Abstract
This article discusses studies that indicate that the best anticorrosion treatment of softened surface waters consists of maintaining a driving‐force index (DFI) of 15‐20 and a momentary excess (ME) of 12‐15, in conjunction with low calcium and high carbonate ion concentrations. In the pH range of 9.5‐10.0, a rough indication of this is a saturation index (SI) of 1.0 or more. As such waters would cause trouble by plugging mains with calcium carbonate scale, 0.5 ppm of a polyphosphate should be applied. Soft surface waters seem to be given insufficient anticorrosion treatment after the pH has been reduced by alum. By increasing the pH to 8.8‐9, a DFI and ME of sufficient magnitude could be obtained to give the desired protection. This could be done by the addition of lime, caustic soda, or sodium carbonate. Aerated and lime‐softened well waters require about the same treatment as lime‐softened surface waters. Oxygen‐free, lime‐softened well waters can probably be treated to ME values of 12‐15, but to a lower DFI.
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