Recycling Technology
By Dr. Bent Urup, UNI-Aqua A/S, October 2004
Contents:
- Definition of recycling
- Water treatment concept
- Introduction to recirculation chemistry
1) Definition of recycling
In general, there is often confusion about what is meant by recycling
and what is meant by reuse.
Recycling is when the water quality is in control of the water treatment system,
not by the inlet water. In recycling systems ammonia is transferred into nitrate
and normally less than 1% of the water has to be replaced with new water per cycle.
Moderate recycling only requires a mechanical filtration, CO2 removal,
Oxygenation and biofilter for degradation of dissolved organic matter and nitrification.
Which means that ammonia is transformed into nitrate, which is then diluted out of the system.
Depending on fish species, 50-200 mg nitrate (N) (885mg NO3-) is acceptable.
High levels of recirculation will require a denitrification filter and potentially a means
to filter phosphorus as well. In the denitrification filter, which is an anaerobic filter,
the nitrate is transferred into free nitrogen. If we include denitrification filters,
we can reach recycling levels where we need less than 0.1% new water and where the
water leaving the system can potentially be reduced to the water content in the sludge,
plus what leaves through evaporation.
2) Water treatment concept
Leaving the tanks, the water will undergo mechanical filtration to remove as much
organic matter as possible, then CO2 will be removed and ammonia
transferred into nitrate. Finally, Oxygen will be added before the water
re-enters the tanks. UV systems are often applied to control the pathogen situation.
Depending on technology, fish species and chemical parameters required, either
a single loop or a multi loop system is applied.
In an old style single loop system, all the water is mechanically filtered,
directed through the biofilter then through CO2 stripping and further
through oxygenation before the water returns to the fish tanks.
In the multi loop system (see figure 1) all the water is mechanically filtered, then the flow
through CO2 stripping, biofilter and potentially denitrification filter
and phosphorus removal are individually dimensioned/optimised to match the
required water chemical specifications.
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Figure 1. Illustration of a multi loop system.
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3) Introduction to recycling chemistry
Food and energy conversion in flatfish species as an example (Figure 2):
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Figure 2. Mass flow for flatfish in recirculation.
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The values are of a general nature, as the actual values will always depend
on a number of parameters related to the actual production situation.
The point is that whatever proportion of the feed that is not absorbed by
the fish will be released, and somehow has to be removed by the water
treatment system.
The key chemical components in recirculation are nitrogen, carbon dioxide,
oxygen and phosphorus.
Nitrogen
Nitrogen will mainly be excreted by the fish as ammonia. Approx. 80% of the
ammonia excreted by the animals will be excreted over the gills, while the rest
will be excreted in the faeces. The ammonia excreted over the gills
will be unionised- NH3, but due to the pH and the high pKa
value of the NH3 - NH4+ buffer system, the
majority will immediately be converted to NH4+, which is not
nearly as toxic to the fish, as it does not pass back into the animals
across the gill epithelia.
NH3 is quite toxic and for the majority of fish species,
the level of NH3 should not be above 0.02 mg/litre under
normal production conditions. If the levels are very stable,
levels twice as high can be accepted as peak values.
Further increases will cause reduced growth of the fish.
The problem with ammonia is more serious in seawater because of the high pH,
see fig. below. Any sudden increase in pH has to be avoided - pH has to be
maintained at a very stable level.
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FIGURE 3. The NH3/NH4+ equilibrium in water.
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The NH3 - NH4+ equilibrium is however not only
dependant on the pH, the salinity also has some influence as well.
Depending on the source, you will find small differences in measured amounts.
Recirculation facilities with poor biofiltration (nitrification) performance,
permanently operate with low pH levels to avoid the consequence of high ammonia
levels. High ammonia levels will alternatively cause poor feeding rate, poor
feed conversion and potentially mortality.
Reducing the pH is though not a suitable way to control the ammonia situation.
The low pH is normally not very optimal for the fish, further the biofilter
will function poorly and finally it is a very risky strategy. Farms applying
this strategy have unexpectedly lost large amounts of fish do to sharp
increases in pH level caused by accident, typically addition of more base than needed.
Carbon Dioxide
The second most important waste product is carbon dioxide.
