Tankers arriving at Alyeska's Marine Terminal carry ballast water to stabilize the tankers during travel. When carried in a ship's cargo tanks, the ballast water becomes contaminated with remnants of the oil. This non segregated ballast must be discharged into the Ballast Water Treatment (BWT) system for processing. The Alyeska BWT system recovers crude oil from ballast water and treats separated water. The oil recovered during treatment is returned to the crude oil system. The separated water is treated and discharged through a pipe to a diffuser approximately 200 feet deep in the water of Port Valdez. The BWT System involves a three step process: Gravity Separation, Dissolved Air Flotation, and Biological Treatment.

Ballast water is initially pumped into one of three gravity separation tanks. Oil floats to the surface of the water where it is skimmed off. The separated water is sent to the Dissolved Air Flotation (DAF) chambers. Microscopic bubbles of air and a polymer are added to accelerate the separation of the oil from the water. The polymer binds the oil together and the air bubbles float the oil to the surface. The oil is then skimmed off the surface. The remaining water moves on to the biological treatment tanks.

The goal of biological treatment is to remove a group of aromatic hydrocarbons known as BTEX: benzene, toluene, ethyl benzene, and xylene. Microbes in the biological treatment tanks (BTT) eat the oil and reduce the amount of BTEX and other hydrocarbons which remain in the oil. The BWT operators add nutrients and oxygen to ensure a proper environment to increase both the population and activity of these oil-eating microbes. If the microbe population diminishes, air strippers are available to ensure that effluent levels of BETX will not exceed permitted levels. This is a permitted treatment step and rarely occurs.

Back to top