FilmTec Technical Manual - Chester Paul Company

With a recovery of, for example, 75% (Y = 0.75), the ionic strength of the concentrate becomes⎛ 1 ⎞I c = 0.0444⎜⎟⎝ 1−0.75 ⎠= 0.178Ic2.4.2 Calcium Carbonate Scale Prevention2.4.2.1 Brackish WaterFor brackish waters with TDS < 10,000 mg/L in the concentrate stream, the Langelier Saturation Index (LSI) is used toexpress the scaling potential for calcium carbonate /6/.The following data are needed to calculate the LSI of the concentrate stream (LSI c ):Ca f = Calcium concentration in feed as CaCO 3 , mg/LTDS f = Concentration of total dissolved solids in the feed, mg/LAlk f = Alkalinity in feed as CaCO 3 , mg/LpH f = pH of the feed solutionT = Temperature of the feed solutionY = Recovery of the reverse osmosis system, expressed as a decimalCalculations1. Calculate the calcium concentration in the concentrate stream, Ca c , as CaCO 3 in mg/L:⎛ 1 ⎞Ca c = Caf⎜ ⎟Eq. 6⎝ 1−Y⎠2. Calculate the total dissolved solids in the concentrate stream, TDS c in mg/L:⎛ 1 ⎞TDSc= TDSf ⎜ ⎟Eq. 7⎝1−Y ⎠3. Calculate the alkalinity in the concentrate stream, Alk c , as CaCO 3 in mg/L:⎛ 1 ⎞Alk c = Alkf⎜ ⎟Eq. 8⎝ 1−Y⎠4. Calculate the free carbon dioxide content (C) in the concentrate stream by assuming that the CO 2 concentration in theconcentrate is equal to the CO 2 concentration in the feed: C c = C f . The concentration of free carbon dioxide in the feedsolution is obtained from Figure 2.2 as a function of the alkalinity and the pH of the feed solution.5. Calculate the pH of the concentrate stream (pH c ) using the ratio of alkalinity Alk c to free CO 2 in the concentrate,Figure 2.2.6. From Figure 2.3 obtain: pCa as a function of Ca c , pAlk as a function of Alk c , “C” as a function of TDS c and temperature(temperature of the concentrate is assumed equal to temperature of the feed solution).7. Calculate pH at which the concentrate stream is saturated with CaCO 3 (pH s ) as follows:pHs= pCa + pAlk+"C"Eq. 98. Calculate the Langelier Saturation Index of the concentrate (LSI c ) as follows:LSI = pH − pHEq. 10ccsAdjustments of LSI cIn most natural waters, LSI c would be positive without pretreatment. To control CaCO 3 scaling, LSI c has to be adjusted to anegative value, except if adding a scale inhibitor (Section 2.3.3) or preventive cleaning (Section 2.3.7) is applied.Page 30 of 180 ® Trademark of The Dow Chemical Company ("Dow") or an affiliated company of Dow Form No. 609-00071