MINERALIZATION OF FISH WASTE IN DECOUPLED AQUAPONICS – THE FIRST STEPS OF A COMBINED AEROBIC & ANOXIC PROCESS PROTOTYPE

INTRO: In commercial aquaponics, waste mineralization is a key in eliminating aquaculture discharges while increasing horticultural yields. Mineralization is a process that allows organic matter digestion by microorganism activity.  A two step process is involved; solid crushing then; organic to inorganic matter conversion. In 2017, preliminary tests (20 buckets during 2 months) focused on dissolved oxygen effects in these two processes. This revealed that solid crushing was more effective in anoxia conditions and the conversion to inorganic compounds was higher in aerobic conditions. Thus, a mineralization system prototype integrating a dual treatment phase was developped (Figure 1.).

METHOD: Ten consecutive tests were conducted with the same input of fish waste but without any system sterilization between each experiment in order to promote organisms colonisation. The first test lasted 24h, the second 48h and so on until the last test, which lasted 10 days, for a total of 65 days (24h between each test). Thus, tests are not properly comparable, but still indicate an noteworthy trend. Each input was 50L of fish sludge collected from the bottom drain of circular tanks containing 120±11 kg of rainbow trout O. mykiss fed daily with 1.2 kg of Skretting LP pellets. After each experiment, anoxic and aerobic tanks were cleaned manualy to collect the residual solids which were then dried in an oven. Electrical conductivity (EC) and Total Dissolved Solids (TDS) were analysed in the supernatant water that goes to plant culture.

RESULTS: These experiments indicate that the mineralization process seems to reach a threshold on EC and TDS of the supernatant water after 4 and 6 days of treatment respectively (Figure 2). The mass of dried solids collected from the aerobic tank seems constant while it tends to decrease in the anoxic tank (Figure 3). Thus, results suggest a roughly 70% in efficiency for the conversion of organic solids into inorganic ions after 10 days in this two-step mineralization system prototype.