ABSTRACT
Produced water is water that is produced as a by-product along with oil and gas during oil exploration. A study of the effect of produced water on microalgae was carried out on natural freshwater and marine ecosystems as well as test algae. The study was conducted in two phases: microcosm experiment using the natural microalgal community and as well as test algae in the laboratory. Different concentrations, 0%, 1%, 5%, 10%, 15%, 20% and 25% of produced water were mainly used for both community microalgae in the natural freshwater and marine environment as well as the test microalgae. Growth responses of freshwater and marine community microalgae as well as test algae were determined by gravimetry and optically. Water samples from the freshwater and marine ecosystems were analyzed for physicochemical parameters. Produced water was analyzed and characterized to determine its physicochemical parameters as well using gas chromatography to determine its poly aromatic hydrocarbon (PAH) and total petroleum hydrocarbon (TPH) content. Descriptive statistics, inferential (one-way analysis of variance (ANOVA), t-test) and post-hoc test Duncan multiple replication (DMR) were done using Microsoft excel 2010, paleontological statistics (PAST) and statistical package for social science SPSS 20. The results indicated that different concentrations of produced water affected the growth of both freshwater and marine community microalgae with the optimal growth occurring at 25%. The experiment involving marine test microalgae showed that Nannochloropsis oculata had an optimal growth at 1%, Isochrysis galbana at 25% and Porphyridium cruentum at 25%. For the freshwater test microalgae, Chlorella vulgaris had an optimal growth at 25%, Dictyosphaerium pulchellum at 15% and Coelastrum microsporum at 10%. There was a seasonal trend in the growth response of xxii the marine and freshwater community microalgae. Optimal growth was recorded in the dry season for freshwater community microalgae and wet season for marine community microalgae while the minimal growth was obtained in the wet season for freshwater microalgae and dry season for marine community microalgae, respectively. A comparison of the growth response of the freshwater community microalgae with that of the marine community microalgae using t- test showed significant differences ( p < 0.05) for all the months except December, April and June where there were no significant differences ( p > 0.05). The marine test microalgae grew very well in different concentrations of produced water. Nannochloropsis oculata had the highest yield followed by Isochrysis galbana and Porphyridium cruentum. The freshwater test microalgae also thrived well in produced water. Chlorella vulgaris had the highest yield followed by Dictyosphaerium pulchellum and Coelastrum microsporum. Therefore, the marine microalgae Nannochloropsis oculata, Isochrysis galbana, Porphyridium cruentum and freshwater microalga Chlorella vulgaris can be used in the bioremediation of produced water. A comparative physicochemical analyses of produced water with that of freshwater and seawater showed that the produced water used for this study had higher values for turbidity, alkalinity, phosphate, ammonium nitrogen, manganese, pH and low salinity. Gas chromatography analyses of produced water revealed the presence of high levels of polyaromatic hydrocarbon (PAH) benzo(a)anthracene and benzo(b)fluoranthene: which are known carcinogens and total petroleum hydrocarbon such as kerosene (C15), diesel(C17, C20) and lube oil(C21, C28) at detectable levels
