Ahead of the 250th anniversary of the Declaration of Independence in the US, an ozone nanobubble system has been used to keep the Lincoln Memorial Reflecting Pool clear.
Months before the celebrations a massive clean up of the pool had taken place, but despite this an algae bloom had turned the water bright green. To deal with this a US$1.7 million (£1.27 million) ozone “nanobubbler” injected microscopic bubbles into the pool.
Nanobubbles are extremely small gas bubbles, often made with oxygen, air or ozone, that can remain in water far longer than ordinary bubbles. In a pool, ozone nanobubbles can act as a strong oxidising treatment, attacking algae and organic matter.
But the more important question is whether this state-of-the-art technology can help solve one of the hardest problems in aquatic restoration: getting oxygen to places where lakes, reservoirs and coastal seas are dying from the bottom up.
Clearing the Lincoln Memorial reflecting pool
The famous Lincoln Memorial reflecting pool in Washington DC is shallow, hard bottomed and man-made. It has no natural sediment bed like that of a natural lake, and its primary goal is to look clear. In this context, ozone nanobubbles can be useful, provided the water is circulated artificially and the treatment is maintained.
This is much easier than restoring a natural body of water, where the main problem is often not obvious, and can be complex to resolve.
In a eutrophic lake or sea, one that has become overloaded with nutrients such as phosphorus and nitrogen, algal blooms are a visible symptom of a wider problem. When algae die, they sink. Bacteria decompose organic matter and consume oxygen. The water at the bottom of the lake can become hypoxic (oxygen-poor) or anoxic (essentially oxygen-free). Under these conditions, sediment can release nutrients, and the nutrients can cause eutrophication.
The worst syndrome of this is algal bloom, which can kill fish and create dead zones within the lake. A vicious circle develops: blooms deplete oxygen, oxygen depletion releases nutrients, and nutrients create more blooms.
This is why oxygen delivery is so important. The challenge is not simply to add oxygen somewhere in the lake or sea. It is to deliver oxygen precisely to the thin layer of bottom sediment, where phosphorus is released, methane produced, and other processes occur.
So, there are two types of nanobubble use.
The first is bulk nanobubbles: bubbles dispersed through the water by machines. These can work well in tanks, aquaculture systems, wastewater treatment, pools and small bodies of water which can be continuously circulated.
But in large natural waters, bulk treatment faces practical limitations. The machines must keep running, and the oxygen distribution depends on pumps, cables and pipes. In a large lake or sea basin, that means high energy demand and uncertain delivery to the bottom.
The second type is what are called interfacial oxygen nanobubbles. These are oxygen nanobubbles attached to the surfaces and pores of solid particles, such as modified clay or other porous natural materials. Oxygen is loaded onto particles that sink. The particles then deliver oxygen directly to the area where the water meets the sediment.
This could reduce energy requirements and avoid some of the ecosystem disturbance associated with large-scale artificial mixing.
The potential impact is significant. If oxygen can be delivered cost-effectively to surface sediments, it may help reduce internal phosphorus release, suppress methane generation, and generally make conditions more favourable for life at the bottom of ocean. These go to the heart of whether a degraded lake or coastal basin can recover.
This is a very different engineering idea. It does not aim to oxygenate the entire volume of the lake.
However, if sewage or fertiliser runoff continue, any oxygenation technology will not be effective.
This strategy for lake restoration is to remove algae and nutrients from the water, lock nutrients into the seabed sediments, and oxygenate the sediment surface to reduce nutrients re-release.
Baltic Sea project
The importance of this targeted approach becomes clearer when we look at the Baltic Sea, one of the world’s best-known examples of oxygen-depleted “dead zones”.
The Baltic is naturally vulnerable due to limited water coming in and out of the sea as it connects through very narrow waterways. It also has distinct deep water and surface layers that don’t tend to mix. Nutrients are constantly released from the sediment into the water, causing oxygen levels to drop dramatically.
One of the most ambitious engineering responses to tackle this kind of dying sea was the deep-water oxygenation project in the Baltic. The principle was straightforward: pump oxygen-rich surface or upper-layer water down into deep oxygen-depleted water.
This process used wind-powered pumping to move oxygen-enriched water from around 50 metres down to much deeper, about 125 metres, using around 100 offshore wind-powered pumps. An alternative to the process used in the Baltic project would be to use nanobubble-clay materials to deliver oxygen onto the the deep water sea bottom by gravity and reduce the energy cost and negative impact to the aquatic ecosystem.
The project, which began work in 2009, showed why fixing the problem is so difficult.
Pumping can increase oxygen levels, but it requires extensive infrastructure and can alter the hydrology and ecology of the whole lake or sea. There are unresolved questions about cost, energy, maintenance, ecological side effects and other environmental effects.
In a shallow hard-bottom pool, nanobubbles are judged by whether the water looks clear. Oxygen nanobubbles attached to porous particles could become a lake or sea restoration tool, but with limitations.
Nanobubbles greatest environmental value may be in helping oxygen reach the dark, thin, neglected layer, the dead zone, at the bottom of lakes and seas. But this may be a costly, and complicated, exercise.
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