BIOLOGICAL FILTERS FOR AQUACULTURE
What are
Biological filters?
Biological filters are devices to culture
microorganisms that will perform a given task for us. Different types of organisms will perform
different tasks. Part of the art of designing
and using biofilters is to create an environment that will promote the growth
of the organisms that are needed.
Why do we need
biological filters for aquaculture?
We use biofilters to help maintain water
quality in recirculating or closed loop systems. Biofilters are also used to improve water
quality before water is discharged from a facility. There are many different
methods of maintaining good water quality and biofiltration is only one
component of the total picture. It is
however, a very important and essential component especially for recirculating
aquaculture or aquarium systems.
How will
biofilters help us?
Depending on design and application,
biofilters have the ability to accomplish the following functions. The first three functions are performed by
biological means and the last four are done by physical processes that do not
depend on living organisms.
1. Remove
ammonia
2. Remove
nitrites
3. Remove
dissolved organic solids
4. Add
oxygen
5. Remove
carbon dioxide
6. Remove
excess nitrogen and other dissolved gasses
7. Remove
suspended solids
In general, there are two types of aerobic
microorganisms that colonize biofilters for aquaculture. Heterotrophic bacteria utilize the dissolved
carbonaceous material as their food source.
Chemotrophic bacteria such as Nitrosomonos sp.
bacteria utilize ammonia as a food source and produce nitrite as a waste
product. Chemotrophic bacteria such as Nitrospira sp.
utilize nitrite as a food source and produce nitrates as a waste product. Nitrosomonos and Nitrospira will both grow and colonize the biofilter as
long as there is a food source available.
Unfortunately, both of these types of bacteria are relatively slow
growing. Heterotrophic bacteria grow
about 5 times faster and will out compete the other two types for space if food
is available. Since most aquaculture
biofiltration systems are designed for the purpose of converting and removing
ammonia from the water this presents a problem.
There are three ways to deal with this
problem. The first is to remove most of
the carbonaceous BOD (biological oxygen demand) before the water enters the
biofilter. The second method is to
provide sufficient extra capacity (surface area) in the biofilter to allow all
of the various bacteria to grow. Another
method is to have a very long plug flow path through the biofilter. This allows different zones of bacteria to
establish themselves in different parts of the biofilter.
There are 4 main types of aerobic
biological filters and several subcategories of each. Here is a listing of the major types.
I. Recirculated
Suspended Solids (Activated
sludge and biofloc systems)
II. Aquatic
Plant Filters
A. Unicellular (Microscopic)
B. Multicellular (Macroscopic)
III. Fluidized
Bed Filters
A. Sand Filters
B. Bead Filters
IV. Fixed
film
A. Rotating Biological Contactors (RBC)
B. Trickling Filters
C. Submerged Filters (with or without
aeration)
1. Up flow
2. Down flow
3. Horizontal flow
4. Moving Bed
Anaerobic filters can also be defined as
biofilters but they are never the main biofilter used for maintaining water
quality in the culture system. There are
two main reasons why they are not suitable.
The number one reason is that they are not capable of effectively
cleaning the water to the level required.
The other reason is that they operate too slowly. Anaerobic filters are sometimes used in
aquaculture for conversion of nitrates into N2.
However, this is a difficult process to control and it is generally less
expensive to remove nitrates by discharging a small amount of water from the
system. The water removed with the
solids is usually sufficient to remove the nitrates as well.
Anaerobic biofilters are best suited for
processing high strength waste. The
sludge produced by the physical filter system is an example of a high strength
waste. Processing plant wastes are
another candidate for anaerobic digestion.
In an integrated production/processing plant these two streams could be
combined. The best feature of anaerobic
systems is the production of methane.
There are specially designed engines that can burn this gas to produce
electricity. Using the gas to heat water
is another obvious possibility. However, the capital cost of these systems
generally limits their use to large operations.
