Aquaculture is a rapidly growing industry in Queensland and around Australia. Its dependency on water resources makes it worthwhile considering the option of recycling the water by incorporating biofilters to improve the water quality. Wastewater from a recirculating intensive aquaculture system contains nitrogen compounds such as ammonia, nitrite, and nitrate that can rise to lethal levels and can affect the growth and survival of the aquatic animals reared. Biofilters are widely used to remove these nitrogenous compounds. However the efficiency of the biofilters is directly dependent on the microorganisms that inhabit them. Typically there is a start-up time ranging from a few days to several weeks for the biofilter before it is capable of oxidising the toxic nitrogenous components in the water. In addition, on some occasions, rapid growth of fish leads to a sudden rise in ammonia which natural rates of nitrification cannot
respond to requiring intervention.
In the present work, we consider an effective method that might help reduce the start-up time of the biofilter, or prevent excessive rises in ammonia following growth spurts of the aquatic organisms, by seeding the biofilter with an active culture containing Nitrosomonas europaea and Nitrobacter winogradsky. However, there is a need for developing an effective and reproducible process for the cost-effective production of these cultures. As there is a large variation in kinetic parameters available from the literature reports, we have estimated the kinetic parameters of the two species by conducting batch and continuous cultures and analysing the data assuming Monod and Haldane equations for their growth. The kinetic parameters obtained for Nitrosomonas europaea were
μmax=0.125 h-1, ks=1.61 mg TAN/L, YMx/s=0.2 mg biomass/mg TAN, ki,s=873 mg TAN/L, and ki,p=4238 mg NO2-N/L and those for Nitrobacter winogradsky were μmax,=0.047 h-1 ks=6.32 mg NO2-N/L, YMx/s=0.035 mg biomass/mg NO2-N, ki,s=794 mg NO2-N/L, and ki,p=7031 mg NO3-N/L, where μmax is the maximum specific growth rate, ks is the substrate half-saturation constant, YMx/s
is the true growth yield of the biomass on substrate, ki,s is the substrate inhibition constant, and k,„ is the product inhibition constant of the organisms.
The estimated parameters as above were then made use of in a series of Matlab simulations that described the Nitrosomonas europaea and Nitrobacter winogradsky pure cultures grown in batch and fed-batch modes under inhibition by substrate and product. Fed-batch culture simulations suggested higher values of productivity for Nitrosomonas europaea and Nitrobacter winogradsky could be obtained compared to those obtained through batch culture simulations. Results from the simulations were then validated by comparing appropriate batch and fed-batch fermentation experiments.
In a bioactivity study conducted with cultures of Nitrosomonas europaea and
Nitrobacter winogradsky obtained from fed-batch cultures, elimination of ammonia and nitrite from an open water system was observed. Thus, a high-density culture resulting from these experiments would potentially lead to a cost-effective solution to overcome effects of nitrogen toxicity in the aquaculture industry. Quick establishment of a Nitrosomonas europaea and Nitrobacter winogradsky biofilm on the biofilters will help to handle unexpected ammonia loading arising from growth of aquatic animals or intensive fish cultivation. Several advantages will result from the present study including lower fish mortality through their lower susceptibility to diseases following better control of water ammonia/ nitrite levels.