Basically there are 3 types of industries that constantly generate oily wastewater which includes the petroleum and edible oil refining, machining and metals manufacturing, and finally food processing. At any point during its processes along the whole systems, generation of oil plus water forming hard-to-break emulsion is often unavoidable and whether it is an O/W or W/O emulsion, all these pose a major problem and is a tough challenge faced by these industries as not only there is a need to recover the oil but at the same time prevent oily wastewater discharge polluting the environment.
In a typical petroleum and edible oil refining, the waste oil can come from different sources and is usually collected in fat traps, skimmers, recovery pits including drainage systems and then diverted to the sewers. These components as they travel along the discharge pipeline are often mixed thus forming additional emulsion which can be hard to deal with later part. The oily discharge is usually diverted to an API separator. Usually the skimming effect taking place over here separates the top oily solid which are recovered and then sent back to the desalter or the coker while the underflow that comes out from the API separator will then travel to the DAF unit. At the dissolved air flotation system, usually the collected oily waste are diverted to the sludge holding tank which are then sent for disposal or reuse in boiler as fuel source. As quality of the recovered material is a concern here, usually the oil coming from the DAF unit is not recycled back again to be used as feedstock. Sometimes in order to get better separation, organic emulsion breakers are added to aid the process and then allow the wastewater to settle within a certain period of holding time before the bottom wastewater are drained off.
In the metal industry and steel mill (represented by the schematic and process route diagram above), usually the oil waste include both emulsified and non-emulsified floating layer. These contain primarily lubricating and hydraulic fluids originating from the machines while the metal rolling process and cold strip operations also generate their portion of oil in water emulsion that are hard-to-break. Typical removal step will start with the equalization process whereby wide variety of contaminants mainly solids and metal chips are removed before going to the DAF unit. Similarly as in the previously discussed topic, the oil will be skimmed off and then pumped to storage tanks for further processing using oil emulsion breakers while the underflow water layer will be sent to oxidation ponds or aeration unit and then to final clarifier before disposal into sewers. The treatment steps in the metal industry can be more complicated as chemical emulsion breaking stage will require addition of acids to dissolve the metal fines and heat has to be applied throughout the whole process. The recovered oil layer is usually of higher quality grade and is suitable for use as boiler fuel blend. Sometimes in order to get a higher degree of separation, high molecular weight polymer can be introduced along the processing steps.Finally for the meat and food processing plants, emulsion generation also pose a big problem and the process route adopted for oil recovery can be similar like the ones used for petroleum refining. In order to recover its value as a fuel source or lower-grade by-products, sometimes additional functional unit has to be installed in order to get better separation. This can be achieved either by using a partially submerged continuous belt system which is fabricated from selectively oil-wetted materials or either way a manometer-type oil-water separator system can be constructed taking advantage of the difference between the specific gravity of oil and water.
