A comparison of buffering species and regimes applied within a research-scale, recirculating aquaponics system

Abstract

Murray Cod, Maccullochella peelii peelii, and Green Oak lettuce, Lactuca sativa, were used to test for differences between four buffering regimes in a research-scale, recirculating aquaponic system consisting of twelve, separate 140L aquaponic units. In the aquaponic system, where plant nutrients were supplied from fish wastes and plants stripped nutrients from the water before it was returned to the fish, the buffers tested were potassium bicarbonate, calcium hydroxide, mixed (an equal mixture of potassium bicarbonate and calcium hydroxide) and a control where the buffer used was sodium bicarbonate. Murray Cod had FCRs and biomass gains that were statistically identical in all treatments (SGR = 1.19%/replicate/day; FCR = 0.86). Lettuce yields were determined over a 21-day trial, with the potassium treatment (yield of 4.75 kg/m2) and mixed treatment (yield of 5.00 kg/m2) providing the highest production. Potassium and mixed treatments also had lower levels of nitrate accumulation (potassium treatment = 7.80 mg/L; mixed treatment = 8.77 mg/L) and the lowest levels of water use (potassium treatment = 1.59 L/day; mixed treatment = 1.60 L/day) compared with the other test treatment and the control. Mixed and calcium treatments yielded the lowest phosphate accumulations (mixed treatment = 2.81 mg/L; calcium treatment = 2.60 mg/L), but the calcium treatment may have been affected by calcium–phosphate complexing which may have led to false identifiable phosphate concentrations. For dissolved oxygen, pH and conductivity, no statistical differences were observed. Overall, results suggest that potassium-based buffer salts were superior to the other buffers tested in the research-scale aquaponic system tested.