Bubble Curtains Catch the Plastic That Floats, Not the Plastic That Sinks

A bubble barrier is a simple piece of equipment. Air is pushed into the water at the bottom of a channel and rises as a wall of bubbles, and plastic drifting down with the current is meant to be pushed up and held back before it can reach the sea or leave a wastewater plant. The hard part is not the bottles.

The authors of a new study describe bubble barriers as a promising passive method for intercepting large plastic, and possibly the small kind, before it gets that far. Microplastics, the tiny fragments plastic breaks down into, are too small to skim off a surface, and a barrier that stops a bottle need not stop a flake. Five researchers in Portugal, Germany and the Netherlands built one into a laboratory channel to find out which fragments it holds. Their answer, published Sept. 19 in Microplastics and Nanoplastics, turns on a single property, and it is not size: it is whether the plastic floats.

The work comes from César Santos and Cristina Fael of the University of Beira Interior in Portugal. Their colleagues are at the University of Bayreuth and the University of Trier in Germany, and at Wageningen University and Research in the Netherlands. The water in their channel ran turbulent but slow, at a Reynolds number near 4,700, a standard measure of churn. The barrier was tested at air pressures from 500 to 1,000 millibars. To follow the water, the team used fluorescein, a dye that travels with the flow; to follow the plastic, they tracked particles on camera. The paper is peer-reviewed, and what is posted is the accepted version, with the journal's final version still to come.

Polyethylene floats. The team used both common grades, low-density and high-density, the plastic of shopping bags and bottle caps. Those particles rode the upward flow that the bubbles create and gathered at the surface, and less than 20 percent of them were recovered at the downstream end of the channel.

Polystyrene does not float; it is a little denser than water. The smallest grains the team tested measured 75 to 125 micrometers across, roughly the width of a human hair, and they crossed the barrier much as the dye did. Eighty percent of them came out downstream. They are too light to break away from the water's path, so they follow it wherever it goes; minimal retention is the paper's own description of what the barrier achieved on them. One fifth of those grains, even so, never came out the far end.

A low count downstream is not the same thing as capture, and the paper draws that line itself. The largest polystyrene particles it tested also turned up in small numbers at the far end, but there the cause was mainly gravity: they settled to the bed early in the channel rather than being held by the barrier. The middle size class fell in between, pushed around somewhat by the churn the bubbles raise.

The bubbles do more than lift. The dye showed that the barrier opens preferred routes through the water and sets up pockets where the flow circles back, and that it held the dye in the channel up to 24 percent longer. At the higher pressures, the upward push near the bubble stream was strong enough that it could lift sinking particles off the bed. The authors treat that as an opportunity rather than a defect: a barrier tuned that way might recover plastic that has already settled.

All of this happened in a laboratory channel. No barrier in a canal or at a treatment plant was tested. The authors' own summary is that the technology is highly effective for large microplastics that float and limited for the denser, smaller kind, and that better designs will have to work on how the bubbles drive water upward. For a technology meant to keep plastic out of the sea, that is a useful line to have drawn. Whether a fragment is held or carried past is largely decided before it arrives, by whether it floats.