General Water Quality Maintenance Principles
Not all aquaculture applications have the
same requirements for biofiltration. Not
only do crops vary in their requirements but different farmers may grow the
same crop under different conditions. The biofilter is only one of several
components of the system used to maintain water quality. The functions that the biofilter must perform
are determined by the presence and effectiveness of other components. Here are some other components and their
effects on the system.
Aeration or
oxygenating systems
If the fish don't have oxygen you are out
of business no matter what else you do.
Aeration is always the first step when increasing carrying capacity over
an open, lightly loaded system.
Mechanical surface aerators, subsurface air bubblers and pure oxygen
injection is the typical progression in terms of technology and
complexity. All aerobic biofilters
require oxygen to operate. If the
biofilter does not provide its own oxygen, it will be limited to the oxygen
carried in with the water.
Particulate
Filters
Once sufficient oxygen is provided, the
next easiest way to improve water quality is to remove suspended solids. This
is a more difficult task since particles come in all shapes, sizes and
densities. Suspended solids consist primarily of uneaten food and feces which
are slightly denser than water. Large particles, above 100 microns, will settle
out quite easily. Particles above 50
microns can be filtered out with a screen.
Particles below 10 microns are difficult to filter and are generally
removed by some other means.
There are many different types of
particulate filters that can remove suspended solids. They generally fall into three broad
categories. The first type are settling
basins, tube settlers, plate settlers, swirl separators and similar systems
that allow the particles to drop out of the flowing stream by gravity. They are relatively simple devices and they
work well on large particles. Settling
systems generally have very low pump head requirements.
The second type are sand filters, sock
filters, drum filters, disk filters, belt filters and similar systems that
mechanically remove the particles from a flowing stream. These types of systems "screen" the
particles. The size of particle removed
is dependent on the size of the screen or sieve. Pump head requirements can vary from low to
very high. Some biofilters such as bead
filters claim to do both particulate filtration and biological filtration.
The third type of particulate filter is
air floatation or fractionation. These
are commonly known as protein skimmers.
In this device, air is bubbled into a column and the fine particles
become attached to the surface of bubbles. The resulting froth or foam is
collected and removed from the system.
These devices require a certain amount of surfactant type compounds in
the water in order to work properly.
Generally speaking, they work better in salt water than fresh water
systems.
Although they are not typically designed
for solids removal, some submerged biofilters will tend to collect fine
particles due to the sticky nature of biofilms.
This can be both a benefit and a maintenance problem. If the biofilter is not designed for easy
cleaning, solids collection can represent a maintenance headache.
Removal of suspended solids is important
since suspended solids comprise the majority of the BOD (Biological Oxygen
Demand). The BOD not removed by the
particulate filtration system must be removed by the biofilter before effective
ammonia removal will occur. Thus the
size of the biofilter is influenced by the effectiveness of the particulate
filtration system.
The way that solids are removed is also
important. The best systems remove
solids quickly without degrading them in any way. If the solid particles are broken or reduced
in size, it makes it easier for nutrients to dissolve into the water. These nutrients must then be removed by
another part of the water treatment system or flushed out by water
exchange. Time is also important because
the longer solids are held in the system, the more degradation will occur. Floating bead filters are particularly bad in
this regard since they hold the solids for long periods of time before backflushing.
Foam
Fractionators
Foam fractionators are very useful but
sometimes optional pieces of equipment.
They are good at removing small particles (under 10 microns) and surface
active compounds. They are sometimes
referred to as protein skimmers. Since
proteins are nitrogenous compounds that degrade into ammonia, foam
fractionators can reduce the load on the biofilters. They are definitely useful in systems where
water clarity is important. Foam
fractionators also add oxygen to the water as a secondary benefit. Unfortunately, foam fractionators do not
always work well in fresh water.