It is excreted over the gills. In addition to that further
degradation of feed and faeces will also produce free CO2.
The fish use oxygen to oxidise fats, sugars and proteins.
The carbon chains are broken to free energy and the carbon atoms
are then released as CO2. As a total breakdown- energy
derived from fat will produce approx. 0.71 mole of CO2
per mole of O2 used. When proteins are the energy
source 0.8 moles of CO2 will be produced per
mole of O2; whereas using glucose as an energy source
there will be 1 mole of CO2 produced for each
mole of O2.
In seawater, we have to pay special attention to the carbon
dioxide because of the pH dependant chemical equilibrium with
HCO3- and CO32-.
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Figure 4. The carbon dioxide system in water.
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In seawater the majority of CO2 will be transformed to
HCO3- (less so in freshwater due to its lower pH),
which is not as big of a problem for the fish as is the CO2,
although it still seems to influence the capacity of the blood to
transport the CO2 from the tissue to the gills.
Clearly HCO3- must not be allowed to build up in system.
The equilibrium between CO2 and HCO3-
in seawater gives some benefits but also some disadvantages.
The benefit is that in a pump-through system the carbon dioxide will
be converted to HCO3- which is less harmful
to the fish. The disadvantage is that it gets more difficult to
get out of the water again in a recycled system. This means if we
don't have an effective system for removing carbon dioxide, we will
have a slow build up of CO2/HCO3-
to unacceptable levels. Optimally, the recirculation system should
keep the levels of CO2 below 10-20 mg/l. This is
clearly one of the major difficulties in marine recirculation technologies,
as higher levels will cause reduction in growth.
Aqua-Partners ApS/UNI-Aqua A/S have developed a system where CO2
levels can be maintained below 10 mg/litre in seawater (also in freshwater),
where recycled plants in general, operate with levels between 60 and 100 mg CO2/l.
High CO2 levels in freshwater systems should also be kept below
20 mg and should never exceed 30 mg; as growth will be reduced and
feed conversion poor.
For salmon smolt produced in CO2 rich water, it seems
the growth is considerably lower even when the fish have been transferred to the cages.
Phosphorus
The production of phosphorus will only at very high recycling levels
be a problem to the fish, if using foam separators, it can
even have some benefits in supporting the foam creation.
Too high levels will have some impact on CO2 removal.
If needed, a phosphorus filter is a relatively simple and well-proven piece of technology.
BOD5
The biological oxygen demand (BOD5) is a very important
factor and one of the reasons why it is important to effectively remove
particulate matter in the mechanical filter. From figure 2 it can be seen
that by removing particulate matter, we also remove nitrogen and phosphorus.
If we don't remove the particulate matter, it alternatively has to be
broken down by biological means in the production water, resulting
in the consumption of oxygen and production of carbon dioxide,
which we then have to add/remove. It is important to notice that the level
of BOD5 in the biological filter has to be relatively low before the filter
starts working at converting ammonia products. The simplest way and the first step in
making the bio filter work effectively, is by having effective mechanical filtration.
In seawater, due to its high pH and consequently high proportion of unionised ammonia,
it is necessary for the nitrification to work without interruption.
Therefore, a biofilter structure normally applied in freshwater,
can not be used for Seawater. Opposite a biofilter designed for
seawater will work very well with freshwater.
Oxygen
The fish will use oxygen for oxidizing their food; see above under carbon dioxide.
We can add oxygen to the animals by adding new water with a natural oxygen content.
Further, we can aerate the water thereby transferring oxygen from the atmosphere
to the production water. Finally we can also use pure oxygen,
which can be introduced in several ways.
Super-saturation of the inlet water to the tanks is often used.
This is often a suitable method, however in the tanks the water should only
be slightly super-saturated with oxygen, as too high levels
(typical levels above 110% -120% oxygen saturation) will reduce growth rates.
In cases where CO2 levels are high, oxygen super-saturation
is often used to improve growth results, the system will then operate
with too high carbon dioxide/bicarbonate levels- as well as O2
levels as these two parameters interact. This is though symptom treatment.
It will give better results and less stress to the fish, if both parameters
are under control, and managed within an optimal range.
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