USE OF ACTIVATED CARBON FOR PESTICIDES AND HEAVY METALS
From: Juan Martinez postlarmar@geo.net.co
To: shrimp@egroups.com
Sent: 13 July 2000
QUESTION:
Even though I still haven't gotten the results on pesticides and heavy
metals, I am pretty sure that one or both problems are affecting my
hatchery production.
I have learned that in using granular activated carbon, it is a must
to have a retention time of 10 to 15 minutes for an effective
filtration. It means that for a flow of 120 GPM, I would need around
6 Mt3 of carbon. Is it right? This seems very expensive and
complicated to implement.
I have also been told that a good activated carbon for heavy metals
is made of bone, due to its ionic exchange capability. Is it right?
Can other carbons get rid of heavy metals? Which carbon is better for
these 2 problems, vegetal or mineral? And small, medium, or large
pores?
Other option to granular carbon is powdered carbon. It can be applied
to the water column directly only the required amount. Moreover, it
is only used once, which guarantees that it works correctly. For
fresh
water I know it is around 3 to 4 ppm. What is the lowest, greatest
and usual amount of powdered carbon applied to salt water?
I have been thinking of filtering the water through sand and bio-
filters. Then, once the water reaches the 120 tons reservoirs, it is
chlorinated and dechlorinated. Then I would pour the powdered carbon
into the reservoir and let it work for around 30 minutes. My problem
is that I do not know a cost-effective system to keep the carbon from
sedimenting and to keep the column in strong motion to have a good
contact between the water and the carbon. I have been thinking on air
lifts with a blower, water recirculation going into the reservoir
through bottom pipes, using venturis, water circulators, etc. Which
is the best choice? Does it keep the powdered carbon mixed in the
water column?
Is it better to use granular or powdered activated carbon?
Juan Martinez
E-mail: postlarmar@geo.net.co
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COMMENTS 1 :
Try 7 gallons per minute per cubic foot of activated carbon
of a 4 x 8 mesh size particle (hence this is granular).
A standard sand filter rated at 140-150 gpm has around 6-7 cubic
feet of capacity. Hence, to properly treat 120 gpm, you need
about three of these sand filters filled with carbon (plumbed
in parallel such that each vessel received no more than
about 40-45 gpm). Note that this is a flow-through design
and the retention is built into the flow to surface area ratio,
but on a once pass through flow. We supply quite a few
operations throughout Central America but use coconut
shell charcoal. Bituminous coal base charcoal is not used a lot
for this application. Bone charcoal may be too costly.
The use of powdered carbon will cause some problems
in turbidity as some of the particles are colloidal and will stay
in suspension for a while.
We have used this formula in several locations for pesticides and the
flow rate formula was supplied to us by the carbon manufacturers for
computing flows in other medical and industrial applications.....seems to
work well.
Leland Lai
Aquafauna Bio-Marine, Inc.
E-mail: lelandlai@aquafauna.com
***************
COMMENTS 2 :
If you have ever worked with significant amounts of powdered carbon,
you will drop that idea. The 10 - 15 minutes seems a little
long but
I would recommend two reactors in series to be able to determine when
you need to change out the carbon in the first one. You could
bioassay or analyze the water between stages. If you don't know
what
is causing the problem, use bioassay.
As far as using bone carbon for heavy metals, it sounds like a bone
head idea. Carbon isn't designed for cation exchange.
If you want
cation exchange capacity use a cation exchange resin and get some
real capacity. Initially load the resin with Ca/Sr at the
correct
ratio for seawater with two columns in series. It should strip
all
heavy metals until breakthrough, when you have to regenerate the
columns (salt/acid followed by Ca/Sr solution). However, we
don't
know which of these heavy metals are required and how much. You
may have a shortage of some heavy metal.
Dallas E. Weaver, Ph.D.
E-mail: deweaver@gte.net
***************
COMMENTS 3 :
Heavy metals can mostly not been extracted by activated carbon, no
matter of what origin.
The method would be to coagulate by iron, bentonite or so.
The minimum retention time in an AC is 20 minutes, according to our
experience.
You should closely watch and analyse the AC you are buying for
inhibiting matter.
AC will not work at all under a pH of 7.0.
Try to use (if available) zeolites to reduce HM.
AC will only work perfectly if used at the end of a water treatment
system that supplies almost clear water to the filter.
Any alga or so will cause fowling and clogging.
As for HM: bentonite will flock out cadmium and lead, but rather
nothing more.
Pesticides you can break up by using UV and/or O3.
Burkhard Klein
E-mail: mantenec@t-online.de
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