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By Nathan Schiff, Ph.D.

Schiff Consulting

If you were to fill an animal's bladder with water, then immerse it into a salt solution, water would pass readily through the bladder's wall and dilute the salt solution. None of the salt would flow backwards into the bladder. The more highly concentrated the salt solution, the greater would be the pressure driving the water into the salt solution. This selective movement of water across the bladder’s, wall - a biological semi-permeable membrane - is referred to as osmosis. The force driving the water across the membrane is called osmotic pressure. If sufficient pressure is applied to the salt-containing side of the membrane, Reverse Osmosis occurs in which the natural direction of water flow has been reversed. Regardless of path taken, water can easily cross a semi-permeable membrane in either direction. Dissolved solids such as salts, or organic matter are unable to.   This article examines how Reverse Osmosis (RO) is used to convert wastewater into high quality water, capable of being reused in certain industrial applications including laundry wash water.

What are Semi-permeable Membranes?

These are membranes that are very selective in what they permit to pass through their sub-microscopic pores.

The biological membranes form the walls of every living cell, and play a major role in regulating the internal environment of all living cells. They do so by allowing water to pass freely on either side of the membrane's pore, but will exclude other molecules, such as urea and inorganic salts. Synthetic membranes, composed of large polymers function in similar manner, and are used in the treatment of large volumes of wastewater.  

Figure 1. Osmosis

Water passes through the membrane by osmosis, which raises the height of liquid in the left arm. This creates a backpressure on the membrane. When a certain height of liquid is reached, the backpressure stops the liquid from passing through the membrane’s pores.

Applying Pressure to the Left Arm of the Tube

When the pressure applied to the left arm is greater than the osmotic pressure, Reverse Osmosis (RO) occurs and the flow of water is in the opposite direction. Only the pure water in the waste is transferred to the right side and can be reutilized.  One very important application of RO technology is in desalinating seawater.   Ocean going ships have their own RO units on board, which provide a ready supply of drinking water from the sea or ocean.   In reverse osmosis, the membrane functions as an ultra-fine filter also capable of separating out a variety or undesirable substances, including toxic molecules.

Figure 2. Reverse Osmosis

The Key to RO is the Membrane

Commercially manufactured RO membranes are composed of polymers composed of either: cellulose acetate, polysulfone or polyamide. The membrane consists of a skin about 0.25* microns thick and a support layer about 100* microns. The skin is the active component of the membrane and only allows water to pass through the membrane. In essence, the RO membrane functions as an ultra-fine filter, capable of separating out the smallest of molecules and even filtering out 99.5 to 100% of most microorganisms, viruses, bacteria and inorganic salts.

To be commercially feasible to treat or recapture several hundred thousand gallons of water within an acceptable time frame, requires a very large membrane surface area. The most desirable configuration occurs in a spiral design. Many sheets of RO membranes are sandwiched with spacers, then connected and wound around a tube, which receives the purified water from the membranes and distributes it for the various applications.

*A micron is 1/1000 th of an mm or 100 times smaller than the diameter of a human hair.

The Real Benefits of RO

Very large laundries or food processing plants can use anywhere from 200,000 to 1,000,000 gallons of water, daily. Expenses are incurred in both acquiring the water and in it’s subsequent treatment and discharge into municipal sanitary sewers.

When laundry wash water is discharged, it contains approximately 5% waste in the form of soluble soil, emulsified oils, spent detergent and water insoluble residue removed from the fabric. The remainder of the liquid consists of pure water. The water treatment costs of 200,000 gallons of wastewater, daily, requires special treatment facilities including large, concrete holding lagoons. Waste materials that contribute towards high BOD levels are reduced there by the action of bacteria and enzymes in digesting organic wastes. Additional treatment to remove heavy metals and other regulated components require further processing before the water can be discharged.

In a RO system, processing 200,000 gallons of water daily, a significant amount of wastewater can be filtered and returned to the plant for re-use. When 150,000 gallons of water are returned for re-use, the daily quantity of fresh input water required by the facility is reduced to 50,000 gallons. In addition, the volume of wastewater requiring treatment is reduced by 75%. Reducing water usage makes good sense, from an economic and environmental viewpoint.


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