Effect of nutrient solution, nitrate-nitrogen concentration, and pH on nitrification rate in Perlite medium

Reconciling water quality parameters in sustainable aquaponic (integrated hydroponic and recirculating aquaculture) systems requires balancing nutrients and pH for the optimal growth of three organisms: the plant, the fish, and the nitrifying bacteria. Nitrifying bacteria convert fish waste into nitrate (NO3-)-nitrogen (N) that may be used by the plants. Fish waste rarely supplies nutrients in adequate amounts for plants without supplementation. Increasing nitrification rate and efficiency would allow greater stocking density for fish and increased nutrient loads for plants. The objective of this research was to determine the nitrification rate response in a perlite trickling biofilter (root growth medium) exposed to hydroponic nutrient solution, varying NO3-N concentrations, and to pH levels optimum for plants (6.5) and nitrification (8.5). The experiment used recirculating tank batch culture and was based on typical startup characteristics for bringing biological filters up to full capacity in aquaculture systems. No significant difference (P value < 0.05) in nitrification rate was found when recirculating system water contained no nutrient solution versus a complete hydroponic nutrient solution or NO3--N concentrations of 0, 100, or 200 mg/L. These results indicate that hydroponic plant nutrient supplementation to concentrations found in plant production systems do not significantly affect nitrification rate in perlite medium. Nitrification was significantly impacted by water pH. Ammonia (NH3) oxidation of initial total ammonia nitrogen (TAN = ammonium (NH4+) - N + NH3- N = 8 mg/L) occurred at the rates of 231 and 300 mu g L-1 d(-1) at pH 6.5 and 400 and 540 mu g L-1 d(-1) at pH 8.5, for experiments 1 and 2, respectively. The rates proceeded 1.75 times faster at pH 8.5 than at pH 6.5. Nitrite (NO2-) oxidation occurred at the rates of 231 and 375 mu g L-(1) d(-1) for pH 6.5 and 267 and 540 mu g L-1 d(-1) for pH 8.5 and proceeded 1.2 and 1.4 times faster, respectively. The increased ammonia oxidation rate (1.75) compared to nitrite oxidation rate (1.3) at pH 8.5 resulted in accumulation of NO2--N to levels near those harmful to plants and fish (observed peaks of 4.2 and 3.8 mg/L NO2--N, respectively). The potential for increased levels of un-ionized ammonia, which are toxic to fish and reduced plant nutrient uptake from micronutrient precipitation, are additional problems associated with pH 8.5. The advantages of increased nitrification efficiency, which averaged 23% in the current trials at the higher pH, when weighed against the potential increased water quality risks to the fish and plant, justify a compromise between pH optima for nitrification and plant production to pH 7 for aquaponic system water. A more flexible management strategy for these systems would be to supplement with plant nutrients, which would permit less reliance on the fish and nitrification to provide optimal plant nutrient levels.