| A finite element, finite difference model was developed to predict nutrient and microbial dynamics in recirculating aquaculture systems. Total ammonia nitrogen, nitrite, nitrate, chemical oxygen demand, and dissolved oxygen concentrations are predicted in addition to growth and decay of algae, Nitrosomonas, Nitrobacter, denitrifying heterotrophs, and aerobic heterotrophs. Inputs to the model include temperature, photosynthetically available radiation, feed application rate, pH, aeration/mixing energy, water exchange configuration, water exchange rate, tilapia biomass, and dissolved oxygen concentration. Up to 10 exchanged units may be included, user-definable as continuous stirred tank reactors or sequencing batch reactors. Demonstration of the model's utility was demonstrated through calibration to three distinct shrimp production systems. The calibrated model was then applied to several simulated systems in order to evaluate optimal design and management strategies. Development of the model led to new understandings and proposals of interactions occurring in intensive recirculating aquaculture systems. Classical wastewater treatment models for heterotrophic growth are not valid due to the carbon cycling that takes place in the absence of solids removal. Model predictions show that mixing energy provided through aeration plays a critical role in defining aerobic heterotroph viability and overall microorganism dominance. A proposal is provided to account for mixing by shrimp, which can also contribute to definition of microbial dominance. The model-predicted mixed liquor suspended solids concentration is used to generate a shading term for light-limited algal growth. Simulations indicate that algal productivity is desirable due to its reduction of aeration requirements, but the unpredictability of cloudy periods suggests that algal contribution to intensive aquaculture systems should be limited. Outputs from the model indicate that compromised water quality is always linked to instability of system inputs. For the simulations conducted, model outputs show that cost of water treatment is lowest and water quality is highest for a staggered stocking, continuously loaded system with aerobic heterotroph dominance. |
