what is reverse osmosis

The water demand of a swelling population and the resultant pollution has made water safety a key environmental issue for this generation and possibly the coming ones. Water treatment systems are now adopting reverse osmosis towards addressing some of these emerging concerns. Reverse Osmosis technology is used for removing contaminants that are found in water.

The technology works by pushing the water through a semipermeable membrane under pressure. In recent decades, it has developed greatly and has moved from being ranked among emerging technologies to become a competitive, consolidated, and efficient process that is widely used around the globe.

How Does Reverse Osmosis Work

Osmosis is one of the most significant naturally occurring processes. During this process, weaker saline solutions tend to migrate towards a stronger saline solution. Good examples of osmosis are human kidneys when they absorb water from the blood or when plant roots are absorbing water from the soil.

Whereas natural osmosis occurs without requiring any energy, the process of reverse osmosis requires energy to be applied to the more saline solution. It works as the water is being forced through a semipermeable membrane at high pressure. This leaves all the contaminants behind which get flushed down your drain. The filtered and clean drinking water is collected in a ready holding tank.

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What Are Reverse Osmosis Membranes?

reverse osmosis membrane

Semipermeable membranes were initially made with cellulose acetate (CA). Later, however, the industry switched to using a thin film composite (TFC) that is placed on top of a substrate that is stronger. Today, TFC reverse osmosis membranes are primarily used.

When pressure pushes the water through the Reverse Osmosis membrane or other filters like carbon or sediment filters, impurities get filtered out and then flushed down the system drain. What you have left behind is clean-tasting water.

A Reverse Osmosis system employs cross-filtration. Here the solution goes through a filter that has two outlets: the contaminated water goes one way and the filtered clean water goes the other way. To avoid possible buildup of contaminants, the cross-flow filtration system has been designed to allow the water to sweep away the buildup and also have sufficient turbulence that keeps the surface of the membrane clean.

Cleaning RO Membranes

Inevitably, because of the nature of their work, RO membranes will require periodic cleaning. This can range anywhere between 1 and 4 times yearly depending on the quality of the feed water. In general, if the normalized water pressure has dropped or the normalized salt passage increases by 15%, it’s time to have the RO membranes cleaned. If the water’s normalized permeate flow decreases by 15% it’s also time to do the membranes cleaning. 

Larger Reverse Osmosis systems like the ones typically used in industrial processes, hospitals, in seawater desalination, or bottled water plants require constant monitoring to verify membrane condition and water quality.

Reverse Osmosis System Key Components

The required amount of pressure will depend on the concentration of salt in the feed water. Generally, the more concentrated it is, the more the pressure that is needed. The desalinated water (deionized or demineralized) is called product (or permeate) water. 

For optimal water quality, most Reverse Osmosis units have a 4 stage process.

Sediment Filter

This is the pre-filter stage designed to strain out silt, dirt, and sediment. It is particularly important as it protects such elements from reaching the delicate reverse osmosis membranes which can be easily damaged.

Carbon Filter

The carbon filter stage removes chlorine as well as other contaminants that can affect the life and performance of the RO membrane.

Reverse Osmosis Membrane

The semipermeable membrane in the RO system allows water through but it filters out nearly all contaminants.

Polishing Filter

A final post filter, usually a carbon filter, “polishes” off the water removing any lingering odor and taste in your water. It is this final filter that ensures you get outstanding drinking water.

Reverse Osmosis Applications

For over a century reverse osmosis has become a recognized technology and commercialized from around the 1960s. It is now being used widely both for domestic and commercial applications.

Reverse Osmosis Home Use

Home RO Systems are being used to produce highly purified water for drinking as well as a continuous supply of fresh and clean water for other domestic applications.

A home RO unit connected to your home where the water enters the house uses 3 to 4 filters to give your household purified and safe drinking water.

The filtered water can then be stored in your storage tank or under the sink. You may opt to install a separate faucet on your sink that is fed from your storage tank. Domestic Reverse Osmosis can eliminate up to 99% of the dissolved solids, microorganisms, fluoride, chlorine, and heavy metals like chromium, barium, cadmium, mercury, and lead from your water.

Besides drinking, RO purified water can be used in cooking & making ice as it has an improved flavor and several health benefits. But RO home units may not be ideal for everybody. This is basically because most countertop RO units tend to be quite slow as a result of lower-flow faucetsreverse osmosys for home

Industrial & Commercial Use

Reverse Osmosis units are being used in the cosmetics industry, food and beverage processing, pharmaceutical production, in industrial boilers, seawater desalination, and lots of other large-scale applications.

Improvements in Reverse Osmosis technology such as advanced process and module design, energy recovery, and membrane material has led to considerable reductions in cost. In turn, this has led to more usage in commercial applications. RO is also being used in applications such as selective purification, separation, and concentration processes.

In the food industry, Reverse Osmosis is being applied in the concentration of vegetable and fruits juices, milk and whey pre-concentration, and in the dealcoholization of alcoholic beverages. For areas with large sources of peat or naturally humic water, RO can be used to give clean water for use by the community.

RO Drawbacks

In general, Reverse Osmosis systems are not fully self-sustaining. The water needs to be pretreated with chemicals so that the fine RO membranes don’t get clogged. The membranes themselves are delicate and need to be cleaned often, and they can still trap some bacteria.

An emerging concern that is unique to the usage of RO systems in desalination plants is the risk of marine life such as small fish being sucked into the system. However, adjusting the water intake velocities and pressures can prevent excessive harm to marine creatures. The biggest impediment to the use of reverse osmosis systems is, however, the cost which remains high.

Summary

Looking into the future, besides being a key source of clean and safe drinking water for many disadvantaged communities, the application of Reverse Osmosis is being seen as a promising final step towards treatment of wastewater for reuse and a valuable component in the recovery and reclamation of wastewater.