Calcium carbonate is the scale RO plants run into most often, and for decades the industry has leaned on old rules of thumb (the LSI and CCPP indexes) to decide how much antiscalant to dose and how hard a system can be pushed. The trouble is those rules are unreliable, especially at higher pH and in complex industrial water, so plants either over-dose chemicals or hit scaling they didn’t see coming.
Abstract
Calcium carbonate (CaCO3) is the most frequently encountered scale in industrial reverse osmosis (RO) systems. The Langelier Saturation Index (LSI) has traditionally been used to predict antiscalant limits to inhibiting CaCO3 scaling, but has been highly unreliable. The lack of reliability of the LSI is in part due to its use of TDS values rather than ionic strength to estimate CaCO3 solubility, but mostly because it doesn’t consider for ion pairing.
Another widely used scale prediction model, the Calcium Carbonate Precipitation Potential (CCPP), accurately predicts the quantity of precipitant in a low ionic strength solution, but cannot reliably predict levels at which antiscalants will fail to inhibit scale; the reason being that severe scaling occurs at very low CCPP values at the upper pH range where the driving force for scale formation is highest. Both calculations are ineffective at reliably predicting antiscalant dosage demands at varying pH levels, however both are commonly used to estimate such dosages in the absence of alternatives.
Antiscalants inhibit crystalline scales like calcium carbonate through threshold inhibition, a mechanism that interferes with the rate of crystal nucleation. A series of lab experiments through the neutral to high pH range validated a relationship between the rate of nucleation and antiscalant demand. This led to the development of a kinetics index used to calculate inhibitor dosages. Furthermore, a saturation calculation using ionic activity and ion complexation was used to predict the maximum saturation that could be controlled by antiscalants regardless of feedwater type. By considering for both carbonate and bicarbonate alkalinity, the calculation was made to be reliable at any pH. Such a calculation was essential for predicting whether pH reduction would be required, and for reliably determining the maximum recovery at which an RO system could be operated. The calculation was converted into an index for ease of use.
It was also recognized that antiscalants were severely limited by calcium and magnesium concentrations when attempting to operate at high recovery or high pH. Calcium and magnesium at high concentrations can form salts with all antiscalants, regardless of whether phosphonate or acrylate based. Higher inhibitor dosages exasperate this phenomenon, frustrating those who adhere to the common engineering practice of applying a safety factor. This creates scenarios where the antiscalant could fail even when a calculated saturation index is well below the maximum limit for a given inhibitor. The failure is twofold; the calcium or magnesium salts act as scales, while the loss of active inhibitor results in CaCO3 precipitation. An index was developed to calculate the point of failure for antiscalants at all pH ranges, with different constants applied for different antiscalant chemistries.
Using the three abovementioned indices, the maximum recovery, optimal operating pH, and minimum antiscalant dosage can all be predicted for complex feedwater chemistries.
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