Biofilters
By Dr. Bent Urup, UNI-Aqua A/S, October 2004
Summary:
A biofilter has potentially three purposes:
- To reduce the level of dissolved organic matter, through bacterial activity.
- To transform ammonia into Nitrate (nitrification).
- To maintain bacterial stability and a disease free environment, by letting the
bacteria in the biofilter out compete pathogenic bacteria.
Several biofilter technologies for use in aquaculture systems exist.
Trickling filters, Fluidised filters, Submerged single chamber
filters and Multistep submerged filters.
At a low pH, the ammonia concentration will be less toxic to the
fish and nitrification (transfer of ammonia into nitrate) will be
less important.
The higher the pH in the system, the more important is the stability
in the performance of the biofilter as fluctuations in ammonia
levels at a high pH will cause growth reduction or mortality.
Only the multi-step filters are suitable/safe for use in
seawater and in freshwater systems, where the pH may reach
levels higher than 6.
The purpose of the biofilter:
The purpose of the biofilter depends on the water quality
desired in the system.
Basically a biofilter can be used for:
- Reducing the level of dissolved organic load in the system.
- Transfer of ammonia into Nitrate.
- Maintaining control of the pathogenic bacteria.
In some recirculation systems, the biofilter is first of
all used for reducing the amount of dissolved organic matter,
which also has an indirect positive effect on reducing
pathogens in the system.
Yet, in other systems, the biofilters are dimensioned/designed
to handle organic load and for the effective transfer of
ammonia into nitrate.
As well as being specifically designed to handle organic
matter and harmful ammonia levels, the biofilter, if designed
and dimensioned correctly, can be a very effective tool to
handle the pathogen situation in the facility as a whole.
1) Reducing the dissolved organic load
First of all the biofilter has to remove all the organic matter
which is dissolved in the water. In the early history of
recirculation technology, all waste was removed in the biofilters,
but with the introduction of effective mechanical filters, a much
smaller fraction of the organic waste has to be removed in the biofilters.
It is important that the organic waste is removed, for two main reasons:
- If the organic waste is not removed in the biofilters,
then the degradation will take place in the fish tanks,
causing a heavy bacterial load, including increased oxygen consumption,
disease risk and poor water quality with low visibility.
- If the organic level is too high, nitrification will
not take place in the biofilter or at least nitrification will
be at a low activity level and total ammonia levels will increase.
2) Nitrification
Three principal waste products from the fish are; (1) organic matter,
(2) carbon dioxide and (3) ammonia. If ammonia is not constantly removed,
it will build up and cause growth reduction or even mortality.
The level where ammonia starts to affect the fish is in the range
of 0.02 - 0.04 mg NH3/ltr and at a level of 0.1 - 0.4 mg/litre it will
potentially cause mortality.
At what concentration total ammonia levels will reach depends on
the pH; as there is a direct relationship between pH and the amount
of NH3 and NH4+ , of which only NH3 is toxic to the fish.
The following figures show the effect that pH level has on the amount
of total ammonia in the system when NH3 is 0.02 mg/litre (16 degree C):
| pH | NH3 | NH3/NH4(total ammonia) |
| 8 | 0.02 mg/litre | 0.7 mg/litre |
| 7 | 0.02 mg/litre | 6.8 mg/litre |
| 6 | 0.02 mg/litre | 68 mg/litre |
| 5 | 0.02 mg/litre | 670 mg/litre |
This means that the higher the pH level of the system, the better
the biofilter has to function due to the lower concentration of total
ammonia that has to be maintained.
On the other hand, to obtain a high nitrification activity, the pH
should be relatively high, optimally above pH 7.
If operating with a pH level below 5.5- 6, a nitrification system is
maybe not absolutely needed from the point of ammonia control
(not considering the pathogen control of the biofilter), as ammonia
then can be diluted through the new water inlet.
And the reality is, that in facilities with old technology, the
biofilter is often mainly a heterotrophic biofilter, which only breaks
down dissolved organic material.
The figures above also explain why a system designed/dimensioned for
freshwater does not function for seawater, whereas, a system designed
for seawater works very well with freshwater. Natural seawater has
a pH level of approx 8.3 whereas the pH level in fresh water
typically would be in the range of pH 6-7.
