant, use of high-silica-rejection RO membranes, and use of CO2 removal technology can lead to successful RO-EDI systems even with high feedwater silica. Even with 1-pass RO.  Even in regions of the world like Mexico and Japan where silica is often present in the 50-100 ppm range.

Introduction

When silica (SiO2) in feedwater to an ultrapure water system is high there are 2 challenges:

  • high silica can scale the RO and even the EDI
  • high silica can prevent achievement of low ppb levels of silica in the ultrapure water product

This paper discusses 3 factors for successfully operating an RO-EDI ultrapure water system with high silica feedwater.

  1. use of premium silica antiscalant
  2. use of high-silica-rejection RO membranes
  3. use of CO2 removal technology

Silica Levels & Silica Chemistry

Silica Saturation: 80-140 ppm, depends on temperature, pH, etc.

Silica in Raw Waters: 5-100 ppm

Silica in RO Concentrate: 20-320 ppm, with antiscalant

Silica in EDI Feed: 100 ppb – 1 ppm

Silica in EDI Product: 2-100 ppb

Silica in natural waters ranges from low ppm levels up to saturation, near 100-120 ppm. Silica saturation depends primarily on pH, as well as temperature, alkalinity, and the other ions and metals present.  Silica is present in solid form (colloids), in monomer form (reactive), and in silicate complexes with metals.  Silica chemistry is complex, as reactive SiO2 exists in numerous hydrated forms, and weakly ionizes at high pH. Solubility of SiO2 begins increasing above about pH 7.8, and rises with temperature at all pHs.

SiO2                       =             SiO2

SiO2*H2O             =             H2SiO3

SiO2*2H2O           =             H4SiO4   =>           H+ + H3SiO4         pKa≈9.5  (“Silicic Acid”)

SiO2*3H2O           =             H6SiO5

… etc

 

High silica levels are prevalent in volcanic areas of the world, such as Japan and Mexico, and in ground- and well-waters. Surface waters generally have less silica, and silica levels in seawater are vanishingly small.

Examples of SiO2 concentrations in high-recovery RO

Example 1: 20 ppm SiO2 in feedwater, pH 7.2, 25°C, 67% recovery, 98% SiO2 rejection RO

RO feed: 20 ppm SiO2

RO concentrate: 60 ppm SiO2

RO average feedside: 40 ppm SiO2

RO permeate, at 98% rejection SiO2:  0.02*40 = 0.8 ppm SiO2 (800 ppb)

In this normal case, silica in the RO concentrate is below scaling potential/saturation. The silica in the RO permeate (electrodeionization feed) is less than 1 ppm, so won’t scale the EDI at 90% recovery. It is though above the nominal limit of 0.5 ppm SiO2 for EDI feedwater.

Example 2: 35 ppm SiO2 in feedwater, pH 7.2, 25°C, 67% recovery, 98% SiO2 rejection RO

RO feed: 35 ppm SiO2

RO concentrate: 105 ppm SiO2

RO average feedside: 70 ppm SiO2

RO permeate, at 98% rejection SiO2:  0.02*70 = 1.4 ppm SiO2

In this marginal case, silica in the RO concentrate is at scaling potential/saturation. The RO will require a special antiscalant to prevent silica scaling. And the silica in the permeate is more than 1 ppm, so will likely scale the EDI.  This case requires a special high-silica-rejection RO membrane like ExcellPureRO™.

Example 3: 80 ppm SiO2 in feedwater***, pH 7.2, 25°C, 67% recovery, 98% SiO2 rejection RO

RO feed: 80 ppm SiO2                                                                                        *** requires use of Avista Vitec® 4000

RO concentrate: 240 ppm SiO2***

RO average feedside: 160 ppm SiO2

RO permeate, at 98% rejection SiO2:  0.02*160 = 3.2 ppm SiO2

In this high-silica case, silica in the RO concentrate is over the scaling potential/saturation, so will require a premium silica antiscalant. Avista Vitec® 4000 will protect up to 320 ppm silica in the RO at 25C and pH 7.0. The silica in the permeate is more than 3 ppm, so would quickly scale the EDI, and is not recommended.  This case requires a special high-silica-rejection RO membrane like ExcellPureRO™.

Discussion: RO silica and premium silica antiscalants

As you can see from the examples, 1-pass RO systems running at reasonable recoveries can only operate with very low silica feedwaters (<20 ppm) without the use of silica antiscalant.

Not all silica antiscalants are equivalent. Avista’s Vitec® 4000 is a premium silica antiscalant. Vitec® 4000 can prevent silica precipitation in an RO up to approximately 320 ppm, at pH 7., which allows RO systems to operate scale-free under a very wide range of conditions, including feedwaters in the 80-100 ppm SiO2 range, at recoveries in the 65-75% range. Other popular silica antiscalants can reach low- to mid-200 ppm range SiO2 in the RO.

SnowPure recommends the use of Vitec® 4000 for RO-EDI systems making ultrapure water.

