Comparing water treatment topologies in Recirculating Aquaculture Plants

Övrigt konferensbidrag, 2018

Aquaculture, the farming of fish and aquatic crops such as
kelp and algae, is traditionally carried out in natural bod-
ies of water. An alternative is land-based farming in tanks
or raceways, which is particularly attractive when coupled
with water treatment to form a recirculating aquaculture
system (RAS). Benefits compared to traditional farming in
open cages include reduced emissions of nutrients, small or
no risk of escapes, and control of pathogens (Thorarensen
and Farrell, 2011).
Water treatment takes place in a series of mechanical
filters and biological reactors, where particulate and dis-
solved matter is degraded by microorganisms similarly to
how municipal sewage is treated. The biological nature
of recirculating aquaculture systems makes experimental
process development troublesome. Contributing factors in-
clude very long time constants, biological variations, and
concerns for animal welfare. This strongly motivates the
use of dynamic simulations, and for that purpose a RAS
simulator – called FishSim – was developed (Wik et al.,
2009). However, the capabilities of that implementation
were limited by numerical problems.
Using Modelica, a high-level object-oriented language for
dynamic systems modeling (Modelica Association, 2012),
we have developed a new simulation tool for recirculating
aquaculture. Like FishSim, it is based on Activated Sludge
Model 1 (Henze et al., 2000), but this implementation
is numerically well-behaved and robust which allows a
much greater variety in the simulated systems. It is also
significantly faster, even after the models have been ex-
panded with many more features, such as energy balances,
different feeding options, and a separation of autotrophic
bacteria into ammonia-oxidizing and nitrite-oxidizing bac-
teria. Since open-source Modelica tools are available, the
software is also free to use.
Water treatment is central in recirculating aquaculture.
Fish excrete ammonia, which is toxic to them. Aerated
bioreactors are typically employed to remove ammonia
and ammonium via nitrifying autotrophs, which require
low levels of biodegradable organics to thrive. Nitrifica-
tion creates nitrite (also toxic to fish at low levels) and
nitrate, the latter which is removed by water exchange
or denitrification. Denitrifcation conversely requires high
availability of biodegradable organics, but only progresses
rapidly in the absence of oxygen. The treatment systems
often further contain particle filters and UV and/or ozone
treatment against pathogens.
While it is reasonably clear to the industry which compo-
nents should be present in the treatment system, the order
in which they are best employed is still an open question.
In the literature and supplier information material there
is a large number of suggested configurations, but few
studies comparing them. Some guesses can be made based
on elementary chemical reaction engineering, but the very
complex dynamics of the biological treatment leads to high
uncertainty.
Using the simulator, we have investigated and compared
several treatment topologies. Through parameter opti-
mization based on a genetic algorithm (Haupt and Haupt,
2003) the minimal reactor sizes in each configuration was
found which could maintain acceptable levels of ammonia
and nitrate. The resulting sizes are an indicator of which
topology is the most effective.