Ozone
Ozone is a powerful oxidizer and sterilant. It is
potentially harmful to fish, humans and most living organisms. It is definitely harmful to biofilters. It is used to improve water clarity and
reduce disease transmission. Ozone should never be used directly before a
biofilter. If ozone is used upstream of
a biofilter, there should be sufficient retention time after the injection point
to insure that no ozone residual enters the biofilter.
UV light
Certain wavelengths of UV (Ultraviolet)
light can be used as a sterilant. UV light is often used with ozone. UV light and ozone are complimentary and
synergistic.
Carbon dioxide
strippers
Build up of CO2 can be a serious problem
in a heavily loaded, intensive recirculating system using pure oxygen. The choice of biofilter has a direct
influence on the degree to which CO2 is a problem. In general, any biofilter other than a
trickling filter or RBC will have a CO2 problem when pure oxygen is used rather
than compressed air for aeration.
Building a CO2 stripper is not a difficult task but it must be included
in the overall design of the system.
In order to remove carbon dioxide, there
must be a large interfacial area between air and water. The interfacial area can be increased through
the use of subsurface aeration, mechanical surface aerators, spray systems or
packed columns. Subsurface aeration is
not very efficient and mechanical surface aerators are difficult to use in an
intensive recirculating systems. Spray
systems can be big energy users and they are not very efficient either. The best choice for intensive and space
limited systems is the packed column.
Packed columns can be either cross flow or counter flow systems. Packed columns for CO2 stripping require fans
to either force (push) air in or induce (pull) air through the packing.
Characteristics of the "Ideal" Biofilter
Before we examine each type of biofilter,
it would be useful to define the characteristics of the ideal biofilter. The following characteristics can be
considered a checklist that we can use to rate each of the different
types. In some cases, different features
may be mutually exclusive but we can use the ideal characteristics as a
yardstick or goal. In practice it may be necessary to trade off one feature for
another but it doesn't hurt to know what the ideal should look like. The following list contains most of the pertinent
features of a good biofilter.
1. Small
footprint - The biofilter should occupy as little space as possible. It is common to have culture tanks and the
biofilters under cover for protection and temperature control. Space allocated for biofilters takes away
area that could be used for culture tanks.
2. Inert
materials of construction - All materials used in the biofilters should be
non-corrodible, UV resistant, resistant to rot or decay and generally
impervious to chemical attack. In
general, marine grade construction materials are required for reasonable
working lifetimes.
3. Low
capital cost - The biofilter must be inexpensive to purchase or build and cheap
to transport to the farm location.
4. Good
mechanical strength - The biofilter and its components must be tough enough to
withstand the normal wear and tear of an industrial/agricultural environment.
5. Low
energy consumption - The energy cost (usually electricity) to operate the
biofilters should be as low as possible.
The largest energy users are the pumps to move water and compressors to
move air.
6. Low
maintenance requirements - The biofilters should be self cleaning with little
or no care required for the normal life of the crop.
7. Portability
- The biofilters should be easily movable to facilitate changes in operation of
the facility.
8. Reliability
- Ideally the biofilters should have no moving parts that could fail at an
inopportune time. If the biofilters does
have moving parts, they should be rugged and designed for a continuous
operating life of several years.
9. Monitorabilty - It should be easy to observe the operation
of the biofilter to insure that it is operating correctly.
10. Controllability
- It should be easy to change operating variables to assure optimum
performance.
11. Turndown
ratio - The biofilters should be able to work under a wide range of water flow
rates and nutrient loading levels.
12. Safety
- The biofilters should not have any inherent dangers to either the crop or the
owner/operator.
13. Utility
- The biofilters should accomplish all of the goals set forth in beginning of
this paper i.e. removal of ammonia, carbon dioxide, BOD, suspended solids etc.
14. Scalable
- A small system should work the same way as a large system. The performance per unit volume should be
constant regardless of the size of the system.
Now that the characteristics of the
"ideal" biofiltration packing have been established, it makes sense
to compare the existing medias to that standard.
