Reverse Osmosis Wastewater Treatment

Wastewater treatment plant

Reverse osmosis is a membrane-filtration technology that purifies water of all forms of dissolved salts, suspended impurities, organic matter and microbial load. This technology has applications in the food and pharmaceutical industries and desalination plants, as well as wastewater treatment plants.

Wastewater Treatment

Effluents from industry include process water containing by-products, water discharged from waste stream, cooling water that comes from boilers and rainwater runoff. This waste material can be treated with reverse osmosis process for reuse in the same cycle, or it can be used for gardening and washing vehicles or as flush water in toilets.

Characteristics of Wastewater

Industrial effluents contain high concentrations of contaminants, which vary by industry, and the levels tend to fluctuate periodically. If the wastewater stream contains a large amount of salts and sediments, organic matter, inorganic substances or heavy metals, it can cause undesirable changes to the wastewater's characteristics. For example, the pH values could change, making the water more or less acidic or alkaline. Or, the biological and chemical oxygen demand of the effluent waters can increase.

Pretreatment

Unless contaminants in wastewater are removed or treated sufficiently, they can cause membrane disruption or plugging. Pretreatment technologies, which are crucial for the effectiveness of the reverse osmosis process, include conditioning to adjust the pH of the wastewater and to prevent membrane plugging from salt precipitation. Pretreatment filtration--ultrafiltration, microfiltration, activated carbon filtration or cartridge filters--prevents clogging from suspended impurities. The type chosen depends on waste characteristics and desired levels of purity. Other pretreatment processes, such as clarification, softening with ion exchangers, chlorination or de-chlorination, can also be employed, depending on need and end use.

Reverse Osmosis

Pretreated wastewater passes through the reverse osmosis chamber; the other fluid in the chamber is pure water. The two fluids are separated by a semi-permeable membrane, which allows only the fluids to pass through, but holds back ions and other dissolved and suspended solids, including microorganisms. When the wastewater is passed through the reverse osmosis chamber under pressure, only its water molecules are forced out through the membrane's pores, while dissolved solids and ionic matter are retained. The pressure applied is usually around 150 to 800 PSI, depending on salt concentration and type of membrane, according to a Merit Partnership fact sheet.

Considerations

The membrane chosen depends on the wastewater quality and characteristics. An effluent without chlorine may only require thin film (TF) membranes, but if pretreatment does not include chlorine removal, a cellulose triacetate (CTA) membrane may be indicated. CTA membranes require another step, pH preconditioning, because they operate at narrow pH range. Once the wastewater has undergone reverse osmosis treatment, it is held in a storage tank until reuse.

References

• Water Treatment Solutions Lenntech: Nano Filtration and Reverse Osmosis
• Water Online: Use Of Reverse Osmosis Increasing In Industrial Sector
• Reverse Osmosis Guide: The Many Applications Of Reverse Osmosis
• Merit Partnership Pollution Prevention Project for Metal Finishers: Reverse Osmosis Applications

Resources

• JSTOR: Application of Reverse Osmosis to Wastewater Treatment