Incoming tankers carry ballast water to improve tanker stability in rough seas. Depending on ship design, ballast water is carried in cargo tanks or in separate tanks whose sole purpose is for ballast water. Ballast carried in cargo tanks requires shore-side processing to remove oil remnants that mixed with the water. Ballast water that is not contaminated with oil remnants can be discharged into the sea without any processing; however, such discharge may introduce non-indigenous or nuisance species at the location of discharge.
Oily ballast water is pumped from the incoming tankers into the Ballast Water Treatment Facility (BWTF) at the Valdez Marine Terminal (VMT) for removal of hydrocarbons. Oil recovered in the treatment process is mixed with crude oil and then loaded back into the tankers as cargo. Treated water is discharged into Port Valdez. Approximately 20,000 barrels of oil are recovered from the BWTF per month. Future ballast water treatment volumes are expected to decrease with the increased use of double-hulled tankers, which can segregate ballast water in the space between the cargo tanks and the outer hull. Unfortunately, the severity of winter weather typically encountered between Valdez and West Coast ports requires more ballast than can usually be carried in the segregated tanks. Consequently, the need for processing of oily ballast water will never decrease to zero.
Three Stages of Ballast Water Treatment
1. Gravity Separation
Ballast water is pumped from the tankers to large process tanks, commonly referred to as the 90s tanks, or gravity separation tanks. Oil is separated from the water in the 90s tanks by the simple process of allowing oil to float to the surface during a holding time of at least 4 hours. The floating oil is skimmed from the top of the 90s tanks into the 80s tanks, or recovered crude oil tanks, and then pumped back into the tankers as cargo. Routine operation of a vapor control system on the 90s tanks is expected in early 2008.
2. Dissolved Air Flotation
From the gravity separation tanks, the water is then drained into the Dissolved Air Flotation (DAF) cells for further hydrocarbon removal. Air and polymers (long chain molecules) are dissolved into the ballast water under pressure as it is piped to the DAF cells. Release of the pressure as the water enters the DAF cells causes the formation of very tiny bubbles that accelerates the process of separating the remaining, suspended oil droplets from the water. The polymer binds the oil together, while tiny air bubbles float the polymerized oil to the surface. Oil recovered from the DAF cells is also sent to the recovered crude oil tanks. Ultimately, all recovered crude oil is again loaded on tankers as cargo.
3. Biological Treatment
Water from DAF cells is then sent to biological treatment tanks (BTT) for the final stage of hydrocarbon removal. The major goal of biological treatment is to remove certain aromatic hydrocarbons known as BTEX (benzene, toluene, ethylbenzene, and xylene). Typically, oil-consuming microbes convert the hydrocarbons to less harmful compounds. Rarely, during an upset condition wherein the microbes have become inactive, an air stripping process volatilizes the remaining hydrocarbons such that they are removed from the effluent. The effluent discharged into Port Valdez typically contains less than 5 ppm total recoverable oil and grease (TROG) and must, as a requirement of an NPDES permit, contain less than 15 ppb BTEX.
Studies by both PWSRCAC and Alyeska indicate that a reasonably efficient treatment process is biologically degrading almost all of the BTEX entering the BTTs; however, significant emissions of BTEX from the DAF weirs and splitter box, which provide turbulent transport of effluent from one processing stage to the next and are open to the atmosphere, have been observed. All available evidence continues to indicate the presence of a continuing emissions problem at the VMT wherein hazardous air pollutants are emitted at a rate greatly exceeding EPA's major source threshold of 25 tons per year.
TROG Analysis of BWTF Effluent
In 2002, council staff noticed that Alyeska data seemed to indicate that while ballast water throughput continued to decrease, concentrations of total recoverable oil and grease, or TROG, appeared to increase. Not only did TROG concentrations increase, the absolute amount of oil being discharged into Port Valdez by the BWTF also appeared to increase. Recently, Alyeska switched its method of measuring TROG. The council continues to monitor this issue and to develop possible recommendations regarding the upgrades to this facilty.