Imagine a filtration system that blocks or catches small stuff while allowing large objects through. If you think about how a filter works it seems impossible. Filters use a mesh or screen to trap everything too large to pass through the holes. How can a filter do the reverse and trap small particles while passing those that are larger?

The answer lies in a clever application of perforated tubes. Read on for details.


Water Filtration

In most US cities and many smaller communities drinking water comes from large treatment facilities. These remove impurities from river or ground water before distributing it through a network of pipes. This cleaning is done by flocculation, (binding contaminants to specific cleaning chemicals,) followed by filtration through sand, gravel and charcoal. After this comes disinfection to kill any remaining microorganisms. Some communities do this by bubbling ozone gas through the water.

While we don’t drink directly from rivers, and never from the ocean, the cleanliness of the water there is a growing concern. Recent years have seen increased awareness of the impact of plastic in the oceans. It creates a hazard for those who want to enjoy the water – divers and surfers for example – and poses a danger to aquatic life.

Unlike with drinking water, there’s no practical way to filter the oceans. However, given that most marine trash gets there from rivers and streams perhaps the answer is to put filters in those. This is where good intentions meet practical realities. To be effective, all the water flowing in a river needs to pass through some form of screen or mesh. Besides the difficulty of cleaning or replacing such a filter there’s another problem: how will boats and marine life pass through?


Perforated Tubes for Filtration

Filtration is one of the biggest uses of perforated tubes. Filters that use perforated tubes are used in purification, for product isolation, to protect equipment, for efficiency and to ensure safety. Machine lubrication, fuel storage and delivery and food processing are just a few of the places you’ll find perforated tube filters being used.

These filters generally look like cylindrical cartridges. An outer metal perforated tube surrounds the filter media, which might be pleated fabric or a fiberglass mesh. Inside the filter media is a second perforated tube. Designs vary but in one filtration method fluid is drawn out from the inner tube. This suction pulls fluid through the outer perforated tube and the filter media.

The outer perforated tube traps particles larger than the holes in the tube, but its main purpose is to distribute fluid flow evenly across the filter media. Likewise, the inner tube supports the filter media while letting the fluid flow through.

Perforation size and spacing or pattern is critical for ensuring low pressure drop and maximum filter life. Large holes are good for flow but could lead to filter media damage. Smaller holes provide good support and protection against direct impingement but impose more pressure drop. Filter designers balance these conflicting requirements to achieve optimal performance.


Perforated Tubes for River Filtration

Noise from underwater activities like pile-driving is very disturbing for marine life. To counter this engineers are making increasing use of bubble curtains.

A bubble curtain is a screen of bubbles rising up through water. The bubbles act as an acoustic barrier, absorbing sound waves and reducing the intensity of what’s left to spread out. Bubble curtains have been used on a number of civil engineering projects and evidence indicates they are an effective way of protecting fish and aquatic mammals.

A bubble curtain is created by blowing air through a perforated tube. Small holes let a stream of bubbles emerge, with hole size and spacing configured to maximize bubble formation and the curtain effect. This same technique is now being used experimentally to remove plastic from rivers.

In one of the best-known trials Dutch engineers have placed a perforated tube diagonally across the floor of a canal. Compressed air pumped through the tube creates a curtain of bubbles the width of the canal. Appearing on the surface as a line of bubbles, this steers floating trash, (plastic bottles, drinks cans and so on,) to one side where it flows into a collector.

Not all river-borne trash floats: some is under the surface and some of this tumbles along the river bottom. The bubble curtain handles this too. The stream of bubbles creates an upward current that brings trash to the surface. Once there, it’s guided towards the collector just like the surface trash. (“Air bubble barrier traps plastic waste in Amsterdam’s canals,” (The Guardian, November 2019,) has more details and links.)

One additional benefit that should be apparent; unlike physical barriers, a bubble curtain is no hindrance to the movement of boats or fish.


Clean Water, Thanks to Perforated Tubes

The perforated tubing used in these trials isn’t the same as that used in cartridge-style filters, but it functions in a similar way. An optimized pattern of holes allows fluid, in this case compressed air, to flow through in a carefully engineered pattern. 

Press reports indicate the potential of bubble curtains for keeping plastic out of the oceans is still being evaluated, but it looks promising. This air-bubble method currently being tested to help contain oil-spills making it easier to clean up. So who knows, perhaps perforated tubes will soon give us a clean ocean!


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