Algal bioproducts derived from suspended solids in intensive land-based aquaculture.

Land-based aquaculture produces suspended solids in culture pond and settlement pond waters that could be harvested as a bioresource. Suspended solids were quantified, characterised and harvested from these two sources to assess their suitability for conversion to bioproducts. The suspended solids of settlement ponds were less concentrated (87.6±24.7mgL(-1)) than those of culture ponds (131.8±8.8mgL(-1)), but had a higher concentration of microalgae (27.5±4.0%) and consequently higher particulate organic carbon (24.8±4.7%) and particulate nitrogen (4.0±0.8%). The microalgal community also differed between sources with a higher concentration of fatty acids in the biomass from settlement ponds. Consequently, biochar produced from biomass harvested from settlement ponds was higher in organic carbon and nitrogen, with a lower cation exchange capacity. In conclusion, we characterised a renewable and potentially valuable bioresource for algal bioproducts derived from suspended solids in intensive land-based aquaculture.

Denitrification and anammox in tropical aquaculture settlement ponds: an isotope tracer approach for evaluating N2 production.

Settlement ponds are used to treat aquaculture discharge water by removing nutrients through physical (settling) and biological (microbial transformation) processes. Nutrient removal through settling has been quantified, however, the occurrence of, and potential for microbial nitrogen (N) removal is largely unknown in these systems. Therefore, isotope tracer techniques were used to measure potential rates of denitrification and anaerobic ammonium oxidation (anammox) in the sediment of settlement ponds in tropical aquaculture systems. Dinitrogen gas (N(2)) was produced in all ponds, although potential rates were low (0-7.07 nmol N cm(-3) h(-1)) relative to other aquatic systems. Denitrification was the main driver of N(2) production, with anammox only detected in two of the four ponds. No correlations were detected between the measured sediment variables (total organic carbon, total nitrogen, iron, manganese, sulphur and phosphorous) and denitrification or anammox. Furthermore, denitrification was not carbon limited as the addition of particulate organic matter (paired t-Test; P = 0.350, n = 3) or methanol (paired t-Test; P = 0.744, n = 3) did not stimulate production of N(2). A simple mass balance model showed that only 2.5% of added fixed N was removed in the studied settlement ponds through the denitrification and anammox processes. It is recommended that settlement ponds be used in conjunction with additional technologies (i.e. constructed wetlands or biological reactors) to enhance N(2) production and N removal from aquaculture wastewater.