Characteristics of Real Biofilters
Activated Sludge
Systems
Activated sludge systems are not common in
aquaculture systems. Activated sludge systems are good at removing carbonaceous
BOD in systems with high nutrient loadings.
They are commonly used in domestic waste water treatment systems.
Activated sludge systems are typically expensive to operate and do not provide
the effluent water quality necessary for aquaculture.
Aquatic Plant
Systems
Plants are not normally used for the
primary biofilter in aquaculture systems.
They do however provide a very good sink for the nitrates produced by a
well functioning biofiltration system.
The marriage of recirculating aquaculture systems and hydroponics is
known as aquaponics.
Aquaponics use the feed resources efficiently
and effectively. In addition to the
valuable plants grown in aquaponic systems, they
minimize the amount of waste that must be disposed of in the environment. Removal of nitrates and phosphorus from waste
water is a big benefit.
Unicellular plants (algae, diatoms etc.)
are sometimes allowed to grow in the culture tanks. Some species such as tilapia are tolerant of
poor water quality and can use the algae as food. Systems operated this way are sometimes
called "green water" or biofloc systems to
distinguish them from the clear water systems that many species require. Green water systems can be a very cost
effective way to culture certain species but they are not recommended for
beginners to aquaculture. Management of
these systems requires some experience and specific knowledge.
Fluidized bed
sand filters
Regular sand filters such as the type used
for swimming pool filters or potable water filters are virtually worthless as
biofilters for aquaculture. The biofilm
quickly fills the spaces between the grains of sand and the pressure drop
across the filter rises rapidly.
Frequent back flushing is required and the active biological film is
removed each time. In contrast,
fluidized bed sand filters have been successfully used for aquaculture
applications. A sand filter becomes
fluidized when the velocity of the water flowing up through the bed is
sufficient to raise the grains of sand up and separate each grain from its
neighbors. In hydraulic terms, the drag
on each particle is sufficient to overcome the weight of the particle and the
particle is suspended in the stream of water.
The velocity required to fluidize the particle is a function of the
shape, size and density of the particle.
Fluidized bed sand filters have several very
good advantages. They pack more
biologically active surface area into a given volume than any other type of
biofilter. In addition, the best shape
for a fluidized bed sand filters is a tall column. Thus they have a small foot print for a given
capacity. They are self cleaning and
relatively tolerant of different nutrient loadings.
There are also several disadvantages and
potential problem areas with fluidized bed sand filters. The fluidized bed sand filter has a
relatively high energy requirement because of the high pressure drop necessary
to fluidize the sand. The other main
problem with sand filters is that the pressure required to fluidize the bed
varies depending on the amount of biofilm on the sand particle. As the biofilm builds on the sand particle
the size of the particle increases while the density of the particle
decreases. This means that the depth of
the bed will tend to increase as the bed ages.
It also means that the bed depth will fluctuate as the loading on the
bed varies. In order to prevent blowing
the sand out of the tank, the tank must be oversized or the flow of water needs
to be regulated.
Another potential problem is the
uniformity of the water flow. In order
to completely fluidize the bed, the water needs to be evenly distributed across
the whole bed. Two things can happen if
the flow is not uniform. One possibility
is that the water will channel and short circuit though the bed. This means
that the treatment capacity will plummet.
Another possibility is that the short circuit will happen near the wall
of the vessel and the abrasive sand will eat a hole through the wall of the
vessel.
Fluidized bed sand filters are limited to
the oxygen carried in with the water.
This means that the water entering the filter should have a high level
of oxygen in order to insure a good level of treatment.
Bead filters
Bead filters are a relatively new type of
biofilter. They are advertised as the
complete solution to water quality for recirculating systems. They consist of a
closed vessel partially filled with small beads of plastic. Usually the vessel is filled with water and
the beads float at the top of the vessel.
Water flows up through the bed of beads.
