Thesis (M.Sc.)--Chulalongkorn University, 2009

The objective of this work is to study the treatment and separation of synthetic oily wastewater with 1 g/l cutting oil in water by combined process of coalescer and decantation. Three shapes of polypropylene (PP) materials were applied as coalescing medium including granular, fibrous, and tubular PP at different operating conditions. Moreover, the concept of stage coalescer (step bed configuration), liquid recirculation, and improvement of decantation performance were applied for increasing the overall treatment efficiency. Finally, the simple model will be proposed and applied for providing a better understanding on the cutting oily-wastewater treatment efficiency obtained with the combined coalescer and decantation processes. The results shown that different shapes of polypropylene (PP) was hydrophobic material and can be used as coalescing medium with contact angle of 68 degrees in average. It can be noticed that the flow velocity and bed height can affect the overall treatment efficiency of the process: the highest treatment efficiency (43.64%) was obtained from 2.0 cm/s flow velocity and 10 cm bed height with tubular PP medium. It also found that the size of oil droplets was increased after passing coalescing medium, which caused the increasing of rising velocity of droplet, thus the separation was enhanced. Furthermore, by applying several methods with the conventional process, it was found that the effects of stage coalescer and wastewater recirculation application were different due to the medium types. The treatment performance of fibrous medium was enhanced, while unchanged and negative effect can be observed in case of granular and tubular PP, respectively. The concept of mixed bed by varying porosity for each bed was applied and found that the mixed step bed provided highest treatment efficiency at 59.12%. From the experimental results, the discrete settling relationship can be created and used for designing effective decanter. In addition, the simple model was applied for defining occurred treatment mechanisms. Ergun’s equation was applied to determine the actual diameter (Dactual) of coalescing medium. The simple model combining filtration efficiency equation with decantation was used to investigate the effects of operating condition, which provided the similar trend with the experimental results. Moreover, by validating this model with the treatment of cutting oil emulsion with fibrous stainless steel medium, it was found that the efficiency obtained from decantation was comparatively much higher than the efficiency in term of αηT. The treatment efficiency of process might be improved by adjusting the optimal operating condition for decantation. However, this model was not considered the effect of turbulence in the process.