Wastewater Treatment Glossary: Oily Wastewater
It mainly includes wastewater from oilfields, refineries and petrochemical plants, ballast water, tank cleaning water and engine room water from oil tankers, and cleaning water from oil tank (truck) vehicles. These are the main sources of environmental oil pollution, especially marine oil pollution (see petroleum pollution). Wastewater from gas works and coking plants contains coal tar, while wastewater from leather factories and food processing plants contains animal and vegetable oils. These also belong to oily wastewater, but account for a small proportion.
Oilfield wastewater has an oil concentration of 5000-10000 ppm. Refinery wastewater oil concentration can be as high as 1000 ppm and as low as only 10-20 ppm. According to statistics, oilfields, refineries and petrochemical plants worldwide discharge about 300,000 tons of crude oil and its products in wastewater each year. Industrial waste engine oil and automotive waste oil total about 1.3 million tons.
After an oil tanker is unloaded, 3 per thousand to 5 per thousand of petroleum generally remains in the tank. When the ship is in ballast, this oil mixes into the ballast water, reaching a concentration of 1000-2000 ppm. During ship repair or cargo change, hot water is used to wash the oil or cargo tanks. The tank washing water contains about 3000 ppm of oil. Engine room water contains oil leaking from ship engine room pipelines and valves, with many types of oil including fuel oil, engine oil and lubricating oil. The petroleum discharged into the ocean with this wastewater worldwide can reach 1 million tons per year.
To prevent pollution and harm from oily wastewater, China stipulates that the maximum permissible concentration of petroleum (including kerosene and gasoline) in surface water is 0.3 mg/L, and the maximum permissible petroleum content in farmland irrigation water is 5 mg/L (recommended value). The maximum permissible concentration of petroleum in fishery water (kerosene, gasoline) is 0.05 mg/L (recommended value). The maximum permissible discharge concentration of petroleum in industrial wastewater is 10 mg/L.
