Fig. 1
Water flow in one replicate of three aquaponic systems as operated between 2017 and 2018 with an anaerobic (marked brown) and aerobic loop consisting of an aquaculture (marked blue) and hydroponic component (marked green), and sampling points (marked with red dots): Using gravity, water from the fish tank was continuously flowing through the solids removal unit to the biofilter. In the solids removal unit i.e. drum filter, the solids (fish feces and feed residues) were mechanically separated from the clear system water with a 40 μm mesh drum filter. A circulation pump (1) was continuously (5 m3 h−1) pumping water from the biofilter through the UV and oxygenation zone. The computer-controlled valve B, opened every 5 min for 2 min, resulting in a water flow of 0.5 m3 h− 1 to the sump. A level sensor-controlled pump (3) then pumped the water back to the fish tank keeping the water level in the sump stable. A different pump (2) was continually pumping (0.36 m3 h− 1) the water to the hydroponic raft table and from there back to the sump over the drainage point. To maintain a constant water level and to control water consumption in the system, fresh tap water was added to the system via a mechanically controlled water valve and analogous water counter. During the automatic drum filter rinsing with clear system water, small amounts of water with solids were rinsed into the solid thickening unit i.e. radial flow settler. Three times per week, 7 L of thickened settled sludge was manually removed and added to the anaerobic digester, at the same time, 7 L of the supernatant from anaerobic digester was added back to the radial flow settler, which returned water full of nutrients to the main water loop of the system