Abstract

ARREDONDO-FIGUEROA, J. L. et al. Ammonia and nitrite removal rates in a closed recirculating-water system, under three load rates of rainbow trout Oncorhynchus mykiss. Rev. Mex. Ing. Quím [online]. 2007, vol.6, n.3, pp.301-308. ISSN 1665-2738.

Nitrification and denitrification rates of inorganic nitrogen were studied in a closed recirculating-water system, comparing three load rates of rainbow trout Oncorhynchus mykiss (89, 156 and 194 kg in each tank with two repetitions). Six self-cleaning water circular fish tanks with a volume of 4.3 m³ were used, maintaining a 3.94 m³/day of average flow rate and constant aeration. A total of 371 rainbow trout, 524 ± 8 g initial wet weight were introduced in the system and fed with a commercial feed that contained 38% of protein. A total study time of 44 days was divided into three phases of 14, 17 and 13 days according to the load fish rate. Temperature, dissolved oxygen, pH, total ammonia nitrogen (TAN), un-ionized ammonia, nitrite and nitrate were daily evaluated at four monitoring sites: fish tank (FT), settling tank (ST), biofilter (B) and reconditioning tank (RT). Water physicochemical characteristics and their fluctuations played an important role in treatment efficiency. Water temperature varied between 18 °C and 20.5 °C and dissolved oxygen from 4.6 to 7.7 mg/l. The lowest values of these two variables were registered in the ST where all wastes accumulate. No significant differences (p<0.05) were observed in pH values (8.3-8.6). These conditions allowed good nitrification and denitrification rates. TAN varied from 0.2 to 1.96 mg/l; however, this value was 80% lower in the outlet (RT) as compared to the inlet (ST). The load fish rate caused a significant difference (p<0.05) in TAN and non-ionized ammonia in the FT with the lowest value for 89 kg load density as compared to 156 and 194 kg respectively. Conversely, nitrite concentration did not show a significant difference (p>0.05) among load fish rate. Nitrate concentration had an accumulative tendency at 156 kg load rate batch up to 30 days with a further decrease. The results showed that a reduction of load rate did not change apparently the equilibrium of bacteria population. Therefore, it is possible to control variables such as TAN, non-ionized ammonia and nitrite concentration, hence maintaining an adequate water quality for rainbow trout.

Keywords : closed recirculating system; denitrification; load density; nitrification rate; rainbow trout.