Distinctive for oil/water mixtures is that we have two immiscible fluids which, given time, will separate spontaneously. Under the influence of gravity the oil will rise to the surface and form a layer on top of the water phase. This is what is called gravity separation.
A mixture that is not (completely) separated yet is called an emulsion. Oil is not dissolved in the water phase but is dispersed throughout the water phase as very fine droplets. The oil "emulsifies"; oil and water build an emulsion. Depending on the characteristics, an emulsion will be more or less stable. The smaller the droplets, the longer it will take for the emulsion to separate under normal gravity conditions. Intensive mixing, which breaks up the oil and creates small oil droplets, will cause the oil to emulsify and separate less readily from the aqueous medium. That is why pumping and transferring oil/water mixtures can have an adverse effect on separation.
Other factors that negatively influence the separation (or de-emulsification) are the presence of other pollutants or chemicals in the mixture. In particular emulsifiers (surfactants, tensides) that are used abundantly in cleaners, metal working fluids and in food stuff, are known for their negative effect on oil/water separation.
In the separation of oil from water, some devices target the removal of the floating oil, some the removal of the dispersed oil. Although some of the technologies for oil removal claim to target both, they often require pre-separation of the free (floating) oil for economical or technical reasons.
There are a number of reasons why it is not possible to "scoop off" an oil layer with a bucket or a weir, but there is one obvious one. Compare the oil layer with a piece of wood. As the wood, the oil layer "sinks" partially into the water. How deep it sinks depends on how thick the oil layer (the piece of wood) is, and how heavy the oil is (what density it has).
When removing floating oil with a simple overflow weir, initially pure oil is separated. This will cause the oil layer to get thinner and the interface between oil and water to move upwards, towards the top of the weir.
Even before the oil is completely gone, water will start spilling over the weir. Any over flow weir construction will result in high to excessive amounts of water, being separated with the oil, unless a relatively thick oil layer is allowed to sit on the surface all the time.
Why this situation is undesirable is explained here. The Suparator® thin-film technology overcomes these problems all the way, offering many advantages.
There have been many attempts to automatically adjust the position of skimming buckets or weirs, all without little to no success. The reason is that there are other factors influencing the skimming process. One is that, due to its lower viscosity, water will more readily flow towards a skimming device than oil. The skimming device quickly creates a hole in the oil layer and starts to remove more water than oil.
Many oil separation devices, as a compromise, settle for removing only the surplus of oil, leaving a more or less thick oil layer behind in the separator all the time. As a result a dirty, messy layer is formed at the interface between oil and water. As nothing gets removed, except for the nice top oil layer, in time more and more dirt accumulates at the interface and gets re-entrained in the water phase. This mechanism causes the performance of oil/water separators to deteriorate in time.