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?
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 three types of aerobic
microorganisms that colonize biofilters for aquaculture. Heterotrophic bacteria utilize the dissolved
carbonaceous material as their food source.
Nitrosomonos sp. bacteria utilize ammonia as a
food source and produce nitrite as a waste product. 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)
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. There is however, a
couple possible uses for anaerobic filters in aquaculture. Theoretically, one could use an anaerobic
filter to convert the nitrates into N2.
However, this is a difficult process to control and it is generally less
expensive to replace a small amount of water to remove nitrates.
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 the
floatation or bubble seperator. 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.
Although they are not typicaly
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
filter.
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 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 a
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 are a good
example of efficient use of resources. In addition to commercially valuable
plants grown in hydroponics systems, aquatic plants such as hydrilla,
cattails, water hyacinths and duck weed can be used to absorb nitrates and
phosphorus from waste water.
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" 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 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 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 more sophisticated
systems incorporate a mechanical stirring devices such
as a propeller on a shaft. Periodically
the water flow is shut off and the bed of beads stirred 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 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 heterotrophs
at the expense of nitrobacter sp. and nitrospira sp.
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 that are
manufactured. 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 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 some systems, air is forced into the filter with a fan. However, most 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 today, the best packing is
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 it's flexibility and ease of use. Structured media can be used to build a
convenient 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.
The most important requirement in the design of
any trickling filter is a good water distribution system at the top. There are
two common 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.
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 and
restore full flow. The problems with these filters is the large size, low void
fraction, tendency to plug and extremely high weight. 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 cross (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.
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 Filters
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
P.O.
Box 51033
Fort
Myers, FL 33994 USA
Author:
Matt Smith
Tel# 239-543-1258
Fax# 239-543-7308
Email:
mattsmith@biofilters.com
rev.
3/04/03