What Is an API Oil Separator? Wastewater Treatment Terminology Explained
The basic principle of treating wastewater with an oil separation tank (API separator) is the same as that of a sedimentation tank: separation is achieved by utilizing the difference in specific gravity between suspended matter and water in the wastewater. The structure of an oil separator is mostly of the horizontal-flow type. Oily wastewater enters a rectangular oil separator with a planar layout through a distribution channel and flows slowly in the horizontal direction. During the flow, oil floats up to the water surface and is pushed by an oil collecting pipe or a skimmer installed on the tank surface into the oil collecting pipe, and then flows into a dehydration tank. Heavy oil and other impurities that settle in the separator accumulate in the sludge hopper at the bottom of the tank and enter the sludge pipe through the sludge discharge pipe.
Oil separators are mostly built of reinforced concrete, and some are built of brick and stone.
On the rectangular plane, the tank is divided into 2–4 compartments along the flow direction, and the width of each compartment generally does not exceed 6 meters to ensure uniform water distribution. The effective water depth does not exceed 2 meters, and the length of the oil separator is generally more than 4 times the width of each compartment. Chain-type oil skimmers and sludge scrapers are commonly used to remove floating oil and bottom sludge respectively. Generally, each compartment is equipped with one set of oil skimmer and sludge scraper and one sludge hopper. If a baffle is added in the middle of each compartment, with oil skimmers and sludge scrapers installed on both sides of the baffle and a sludge hopper provided, it is called a two-stage oil separator, which can improve the oil removal efficiency, but increases the number of equipment and energy consumption. If several inclined plates are added inside the oil separator, the oil removal efficiency can also be improved, but the construction investment is higher. In cold regions, to prevent oil from solidifying in winter, a steam pipe can be installed at the bottom of the oil collecting pipe for heating. Oil separators generally need to be covered, with steam pipes installed under the cover for thermal insulation, so as to prevent fire in the separator and volatilization of oil, and to prevent dust and sand from entering.
The specific gravity of oil in wastewater is generally lower than that of water, and it exists in three states: ① Suspended state: the oil particles are relatively large, with a droplet diameter of more than 0.1 mm, floating on the water surface and easy to separate from water. In the petroleum industry, this type of oil accounts for about 60–80% of the oil content in wastewater. ② Emulsified state: the dispersed particle size of the oil is small, with a droplet diameter of less than 0.1 mm, in an emulsified state, and it is difficult to float and separate from water. This type of oil accounts for about 10–15% of the oil content in wastewater. ③ Dissolved state: the solubility of petroleum in water is extremely small, and the oil dissolved in water accounts for 0.2–0.5% of the oil content in wastewater.
Performance Comparison of API, PPI and CPI Oil Separators
| Item | API | PPI | CPI |
|---|---|---|---|
| Oil removal efficiency / % | 60 ~ 70 | 70 ~ 80 | 70 ~ 80 |
| Floor area at the same treatment capacity | 1 | 1/2 | 1/3 ~ 1/4 |
Installation drawings and parameters: refer to the atlas of small drainage structures (Jianshe [2001] No. 163).
The oil separator product needs to be installed directly on the channel through which oily sewage and oil-water flow, with the sewage outlet aligned with the barred inlet of the oil-water separator, and it can be connected to other equipment by pipelines. During installation, the oil-water separator (oil separation tank) must be adjusted to a horizontal position.
Before the first use, the equipment should be filled with tap water, and the water level adjustment pipe should be adjusted so that the top of the water level adjustment pipe is at the same level as the upper edge of the oil overflow groove. The position of the water inlet pipe should keep a certain distance from the debris separation box, so that the debris separation box can be taken out easily.
After oily wastewater is fed in, adjust the water level adjustment pipe again until the oil drain pipe only discharges oil without draining water.
If the position of the oil drain pipe is too low, the equipment cover can be opened and the oil can be taken out with tools.
1. Oily sewage from canteens and restaurants must pass through an oil removal device before being discharged into the sewage pipeline.
2. The sewage flow should be calculated according to the design second flow rate.
3. The flow velocity of sewage containing edible oil in the tank must not exceed 0.005 m/s.
4. The retention time of sewage containing edible oil in the tank water is 2–10 min.
5. For oil separators with manual oil removal, the volume of the oil storage part must not be less than 25% of the effective volume of the tank.
6. The oil separator should be equipped with a movable cover plate, and the water inlet pipe should allow cleaning.
The main types are automatic oil separators and gravity oil separators. According to different placement methods and materials, they are divided into buried FRP oil separators, above-ground FRP oil separators, stainless steel oil separators, and reinforced concrete structures.
