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- Bead filters: Open style on the left, closed style filters in the back
- Biofiltration
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- Bio Balls Bio-FillTM
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- A simple degassing column
In the world of aquaculture, filtration and
biofiltration are very distinct and separate entities and they must be
treated as such. Filtration is the removal of solid waste, whereas biofiltration
is the biological process which eliminates toxic nitrogenous wastes.
This page will cover these differences, as well as delving into some of
the numerous types of filtration devices.
- Filtration
solids are those solids which have a relatively high specific gravity compared to
the water in which they exist. They will settle to the bottom. Suspended solids
are those in a category which have a specific gravity the same as or slightly
higher than the water. They tend to stay in suspension and will only "drop-out"
over a long period of time. Dissolved solids are those which actually become a
part of the water. The dissolved solids are eliminated by reverse osmosis, anion
and cation resins, activated carbon, etc.
One method
of removing solid waste from a round fish tank is to use a double
drain. It will direct the settled
solids to a separate area from the suspended
solids. The settled solids can
be directed into a small clarifier, much smaller than
one which had to be sized
to handle the entire flow of recirculating water. The
other drain takes the suspended
solids along with the nitrogenous waste.
Suspended
solids can be removed by several methods. One is the affinity bead
filter which incorporates the
use of small polyethylene beads that have an
electrostatic charge. These beads
have an affinity for the negatively charged
suspended solids. As the particles
pass these beads, they are "statically" drawn
to them. When the beads are loaded
with solids, it is time to backwash them.
(Too often, backwashing is not
done frequently enough.) Suspended solids can
also be removed by mechanical
means such as bag filters, drum filters and
vegetable filters.
waste (ammonia). Fish will excrete about 14 grams of ammonia for each pound of food eaten
(at 35% protein). The nitrification of ammonia is accomplished by two strains of autotrophic
bacteria. These bacteria are naturally occurring and will ultimately colonize the bio-media in
your biofilter as well as the tank walls etc. The speed of this process is dependent on
temperature, pH, salinity surface area, flow rate, etc.
These bacteria
use oxygen in a two-step process. The first strain of bacteria,
Nitrosomonas, converts the toxic
ammonia NH3, to NO2- which is nitrite.
Nitrobacteria is the second strain
of bacteria that converts nitrites NO2- to
NO3- which is
nitrate. Nitrate is typically
tolerated by most cultured species until it reaches very high
levels. Controlling nitrate is
accomplished by diluting with clean water or by using a
denitrification chamber which
converts nitrate into nitrogen gas. (This is an anaerobic
process that uses a group of chemoheterotrophic
bacteria.) A third method that can be used
to keep nitrate levels in check
is by using plants. You can have a green water system (using
algae), a vegetative filter or
even use a hydroponic plant filter.
For those of you who are more chemistry minded, here are the nitrification process calculations:
Nitrosomonas calculation - conversion of ammonia to nitrite
55 NH4+ + 76 O2 + 5 CO2
===> C5H7O2N + 52 H2O +54 NO2-
+ H+
Nitrobacteria calculation - conversion of nitrite to nitrate
400 NO2- + NH4+ +
5 CO2 + 195 O2 + 2 H2O ===> C5H7O2
+ 400 NO3- + H+
Rotating Biological Contactor: A biofilter converting ammonia to nitrate
- Biofiltration Media
Biofiltration media is merely a mass of surfaces serving as the attachment basis
for micro-organisms. The spacing between these surfaces is important, both for
the passage of water and to provide sufficient room for bacterial growth. The
biggest cause of biofilter fouling is solid waste, which grows heterotrophic
bacteria. Always try to filter out all solids prior to your biofilter. Bio Barrels, Bio
Balls, Bio Strata, Bio-FillTM, scrub pads, and even sand can be used as biofilter
media. It is recommended that you use approximately 300 sq. ft. of surface area per 100
lbs. of fish in a warm water recirculating system. To give a general idea, these manufactured
media range from 26-370 square feet of surface area per every cubic foot of media
- Stages of Filtration
Regardless
of which type of filtering equipment you decide to use, the important
thing to keep in mind is to always
stage your filtration. It is an all to common
mistake to design a system that
relies too heavily on a single filtering device to
provide all of the filtering demands
that a recirculating system has. By staging your
filtration, you will find your
system performing at or near its peak.
Sedimentation - removal of solid wastes from the water
1. Do whatever
is possible to allow fish feces to drop intact into the waste collection
area or self-cleaning bottom with minimal damage. Minimize the use
of pumps,
aerators and air diffusers wherever feces is present.
