The current worldwide growth rate o

The current worldwide growth rate of the aquaculture business
(8.9–9.1% per year since the 1970s) is needed in order to cope with the
problem of shortage in protein food supplies, which is particularly
situated in the developing countries (Gutierrez-Wing and Malone,
2006; Matos et al., 2006; Subasinghe, 2005). However, environmental
and economical limitations can hamper this growth. Especially
intensive aquaculture coincides with the pollution of the culture
water by an excess of organic materials and nutrients that are likely to
cause acute toxic effects and long term environmental risks
(Piedrahita, 2003). For long, the most common method for dealing
with this pollution has been the use of continuous replacement of the
pond water with external fresh water (Gutierrez-Wing and Malone,
2006). However, the water volume needed for even small to medium
aquaculture systems can reach up to several hundreds of cubic meters
per day. For instance, penaeid shrimp require about 20 m3 fresh water
per kg shrimp produced (Wang, 2003). For an average farm with a
production of 1000 kg shrimp ha−1 yr−1 and total pond surface of 5 ha,
this corresponds with a water use of ca. 270 m3 day−1
. For a mediumsized
trout raceway system of 140 m3
, even a daily replacement of 100
times the water volume is applied (Maillard et al., 2005). A second
approach is the removal of the major part of the pollutants in the
water as is performed in recirculating aquaculture systems (RAS) with
different kinds of biologically based water treatment systems
(Gutierrez-Wing and Malone, 2006). The amount of water that
needs to be replaced on a daily basis generally is reduced to about
10% of the total water volume (Twarowska et al., 1997). However, this
technique is costly in terms of capital investment. While capital
investment costs for normal flow-through ponds systems are ca. 1.3 €
kg− 1 annual production, they may increase to 5.9 € kg− 1 in
recirculating systems (Gutierrez-Wing and Malone, 2006).