Reconciling pH for ammonia biofiltration in a cucumber/tilapia aquaponics system using a perlite medium

Integrated hydroponic and aquaculture (aquaponic) production requires balancing pH and water quality for the growth of 3 organisms: plants, fish, and nitrifying bacteria. To improve systems integration, a series of trials were conducted to (1) determine the optimum pH for nitrification and evaluate performance of perlite as a biofilter, (2) determine the effect of hydroponic nutrients on nitrification, (3) make predictions about the contribution of plants and nitrifiers to ammonia biofiltration, and (4) establish a reconciling pH for ammonia biofiltration and cucumber yield in aquaponics. Total ammonium nitrogen (TAN) removal and NO2--N accumulation in a trickling perlite biofilter increased as pH increased from 5.5 to 8.5. Aquaponic biofilter TAN removal rates were 19, 31, and 80 g/m3/d for pH 6.0, 7.0 and 8.0, respectively. Nitrification was unaffected by plant nutrients in solution at optimum levels for hydroponic production. Nutrients may be tailored for plant production (with consideration for fish waste contributions) with no adverse impact on nitrifiers. Most probable number (MPN) sampling of biofilter cores indicated that aquaculture control at pH 7.0 with no plants had a higher (0.01% level) number of Nitrosomonas sp. biofilter bacteria compared to treatments containing plants in the biofilter. However, the highest ammonia biofiltration rate was aquaponic production (plant, fish, bacteria) at pH 8.0. pH was a more important factor than bacteria population in the rate of ammonia biofiltration---most likely due to pH induced increases in unionized ammonia, the substrate for the nitrification reaction. Ammonia biofiltration increased 3.7 times at pH 6.0 when bacteria and plants were in the biofilter compared to plants alone. The vigor of tilapia ( Oreochromis niloticus) feeding increased and mortality decreased as water pH increased from 6.0 to 8.0. Early marketable cucumber fruit yield decreased linearly as pH increased from pH 5.0 to pH 8.0. However, total marketable yield was unaffected by pH. The reconciling pH for ammonia biofiltration and cucumber yield should be pH 7.5 to 8.0 given the importance of pH and bacteria to the ammonia biofiltration rate, differences in fish vigor, and given that no difference in total cucumber fruit yield among treatments was found.