We know that the water molecule is made of one oxygen and two hydrogen atoms (H2O). By using an electrolyzer, one can split the hydrogen and oxygen atoms (H2 and O). There is another way to split water — into hydrogen and hydroxyl radicals (H and OH). A radical is an atom or an ion or a molecule which has at least one unpaired valence electron in its outermost shell, which makes it highly reactive and therefore, short-lived.

As it turns out, hydroxyl radicals have a special property — they can degrade a wide range of pollutants. So, in recent times, scientists have been burning the midnight oil to find out a way to use hydroxyl as a water cleaner.

But how do you split water into hydrogen and hydroxyl radicals? The answer to this question is an interesting branch of physics called ‘hydrodynamic cavitation’. In simple words, creating bubbles.

Bubble power

Bubbles in water, or soap water, is something that everyone is familiar with, but few think about how they form. Bubbles appear when a liquid flows quickly through a narrow space, like a small tube. These bubbles, also called cavities, is filled with the liquid’s vapour. When they move to an area of higher pressure, they collapse, generating extremely high temperatures (over 10,000 degrees K) and pressures (1,000 bars). When this happens in water, it breaks the water molecule into hydrogen and hydroxyl radicals.

As mentioned earlier, radicals are highly reactive, eager to bond with other atoms or molecules. The ‘reactive’ hydroxyl radicals fling themselves upon both organic and inorganic pollutants such as those of those of dyes, pharmaceuticals and pesticides, breaking them down into simpler molecules. They can even mineralise organic pollutants, turning them into carbon dioxide, water and simple salts. And, they are ‘non-selective’, meaning they can degrade a wide variety of pollutants, making them very useful for cleaning water. This method is absolutely eco-friendly, as it uses no chemicals — though it does require electricity to run the reactor.

By the way, bubbles can also be created by passing sounds of very high frequency (ultrasonic cavitation) or light from a pulsed laser (photo-induced cavitation), but hydrodynamic cavitation is considered more efficient in producing bubbles, and hence radicals.

The green solution

While the science of hydrodynamic cavitation (HC) has been known for a long time, research into its use for tackling pollution is not very old. “HC has emerged as a promising technology since it offers several advantages over conventional methods making it a scalable solution for large-scale wastewater treatment,” says a scientific paper by Shishir Raut et al of the Department of Chemical Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar.

Prof Dhiman Chatterjee from the Department of Mechanical Engineering at IIT Madras is one of the scientists who has been researching hydrodynamic cavitation for wastewater treatment. He told Quantum that although research started in the late 20th century, HC “is yet to become a regular industry solution.”

That said, there is an operating hydrodynamic cavitation reactor at the Nandesari Industries Association in Gujarat. This facility treats 20 million litres per day, requires 5.5 acres of land and has a treatment time of 6-8 hours per batch, compared to 4-5 days for biological processes. The cost is 8 to 14 paise per litre.

Yet, many scientific papers that Quantum checked indicates that the HC reactors are still emerging and are yet to be optimised for efficiency. In this direction, Prof Chatterjee’s recent work, that has been described by another expert, Prof Matevz Dular, from the Faculty of Mechanical Engineering, University of Ljubljana, Slovenia, as “ingenious”, has taken the matter forward.

In a paper co-authored with Jahidul Haque Chaudhuri of IIT Madras, Chatterjee emphasizes that while designing a HC reactor, ‘forget about the volume of cavitation, look at other parameters such as local pressure variation and cavitation volume fluctuations’.

The ‘cavitation number’ is a measure of the possibility of the flow of water to cavitate (make bubbles). The number is based on the pressure difference between the inside and outside of a bubble on the one hand and the kinetic energy per volume on the other.

In essence, Chatterjee has devised a method for predicting the efficiency of a HC reactor, leading to better, more efficient reactors. “The proposed numerical strategy helps to improve cavitation reactor geometry. This improved geometry then needs to be tested at the laboratory scale and then for field testing before a successful launch of the design as a commercial product,” he said.

To sum up, hydrodynamic cavitation is emerging as a climate-friendly method for treating wastewater, especially industrial wastewater.