Nitrogen removal from aquaculture pond water by heterotrophic nitrogen assimilation in lab-scale sequencing batch reactors
Introduction
In intensive land-based aquaculture systems, densities of animal biomass can easily reach high values. For example, fish is cultured at densities up to 90 kg of living biomass m−3 (Conte, 2004). In these feed-driven pond systems, only 20–30% of the nitrogen input is converted into harvestable products (Azim et al., 2003, van Dam et al., 2002). The remainder is excreted and typically accumulates as inorganic nitrogen within closed systems, reaching concentrations toxic to aquatic culture species (Avnimelech, 2007). In contrast to recirculating aquaculture systems (RAS) which have a limited water exchange of about 10% of the pond volume each day (Twarowska et al., 1997), bio-flocs technology (BFT) with zero or minimal water exchange was developed to cope with this accumulation of toxic nitrogen species.
BFT is based on the assimilation of inorganic nitrogen species (ammonia, nitrite and nitrate) by the microbial community present within the pond water. This can be accomplished by aiming at a high C/N ratio in the water (Azim et al., 2007). Either the use of food formulated with a lower protein content (Avnimelech, 1999, Hargreaves, 2006) or supplying additional carbon sources like glucose or starch to the pond water (Crab et al., 2007) can be used as control methods. The bacterial biomass grown on the metabolized organic substrate can be taken up by the fish or shrimp as an additional food source. As such, the nutrients excreted after primary consumption by the aquaculture species and the nutrients originating from non-eaten food are recycled into harvestable microbial biomass. It was shown that a doubling of the nitrogen use by tilapia could be obtained through application of this technology (Avnimelech, 2006). Overall, BFT offers the advantages of lower impact on the environment due to lower external water requirements, the removal of toxic inorganic nitrogen species and the in situ production of additional feed for the culture species. It is estimated that the decrease in feed costs per kg annually produced live weight using BFT can be in the order of 15%. Based on investment and operation costs, nitrogen control can be performed at about half the price of normal pond aquaculture with an external trickling filter (De Schryver et al., 2008).
The microbial metabolism for the decomposition of organic matter necessitates the continuous presence of oxygen (Azim et al., 2007). The addition of carbonaceous substrate to the water may result in sudden and temporary lower dissolved oxygen (DO) concentrations. Anecdotal data suggest that such DO drops can result in fish mortality (Colt, 2006). Secondly, excessive turbidity may have negative effects on sensitive fish species and not all are adaptable to growing in turbid water (Avnimelech, 2006). Although preference should be given to basic BFT, i.e., co-culture of aquaculture species and heterotrophic bacterial biomass within the same solution, the considerations mentioned above sometimes require the application of BFT in a compartmental design as was described by Avnimelech (2006). In such applications, the culturing of fish and the microbial production are performed in separate compartments. This allows for a better control of turbidity, oxygenation and return of microbial protein back into the fish compartment.
In this lab-scale research study, the treatment of simulated fish pond water was explored by means of sequencing batch reactors (SBRs) as an external compartment of the bio-flocs technology. This approach was set up with the goals (1) to provide a simple reactor design for application in intensive aquaculture, (2) to determine the effect of the C/N ratio on the assimilation of the ammonia nitrogen in microbial biomass rather than nitrifying it into nitrate and (3) to assess floc quality (protein and poly-β-hydroxybutyrate) and floc morphology.
Section snippets
Reactor design and operation
The experiments were performed in two 4.5 L sequencing batch reactors. Both had an internal diameter of 8 cm and a liquid filled height of 60 cm (=3 L working volume) and were maintained at a temperature of 24–26 °C (optimal temperature for tilapia culture). The microbial biomass was intensively mixed by means of two fine bubbling diffusive airstones. The first was placed at the bottom of the reactor and the second was placed at half of the water height. Each reactor was supplied with air by means
Reactor performance for nitrogen and carbon removal
An increase in the C/N ratio resulted in a significantly higher nitrogen removal efficiency for both SBRs (Table 2). The maximum concentrations in nitrite and nitrate nitrogen detectable at the end of the sampled cycles were 0.4 ± 0.9 mg L−1 and 0.4 ± 0.2 mg L−1 for the glycerol SBR whereas these were 1.3 ± 0.8 mg L−1 and 0.5 ± 0.5 mg L−1 for the acetate SBR. In additional batch tests, no autotrophic nitrification was observed (data not shown). Without the presence of carbon, the biomass of both the glycerol
Reactor performance for nitrogen and carbon removal
In BFT, intensive heterotrophic growth in the culture water results in turbidity and may induce variations in DO levels due to increased microbial metabolism resulting from carbon dosing. Such varying DO levels may have a negative influence on the aquaculture organisms. The implementation of external heterotrophic growth reactors in which high turbidity and large variations in DO do not influence the species in the aquaculture pond, may offer a solution. In such a concept, the water treated in
Conclusions
In this study, the lab-scale SBRs envisaged as external growth reactors for bio-flocs technology aquaculture ponds were able to provide adequate ammonia removal. Significant nitrogen assimilation was exhibited by the heterotrophic microbial biomass present using both glycerol and acetate as carbon source. This could be accomplished without leaving excessive amounts of carbon in the effluent, provided that an optimal C/N ratio ranging between 10 and 15 was maintained. Moreover, the quality of
Acknowledgements
This work was funded by the “Fonds voor Wetenschappelijk Onderzoek” in Flanders on the project “Probiont-induced functional responses in aquatic organisms”. The authors would also like to thank ir. Bart De Gusseme, Dr. ir. Tuba Hande Ergüder and ir. Siegfried Vlaeminck for the critical reading of the manuscript and helpful suggestions.
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