Bioassimilation versus nitrification for the removal of ammonia in aquaculture biofilters

Abstract

Three experiments were performed with 3-mm floating plastic beads as substrate in 15 miniature aquaculture biofilters. The biofilters were fed nutrients to simulate aquaculture conditions and manipulated to encourage eitherautotrophicnitrification or heterotrophic bioassimilation of ammonia. Treatments in the first experiment were biofiltration without beads, biofiltration with smooth (as manufactured) beads, and biofiltration with severely scratched beads; each treatment was replicated 5 times. For the second experiment, the treatment involving biofiltration without beads was replaced by smooth-bead biofiltration and subjected to organic loading with glucose to give a C:N molar ratio of 19.9:1. It was hypothesized that this would effect the bioassimilation of the ammonia into heterotropic microbial biomass. The third experiment repeated the second, except the C:N molar ratio was decreased to 6.6:1. Results indicated that the presence of beads (either smooth or rough) in the biofilter significantly enhanced nitrification over that with empty biofilters (p<0.01). Rough beads provided a small, though statistically significant, improvement in nitrification over smooth beads in all three experiments (p<0.01). When compared with biofilters which did not receive an organic load, the glucose-loaded biofilters tended to accumulate more biomass. However, that the difference in the fraction of ammonia converted to biomass was non-significant indicates that a third process, heterotrophic nitrification-denitrification, was occurring as well. A C:N ratio of 19.9:1 greatly accelerated the removal of ammonia without accumulation of nitrate. A C:N ratio of 6.6:1 resulted in a small, non-significant improvement in ammonia conversion and a lesser accumulation of nitrate.