The beads are small enough to trap most large suspended solids. In addition, the surface of the beads
supports the growth of a biofilm. The
small size of the beads means that they have a relatively large surface area
per unit volume. The larger systems
incorporate a mechanical stirring device such as a propeller on a shaft. Periodically the water flow is shut off and
the bed of beads is agitated to dislodge the suspended solids. The solids are allowed to settle into the
bottom of the vessel and then drained off.
This ability to remove suspended solids and act as a biological filter
is advertized as the main advantage to bead filters.
The difficulty in successfully operating
bead filters lies in striking a balance between the competing functions. Too frequent washing to remove solids
dislodges the biofilm and disrupts the nitrification process. If the beads are not washed enough however,
the solids start to plug the bed. The
other potential problem is the presence of large amounts of carbonaceous solids
which tends to encourage the growth of heterotrophic bacteria at the expense of
the autotrophic bacteria that work on the ammonia and nitrites.
Another drawback to bead filters is their
relatively high energy consumption due to their high pressure drop. Also, the water flow and pressure drop are
not constant. As the bed of beads
becomes loaded with solids, the pressure drop rises and the water flow
decreases. This leads to cyclic rather
than constant performance.
Since bead filters are not aerated, they
are limited to the oxygen carried in with the water. In general this is not a problem since
retention times are low. Bead filter
systems are probably suitable for small, lightly loaded systems where labor
costs are low. At this time they are not
available for large systems except as multiple units.
RBC (Rotating
Biological Contactors)
Like much of the equipment used in
aquaculture, RBC's were first used in domestic sewage treatment
applications. There are several
different types available for aquaculture.
A typical design consists of plates or disks that are attached to a
horizontal shaft. The shaft is located
at the surface of the water and it is turned at a very slow speed (1-5
rpm). The disks are half submerged in
the water at all times. As they rotate,
the biofilm attached to the surface of the disk is alternately exposed to air
and then submerged in the water. The
original designs used an electric motor to turn the shaft. There is a new design specifically for
aquaculture that uses compressed air or pumped water to drive a paddle wheel in
the center of the cylinder. These RBC's
float in the water and do not require bearings or elaborate mechanical
supports.
RBC's have many advantages. They offer
excellent treatment efficiencies. They
require very little energy to operate and can be located in the culture tank to
save space if necessary. They do not
require additional oxygen and are not limited to oxygen contained in the
incoming water. They can remove
dissolved BOD or ammonia depending on nutrient levels. They are biologically robust and handle shock
loads well. It is easy to observe their
operation and visually monitor the biofilm.
They only have one major drawback besides cost and that relates to
reliability. If there is a power failure
or the cylinder stops turning for any reason, the biofilm exposed to the air
can dry out. When this happens, the
cylinder will be unbalanced and can become difficult to turn.
Trickling
Filters
Trickling filters are one of the oldest
types of biological filters. Trickling
filters filled with rock or coal were built in the late 1800's for sewage
treatment. Trickling filters typically consist of a packing or media contained
in a vessel. The water to be treated is
sprayed over the top of the media and collected in a sump underneath the
media. The surface of the media or
packing provides the substrate for the growth of a biofilm. In large systems, air is forced into the
filter with a fan. However, small can
filters rely on natural convection and diffusion to move air throughout the
filter.
Trickling filters are rugged and easy to
operate. They have the ability to treat
a wide variety of nutrient levels.
Properly designed systems can handle solids very well. One of the big advantages
of a trickling filter is that the water can leave with more oxygen than it
entered. Because trickling filters have a large - air water interface, they
also act as strippers to remove CO2, H2S, N2 or other undesirable volatile
gases. The only major drawback to
trickling filters is the energy cost required to pump the water to the top of
the filter. A high narrow filter will
save space but take more pumping energy.
A wide low filter will use less energy but take up more space.
