How can harmful substances be removed from waste water?

There are options. Choosing the right one depends how clean you need the water to be…

By Sarah Jensen

It’s not a pretty sight: Water discharged by home toilets, restaurant dishwashers, agricultural runoff, manufacturing processes, and other large-scale industrial activities contain harmful petrochemicals, phosphates, nitrates, salt, mercury, lead, traces of pharmaceuticals, and a variety of other potentially harmful substances. Fortunately, a number of effective methods exist for purifying all that water. Which one to employ depends on which impurities are present, the contamination level, and how the treated water will be used. “If it will be used in industrial applications, complete purity may not matter as much; but if we want to drink it, it must be free of bacteria and other harmful substances,” says Anurag Bajpayee, a post-doctoral researcher in MIT’s NanoEngineering Group. “For use in research, hospitals, or manufacturing of semiconductors in clean rooms, you need ultra-pure water.”

The most basic and inexpensive ways to purify water involve adding substances that allow the harmful bits to be easily removed. When chlorine gas or tablets are added to waste water, the chlorination kills the bacteria, and simple charcoal filters can remove some of the suspended solids, which might often be enough, explains Bajpayee. Other methods involve removing suspended solids using coagulating agents such as alum. The free bonds in the alum’s potassium and aluminum ions attach themselves to charged particles in the water, allowing them to coagulate and be easily removed.

Ultrafiltration and nanofiltration are processes through which contaminants are removed by using membranes with tiny pores. Made of organic compounds or polymers, the membranes allow water to pass through but trap the larger solids. Reverse osmosis is a popular process used for home water purification, brackish water treatment, and seawater desalination. In an RO system, Bajpayee explains, a semi-permeable membrane separates contaminated water from pure water. Applying pressure to the contaminated water drives it to the other side of the membrane, leaving behind the salts in a highly concentrated brine. “You still have to dispose of that waste brine, but the purified water is often up to drinking standards,” he says.

Membrane systems they have their limits. “RO can effectively only treat water with a maximum of 5 percent of total dissolved solids,” says Bajpayee. “When it’s higher than that, greater pressure is needed to push the water through to the pure side, which means higher energy consumption, which means high cost, not to mention membrane-fouling problems that arise with highly contaminated streams.”

Then there are distillation processes that essentially boil the water and condense it. Boiling, one of the earliest forms of water treatment, heats the water, which kills the bacteria. As the water boils off, impurities remain behind as a solid, and the pure water is re-condensed. Distillation, however, requires high energy consumption to supply the water with the latent heat of vaporization. Some practical distillation techniques are multi-stage flash (MSF), multi-effect distillation (MED), and mechanical vapor compression (MVC).

Recently, Bajpayee and his colleagues at MIT demonstrated a technology that dissolves the water in a directional solvent when it is heated. The water is separated from the contaminants that don’t dissolve in the solvent, and after it cools, pure water is recovered and the solvent is reused. Bajpayee’s technology, which was named a Top 10 World Changing Idea of 2012 by Scientific American, has enormous implications for treatment of highly contaminated waters such as those produced by unconventional oil and gas extraction methods.

Thanks Rafael Orta of New York City for this question.

Posted: December 11, 2012

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