Since the size of the used oil sample acquired from different source is individually different, it is important to decide how to take representative subsamples each time from the original big sample bottles (at least with volume size of one gallon) before conducting any experiment. A procedure was determined ⎯ the big sample bottles were shaken with inversion for 30 seconds and then allowed to settle for 15 seconds. This allowed the coarsest particles (if any) to settle, but included a representative sample of the other 102 constituents at most homogeneous state. Then, appropriate amounts of used oil samples were quickly poured out from the big bottles directly to smaller bottles (125mL) for ultrasonic irradiation treatment or to the blender to make emulsion samples. The experimental design for the preliminary studies is illustrated in Figure 4-1. The experiments were conducted in a way to study how the factors, mentioned in previous section, would affect the feasibility or efficiency of the reaction. For each different used oil sample, in order to know if the emulsifying condition is good enough for effective ultrasonic irradiation reaction, the water-in-oil emulsion state first has to be observed under microscope (S0). Also the water content in original used oil sample has to be measured first (S0). According to information from many sources, water content reported in used oil ranges from 5% to 20%. The four used oil samples acquired for this study, however, all contain very low water content, e.g., below detection limit of 0.15v/v% for the two automotive used oil samples and the hydraulic fluid sample, and merely 3.99v/v% for the mixed used oil sample. It is very possible that when we acquired the samples from the sources, the samples were just taken randomly from their big holding towers. Depending on from what level or what position they were drawn, the oil samples may not be so representative already. Ideally, we need to have an optimal water content to form effective-size vesicles and micelles in the water-in-oil emulsions so that the ultrasonic irradiation is then able to separate or break down toxins. Since water is considered as a contaminant for any used oil reuse process, extra water should be added as little as possible in order not to create too much trouble in water/oil separation step later on.