Rotating Biological Filter

As we took a long hard look at the range of external canister filters available in the UK it occurred to me that despite the huge improvements in old favourites like the Fluval and the excellent design of Interpet's prime series, little had really changed in the world of externals since their inception.

They will still need careful cleaning out and are only ever as good as the media in them, which must be maintained carefully.

New "externals" like the fluidised sand filters have come onto the market in recent years, but the old combo of electric pump, and a container of immobile media is common to both external, and internal filters and is even found in the old hang-on models, too.

Trickle filter design has also changed little since its inception. Yes, there are more marine fishkeepers using totally submerged (usually sintered glass) media to reduce nitrate in the sump, but the same various types of trickle-fed plastic media can be found in all of the units.

Yes there are chemical units and reaction chambers - yes, you can reduce nitrates electrically - but the basics of water flowing through and over media remain the same in most units.

I wondered if there were other ways left, not exploited by commercial filtration systems (I lie - I knew there were other ways of setting-up biological filtration but I wondered if there were state-of-the-art commercial systems that were set to follow the fluidised sand filter into hobby use). A timely piece of copy faxed its way onto my desk about the same time...

Sparsholt college lecturer Craig Baldwin has been writing a series for PFK, taking us in detail through the whole process of filtration. In a yet-to-be published part of the series (it will appear in the October edition) he writes about filtration systems with the benefit of access to "industrial" systems. Some of these are still remote from the hobby in this country, though they must have attracted the interest of most of the bigger research-minded aquatic equipment companies.

A revolving filter (or Rotating Biological Contactor) is not available to hobbyists in the UK (though something similar IS in the USA). Here the media is rotating discs or slats constantly turning in a partially-submerged setting, so that they pass alternately through air and water. This feeds the filter bacteria a 50% mix of air and the nutrient-laden water they require.

There are problems and drawbacks to the system as Craig explains, but there's food for thought here. He outlines another system, too, a relative of fluidised sand called the floating bead system. Here beads of 3 to 5mm float in a pressurised system which can be flushed regularly. Apparently these are hugely successful in systems with large greedy fish. Craig reckons that it's just a matter of time until systems like these become available on the hobbyist market.

© Steve Windsor. May not be reproduced in part, or whole, without permission.