BV,

Lava rock and gravel both provide a place for the colonization of the
bacteria, which is a matter of surface area, and provide some mechanical
filtration. The biofilm is sticky, and the water passing over it causes the
fine materials to come in contact with the sticky stuff and stay behind.
That is the main problem with these. The build up of solids in the lava
rock or gravel means that they will clog in areas causing channelization in
other areas. The clogged areas go anaerobic. To keep this from happening,
or as soon as it does happen, the lava rock or gravel has to be removed and
cleaned. Lava rock has a ton of sharp abrasive edges and both materials are
heavy. If you have very much, I have about 2 tons, my brother-in-law has
about 10 tons, then it becomes a question of where do you put it when you
take it out of the filter. We started with lava rock filters, then gravel
filters, and even though I have moved to bead filters, DW won't let me get
rid of the rock. Oh well, maybe this year I will try to get her working the
shovel..
--
RichToyBox
http://www.geocities.com/richtoybox/pondintro.html


"BenignVanilla" wrote in
message ...
"zookeeper" wrote in message
...
When we inherited our pond (from previous homeowner), two 50gal barrels
were full of red lava rock, and the third had a large rolled piece of
filter "batting." We kept the rock through the first year, but it was
too heavy to remove or stir when trying to clean. We refilled the "rock"
barrels with strapping tape (courtesy of the boys' paper route bundles)
which is much easier to stir and rinse out once a year.

Forgive me if this is a naive question...is the material in the filter
just
a matter of surface area? I thought the lava rocks had some filtering
property.

BV.