Friday, May 20, 2011

Reverse Osmosis: A Primer

Our client, Sarvajal, uses reverse osmosis technology to treat groundwater to drinking water quality. In order to better understand our project and the challenges it presents, here is a primer on the technology and some context for its application to drinking water treatment.

Reverse osmosis, also known as RO, gets its name from the basic concept of osmosis (i.e. diffusion). Whereas conventional osmosis concerns the movement of water from lower to higher solute concentrations, RO works in the opposite direction by using external pressure to pump water across a selectively-permeable membrane. The membrane acts like a filter, leaving large particles and ions behind and allowing the water to pass through. The final product of this process is thus two-fold: a purified water stream that can be used for drinking water (called the product water or permeate), and a highly-concentrated waste stream that can be routed for disposal (the brine).

Brine water is highly concentrated and presents both the challenge of disposal and the opportunity of reuse. The brine water we are working with has high concentrations of total dissolved solids, including sodium, bicarbonate, and chloride ions. This water is not necessarily unsafe for handling, but is far too concentrated to use in most applications. Finding options for reuse thus requires a great deal of creativity, and our goal is to develop a basket of options to provide flexibility to different franchise locations. Presently, the majority of Sarvajal franchisees use ground application to recharge the water (and dissolved solids) to the groundwater table. In our development and assessment of reuse options, we are especially looking for alternatives that conserve water resources, develop business opportunities for local entrepreneurs, or provide other benefits to the local community.

Prior to passing through the RO unit, the Sarvajal treatment process begins with a carbon cartridge filter followed by a microfiltration cassette. The main treatment unit, RO, then purifies the water to a very high quality, and treatment is concluded with ultraviolet disinfection. RO technology is commonly used in drinking water treatment applications throughout the world, ranging from small flow quantities like those in our project(around a thousand gallons per day) to much larger quantities (several hundred-million gallons per day). Brine water throughout the world is typically discharged to a surface water body or a nearby wastewater treatment plant.

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