This reactor is a cylindric anoxic

This reactor is a cylindric anoxic (no free dissolved oxygen; NOx present) reactor fed with dissolved and particulate faecal organic waste, bacterial flocs and inorganic compounds trapped by the solids removal unit. The waste flow enters the reactor at the bottom centre. The up flow velocity in the reactor is designed to be smaller than the settling velocity of the major fraction of the particulate waste in order to create a sludge bed at the bottom. In the sludge bed the faecal particulate waste is digested by the denitrifying bacteria.
As an example, since 2005, a denitrification reactor using internal carbon source, was integrated into a conventional RAS (Fig. 2) in The Netherlands. In a 600 MT/year Nile tilapia Oreochromis niloticus RAS farm the water exchange rate was as low as 30 L/kg feed, corresponding to 99% recirculation (Martins et al., 2009b). Compared to a conventional RAS, this latest generation RAS thus reduces water consumption, and NO3 and organic matter discharge. The costs for installation and operation of the denitrification reactor are outweighed by the reduction in costs for discharge to the local sewer, groundwater permits restricting groundwater extraction at one production location and the increasing energy costs for heating groundwater to 28◦C(Martins et al., 2009b).
Considering the nutrient balance before and after on-farm implementation of denitrification on an hypothetical 100 MT/year tilapia farm (Eding et al., 2009), performance of a 100 MT/year
tilapia RAS with and without denitrification was compared for the sustainability parameters nutrient utilization efficiency (%),resource use and waste discharge per kg fish produced (Table 4). It can be seen that the RAS with denitrification has substantially lower requirements for heat, water and bicarbonate. Although the RAS with denitrification has somewhat higher requirements for electricity, oxygen and labour (and investments), the actual production
costs per kg harvested fish are approximately 10% lower than for the conventional RAS. Waste discharge is reduced by integration of denitrification by 81% for nitrogen (N), 59% for chemical oxygen demand (COD), 61% for total oxygen demand (TOD), 30% for CO2 and 58% for total dissolved solids (TDS).
Integrating a USBR in a conventional RAS allows to (1) reduce the make-up water volume necessary for NO3control, (2) reduce NO2 discharge, (3) reduce energy consumption due to heat production by the bacterial biomass in the reactor and a reduction in the volume of make-up water that needs to be heated, (4) concentrate and reduce the drum filter solids flow, by digesting the solids in situ, reducing fees for discharge of TAN, NO3, organic nitrogen,
and organic matter (measured as COD), and (5) increase alkalinity allowing a pH neutral fish culture operation.
Despite the considerable advantages of introducing a denitrification reactor in a conventional RAS, its use in commercial farming is still limited. Major reasons include the higher investments, the required expertise and the accumulation of TDS on farm or the alternative use of an external carbon source. In most EU countries (to our knowledge only the Netherlands is an exception), the economical feasibility of using a denitrification reactor still has to be
demonstrated.
One of its major contributions to environmental sustainability of integrating denitrification in RAS is the reduction in water use. However, a small water exchange rate might also create problems. As pointed out by Martins et al. (2009a,b)such reduction may lead to an accumulation of growth inhibiting factors originating from the fish (e.g. cortisol), bacteria (metabolites) and feed (metals). Using a bioassay,Martins et al. (2009a)showed that with a low water
exchange of 30 L/kg feed, the accumulation of phosphate (PO4), NO3 and of the heavy metals arsenic and copper is likely to impair the embryonic and larval development of common carp and therefore deserves further research. Also,Davidson et al. (2009)suggested a negative impact on survival of reducing water refreshment rates in trout cultured in RAS, mainly due to the accumulation of copper. Nevertheless, in grow-out,Good et al. (2009)andMartins et al .(2009b)showed no impact on growth performance of fish cultured in low water exchange RAS. In turbot RAS no growth retardation could be detected compared to re-use of flow-through systems during long term experiments (about 550 days) running those systems under commercial conditions (Schram et al., 2009).