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.

Discharge of produced water into the aquatic environment may release chemicals that are highly toxic to sensitive marine species even at low concentrations, causing bio-degeneration/transformation of the biota. Some metals and hydrocarbons may accumulate in sediments, bio-accumulate in bottom living biological communities which may pose threat to humans and animals. Physicochemical quality of untreated and treated produced water were assessed.Toxicity and pollution levels as well as human health risk assessment of treated and untreated produced water were also determined to ascertain the level of environmental safety vis-à-vis effects of treatment, in order to identify potential environmental concerns from existing treatment practices. Microbial density and concentration of eight heavy metals in produced water samples were assessed following standard procedures and different indices were used to assess the health risk.Concentration (in mg/L) of iron was 6.9 (untreated) and 0.001 (treated wastewater), arsenic 0.001 for both untreated and treated wastewater, zinc (0.002 for both untreated and treated wastewater), mercury (0.001 for both untreated and treated wastewater), chromium (0.001 for both untreated and treated wastewater), cadmium (1.1 for untreated and 0.001 for treated wastewater), lead (0.9 for untreated and 0.001 for treated wastewater) and nickel (0.005 for both untreated and treated wastewater). Further, statistical analysis showed correlation between physicochemical parameters of untreated and treated wastewater as well as heavy metal concentrations. Health risk assessment showed major potential non-carcinogenic risk was via ingestion, with led as the main contributor. Overall non-carcinogenic risk evaluation of produced water showed that humans are not susceptible. Similarly, chromium was the major contributor to the carcinogenic health risk but values for lethal average daily doses and cumulative carcinogenic risk were within permissible limits.

Introduction: The discharge of produced water into the aquatic environment presents a risk to the environment. This is a form of pollution and may release toxicants that are highly noxious to sensitive marine species even at low concentrations, which causes bio-degeneration/transformation and biodiversity loss. Research Gap: There are insufficient literatures on extensive monitoring of the physicochemical and microbial characteristics of produced water. Existing literatures concerning analysis of treated and untreated produced water are not comprehensive with respect to number of physicochemical variables analyzed. Aim: To ascertain if the physicochemical properties and microbial population of produced water were within acceptable regulatoy specifications.  Place and Duration of Study: The study was conducted in the Federal University of Petroleum Resources, Effurun, Delta State, Nigeria from July, 2021 – April, 2022. Methodology: Electrical conductivity, dissolved oxygen, pH, temperature, turbidity, total petroleum hydrocarbons, nitrates, phosphates, sulphates and others were assessed using standard methods. Microbial counts were carried out for total heterotrophic bacteria and hydrocarbon utilizing bacteria using the pour plate method. Results: Values obtained for some key physicochemical parameters monitored in the produced water are as follow:  pH (8.10±0.03 for untreated and 8.27±0.01 for treated); electrical conductivity (35700±11 µs/cm for untreated and 41600±17 µs/cm for treated); total dissolved solids (22848±14mg/l for untreated and 26629±9mg/l for treated); dissolved oxygen (2.05±0.01mg/l for untreated and 4.23±0.03mg/l for treated); biochemical oxygen demand (28.90±0.7mg/l for untreated and 18.4±0.1mg/l for treated); phosphate (2.05±0.01mg/l for untreated and 4.23±0.03mg/l for treated); nitrate (54.82±1.9mg/l for untreated and 50.21±0.9mg/l for treated); total hydrocarbon content (118.00±0.00mg/l for untreated and 34.00±0.00mg/l); total heterotrophic bacteria (4.1e+04 CFU/ml for untreated and 2.8e+04 CFU/ml for treated); and hydrocarbon utilizing bacteria (2.44e+03 CFU/ml for untreated and 1.58e+03 CFU/ml for treated). Statistical analysis of the produced water showed varying forms of correlation between physicochemical parameters of untreated and treated produced water. Conclusion: It is important to assess properties of produced water before disposal into aquatic environment as chronic impact associated with long-term exposures may pose potential ecological risks.