Dissolved CO2 for Scale Removal in Reverse Osmosis

Membrane fouling is a major operational issue in reverse osmosis desalination plants. In particular, plants treating brackish surface waters or groundwater can encounter troublesome inorganic scales that are difficult to remove. Here, dissolved CO₂ is proposed as a novel cleaning method to remove scales from inorganically fouled membranes. The use of dissolved CO₂ differs from conventional two-phase gas cleaning. While gas cleaning uses bulk shear to remove foulants, dissolved gas applies shear as tiny bubbles that form by nucleation. Since the membrane provides nucleation sites, this results in high surface area for shear applied to the scales. Additionally, when CO₂ is dissolved it causes acidification due to the formation of carbonic acid. The lower pH helps remove scales similar to often-used acidic cleaning agents. These two aspects of dissolved CO₂ solutions make it a good consideration for membrane cleaning applications.

Two scales were chosen for testing – calcium carbonate and calcium silicates. Calcium carbonate is a very prevalent scale, though typically controllable. Silicates are very difficult to remove from membranes and are usually controlled by pretreatment of the feed water. The dissolved CO₂ cleaning solution was produced by bubbling CO₂ into a water column within a pressure chamber. Cleaning was done by releasing the contained water through the membrane cell by pressure from the headspace. Non-gas cleaning solutions were cycled through the membrane system for 30 minutes at low pressure to approximate industrial cleaning conditions. Reverse osmosis membranes fouled with calcium carbonate were effectively cleaned using a dissolved CO₂ solution, however, membranes fouled with calcium silicates were not effectively cleaned. The only solution able to remove some of the silicate fouling was a basic solution containing surfactant – neither acid nor clean water had any effect on removal of this scale. The calcium carbonate scale was removed fairly well with an acidic cleaning solution of pH 3 and had limited removal using deionized water. Experiments were also performed with nitrogen gas as a substitute. Nitrogen gas alone resulted in limited removal of calcium carbonate scale. Bubbling nitrogen gas into a low pH solution – pH 4 to match the pH of the carbonated solution – resulted in better removal, but still much less than the dissolved CO₂ solution. It is likely a combination of the solubility of CO₂ for enhanced nucleation and carbonate reactions to lower the pH that make dissolved CO₂ good for scale removal.

The applicability of this technology has several avenues. It is a green alternative to existing costly antiscalants and toxic cleaning solutions, since CO₂ naturally volatilizes after the solution is depressurized. Dissolved CO₂ can be used either as a periodic cleaning, or perhaps as an in-line addition to prevent scale formation and enhance flux. Additionally, there may be a value in carbon sequestration if the RO concentrate is disposed of in underground wells.