Water that contains mineral salts of calcium and magnesium, principally as bicarbonates, chlorides, and sulfates, and sometimes iron. Hardness caused by calcium bicarbonate is known as temporary, because boiling converts the bicarbonate to the insoluble carbonate; hardness from the other salts is called permanent. The calcium and magnesium in hard waters form a hard, adherent scale on boiler plates, increasing fuel consumption and leading to deterioration through overheating.
The langelier index, published in 1936 by the American Water Works Journal, is an accepted method to determine the corrosive effects of some of the dissolved materials in water. Langelier developed a method for predicting the pH at which water is saturated in calcium carbonate (called pHs). The LSI is expressed as the difference between the actual system pH and the saturation pH. LSI = pH (actual) – pH (saturated)
If the actual pH of the water is below the calculated saturation pH, the LSI is negative and the water has a very limited scaling potential. If the actual pH exceeds pHs, the LSI is positive, and being supersaturated with CaCO3, the water has a tendency to form scale. At increasing positive index values, the scaling potential increases.
Avoiding Scale Problems
If the LSI calculation yields a positive index value for an animal drinking water supply, scaling can occur, and reduce the efficiency of the system. Scaling problems can be avoided by:
- Demineralization Reverse Osmosis, distillation, or deionization are all water purification methods that will remove dissolved minerals from water. Water purification will lower the TDS of the water supply.
- Softening Water softening or cation exchange will reduce the calcium and magnesium concentrations in water.
- Acidification lowering the actual pH of the water to below the saturation pHs will prevent scale deposits.
- Low Temperature Avoiding high temperatures where scale formation is more likely will prevent scale deposits.
Concentration and Temperature Effects
Higher concentrations of calcium, total dissolved solids, and alkalinity all promote a greater tendency for scale. This explains why scale or hardness spots form where water has evaporated.
Scaling potential increases with increasing temperature. This explains that while water inside a manifold may not form scale at room temperature, scale could deposit during a cage wash cycle.
RTW Corrosivity Index Calculator
Rothberg, Tamburini & Winsor, Inc. along with the American Water Works Association (www.awwa.org) , are pleased to offer a tool for water operators. The RTW Corrosivity Index Calculator calculates two useful corrosion indices: the Langelier Saturation Index, and the Ryznar Stability Index. Each can be independently used to determine the corrosive nature of a given influent water.
The Ryznar Index is based on the Langelier index and incorporates an empirical correlation observed in municipal water systems between calcium carbonate buildup and water chemistry.