Reverse Osmosis (RO)
Reverse osmosis (RO) is a process water purification technological innovation that uses a semipermeable tissue layer to eliminate ions, elements and larger contaminants from mineral process water. In opposite osmosis, an applied stress is used to overcome osmotic stress, a colligative property, that is driven by chemical prospective variations of the remedy, a thermodynamic parameter. Ro can eliminate many types of demolished and revoked species from process water, including bacteria, and is used in both industrial processes and the production of safe and clean process water. The result is that the solute is maintained on the condensed part of the tissue layer and the genuine remedy is allowed to move to the other part. To be “selective“, this tissue layer should not allow large elements or ions through the pores (holes), but should allow smaller components of the remedy (such as remedy molecules) to move freely.
Problems With Reverse Osmosis
Scaling occurs on RO membranes when the concentration of scale-forming species exceeds saturation, producing additional solids within the RO feedwater. Scalants include such chemical species as calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, and reactive silica. Since these species have very low solubilities, they are difficult to remove from RO membranes. Scaling decreases the effectiveness of the membranes in reducing the solids and causes more frequent cleanings. A scale on a membrane provides nucleation sites that increase the rate of formation of additional scale.
Methods to minimize scaling
Magnesium hydroxide tends to absorb silica, another scalant. Another softening procedure involves zeolite in an ion exchange process.
Often used with acidification, or by itself, are antiscalants. Antiscalants are chemicals added to wastewater to minimize scale carbonate or sulfate based scale. They consist of acrylates and phosphonates which inhibit the precipitation of carbonate or sulfanates.
Methods to prevent fouling
The second problem with reverse osmosis is with the fouling of membranes. Fouling occurs when suspended solids, microbes and organic material deposit on the surface of the membrane. Another problem is from colloidal sulfur, which when oxidized from H2S can foul RO membranes.
Coagulation is one technique that neutralizes the negative surface of the suspended solids, allowing the particles to cometogether. These large particles are then easy to remove from the water using filtration. The most common coagulants used are cationic polymers, inorganic salts, and aluminum and iron salts. Inorganic solvents tend to form large particles, while catonionic polymers require much less product for coagulation. Similar to coagulation is the clarification method, which destablizes suspended particles through charge neutralization. This is generally done by oxidizing iron and manganese and physically removing the precipitates in the manganese greensand bed.