2. Do not pump
the waste prior to separation. Design for gravity flow (or siphon)
into a sedimentation tank or basin. Splashing and turbulence can
attach air bubbles
and break apart solids. Feces and food particles smaller than 40
microns may not
settle without chemical floculents.
3. Always locate
your biofilter after the solids removal system. Solids provide
carbon for heterotrophic bacteria which can foul a biofilter and/or
reduce its
performance.
4. Clean
both the settling area and filters at least once a day even if they
contain little
waste.
5.
If further filtration is required after sedimentation, pump the water
to an affinity
bead clarifier or particulate filter.
Sedimentation Basin Design
velocity), a surface area of .7 to 1.4 sq. ft. of basin per gpm flow (for
feces with a specific gravity of 1.01 or greater), wide outlet weir (never
a stand pipe), no baffles (which increase velocities) and a simple waste
drain. A depth of just a few inches is enough for most designs.
Degassing - removal of dangerous gasses from the water
- Degassing is a process which is used to remove undesirable gasses,
that are present in greater concentrations in the water than would otherwise
naturally be found. When they are in this state, such gasses are called
supersaturated gasses. A supersaturated gas has a natural tendency, when
exposed to an interface between air and water to escape out of the water
and into the surrounding air. (This is like in fizzy drinks, where while
enclosed in a can and under pressure the gas stays in solution, but when
the can is opened the pressure of the liquid reduces suddenly. The ability
of the drink to hold such an amount of gas is reduced and the gas has a
route to escape through the surface of the drink. The gas forms bubbles
in the glass as it tries to escape.) The main function of a degasser
is to create a large interface between the water and the air. This
is achieved in a variety of ways. One way is by heavy aeration, where
the surface area of the bubbles creates a large interface as they rise
through the water. Aeration also creates a lot of turbulence, bringing
water to the surface where the gas can escape directly to the atmosphere.
Another degassing method is letting the water fall over weirs and cascades,
where it is broken up into droplets, thereby increasing the interface.
Also commonly used are packed columns, which are vessels filled with a
type of media over which the water runs.
Protein Skimmers (Foam Fractionators)
- Protein skimmers are helpful in removingdissolved
organic material from water, and are beneficial for improving water clarity,
aeration and redox potential in marine systems. These units are often
used in conjunction with ozone generators.
Ozone Generators
The use of ozone is increasing in popularity for several reasons:
1. It is highly effective in removing organics, pesticides, color and nitrites.
2. It reverts back to oxygen quickly. Unlike chlorine, there are no detrimental residuals
(except in saltwater).
3. It is produced on site, with no electricity near the water.
4. It is economical and nonpolluting, when used correctly.
5. It can be used as a sterilizer, before, during and after water is used for aquaculture.
6. Ozonization improves biological filtration and particulate filtration.
7. It can remove the biological oxygen demand in the water.
8. It oxidizes long chain molecules, which biofiltration cannot do.
Ozone is generated by passing air or oxygen
through a reaction vessel, where either
an electric arc, corona discharge (CD), or an ultraviolet (UV) lamp
"excites" the
oxygen. In this reaction oxygen molecules separate into atoms of oxygen
which
then temporarily recombine with each other to form ozone. When ozone
oxidizes
organics only one atom of oxygen is used, leaving one molecule of oxygen.
Different Types of Ozone Generators
Ultraviolet lights with a specific ozone generating
wave length are generally used to
produce low levels of ozone. The slower the gas moves through
the reaction vessel,
the higher the percent of ozone.
The corona discharge (CD) type of uses an electric
arc similar to sparks or lightning
to produce higher percentages of ozone by weight. A relatively
small CD reaction
vessel can produce a relatively large volume of ozone. The greater
the percentage
of ozone, the faster the oxidizing reactions take place.
Ozone can be used in a protein skimmer (foam fractionation
device), where it helps
the process while the vessel allows capture of the off gas for
venting or ozone
destruction. Ozone works very well in oxygen saturators for the
same reasons.
UV Sterilization
Ultraviolet light can be very effective at eliminating viruses, bacteria, algae and fungi.
Since it is the intensity of light that is doing the killing, we must know how much light
energy to use and how much is reaching the target. Just as some sunglasses and
sunscreens reduce UV intensity, so does discolored water, temperatures, turbidity,
dirty quartz sleeves, and even some dissolved salts such as sodium thiosulfate. Old
"run down" lamps also affect the intensity. Even lamp temperatures can reduce
output when operated in cold water temperatures (110°F is maximum UV output).
To insure sterile water using UV light, you must first start with clear water, have a
lamp and flow rate that are sized to deliver the correct amount of irradiation for the
target organism (see an exposures list). If a UV light is flow rated for 15,000 mws and
you want 30,000 you can either double the amount of lamps or reduce the flow by half.
