Bubbles breathe life into troubled waters

While bubble curtains are becoming an established silt mitigation technology, they offer a lot more. Frog Environmental, which won the Diving and Underwater Innovations Showcase award at Seawork in June, has been called in to protect the salmon swimming through a £20 million (€23 million) redevelopment at South Shields on the River Tyne in northern England.

Technical director Leela O’Dea says that while everyone winces at the noise from pile driving, the underwater reverberations don’t just deafen fish – it can kill them. Furthermore nature – often a harsh taskmaster – can actually pull these migrating salmon into the danger zone around the old dockyards.

Installation on the Tyne

In a first for the UK, Frog Environmental worked with piling contractor John F Hunt Regeneration to install a 91m bubble cordon around the operation.

It’s a case of size mattering: the smaller the better.

The perforated hosing (by Canadian company Bubble Tubing) releases micro-bubbles between 1mm and 7mm. which yield a substantial amount of surface to air volume, enhancing oxygen transfer rates.

Deployment is not always straightforward, as every location comes with its own characteristics.

“We can manage 20-30 metres no problem,” O’Dea says, “but it adds an extra bar of pressure on the tubing for every 10 metres you go down… and the compressor has to be sized to account for this.”

The installation at the Tyne site also had to be capable of delivering 360 cfm at 100 psi: that’s not unusual, but it still means a disconnected link could whiplash upward with enough force to kill someone.

It is therefore vital that lines have whip socks covering every connection.

“You also have to think about the location and the trajectory of the line in relation to the water flow or tidal current,” says O’Dea. “If it disperses the bubbles before they reach the surface, then the curtain will be inefficient. Likewise if you’ve got a steep decline followed by an incline, the bubble flow is not going to be evenly distributed – there’ll be gaps.”

That means reviewing location or adding an extra line to yield the necessary uninterrupted curtain.

There’s also length to consider: after all, the system is designed to leak air. Pressure can usually be maintained across an extended area by balancing the demand between a series of linked curtains, but as with the Tyne project, the pragmatic answer is an installation that can be moved to wrap around the active hammer zone, rather than one over a kilometre long.

In short, energy efficiency needs to be kept up and compressor size down or the whole thing quickly becomes unfeasible, says O’Dea.

So, did it work for the Tyne operation?

Independently verified results by Subacoustic returned a 99% reduction in noise at 750 metres from the piling, well within the UK’s Marine Management Organisation demands. It also saved thousands of pounds in time costs for John F Hunt Regeneration, as work could continue on both tides, keeping the salmon’s migratory route open for the duration of the project.

Restoring water health

However, this nascent technology can do more than deaden sound: it’s also been used to keep litter, fish and jellyfish out from where they’d do damage or come to harm, such as a power station inlet. The air bubbles simply lift the jellyfish, seaweed and other debris to the surface, out of the way of the biggest suction forces.

It could also help bring back to life some of our most troubled waters. Many lakes and reservoirs are, beginning to succumb to a mixture of warmer temperatures with reduced oxygen capacity and nutrient rich run-off from intense farming practices. In the UK there has also been the release of raw sewage into places like Lake Windemere.

Increased nitrogen and phosphates from fertilisers and sewage super charge the growth of smothering weeds and toxic algae blooms such as cyanobacteria, depleting the water’s oxygen content. Unfortunately, anoxic environments also ‘unlock’ the phosphate in the sediment, says O’Dea, adding to the burden in the water column.

At this point, it’s all too easy to enter a catastrophic downward spiral as plant matter, dead fish and other aquatic organisms start to rot, leading to eutrophication – waters incapable of supporting life.

The answer is oxygen. Creating an oxygen-rich environment, especially at the sediment boundary, supports the organisms that thrive on organic matter, which in turn cycles the nitrogen out of the system. Further, it stops phosphates dissolving into the water, keeping it locked up in the sediment, says O’Dea.

Different systems

Not all aerators are equally efficient. The more common pumps, paddles and fountains can only modify the upper reaches of the water body, and there are substantial energy costs associated with pushing water around or pumping it into the air.

Bubble Tubing’s position on the bottom roughly halves the OPEX and carbon spend of a typical dual-fountain system. It’s also the most effective option.

“As the air bubbles travel upward, you get really amazing oxygen transfer rates because they’re moving through the whole water column,” says O’Dea. “As a result, you’ve got active, aerobic microbes.”

It’s a win for them, for us, and even for the water companies.

“This way, there’s not so much demand for intensive treatment - and it leads to better management of our reservoirs,” she says.

Finally, could it mitigate the poisonous effects of sewage dumping?

Possibly: it won’t work if the pollution continues unabated, “but the answer is yes”, says O’Dea: an aerobic microbial community that makes a meal out of the organic matter could literally breathe life back into poisoned waters.