Food security, specifically in water-scarce regions, is a local and global aim that requires innovative solutions. Aquaponics, the integration of a conventional recirculating aquaculture system (RAS) with hydroponics in a symbiotic arrangement, can be such a solution. In aquaponics, fish excretions are assimilated as a nutrient source for vegetable production. As a result, the assimilation of the fish waste by the plants, ‘treat ’ the water, and enables its recirculation back to the fish tank. This practice allows for extremely high efficiency in the use of water and nutrients and greatly limits the discharge of pollutants.
Basic aquaponics systems consist of four major components: fish tank, solid filter, nitrification biofilter, and plant beds. The water is continuously recycled through one loop. In these systems, the suspended solids accumulate in the solid filter (sludge), which can reach up to 50% of fish feed, and are washed out from the system, leading to loss of water and the need for further treatment or potential pollution.
Recently, we developed a novel multi-loop near-zero waste aquaponic system. The system includes separate loops for fish production (RAS) and for plant growth (hydroponic) which facilitate optimal conditions per each crop. In addition, two separate loops are used to treat the solid waste (fish and inedible plant bits) by anaerobic digestion, producing nutrient-rich liquid good for plant growth and energy.
Following a stabilization stage, the system was operated for more than 3 years. Fish stocking density reached approximately 50 kg/m3. Feed (45% protein content) was applied daily at 2% of body weight. Typical fish performance was observed with a survival rate >97% and a feed conversion ratio of 1.33. Lettuce production was up to 5.65 kg/m2, significantly higher than previous reports, largely because of high nutrients reuse efficiency from the anaerobic supernatant. Of the feed carbon, 24.5% was taken up by fish biomass. Fish solid wastes contained 38.2% carbon, of which 91.9% were recovered as biogas (74.5% CH4). Biogas production was 0.84 m3/kg for fish sludge and 0.67 m3/kg for dry plant material. CO2 sequestration was 1.4 higher than the feed carbon, which reduced the system’s carbon footprint by 64%. This study is the first to demonstrate highly efficient fish and plant production with near-zero water and waste discharge and with energy recovery that can potentially supply the system’s energy demand.
