Nitrogen treatment in closed-system tilapia cultivation using submerged nitrifying biofilter

Abstract

This work focused on a development of simple yet effective aquaculture systems for an adoption among budget-limited Thai farmers. The first part of the work investigated the parameters that influenced an acclimation of nitrifying biofilters while the second part tested for the performance of the proposed aquaculture systems integrated with acclimated nitrifying biofilters at different initial aquaculture densities. Shrimp diet addition was able to establish nitrification on Biocord[superscript TM] biofilters within a period of 3-4 weeks. An initial nitrifying rate of Biocord[superscript TM] biofilters after acclimating period was determined at 24.1 mg N m[superscript -2] day[superscript -1]. The zero-water exchanged tilapia cultivation was carried out in the proposed aquaculture systems integrated with shrimp diet acclimated Biocord[superscript TM] biofilters. Design of the proposed system combined aquaculture production, water treatment, and solid-liquid separation within single tank, thereby offering a possibility of process simplification and reduction in operational costs. Series of experiments indicated that the proposed aquaculture systems were capable of maintaining ammonium and nitrite concentrations below 1.0 mg N L[superscript -1] despite increasing inorganic nitrogen loadings more than 30-folds from 5-18.3 mg N L[superscript -1] despite increasing inorganic nitrogen loadings more than 3-folds from 5-18.3 mg N L[superscript -1] day[superscript -1]. Efficient performance of the systems was linked to a proper biofilter preparation to achieve fully nitrification before an initial deployment was taken placed as well as microbial immobilization that successfully retain slow-growth nitrifying facteria in the systems. Besides nitrification, direct assimilation of inorganic nitrogen and heterotrophic denitrification were other mechanisms responsible for inorganic nitrogen control. Teh excellent capability of the chosen Biocord[superscript TM] biofilters in retaining suspended solids was crucial for the onset of heterotrophic denitrification. Frequent suspended solid removal from Biocord[superscript TM] biofilters was able to reduce the likelihood of heterotrophic denitrification and sulfate reduction, which produced toxic H₂S. Regular cleaning of Biocord[superscript TM] biofilters also enhanced nitrifying rates in comparison to unclean biofilters. With biofilter cleaning (i.e., once every 2 weeks), the proposed aquaculture systems were able to increase their harvesting density to as high as 25-30 kg m[superscript -3], which is comparable to unenvironmental-friendly caged production. In this work, nitrification rates obtained (i.e., 27.6-41.2 mg N m[superscript -2] day[superscript -1]) were in the low-range compared to other works. In spite of that, the proposed aquaculture systems should remain attractive for Thai farmers due to their design simplicity.