Effects of Biofilter/Culture Tank Volume Ratios on Productivity of a Recirculating Fish/Vegetable Co-Culture System

Abstract

The effects of four biofilter volume (BFV)/culture tank volume ratios (0.67/1, 1.00/1, 1.50/1, and 2.25/1) on biofilter function were examined in a recirculating fish/vegetable production system in a greenhouse. Sand beds served as biofilters, as substrate for vegetable growth, and as location for decomposition of waste solids. No fertilizer was used. Three experiments were conducted over the course of one year. In Experiment 1, as the BF V/tank volume ratio increased, total ammoniacal nitrogen (TAN) and nitrite concentrations decreased (9.0 to 3.6 mg/L and 0.39 to 0.20 mg/L, respectively), and biomass increase over the culture period and oxygen levels increased significantly (13.34 to 16.03 kg/m³ and 6.03 to 6.47 mg/L, respectively). pH was maintained at 5.8-6.2 without the addition of lime. Yield per plant of the tomato variety ‘Laura’ tended to decrease (3.4 to 2.3 kg/plant), and yield per plot increased (13.6 to 31.6 kg/plant) with increasing BFV/tank ratio. In Experiment 2, the system was operated for 42 days without plants. pH dropped rapidly to near 4.0. Cucumbers were then planted, and weekly additions of lime and CaO were made. Significantly less CaO was required to achieve target pH in systems with the largest BFV/tank ratios. pH levels conducive to good plant growth were only slowly stabilized, and cucumber yields were erratic. TAN and nitrite levels were not measured, but fish grew well (5.2 to 7.2 kg/m³ with increasing BFV/tank ratio). By Experiment 3, with the tomato variety ‘Kewalo,’ TAN and nitrite concentrations decreased from 0.96 to 0.48 mg/L and from 0.06 to 0.02 mg/L, respectively, with increasing BFV/tank ratio, and in the latter part of the experiment, pH was stabilized at 6.3-6.5 without lime. Yield/plant decreased from 5.0 to 2.4 kg/plant and yield per plot increased from 19.9 to 33.1 kg/plot with increasing BFV/tank ratio. Daily water exchanges averaged 2.8%. Nutrient concentrations of the irrigation water after a year's operation were low overall. Although plants showed no deficiency or toxicity symptoms, K⁺ was found to be low and Zn⁺⁺ high relative to other ions. No clogging was observed in the sand beds. Carbon measurements ± SEM of the sand medium at the wastewater inlet of the smallest and largest BFV/tank ratio systems were 0.23 ±0.03%, and 0.15 ±0.01%, respectively. Nitrogen was below detectable levels (<0.04%). The enhanced biofil-ter/culture tank ratios used here resulted in a functionally well balanced fish/vegetable co-culture system. While needing refinement, this design represents a step towards a highly productive, low-tech system with efficient use of water, chemical, and labor resources.