The first step in the design of a
trickling filter is to pick the right packing or media. Over the years many
different materials have been used for trickling filters but for the last 40
years, the best packing has been structured media. Structured media is composed of sheets of
rigid PVC that are corrugated and glued together to form blocks. For an in
depth review and analysis of packing materials, refer to the paper "A
Review of Biofiltration Packings".
One of the advantages of structured media
is its flexibility and ease of use.
Structured media can be used to build a small biofilter without a
vessel. Since the vessel is typically the major cost of a biofilter, a
biofilter with no vessel can be a real money saver. Structured media can be
stacked on a frame work or any flat surface.
It can be located over a culture tank or have its own water collecting
sump. No sides are required because the packing is self supporting. Of course, large systems are typically built
with walls and fans to move air through the media.
The most important requirement in the
design of any trickling filter is a good water distribution system at the top.
There are several ways to do this. A
pressure spray system with splash guards at the top is probably the
simplest. The only drawback is the
additional pressure drop required to operate the nozzle. The other system involves the construction of
a shallow water distribution pan with several gravity flow target nozzles in
the bottom of the pan. Here are some typical arrangements for a
"vessel-less" trickling filters.
Figure 1. Trickling filter with pressure nozzle
distribution system.
Fig. 2.
This is a trickling filter with gravity flow target nozzles in a shallow
water distribution pan.
Part of the art of designing a trickling
filter is to balance the competing requirements on the design.
1. In
order to keep the energy costs to a minimum, the pumping head for the filter
should be as low as possible. The
maximum plan area covered by the filter is determined by the minimum water
loading.
2. In
order to minimize the floor space used by the filter, the filter should be as
tall as possible. The practical
limitations are the height of the building, the head limits on the pump and the
structural and stability considerations of the vessel.
3. A
taller filter will have a longer flow path for the water. This means a more complete treatment of the
water with each pass.
4. Taller
filters will have higher specific water loadings. This means better flushing action, more
turbulent water films and higher ammonium removal rates.
Trickling filters for industrial applications
are sometimes 30 ft. tall. This is not
practical for aquaculture systems. In
general, trickling filters for aquaculture are between 4 and 10 ft. tall.
Submerged Bed Filters
Submerged
Bed Filters are familiar to anyone who has owned an aquarium. An under gravel
filter is a classic down flow submerged bed filter. Submerged bed filters have been used
extensively for small scale aquaculture and backyard water feature
systems. These filters can be operated
in up flow, down flow or cross (horizontal) flow. The classic (old) systems consisted of gravel
with an under drain system. An
improvement to these systems was the addition of air piping underneath. The air was used to 'bump' the filter to
dislodge solids that plugged the gravel and restore full flow. There are numerous problems with these types of filters. Their large size, low void fraction, tendency
to plug and extremely high weight make them expensive to build and
maintain. In general, these old gravel
based systems are not suitable for modern aquaculture.
Modern submerged bed filters are very
efficient, have low head loss and are very easy to build and maintain. The key difference is the type of media and
the water flow path. A modern submerged
filter uses structured media in a horizontal flow mode. This type of biofilter probably comes closer
to the ideal biofilter than any other type.
A typical installation would be configured
similar to a raceway. The filter media is
installed in a long trough. The length
of the flow path can vary based on the retention time required. By using a relatively high velocity, it is
possible to insure plug flow. This is a
big advantage over well mixed systems or systems with short retention
times. If it is not possible to remove
all of the BOD before the biofilter, one will establish different zones in the
filter. As nutrients are absorbed or
removed in the first sections of the filter, different types of organisms will
establish dominance in the zones where they enjoy optimum conditions. There are a variety of ways to configure a
raceway type system. Here are a few
examples
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Submerged filters can operate with or
without aeration. If the flow path is
long and the nutrient loading is high, it is wise to have aeration in the
filter. One of the easier methods is the
traditional aeration system with large silica air stones.
