A feasibility study on using a simplified mass balance to predict potassium in integrated freshwater aquaculture

This study assessed whether simplified mass balance calculations that neglect retention could be used to predict the concentration of potassium (hypothesis: yes) and phosphorus (hypothesis: no) in aquaculture systems, irrespective of feed ingredient choice Three feeds with distinct formulations and graded levels of potassium and phosphorus were fed to Nile tilapia ( Oreochromis niloticus ) in recirculating aquaculture systems. All inputs (feed, water, caustic) and outputs (water, sludge, fish) were throughout the 91-day experiment. Mass balance calculations with and without retention term (fish, sludge) were then used for prediction. The steady state concentration based on a simplified mass balance was systematically about 20% lower than predicted. No unambiguous statement could be made about phosphorus due to unaccounted inputs that led to considerable deviation of observed from predicted concentrations. The results of this study indicate that the simplified mass balance might be appropriate to estimate potassium concentrations. Additional studies are, however, necessary to validate the outcomes for a wider range of feed ingredients and fish species. • K dynamics are independent of feed ingredient type but depend on K mass fraction in ingredient. • CSTR model based on simplified mass balance ( m out = m in ) can predict dissolved K accurately. • 80-90% of K entering the system can be recovered in dissolved form. • K management could improve the efficiency of aquaponics or constructed wetlands. • P concentrations cannot be predicted using the same, simple approach.