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Feed Water Conductivity Equivalent (FCE)

The concept of EDI feed water conductivity equivalent arose from the need for a simple field method of estimating the ionic load on a EDI device. Certainly the best way to determine the ionic load is to perform a complete water analysis and determine the concentration of all ionized and ionizable constituents, but in some cases this is not practical. What had frequently been substituted for a complete water analysis was a simple measurement of the conductivity of the EDI feed water. This could introduce considerable error, as a conductivity measurement does not detect the full amount of weakly ionized species such as carbon dioxide (CO2) and silica (SiO2). For example, 10 µS/cm water could contain 4 ppm of NaCl or 60 ppm CO2. Just as this would have a huge impact on the service cycle of a conventional demineralizer, it can have a major impact on sizing a EDI system. For this reason we have developed the concept of EDI feed conductivity equivalent, which attempts to take into account weak ions such as CO2 and silica.

Calculating FCE

  • Measure EDI feed water conductivity (µS/cm)
  • Measure the ppm CO2
  • ppm as CO2 x 2.79 = µS/cm
  • Measure the ppm SiO2
  • ppm SiO2 x 1.94 = µS/cm
  • Add measured conductivity to CO2 & SiO2 µS/cm

In the field, the CO2 concentration in the EDI feed water can be measured using Hach test kit Model CA-23 (#143601). The smallest increment for this test kit is 1.25 mg/l.  The EDI feed water conductivity can be measured with a hand-held conductivity meter such as the Myron L Model 4P or with the permeate conductivity meter on the RO system.

Example FCE Calculation

  • Measured conductivity = 6 µS/cm
  • Measured CO2 = 5 ppm as CO2
  • Measured SiO2 = 0.5 ppm as SiO2
  • FCE = 6 + (5 x 2.79) + (0.5 x 1.94) = 20.9 µS/cm

DC Volts and Amps

  • Voltage (potential) causes current to flow
  • Current causes transfer of salt, regeneration of resin
  • Amount of current required proportional to product water flow, amount of salt being removed
  • Use Faraday’s law to calculate current required

Calculating Amps Required

  • Faraday’s Law
    • I = 1.31 (Q)(FCE)/(# cells)(eff)
    • I = DC current, amps (per module)
    • Q = product flow rate, liters/min/module
  • FCE = feed conductivity equivalent, µS/cm
  • eff = current efficiency, % (assume 10% current efficiency)

Example DC Current Calculation

  • Product flow = 50 lpm
  • Feed = 5 µS/cm + 3.75 ppm CO2
  • FCE = 5 + (3.75 x 2.79) = 15.5 µS/cm
  • I = 1.31(50 lpm)(15 mS/cm)/(24 cells)(10%)
  • = 4.2 amps per module

Setting DC Amperage

  • Use constant current power supply
  • Estimate amps required per module
  • Set amperage to calculated value
  • Power supply adjusts voltage to maintain current
  • Important to record (trend) voltage & amperage

Definition of EDI Recovery

  • % Recovery (R) = (QP)(100)/(QP + QR)
  • QP = Product (Dilute) flow rate
  • QR = Reject (Concentrate) flow rate

Calculating Reject Flow

  • Reject = ((100 - R)/R) x product flow
    • where R is the percent recovery
  • Example: Product flow = 50 m3/h
    • Recovery = 90%
    • Reject = ((100-90)/90) x 50
    • = 5.6 m3/h
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