Incoming tankers carry ballast water to improve tanker stability in rough seas. Depending on ship design, ballast water is carried in cargo tanks and 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. The volume of dirty ballast water delivered to the VMT has decreased significantly over time as crude oil production has decreased and with the tanker fleet being all double-hulled, 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 Tank 81, or recovered crude oil tank, and then pumped back into the tankers as cargo. Hydrocarbon vapors from these tanks are either incinerated or mixed with diesel fuel to generate electricity at the VMT.
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. Polymers (plastic molecules) are dissolved into the ballast water as it is piped to the DAF cells. The polymers bind with oils and other particles and help float them to the surface of the water. After the polymers, pressurized air is forced into the water inside pressure retention tanks. Water from the pressure retention tanks is returned to the DAF cells where microbubbles form in the water, due to the pressure differential between the dissolved air and the atmosphere (like when you open up a soda).Those microbubbles help float the polymerized oil to the surface. Eventually the polymerized oil is skimmed from the DAF cells and piped to a crude oil recovery tank. Ultimately, all recovered crude oil and polymer is 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.
Studies by both PWSRCAC and Alyeska indicate that a reasonably efficient treatment process is biologically degrading almost all of the BTEX entering the BTTs.
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 facility.