The final stage in the process that reduces the oil concentration in ballast water at the Valdez Marine Terminal’s Ballast Water Treatment Facility (BWTF) removes monocyclic aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and xylene (BTEX) by means of microbial degradation and aeration.
Microbial degradation is the preferred method of eliminating BTEX because it is environmentally benign; conversely, aeration converts water pollution into air pollution. The BTEX compounds are of great concern to PWSRCAC. The purpose of this project is to identify the contributions made by microbial activity and those made by aeration to remove hydrocarbons, primarily BTEX, from the effluent that is discharged into Port Valdez.
The extent to which BTEX is being consumed by bacteria or is being aerated out of the biological treatment process has been a matter of speculation and controversy. The process for removal of BTEX from the BWTF effluent takes place in the Biological Treatment Tanks (BTT)—large, aerated, open, concrete-lined ponds. The aeration and mixing action in each pond compete with the bacteriological action in removal of hydrocarbons from the effluent.
PWSRCAC commissioned Payne Environmental Consultants, Inc. to identify the contributions made by bacteriological action and by aeration for removal of hydrocarbons from the BWTF effluent in the BTTs.
Field sampling of the process liquids took place in March and September 2004 and again in January 2005. Coincident with PWSRCAC’s first field sampling, Alyeska announced and started a comprehensive study to characterize the effluent and emissions from the dissolved air flotation and biological treatment processes at the BWTF. Based on the results of PWSRCAC’s March sampling and coordination with Alyeska’s July sampling for its BTEX Fate Study, PWSRCAC revised its sampling protocols and conducted its second and third field sampling using revised protocols.
Typically, oily ballast water is subjected to a three-stage treatment process to remove the oily residue before discharge into Port Valdez: a) gravity separation, b) dissolved air flotation (DAF), and c) biological treatment. The primary results from PWSRCAC’s microbial efficiency study are 1) Evaporative loss to the atmosphere is the dominant removal mechanism of BTEX and naphthalenes from effluent in the DAF tanks, DAF weirs, and splitter box; and 2) microbial degradation is the mechanism by which BTEX is removed from effluent in the biological treatment tanks (BTTs).
An examination of the BTT measurements reveals that 1) microbial degradation of BTEX is very efficient in both summer and winter; 2)microbial BTEX biodegradation appears to be complete in the first half of the BTTs; 3) microbial degradation of SHC (saturated hydrocarbons, n-alkanes) is very efficient during summer, but is less so in winter; 4) microbial degradation of polycyclic aromatic hydrocarbons (PAH) is only partially complete before discharge during summer and even less complete during winter; 5) active biodegradation of BTEX, SHC, and selected PAH was demonstrated in the biorate and microbiological most probable number (MPN) studies; and 6) dilution and flow dynamics cause a large drop in BTEX concentration upon entry to the BTTs.
Although very efficient microbial degradation of BTEX in the BTTs has been confirmed, the observation of significant emissions from the weirs and splitter box, which provide turbulent transport of effluent from one processing stage to the next and are open to the atmosphere, indicate the presence of a continuing emissions situation at the VMT that is worthy of regulation. All available evidence continues to indicate that emissions of hazardous air pollutants from the processes at the BWTF greatly exceed EPA’s major source threshold (25 tons).
An important part of PWSRCAC’s microbial efficiency study was coordination with Alyeska’s BTEX Fate Study. Generally, PWSRCAC and Alyeska BTEX measurements are in very close agreement; and PWSRCAC and Alyeska biorate measurements in very close agreement. However, differences in interpretation of analytic results exist with respect to kinetics rate order (a key parameter in any emissions modeling) and may affect the results of Alyeska’s emissions modeling.
Papers describing the microbial studies were presented at the Arctic and Marine Oilspill Program (AMOP) in June 2005 in Calgary and at the Alaska Section of the American Water Resources Association meeting in Cordova in April 2005.
View final reports and papers:
- Hydrocarbon Biodegradation in the Ballast Water Treatment Facility, Alyeska Marine Terminal – 2005
- Tracking Degradation And Fate Of Oil Discharges – 2005
This project is complete.