It is possible to operate a recirculation system without nitrification
in the biofilter, only if the pH is constantly low, and the
total ammonia level in the system is diluted through water exchange.
It requires that the pH is permanently below 5.5 - 6 or lower
depending on the recirculation degree, and in reality, many
existing recirculation systems have to operate with a pH
close to 5 to avoid mortality and obtain a reasonable
feeding activity and feed conversion - It is often told
that the fish like low pH in recirculation systems - this
is obviously wrong, they can just not handle high levels of NH3.
Such a low pH is not optimal to the fish and as a consequence
many systems, especially those with circular tanks, where only
a modest water exchange is possible, can be difficult to
operate with low levels of CO2, even if the system
has an effective CO2 stripper.
A low pH is in general reducing the effectiveness of the biofilter,
so that a relatively larger biofilter is needed. This situation
creates an ineffective system overall.
The situation with a low pH and high total ammonia levels also
involves a high risk.
It has happened in such systems, that sudden accidental increase
of pH has caused high mortality due to rapidly increased levels of NH3.
Finally, for some species such as salmon smolt, it is not
desirable to produce the smolt at a low pH; as this,
especially in the combination with high CO2
levels, will cause reduced growth, maybe not so much in
the smolt facility. But especially when the fish have
been transferred to the sea cages where the pH is
typically 8.2-8.4.
Often this growth reduction, will have important economical
consequences, as the fish when harvested might not have
reached a size to obtain premium price.
3) Pathogen Control
The pathogen control of the biofilter is related to:
- The biological stability of the biofilter.
- The minimum retention time of the production water
in the biofilter.
- The media surface area per m3 in the biofilter.
Some filters have retention times of only a few minutes or even
less than a minute, this includes all trickling types of
filters, such types of filters have little impact on
the pathogen situation in the system.
In some filters all the water is mixed, so that a fraction
of the water will leave the filter quickly and a fraction
will have a prolonged retention time. Filters that
incorporate submerged single chamber biofilters will also
typically fluctuate between heterotrophic and nitrification activity.
The pathogen control influenced by such filter types will be
important but not optimal.
Optimal pathogen control will require a multi-flow system,
with a multistep biofilter dimensioned with a low enough flow
to secure sufficient retention time of the water for pathogen
control but also have the necessary flow to remove the desired
amount of ammonia. The remaining flow required to secure a
suitable water exchange in the tanks, supply sufficient oxygen
and remove CO2, bypasses the biofilter.
The multistep biofilter, where the water will pass through a
series of sections, will minimise mixing within the filter,
and secure a certain minimum retention time in the biofilter
over the entire water column.
How does the biofilter work:
Normally, the overall purpose of the biofilter is to create
an environment where a biological transformation of the ammonia
products into nitrate can take place; this process is
called nitrification.
The biofilter consists of a tank with material having a high
surface/volume ratio where the necessary bacteria can
concentrate in high numbers. However, it is not only the
total area that counts, but also how efficiently the water
containing the ammonia products is distributed over the
surface of the filter material and the contact time/retention
time in the biofilter.
Optimally, in the first part of the filter, organic material,
which has not been removed by the mechanical filter, will be
broken down through bacterial activity. This activity
out-competes the relatively low energy yielding
nitrification process. Consequently, the bacteria involved
in the nitrification won't be carrying out the nitrification
while the organic load is too high.
If the water is mixing too much within the filter, then
it is either necessary to control the feeding so that the
organic load will never inhibit the nitrification, or
alternatively, the biological activity within the filter
will fluctuate between heterotrophic activity and nitrification.
Trickling filters and fluidised filters have no separation
between these processes. Submerged filters, have some
separation but multi-step filters have
optimal separation.
The breakdown of organic compounds will consume oxygen
and produce carbon dioxide. Aeration is therefore needed
in the first part of the filter to recover oxygen levels
and to increase water velocity within the filter.
Generally speaking, the nitrification process can be
separated into two processes:
- NH4+ + 1½ O2 →
2H+ + NO2- + H2O
- NO2- + ½O2 →
NO3-
The first process is mainly carried out by a group of bacteria called
Nitrosomonas, and the second process by a group
of bacteria called Nitrobacter.
