Treatment of oil-in-water emulsions by coagulation and dissolved-air flotation

The treatment of oil-in-water emulsions containing n-octane (used as simulated wastewater) was investigated by means of dissolved-air flotation jar-tests. The effect of several parameters on flotation efficiency for separation of the emulsified oil was examined, namely, (a) the presence the nonionic surfactant Tween 80, used for the stabilization of the emulsions, (b) the initial pH value of the emulsions, (c) the concentration of chemical additives, such as polyelectrolytes (organic flocculants of cationic or anionic type) or ferric chloride (inorganic coagulant), (d) the concentration of sodium oleate (used as flotation collector) and (e) the recycle ratio. Zeta-potential measurements were also performed in order to interpret the obtained results. The use of polyelectrolytes was not able to effectively treat the studied emulsions, while the addition of ferric chloride and the subsequent application of dissolved-air flotation was found very efficient. At the optimum defined experimental conditions (recycle ratio: 30%, pH: 6, [Fe3+]: 100 mg l−1 and [sodium oleate]: 50 mg l−1) more than 95% of the emulsified oil was effectively separated from an initial concentration of 500 mg l−1.