Nutrient reclamation from brackish and marine aquaculture wastewaters as fish feed additives using algal-bacterial granular sludge technology

Abstract

The potential of algal-bacterial granular sludge (ABGS) technology for converting the nutrient elements in brackish (1.5 % salinity, R1) and marine (3 % salinity, R2) aquaculture wastewaters into nutrient-rich fish feed products were evaluated. During 140 days’ operation, the average removal efficiencies of permanganate index (COD_Mn), total inorganic nitrogen (TIN) and total phosphorus were 98.2 %, 69.6 %, and 64.9 % in R1, 92.8 %, 65.2 %, and 57.4 % in R2, respectively. An increased salinity from 1.5 % to 3 % greatly inhibits the nitrification–denitrification pathway, especially the nitrification process, and the relative abundances of nitrification and denitrification bacteria decreased by 64.2 % and 12.9 %, respectively. In comparison, the ammonium assimilation metabolism was improved at the higher salinity of 3 %, and the relative abundance of microorganisms involved in biosynthesis of amino acids increased by 9.4 %. At the end of the experiment, the contents of crude proteins in the biomass of R1 and R2 were 33.1 % and 41.3 % (dry mass based), and the corresponding conversion efficiencies of TIN to biomass organic nitrogen in R1 and R2 were 5.5 % and 7.1 %, respectively. Both granules cultivating with brackish and marine aquaculture wastewaters had high crude protein contents and desirable amino acid compositions, and they are rich in Mg, K, P, and Fe elements. The nutrient-rich ABGS can be utilized in-situ as low-cost and carbon neutral feed additives for fish.