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Bead Filters

BY Leslie Ter Morshuizen

No discussion of mechanical filters in aquaculture would be complete without the mention of bead filters because, although they are fairly expensive, they are effective at cleaning aquaculture water.

 

A bead filter is basically a pressure sand filter housing that is plumbed with a screen at the top as well as the normal screen in the bottom of the unit. 3mm plastic beads are added to 40% of the volume. As the beads are positively buoyant they float, forming a layer in the top ½ of the filter. The beads must be made of non-toxic plastic, so virgin material is often used.

 

Water enters the bottom of the bead filter and travels up through the bed of beads. Particles being transported by the water get stuck in the bead bed and the clean water exits the top of the bead filter.

 

To clean the filter we stop the flow through the unit and open a drain valve so that the water inside the filter can drain away to waste. (Use this water on your garden it is fantastic fertiliser!) While the water is draining out you should open the port on the top of the filter and use your hand to give the beads a good stir to dislodge trapped particles and break up any clumps of beads that may be forming. A few buckets of water are thrown in once the water has drain out to remove the last dirt and then the filter is closed up and normal flow resumes, except that the first water coming out the top of the bead filter should again be dumped as it also contains fine particles which we want to get rid of.

 

Bead filters are excellent in low solids load applications, such as koi ponds of ornamental fish RAS. However, the fairly small bead volume tends to block too quickly in food fish farming, requiring frequent backflushing. However, multiple bead filters can be used in parallel with great success even with food fish farming.

 

Another major benefit of bead filters is that they are simultaneously effective at both mechanical and biological filtration, reducing the need for additional filters. Scientific studies have shown that the wastes from up to 32kg of feed can be trapped and the resultant ammonia converted to nitrate per m3 of beads per day!

 

 January 06, 2014
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