Impact of operating disturbances on ammonia removal by trickling and submerged-upflow biofilters for intensive recirculating aquaculture

Biofilters are regularly used in freshwater aquaculture production systems to remove ammonia and nitrite and are the only economically feasible ammonia removal devices in saltwater systems. Such filters are very sensitive to environmental conditions and can be severely compromised when conditions vary too far from optimum and, in fact, may lose removal efficiency or “crash” altogether. Very little experimental data is available on optimum biofilter operating conditions, and what is available is usually not universally applicable due to the empirical nature of the data.; Six identical 5.23 L biofilters, i.e., three trickling and three submerged upflow filters, were operated at predetermined water quality conditions (pH-7.5, temperature –25°C, salinity-5 ppt, TAN-1mg/L), and then perturbed to simulate a number of possible operating disturbances, e.g. increased fish load, flushing of culture tanks, closing off of valves. Each disturbance was either a controlled continuous or impulse change in ammonia concentration, temperature, pH or salinity. The maximum ranges of the variation in water quality were pH 6–9, TAN 0.5–4.0 mg/L, temperature 15–35°C, and salinity 0–35 ppt. After each disturbance, the water quality was immediately returned to the predetermined (baseline) values. The ammonia removal across the filters was monitored during the “crash” and then during the recovery period for loss and subsequent restoration of biofilter removal efficiency.; Impulse shocks were the most detrimental disturbances to both filter types—especially low pH (−20% to −40% efficiency), low salinity (−15% to −40% efficiency), and low temperature (−15% efficiency). Temperature (+10% efficiency) and TAN concentration (+0.3 to +0.5 mg/L additional TAN removal) increases appear to be beneficial to the biofilter. Gradual increase in salinity also reduced biofilter efficiency (−10% to −30% efficiency). Gradual increases in temperature (∼+5% efficiency) and TAN (approximately +1 mg/L additional TAN removal at the highest concentration for the slowest rate of addition) appear to be beneficial to the biofilters. Gradually raising of pH or TAN concentration did not appear to significantly change the biofilter efficiency. When the filters were returned to baseline conditions, baseline nitrification rates resumed within one to two hours after cessation of the shock.