Silica Rejection in Various RO Membranes:

Few RO membrane manufacturers broadcast the silica (SiO2) rejection of their membranes. Many promote the use of “low-energy” membranes. These low-pressure membranes are good for drinking water but are not suitable for industrial applications where silica and boron rejection are important. Yet they are marketed to these users anyway.

Most low-energy, low-pressure RO membranes reject only 97-98% SiO2. This is too low for practical industrial systems.

SnowPure recognized this as a problem, and has worked with a Japanese membrane manufacturer to offer the highest silica rejection RO membranes available. SnowPure sells high-silica-rejection RO membranes under the brand ExcellPureRO™.

ExcellPureRO™ membranes achieve 99.2-99.4% rejection of SiO2 and 99.8% rejection of TDS.

Further, the membranes are “right-sized” for Electropure™ EDI. One of the sub-optimizations of an RO-EDI system is to oversize the RO, and when it is run at lower pressure to match the flow needed for the EDI, it provides lower rejection overall. These membranes are designed to operate at 10 bar (150 psi) net operating pressure.

Comparison of “low-energy” membranes with ExcellPureRO™ membranes in an RO-EDI system:

ExcellPureRO™ RO membranes Low-energy RO membranes
99.2-99.4% SiO2 rejection 97-98% SiO2 rejection
RO Example 1:

0.006*40 ppm SiO2 = 0.2 ppm SiO2

RO Example 1:

0.02*40 ppm SiO2 = 0.8 ppm SiO2

RO Example 2:
0.006*70 ppm SiO2 = 0.4 ppm SiO2
Example 2:
0.02*70 ppm SiO2 = 1.4 ppm SiO2
RO Example 3:
0.006*160 ppm SiO2 = 1.0 ppm SiO2
Example 2:
0.02*160 ppm SiO2 = 3.2 ppm SiO2

 

High Silica Rejection RO Conclusion:

With low-energy, low-pressure, “low-silica-rejection” RO membranes, the RO permeate levels of silica have high scaling potential in the EDI. With ExcellPureRO™ membranes, even a 1-pass RO can achieve in-spec SiO2 for EDI feed over a wide range of high-silica feedwaters.

Achieving low SiO2 levels in EDI Product Water:

To achieve ppb levels of SiO2 in EDI product water, two things are necessary. First, the silica levels fed to the EDI should be as low as possible. Second, the FCE (feed conductivity equivalent) fed to the EDI should be as low as possible so that the EDI can effectively polish the silica. Electropure™ EDI can remove 97% of the feed silica as long as FCE is low (<9 uS/cm).

FCE accounts for all species that must be removed from the EDI, including ions (conductivity), CO2 (CO2 & HCO3), and SiO2.

              FCE (μS/cm) = COND (μS/cm) + 2.79*(ppm CO2+HCO3) + 1.94*(ppm SiO2)

FCE Example: 6 μS water, pH 6.4, CO2=2.5 ppm, HCO3=2.5 ppm, SiO2=0.5 ppm

FCE = 6 + 2.79*5 + 1.94*0.5

FCE = 6 + 14 + 1

FCE = 21 μS/cm

To lower the FCE of the EDI feed, removal of CO2 and HCO3 is critical. Inside the EDI, SiO2 and CO2 compete for removal, and since the pKa of CO2 is closer to neutral it is preferentially removed.

There are 2 strategies to lower CO2/HCO3.

  1. Raise the pH of the RO feedwater to the range 8.4-8.7 (often interstage injection of NaOH).
  2. Use Liqui-Cel™ GTM membrane degasser in the RO permeate, pH should be less than 6.1.

For technique #1, at the high pH all CO2 has been converted to HCO3 and CO3-2, which are well-rejected by the RO membrane. The CO2 form passes the RO membrane.

For technique #2, at pH lower than 6.1 most of the HCO3 has been converted to the CO2 form, and when a combination sweep gas (air or N2) and vacuum are used CO2 can be reduced up to 90% or so. SnowPure can run a projection for this, and recommend process design and process conditions.

The FCE example above, assuming CO2+ HCO3 = 1ppm:

FCE Example: 6 μS water, pH 6.4, CO2=0.5 ppm, HCO3=0.5 ppm, SiO2=0.5 ppm

FCE = 6 + 2.79*1 + 1.94*0.5

FCE = 6 +3 + 1

FCE = 10 μS/cm

With this FCE, removal of SiO2 by the EDI should be up to 97%.

Summary

When silica (SiO2) in feedwater to an ultrapure water system is high there are 2 challenges:

  • high silica can scale the RO and even the EDI
  • high silica can prevent achievement of low ppb levels of silica in the ultrapure water product

This paper discussed these 3 factors for successfully operating an RO-EDI ultrapure water system with high silica feedwater:

  1. use of premium silica antiscalant, such as Vitec® 4000
  2. use of high-silica-rejection RO membranes, such as ExcellPureRO™
  3. use of CO2 removal technology, such as Liqui-Cel™ GTM

Michael J. Snow, Ph.D.