Does the biofloc size matter to the nitrification process in Biofloc Technology (BFT) systems?

Abstract

In addition to environmental factors, particle size can interfere with the nitrification process because smaller flocs are less efficient in terms of ammonia and nitrite oxidation. The objective of the present study was to evaluate, in three experiments, the effect of the biofloc size on the nitrification process during the production of the white shrimp Litopenaeus vannamei in a BFT system. In all experiments, 1% of biofloc inoculum from production tanks was used. Water samples were taken to photograph and measure the flocs. Measurements of temperature, salinity, nitrogen compounds and total suspended solids were also carried out in the experiments. In experiment 1, the flocs were separated by sifting the water through mesh nets of 50, 150 and 300 μm in size. In addition, there was a control treatment with integral flocs. In experiment 2, the bioflocs were separated into 300 and 600 μm size classes and were compared with an integral control. Experiment 3 aimed to evaluate the nitrification process in treatments with flocs smaller than 150 μm, in flocs without handling (Control), and in the treatment where flocs were partially filtered in a 50 μm mesh for five days. This treatment resulted in a reduction in the particle size; however, when the meshes were later removed, and the biofloc size increased again. Different from our original hypothesis, the results of these three experiments indicate that the size of the biofloc, as well as the amount of total suspended solids, do not affect the nitrification process in the BFT system. However, all treatments that included manipulation through sifting showed a delay or a decrease in the nitrification activity. Thus, it is likely that, more than the particle size or floc abundance, the rupture of the floc structure affects the distribution and interaction of nitrifying bacteria with reflections in the nitrification process.