Sometimes it is not possible to use a
raceway type biofilter system. If
existing tanks must be used, it might be easier to build a system with internal
recirculation. The advantage of internal
recirculation is that it increases the velocity of water past the media and
adds oxygen to the water. Increasing the
velocity helps insure a more even distribution of water throughout the filter
media and reduces the possibility of dead zones that are not receiving
nutrients and oxygen. It also helps to
keep particles in suspension. Suspended
solids tend to settle out in areas of low water velocity. This is a problem because accumulations of
solids can become anaerobic and contribute to poor water quality. Here are a couple of examples of internal
recirculation systems. The cone bottom
tank is preferred over the flat bottom tank because any solids that settle out
will be removed immediately.
Figure 7
Figure 8.
There is always the possibility to install
the submerged biofilter media in the culture tank. This has the advantage of saving the cost a
separate vessel and associated piping.
The big disadvantage to this system is that it is difficult to remove
the suspended solids before the water enters the biofilter. Because there are too many different
configurations to draw them all, here is a brief description of a few of the
possibilities.
1. Air
lift the water into one end of a filter designed as a raceway and air lift it
back into the culture tank at the other end.
2. Pump
the water into a particulate filter such as a rotary drum and then flow through
the biofilter.
3. Locate
tubes or columns of packing throughout the culture tank and induce a flow
through them with air stones.
4. Locate
the filter media around the walls of the culture tank and induce a flow up
through the media with air stones.
The number of possible configurations is limited
only by one's imagination.
Submerged filters are excellent choices
for small systems because they are very versatile. They can be located in a separate tank or in
the culture tank. They can be horizontal flow, up flow or down flow. They can be aerated or not. The most important consideration for the design is the even
distribution of water to the packing. It
is very common for submerged filters to be designed as large, flat and thin
sections of packing with water direction being up flow or down flow. There is typically no provision for
distributing the water to all areas of the media. The length of the water path through the
media is very short and the resistance to flow is very low. This is a recipe for disaster. The water flow will short circuit though a
small section of the media
and the rest of the biofilter will become anaerobic.
Ideally the flow path through a submerged
filter should be as long as possible. A
long thin raceway is the best. This type
of biofilter is known as a long path, plug flow submerged filter.
Another possible
alternative is the use of aeration to induce a circulating flow
around a tank. The goal should always be
to provide sufficient velocity through the media to insure a fresh supply of
oxygen and nutrients to the bugs on the surface of the media.
NOTE:
This paper and other useful information for those interested in
aquaculture, aquariums or related topics can be found on the web at
http://www.biofilters.com
References
Greiner, A. D., Timmons, N. B., 1998.
Evaluation of the nitrification rates of microbead
and trickling filters in an intensive recirculating tilapia production
facility. Aquacultural Engineering pp 189 - 200
Kamstra,
A., Van der Heul, J.W., Nijhof,
M., 1998.
Performance and optimization of trickling filters on eel farms. Aquacultural Engineering pp 175-192
Saucier, B., Chen, S., Zhu, S.,
“Nitrification Potential and Oxygen Limitation in Biofilters” presented at the Third International
Conference on Recirulating Aquaculture July 2000.
Timmons, M.B., Losordo,
T. M., 1994. Aquaculture Water reuse Systems: Engineering Design and Management Elsevier
Science B.V.
Zhu and Chen “An experimental study on
nitrification biofilms performances using a series reactor system” Aquacultural
Engineering 1999 Vol 23, p. 245 – 259.
©1995-2003
by L. S. Enterprises. All rights
reserved. No part of this publication may be reproduced or transmitted in any
form or by any means electronic or mechanical, including photocopy, recording, or
any information storage and retrieval system, without permission in writing
from the publisher.
Published
by L. S. Enterprises
PO Box 13925
Gainesville,
FL 32604 USA
Author:
Matt Smith
Office 1-352-379-5626
Mobile
1-239-851-1175
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Email:
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rev.
8/15/2013