Both groups are chemolitho-autotrophic, which means that they extract
energy from the above chemical reactions and mainly use the energy to
build carbohydrates from fixed carbon dioxide.
Both processes use oxygen, so to get the processes working optimally the
oxygen saturation in the entire filter should be above 3-4 mg/l. Levels
below 2 mg/l will be risky, as then anaerobic processes can take place,
which could result in toxic compounds- this is more dangerous in seawater
as compared to freshwater, mainly because of the higher level of
sulphur (creation of H2S). It is due to this same reason
that we often prefer not to use denitrification in seawater
(anaerobic process where NO3- is reduced to
N2O, NO, and N2).
We have to dilute the system to rule out of problems of high
nitrate levels or we can add an anaerobic Nitrate filter, which
will transform Nitrate into free nitrogen.
During upstart of the biofilter high levels of ammonia will
initially be seen, followed by a high level of nitrite before
both these levels drop to an acceptable level. A steady
increase in nitrate will then be observed.
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Figure 1. Temporal distribution of ammonia, nitrite and nitrate during biofilter start-up.
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The thinner the bacterial skin on the biofilter material
the more efficient it is. An old, thick bacterial layer is
inefficient and behaves unpredictably. Following the first
start-up of the biofilter, it will typically work most
efficiently at approximately five weeks of age.
A cleaning schedule will be applied for the filter type used,
to secure its optimal performance.
To avoid risk of harmful effects cause by nitrite levels, salt
can be added to the water, which will prevent the nitrite
from entering the fish through the gills.
Different types of biofilters:
In the following we will focus on the four biofilter concepts
normally applied in aquaculture.
Trickling filter:
The trickling filter consists basically of a volume of media,
normally out of plastic or stones, which is not submerged.
The water will then be pumped to the top of this filter and
will trickle down the filter media.
The advantages of this filter type, is that the investment is
low and the filter is also good for removal of CO2.
The down side is, that the contact time between water and
media is short, which has a number of consequences:
- The effectiveness of this kind of filter is typically
only in the range of 30% of the nitrification capacity
per m2 of a submerged filter.
- Nitrification activity will fluctuate highly, as
will then the ammonia levels.
- High organic levels will inhibit the nitrification,
and the very limited contact time will typically cause
the filter to switch to heterotrophic activity, making
nitrification potentially almost non-existent.
Pathogen control is related to separation of bacterial
activity and especially to retention time before the
water re-enters the loop. Regarding pathogen control,
the trickling filter is the worst possible solution,
as there is no separation of the biological processes
and the retention time is minimal; often less than 30 seconds.
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Figure 2. Trickling filter.
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Fluidised filter:
The fluidised filter is a modern filter type, with a
potentially very high heterotrophic and nitrification
rate per m2.
It is basically a container with a floating media,
which is stirred/moved in the container, normally by
using air. Special tank designs have been developed
to optimise this moving of the media. The air secures
the oxygen supply for the bacterial processes and
further creates an effective contact between the
media and the water. Due to the moving media, the
filter is continuously self-cleaning allowing the
bacterial film to remain thin and effective.
The disadvantage here is that there is no separation
of the water in this type of filter, which means that
the biofilter in a normal situation will switch
between nitrification and heterotrophic activity.
Where only organic removal is required, for example
if the system is supposed to operate at very low pH only,
this biofilter type can be a very good solution.
Energy wise it is cheaper to operate than the trickling filter,
but more expensive than the submerged filter with fixed filter bed.
The filter is generally causing a high turbidity in the water,
which is seldom desirable.
In UNI-Aqua we are in general a little reluctant to use the
fluidised filter in spite of its high performance per m2,
as the instability of its performance makes it to difficult to dimension.
We would normally have to guarantee a certain water quality,
which is to risky with this biofilter concept.
The safety margin when dimensioning such a filter would
have to be so high that the benefit of the high
average performance would be lost.
Single chamber submerged filter:
The Single chamber submerged filter, unlike the fluidised filter,
contains media that does not move. It is a fixed media,
often but not always a type of material called bio-blocks is used.
