Complete elimination of acid injection in reverse osmosis plants

Antiscalants with broad activity spectra are available today. When properly chosen, a single antiscalant can efficiently and simultaneously control calcium carbonate, calcium sulfate, strontium sulfate, barium sulfate and calcium fluoride scales as well as inorganic foulants resulting from iron, aluminum and reactive silica present in any given water or wastewater. The effectiveness of many antiscalants towards controlling calcium carbonate scaling in reverse osmosis (RO) plants has allowed us, in the past five years, to successfully help eliminate the continuous injection of acid in all RO systems operating with polyamide membranes. A minority of systems with cellulose acetate membranes however still require acidification due to hydrolytic sensitivity of cellulose acetate towards feedwater pH higher than 6. The chemistry behind the main reason for acidification — the prevention of scaling by calcium carbonate — is reviewed. The mechanisms of calcium carbonate scaling and its avoidance by acidification or with antiscalants are discussed. Major seawater and brackish water RO plants around the world are designed and operated with the continuous injection of concentrated sulfuric or hydrochloric acid for scale control, sometimes simultaneously with an antiscalant. We present a case study in southeastern USA of a 5 million gallons permeate per day plant as a typical example among many for the conversion of major municipal RO plants. By suitable selection of an antiscalant, acid elimination was successfully attained with a simultaneous reduction in the antiscalant dosage, in this case to 2–3 mg/1 in the feedwater. While process optimization continues in the plant, current chemical cost savings due to the deletion of 93% sulfuric acid and the associated caustic soda neutralizer alone amounts to 67% of the annual cost of all chemicals used. This does not include savings through benefits accrued in equipment, operation, maintenance and safety in the plant.