Toxicity Assessment of Produced Water Using Nitrosomonas sp.

Aim: Most studies on the effect of produced water are centred on the effects on the physicochemical parameters of the recipient environment, but not much has been conducted on biogeochemical processes such as the nitrogen cycle. This study aims to determine the median lethal concentration (LC50) of the produced water that would cause mortality of 50% of the exposed population of Nitrosomonas sp. (test organism) which is involved in the ammonification step of the Nitrogen cycle.

Study Design: For the purpose of this study, acute toxicity testing was done for 48 hours to determine the effect of the untreated and treated produced wastewater on the test organism. The LC50 is the median lethal concentration that would cause mortality of 50% of the exposed population of the test organisms.

Methodology: Toxicity effects of treated and untreated produced wastewater were determined following standard procedures against the test organism (Nitrosomonas sp). The resultant output of the probit analysis was generated from the SPSS statistical programme.

Results: Results showed that the mean LC50 for the untreated produced wastewater at the 48-hour test period of bioassay was 3.27mg/L, while the mean LC50 for the treated wastewater was found to be 6.69mg/L. The mean percentage mortality of Nitrosomonas sp. for the untreated produced waster water ranged from 37%(0.01mg/l) to 90%(100mg/l), while for the treated, percentage mortality ranged from 27%(0.01mg/l) to 84%(100mg/l) at the end of the 48 hour test duration.

Conclusion: Toxicity of treated and untreated wastewater against Nitrosomonas sp increased with time and decreased considerably with treatment, corresponding to positive correlation as observed statistically between toxicants with respect to concentration and time of exposure for untreated and treated wastewater against Nitrosomonas sp. These results indicate a likey inteference of the nitrogen cycle by contamination of untreated produced water in the environment and by extension impairment of the productivity of the environment.