Re: Aquatic Plants Digest V5 #40

[rgrmill at rt66_com]

Scott H. wrote:

> Allow me to speak purely from naivete -- I invite comments.  I have
> always suspected that bioballs where something of a "scam."  They 
have
> very little surface area per volume.

Surface area seems to be the stat that the sellers like to quote.  It is 
just about useless.  Take the large surface area in a scintered glass 
product for example.  That big surface area is mostly lining the pores 
inside the medium.  The pores are very small and there is little 
tendency for water to enter and move through those pores; it only 
happens if it's forced.  The big surface area around the pores is 
useless if there is no flow through the pores.  The glass may as well be 
solid.  Add a biofilm around the outside of the product, maybe 
penetrating a millimeter or so into it's porous surface.  Now not only 
is there no tendency for water to move through the pores,but the water 
can't even get to the pores.  The pores are all sealed.

You can get a similar effect in biowheels.  The big surface area that is 
cited for biowheels is created by the surface of all the little fibers 
in the fabric.  As soon as you get any biofilm on those fibers the space 
between the fibers gets filled and most of that surface area is lost.

Biological media, in order to be useful, has to be non-clogging.  
Bioballs and similar products don't have nearly as much surface area as 
some other products, but a stack bioballs doesn't clog and the surface 
area remains useful for colonization.

> But if the idea is to get as much surface as possible into a volume
> while ensuring plenty of air exposure, I can think of lots of things
> better than bioballs and wonder why people buy them for use in
> biofilters.

Getting the most surface area/volume is not the idea.  The surface area 
has to be accessible to flowing water or it's just about useless.  The 
smaller the open space in the media, the less likely it is to allow free 
flow.  The best media provides a compromise between surface area and 
pore size.

A biofilter needs to provide surface area, allow free flow past the 
surfaces and get enough contact time between the water and the surface 
to allow bacteria to do their work.  As a result I think it's a fact of 
life that an effective biofilter requires a fairly large volume of 
media.

I'm sure that out there in the environmental engineering literature  
there are formulae for optimizing those factors.  There's probably even 
a study or two specific to aquaculture.

Incidentally, filter floss is a great mechanical medium.  Floss will 
provide some biological activity but I wouldn't promote it as a 
biological medium.  It plugs too quickly and the water flowing through 
tends to channel along paths of least resistence.  Little of its large 
surface area is actually exposed to flowing water at any one time.


Roger Miller