WWW.WA S .ORG • WORLD AQUACULTURE • MARCH 2022 45 higher pH compared to the 1:1 and 5:1 treatments. It is likely the ORP was driven down by microorganisms while the higher pH may be explained by production of alkalinity. A significant difference in alkalinity was found between the control treatment and the carbonadded treatments, but not among the carbon-addition treatments (Table 1); however, the higher alkalinity in the 3:1 treatment, while not significant compared to other treatments, may have been sufficiently high to cause the difference in pH. An acclimation process analogous to that which occurs during the development of nitrification was observed, with nitrite peaking at the halfway point of the study (Fig. 5). The nitrite spike was highest in the 5:1 treatment, followed by the 1:1 treatment and then the 3:1 treatment. The 14-d nitrite peak concentrations were 179, 159, and 130 mg/L respectively. Nitrite accumulation at the end of the experiment followed this same trend with the 5:1 treatment having a final average of 49 mg/L, the 1:1 having a final average of 5.8 mg/L and the 3:1 treatment having a final average of 0.1 mg/L. No apparent denitrification or nitrite spikes or accumulation was observed in the control treatment. Qualitatively, distinct differences in bubble formation and solids accumulation on the water surface were observed. Little to 95 percent ethanol (Beam Suntory Inc., Chicago, IL) and no carbon was added to the control treatment. Water for the experiment was collected from a local, commercial, white shrimp Litopenaeus vannamei, hybrid biofloc-RAS production system. The water had an initial salinity of 15 g/L and an initial nitrate concentration of 296 mg/L which comprised all but 3 mg/L of the total inorganic nitrogen (TIN). The experiment was conducted in 150-L tanks, each with a polystyrene cover (Fig. 2) for the prevention of evaporation, airwater oxygen exchange and light penetration, which may inhibit denitrification. Water temperature, dissolved oxygen (DO), pH, salinity, and oxidation-reduction potential (ORP) were measured once daily, while nitrate, nitrite, total ammonia nitrogen (TAN), sulfide, alkalinity, TSS/VSS, turbidity, and phosphate were measured weekly. This study was designed to end when TIN concentrations approached 0 mg/L. Daily ethanol additions to each tank were calculated on the basis of the most recent TIN concentration. The duration of the experiment was 27 days, when nearly total nitrate removal was measured in some tanks. Denitrification in all C:N ratio treatments was significantly greater than the control (Figs. 3 and 4), with the 1:1 and 3:1 achieving significantly higher rates of nitrate removal than the 5:1 treatment (average reductions were 287, 294 and 280 mg/L TIN, respectively). The 3:1 treatment had significantly lower ORP, and ( C O N T I N U E D O N P A G E 4 6 ) FIGURE 2. Tanks used in the denitrification experiment, with ORP meters visible (Photo: Mark E. Johannemann). TABLE 1. Water quality results in a study to evaluate the effect of input C:N ratio on denitrification in a hybrid biofloc-RAS. Nitrite and nitrate values are final measurements, all other parameters shown as weekly averages. Tr ea tmen t Turb i d i t y TAN Ni t r i t e Ni t ra t e Al ka l i n i t y Pho s pha t e Su l f i de (NTU) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) Control 1c 0.2 0.2d 298a 129b 30.7 0.0b 1:1 24a 0.2 5.8b 2.3c 535a 22.5 0.1a 3:1 26a 0.2 0.1c 2.3c 597a 22.7 0.1a 5:1 18b 0.2 48.6a 15.5b 498a 23.0 0.0ab D i f f e r e n t s up e r s c r i p t l e t t e r s d e no t e s i g n i f i c an t d i f f e r e n c e s amon g t r e a tme n t s .
RkJQdWJsaXNoZXIy MjExNDY=