Dosing sulfuric acid into RO feedwater is a common “safety net,” but at this Florida plant, it backfired: the extra sulfate drove irreversible strontium sulfate scale that normal cleaning couldn’t touch. This case study walks through how the scale formed and the cleaning chemistry and procedure that finally restored the membranes.
Abstract
Many consultants in the RO water treatment industry recommend sulfuric acid dosing into the feed water as a safety measure. This case study reviews a situation where acid dosing was used with the intention of improving system performance, but resulted in severe sulfate scaling. The difficulties in cleaning sulfate scales are discussed and after numerous lengthy trials, an effective cleaning procedure is found and implemented.
One of the “rules of thumb” recommended by most industry experts is the dosing of sulfuric acid into RO feed‐water as a precautionary measure. Many caution to always dose enough acid to maintain a reduced pH. They argue that sulfuric acid will act as a back‐up scale inhibitor should the antiscalant dosing pump fail. While pH reduction provides excellent control for calcium carbonate and calcium phosphate scale formations, both are reversible scales that are among the easiest to clean. The advantages provided by sulfuric acid dosing have to be weighed against the risks of introducing excess sulfates into the feed water. Sulfates at high concentrations can combine with certain cations to form sparingly soluble salts at levels beyond their saturation limits. Sulfate scales are not pH dependent; they are formed by irreversible reactions, which make them very difficult, if not impossible, to clean.
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