Separation of oily emulsion by flotation and coalescer processes for wastewater treatment

Abstract

The objective of this work was to study the separation of the stable oily emulsion by coalescer and flotation processes. The commercial Castrol Cooledge BI cutting oil was applied for synthesizing the oily wastewater since it can easily form a stabilized emulsion with water. The coalescer experiments were conducted by using polypropylene media with different shapes including granule, fiber, and tube. Effects of emulsion flow velocity and bed height as well as the bed packing were considered. For the flotation, both the dissolved air flotation (DAF) and the induced air flotation (IAF) processes in the pilot scale were employed for the emulsion separation with the addition of aluminium sulfate (Al2(SO4)3) as a coagulant. Influences of operating conditions on the separation efficiency were investigated.

The results indicated that the prepared emulsion was highly stable suggested by its small droplet sizes and high negative zeta potential. The emulsion was partly separated by the coalescer with the highest efficiency of 44% from the 10 cm bed of tubular polypropylene with 2 cm/s flow velocity. In the case of the separation by flotation, the highest efficiency of 85% can be achieved from both DAF and IAF. However, the separation by flotation was ineffective without the coagulation. Therefore, the destabilization of the cutting oil emulsion by aluminium sulfate was further investigated. The main destabilization mechanism was the sweep flocculation occurred at the Al3+ concentration of 1.0 mM and pH of 6.5 – 7.5, where solid flocs can be observed. At lower Al3+ dosage, the destabilization was inefficient suggesting that only droplet coalescence was insufficient for the separation. The flocs were analyzed for their chemical composition and crystalline structure to confirm the formation of aluminium hydroxide (Al(OH)3) that plays a role in the sweep flocculation. Furthermore, the results from the bench scale flotation carried out by the Flottatest were correspondent to those obtained from the pilot scale experiments. The addition of coagulant was needed for the effective separation. However, it was also found that the increase of Al3+ dosages further the 1.0 mM was unable to enhance the separation efficiency. Finally, the interactions of droplet-bubble and floc-bubble were observed in the special made observation cell. No interaction between oil droplets and a bubble can be seen contrasting with the case of oil flocs, which can attach on the bubble surface. This affirmed the difference between the separation of cutting oil emulsion by flotation with and without the formation of flocs.