• Calcium carbonate scale that has been detected early can be removed by lowering the feed water pH to between 3.0 and 5.0 for one or two hours.
• Longer resident accumulations of calcium carbonate scale can be removed by recycling of 2% citric acid and pH not less than 4.0 through the membrane elements.
• Silica can be removed from water by precipitation with multivalent metal hydroxides, such as Fe(OH) 3 , Al(OH) 3 and Mg(OH) 2 .
• This treatment removes both soluble and colloidal silica [3], [7], [8]. ou can dissolve silica with hot *concentrated* sodium or potassium hydroxide solution.This will dissolve alumina as well, as HF does too.