Contact between the water and the media is secured through aeration,
which is also securing the oxygen supply for the bacterial
processes in the filter. Aeration is also cleaning the filter,
although this type of filter is normally not fully self-cleaning.
Theoretically this filter should work well. If the inlet is at the
bottom of the filter, then the lower part of the filter should deal
with heterotrophic activity and the last part of the filter,
when the organic load has been reduced, should deal with nitrification.
The problem is though, that the aeration which is necessary
to secure effective contact between media and water,
and for oxygenation, is also mixing the water in the filter.
So even though the bacterial activity is better
vertical structured, compared to the fluidised filter,
the filter will still switch between heterotrophic
and nitrification activity causing some fluctuation in
the ammonia level. This has little effect if your total
ammonia level is already high, which can be acceptable
with low pH levels. But if the pH is high, which will require
operation with a low total ammonia level, such fluctuations
are dangerous. Even if not lethal, the fluctuations will
then influence the growth and feed conversion of the fish.
The single chamber submerged filter is in general a very
effective filter, which is also relatively easy to dimension.
The filter can be used safely for operation in systems
at pH below 6. At higher pH, when using a single chamber
submerged filter, a much bigger filter would be required
and then a multi-step filter would be a much better option.
UNI-Aqua is using the UNI-Aqua media for the single chamber
submerged filters. This is a well-proven biofilter media,
with a surface area of nearly 600 m2 per m3.
It has been used with success for salmon smolt in the recirculated
facilities on the Faroe Islands, where the majority of the
farms are equipped with The UNI-Aqua media and biofilter technology.
In Denmark, more than 30% of all eels produced in Denmark,
are produced with this filter media and the UNI-Aqua
biofilter single chamber submerged biofilter technology.
Multi-step submerged filter:
The multi-step submerged filter is constructed as a series
of single chamber submerged filters. It is integrating
the multi-step concept developed by Aqua-Partners and
the biofilter technology UNI-Aqua bought after the closure
of Cimbria Aquatec. Further improved now into a long lasting
solution, which in reality out-competes all other technologies.
The concept is that each chamber can be individually optimised
for the bacterial activity that will take place in the chamber.
By allowing the water to pass through a number of sections/chambers,
it can be ensured that the water will not be mixed; thereby
eliminating the risk that organic rich water will
interfere with the nitrification.
The multi-step filter is the most stable filter technology
available today because it ensures minimal fluctuations in
ammonia level and as a result is very suitable for
operation with high pH levels.
This filter has a high nitrification capacity per m2
filter area, even at low total ammonia levels. As mixing will
not take place, there will be a more specialized bacterial
film in each section of the filter, which can continuously
work fully effective.
The pathogen control is optimal, as the retention time
will be long and almost the same for the entire water flow.
The various groups of bacteria in the different sections
of the filter will effectively out-compete the
pathogenic bacteria. For reasons unknown, somehow
this filter type also seems to defeat viruses in the system.
The disadvantage is that the multi-step filter, is slightly
more expensive to build than the single chamber submerged filter.
UNI-Aqua is using three different bio media types in the multi
step biofilter, including the UNI-Aqua media with a surface
area of nearly 600 m2 per m3 - the same media
which is also used by UNI-Aqua for single step filters.
This bio-media has been developed over a period of more
than 10 years, to stand the stress for a long life in
the biofilter. It has been further designed to have
the characteristics to ensure a high surface area
and at the same time, not congregating particle matter,
that could cause the risk of blocking up the biofilter.
The bio-media is held in a fixed position during operation
and will only move when the filter is deep rinsed.
Weekly rinsing is simple. It takes 30 minutes, by turning
a number of valves. Each section of the filter is deep
rinsed approximately once every second month.
Even deep rinsing will have very little effect on the capacity
of the filter, as only a single section in a filter will be
deep rinsed during the same week. Neither will the weekly
rinsing of the filter have any measurable effect.
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Figure 3. Multi-step biofilter. Biofilter designed by
Dr. B. Urup in operation for
production of the flatfish
halibut in Norway - 99.5% recirculation,
900 kg of feed
per day capacity.
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