POTENTIAL OF MARINE MICROALGAE IN WASTEWATER TREATMENT AND SIMULTANEOUS BIOFUEL FEEDSTOCK

Abstract

Rapid population growth, urbanization and industrialization have resulted in increased use of water, discharge of wastewater; massive use of fossil fuels and depletion of this energy source. Discharge of wastewater into the ocean introduces, high loads of degradable organic matter, suspended particulate matter and nutrients. In this study marine microalgae from a eutrophic environment were used to evaluate and demonstrate their potential in wastewater bioremediation and production of biodiesel. Nutrients analysis was done using a flow injection analyser, oil was extracted gravimetrically and biodiesel fatty acid composition was determined using a GC-MS instrument. The effects of physicochemical parameters on abundance and distribution of microalgae was determined in the study sites. Microalgae composition in Mombasa (Tudor and Makupa Creeks) comprised of 97 species while Gazi Bay, a relatively uncontaminated environment, had 79 species. 10 classes were recorded in the study sites namely Bacillariophyceae, Dinophyceae, Cyanophyceae, Coscinodiscophyceae, Mediophyceae, Fragilariophyceae, Euglenophyceae, Chlorophyceae, Dictochophyceae and Zygnematophyceae. The abundance of microalgae in Mombasa ranged from 905±112.5 to 26442±375 cells/L while in Gazi Bay it ranged from 942±50 to 14990±427 cells/L. The concentration levels of nutrients in the Mombasa sampling sites were higher compared to Gazi Bay. In Gazi Bay, the concentration of phosphates and nitrates ranged beween ranged between 0.19±0.01 to 0.35±0.07 µg/L and 0.22±0.01 to 0.36±0.1 µg/L respectively. In Mombasa concentration of phosphates and nitrates ranged beween 0.95±0.79 to 406.0±11.91 µg/L and 4.08±0.74 µg/L to 427.32±17.11 µg/L respectively. Microalgae from Tudor and Makupa Creek were co-cultured in wastewater in the laboratory so as to establish optimal conditions for effective removal of nitrates and phosphates, and production of biomass. The dominant genera of microalgae in the co-cultures were cyanobacteria (Oscillatoria), chlorophyte (Chlorella) and diatoms (Entomoneis). It was established that the co-cultures dominated by Oscillatoria thrived well in 20:80 wastewater:seawater (v/v), Chlorella in 40:60 wastewater:seawater (v/v) and Entomoneis had high biomass productivity at 20:80 wastewater:seawater (v/v). The pH range was between 7.83 to 8.34 for all cultures. Co cultures containing Oscillatoria had the highest percentage decrease in the levels of phosphates with a 93.63% decrease after 14 days and corresponding decrease in nitrates of 92.70% by the 7th day. Co-cultures dominated by Chlorella had the highest ammonium removal within the first 7 days (84.67%). The Biomass productivity was highest in the co-cultures dominated by Oscillatoria (0.155±0.07 gL-1day-1) but was lowest for those dominated by Entomoneis (0.086±0.05 gL-1day-1). Co-cultures containing Chlorella had the highest oil content (55.7±2%), however, amount of oil extracted varied with method of extraction. The biodiesel produced from the microalgae biomass had a high percentage composition of palmitic acid (C16:0) (51.4- 60%) and stearic acid (C18:0) (17.1-30%). These fatty acids were present in all xix microalgae biodiesel samples, an indication of good biodiesel properties. Overall, biodiesel made from co-cultures containing Oscillatoria and Entomoneis had the best quality biodiesel in terms of high MUFA and saturated fatty acids, low levels of acid value and specific gravity. This study has established that marine microalgae are viable for the bioremediation of nutrients in wastewater and feedstock for production of biodiesel. Key word: marine microalgae, wastewater treatment, biodiesel, nutrients bioremediation