There exists a potential for calcium, barium, strontium, fluoride and silica compounds to precipitate in the concentrate or brine channels of the reverse osmosis membranes. Precipitation occurs when the solubility limits of these various salts and silica is exceeded. 

Related Blog: What is the Reverse Osmosis Equipment Manufacturer's (ROEM) Role in Membrane Water Treatment?

How Scale Deposits Form In Your Reverse Osmosis System

As pure water is forced through the membrane under pressure, the salt concentration on the brine side of the membrane increases since the solvent (pure water) is being transported through the membrane, which acts as a barrier to the salts. As the salt concentration increases, the least soluble salts will begin to form micronuclei particles.

These combine to eventually form insoluble salt crystals. Should these crystals be permitted to form unchecked they will eventually form scale deposits on the reverse osmosis membrane surface. Excessive scale build up will cause the R/O membrane element to lose efficiency due to increased polarization or, in extreme cases, plug the membrane channels.

Stopping Scale Formations

To inhibit mineral scale formation a specially formulated organic polymer (scale inhibitor) is injected into the raw water stream. This polymer will help disperse suspended matter present in the raw feed water stream, alter crystal growth (morphology) of mineral scales, and sequester (chelate) micro-nuclei to prevent crystal growth.

Dosage Requirements

The amount of scale inhibitor needed to prevent scale formation in the R/O membrane is determined by the concentration of various constituents in the brine (concentrate or reject) solution. The requirements for scale inhibitor addition is site specific and is dependent on feed flow, raw water analysis and computer generated design projections for the specific R/O membrane being used. Typically, a scale inhibitor/dispersant dosage rate of 2 - 5 milligrams per liter (mg/L) is adequate to prevent scaling.

Dosing Calculations

For sample calculations we'll make the following assumption in order to properly do these calculations.

     Feed Flow: 510 gpm
     Specified Dose: 2.21 mg/l  neat solution (provide by chemical supplier)
     Anti-Scalant Density: 10.3 lb/gallon
     
     510 gal/min * 1440 min/day = 734,400 gal/day   
     2.21 mg/l (ppm) * 8.345 min/day * 734,400/1,000,000 = 13.54 lb/day  
     13.54 lb/day / 10.3 lb/gal = 1.31 gal/day per train
     1.31 lb/day * 3789 ml/gal = 4,963.6 ml/day
     4,963.6 ml/day / 1440 min/day = 3.45 ml/min

Therefore, your dosage should be set at the dosing pump to 3.45 ml/min for this particular example. 

Pump Settings

After diluting scale inhibitor solution (if required), it is necessary to set the feed pumping rate. This is done by adjusting the stroke length and/or stroke frequency (speed) knobs located on the chemical feed pump face. A graduated cylinder should be used to assist the operator in making the necessary stroke/frequency adjustments.
This is done by adjusting the capacity control (stroke) knob located on the chemical feed pump face and the motor VFD speed. A calibration column is available to assist the operator in making the necessary stroke/speed adjustments.

Fill the calibration column with 100 ml of scale inhibitor solution. Close the Scale Inhibitor pump suction valve and open the valve on the calibration column. The feed source to the pump is now the solution in the calibration column. The column is mounted in such a way that the suction to the chemical pump from the calibrated flow column will be flooded and priming should not be necessary. With an RO train running, measure the change in volume of the calibration column during a one-minute period. Adjust the pumping rate by turning the pump capacity control knob clockwise (to increase) or counterclockwise (to decrease).

Calibration of the scale inhibitor pump should be checked weekly to assure proper dosage of the scale inhibitor solution. The procedure above is for the initial pump calibration. For additional operation and maintenance information on the chemical feed pumps, refer to the manufacturer’s equipment manuals.

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