The Lincoln Memorial Reflecting Pool is treated with ‘nanobubbles’. What are they and how do they work?

As the United States approaches 250th birthday celebrations on July 4, Washington DC’s monuments, statues and fountains are being prepared to put on a show.

However, renovations of the Lincoln Memorial Reflecting Pool haven’t gone to plan.

Soon after the renovations were complete, an algal bloom turned the pool’s water a slimy green. Officials are in a hurry to address the issue and, among other treatments, deployed ozone nanobubble technology to clean the water.

But what are nanobubbles and why would they be used to address an algal bloom?

What are nanobubbles?

Nanobubbles, also known as ultrafine bubbles, are gas-filled bubbles with diameters typically around 100 nanometres. That makes each nanobubble about one thousandth of the thickness of a human hair.

When deployed with specialised equipment that injects gases like air, ozone or oxygen into water, nanobubbles occur in enormous numbers – typically around 10 billion nanobubbles for each teaspoon of water. Despite their huge numbers, solutions of nanobubbles look and feel just like water.

Looking at a glass of water with nanobubbles, you would not know they’re there. Yet they have remarkable properties that are used in a number of technologies, including water treatment.

Nanobubbles are surprisingly stable

A fundamental rule is that the pressure inside a bubble – any bubble – is always greater than its surroundings. And the smaller the bubble, the greater the pressure within the bubble.

A typical nanobubble can have an internal pressure as high as 15 atmospheres. That’s the same pressure a diver experiences during a deep dive to a depth of 150 metres.

How much a gas can dissolve into a given liquid increases with pressure. Because of this, you’d expect that nanobubbles, once produced, should rapidly dissolve in much less than a second. However, research has shown that nanobubbles can last for hours, days and even weeks.

Why nanobubbles are so surprisingly stable remains a mystery and the subject of much research. But their strange longevity is also what makes them very useful, because nanobubbles can be produced, stored and used before they disappear.

Nanobubbles last a long time

The small size of nanobubbles provides other advantages too. Small bubbles have very little buoyancy, allowing them to remain in solution for a long time. In comparison, larger bubbles rise quickly to the surface and burst. You can see this at the beach when a wave breaks – the bubbles that form the whitewater typically disappear in less than ten seconds.

If you need to dissolve a lot of gas into solution, this long residence time is very important – it prevents the gas from being lost to the atmosphere, and as we already noted, the high internal pressure of the nanobubbles increases gas solubility.

Another advantage – because nanobubbles are so tiny, they have a very large surface area. In many industries, such as mineral processing or waste-water treatment, the surface area of larger bubbles is used to collect materials dispersed in a liquid. A process known as froth flotation is used to purify millions of tons of minerals in Australia each year.

Similarly, the surfaces of nanobubbles can be used as highly effective, environmentally friendly cleaning agents, replacing the use of detergents such as soap.

As you can see, nanobubbles have a diverse range of applications. Oxygen nanobubbles even increase yields when nanobubble water is used on crops and in fish farms, possibly by concentrating nutrients.

How do nanobubbles keep algae at bay?

Green algae, such as the type found in the Lincoln Memorial Reflecting Pool, thrive in still or slow-flowing water with abundant sunlight.

In water treatment, nanobubbles are used primarily to oxygenate water. This promotes the growth of aerobic bacteria which consume organic nutrients, outcompeting the algae and promoting a healthy aquatic environment.

Nanobubbles can also be used to deliver ozone gas, which is a highly reactive, toxic gas composed of three oxygen atoms. Ozone plays an essential role in the stratosphere in shielding Earth from ultraviolet rays, but it plays a very different role in water treatment.

Ozone reacts with organic materials in water and will therefore break down and kill the algae and nutrients. However, ozone nanobubbles also attack and kill the favourable aerobic bacteria.

The strong action of ozone nanobubble treatment is highly suitable for the treatment of heavily contaminated water – for example, if it contains animal waste.

In the case of the Lincoln Memorial, ozone nanobubbles can quickly resolve an algal bloom, but that comes at the cost of removing aerobic bacteria which could naturally keep the algae in check.

This would then make the pool susceptible to later algal blooms if measures are not taken to prevent a reoccurrence, such as preventing the build-up of nutrients the algae depend on.

The Reflecting Pool at the heart of Washington, DC is now set to be drained again to remedy the rapidly peeling new coating and the algae bloom. It remains to be seen what colour the water will end up being in time for July 4, and whether authorities will switch to oxygen nanobubbles to promote a healthier ecosystem in the water.