A 6-million-gallon-a-day RO plant in South Texas started chlorinating its water to remove arsenic, and unintentionally created a worse problem. The chlorine turned dissolved (ferrous) iron in the well water into insoluble ferric hydroxide, which coated the filters and membranes. The fix was a specialty iron-control antiscalant that stops the ferric hydroxide from forming in the first place.
Abstract
A 6 MGD reverse osmosis plant in South Texas that was using chlorine to oxidize arsenic for improved membrane rejection was experiencing severe membrane fouling. The ferrous ions in the well water were being oxidized by the chlorine, resulting in ferric hydroxide fouling of the cartridge filters and membranes throughout the system. The RO facility did not have an acid dosing system and the antiscalant in use was not designed for such high ferric concentrations.
Despite numerous cleanings, the productivity of the system would not recover even while differential pressures would return to normal. A membrane autopsy revealed that the ferric hydroxide was seeding silica polymerization on the membrane system, so that even while the ferric hydroxide was removed, the silica scaling was continuing to grow.
A cleaning study was performed to determine the best cleaning chemicals and methodology for recouping the membranes’ performance. Lab simulations were then performed to determine the optimal product and dosage for inhibiting scale formation in the presence of high concentrations of ferric ions in the feedwater. Finally, the antiscalant was used on-line at full scale. The normalized data showed excellent control of the scaling and significantly extended times between cleaning. Loss in membrane performance that occurred while the specialty antiscalant was online was found to be due to biological and suspended solids fouling.
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