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