Dyes are often recalcitrant organic molecules that produce a colour change and contribute to the organic load and toxicity of textile industrial wastewater. Untreated effluent from such sources is harmful to aquatic life in the rivers and lakes due to reduced light penetration and the presence of highly toxic metal complex dyes. The use of alum as flocculant/coagulant in wastewater treatment is not encouraged as it induces Alzheimer’s disease in humans and results in the production of large amounts of sludge. Therefore, the development of safe and biodegradable flocculating agents that will minimize environmental and health risks may be considered as an important issue in wastewater treatment. Bioflocculants are extracellular polymers synthesized by living cells. In this study, bacterial bioflocculants were assessed for their ability to remove dyes from textile wastewater as well as reducing the microbial load in untreated river water. The bacteria were isolated from a wastewater treatment plant and identified using standard biochemical tests as well as the analysis of their 16S rDNA gene sequences. Six bacterial isolates were identified viz. Staphylococcus aureus, Pseudomonas plecoglossicida, Pseudomonas pseudoalcaligenes, Exiguobacterium acetylicum, Bacillus subtilis, and Klebsiella terrigena. The flocculating activities of the bioflocculants produced by these isolates were characterized. The effect of temperature, pH, cations and bioflocculant concentration on the removal of dyes, kaolin clay and microbial load was also determined. The amount of bioflocculants produced by the bacterial isolates ranged between 5 and 27.66 g/l. According to the findings of the present study, bacterial bioflocculants were composed of carbohydrates, proteins, uronic acid, and hexosamine in varying quantities. The bioflocculants were effective to varying degrees in removing the dyes in aqueous solution, in particular whale dye, medi-blue, fawn dye and mixed dyes, with a decolourization efficiency ranging between 20-99.9%. Decolourization efficiency was influenced by the bioflocculant concentration, pH, temperature, and cations. The bacterial bioflocculants were also capable of reducing both the kaolin clay and the microbial load from river water. The flocculating activity ranged between 2.395–3.709 OD-1 while up to 70.84% of kaolin clay and 99% of the microbial load from the river water was removed. The efficiency of kaolin clay flocculation increased with higher concentration of bacterial bioflocculants. The optimum pH for the flocculating activity was observed between 6 and 9. The best flocculating activity was observed at 28oC. Divalent cations such as Mg2+ and Mn2+ improved the flocculation while salts such as K2HPO4, CH2COONa, and Na2CO3 did not. The findings of this study strongly suggest that microbial bioflocculants could provide a promising alternative to replace or supplement the physical and chemical treatment processes of river water and textile industry effluent.

Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2008.