Draft permit for Pikka seawater treatment plant out for comment
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The Alaska Department of Environmental Conservation has issued a draft permit for wastewater discharges from Oil Search Alaska’s proposed seawater treatment plant at Oliktok Point on the North Slope. The STP “will function as a treatment and distribution point for waterflood used to maintain formation pressure and enhance oil recovery,” DEC said.
The OSA STP will be next to the existing STP operated by ConocoPhillips Alaska.
The department said it has determined to issue the wastewater discharge permit, which allows mixing zones for “Outfall 001 - Strainer/Filter Backwash.”
The Alaska Pollutant Discharge Elimination System permit “authorizes and sets conditions on the discharge from this facility to waters of the United States,” DEC said in the draft permit fact sheet, dated Dec. 28. The public comment permit closes Jan. 28.
DEC said the permit limits “types and amounts of pollutants that can be discharged from the facility and outlines best management practices (BMPs) to which the facility must adhere.”
DEC received the application from OSA for an APDES individual permit in July. (Oil Search merged with Santos in December and Oil Search is now part of the Santos Group.)
The STP facilityDEC said the proposed STP at Oliktok Point will include a make-up water (waterflood) pipeline, supporting development of the Pikka unit, with the STP on a gravel pad adjacent to the mainland shoreline at Oliktok Point next to the existing ConocoPhillips Alaska-operated STP.
The reservoirs at Pikka require a supply of water for waterflood “to optimize pressure in the reservoirs for enhanced oil recovery,” DEC said.
“The STP will strain, filter, heat, bio-treat, and de-aerate seawater drawn from four intake bays located on the face of the OSA sheet pile dock facility for waterflood and other industrial uses,” the department said.
Wastewater will be discharged from a single outfall with the outfall line estimated to terminate 140.2 meters offshore with outfall oriented perpendicular to prevailing currents and the shoreline at a depth of some 2.44 meters below the sea surface.
Strainer/filter backwashDEC said it is necessary to remove suspended sediment and microbes from the seawater prior to waterflood “to prevent clogging of pore spaces in the formation,” typically done using sand filtration and “hypochlorite to prevent biofouling filters, vessels, and pipes.”
The OSA STP will use membrane ultrafiltration and nanofiltration “to remove a greater percentage of sediment and microbes to be commensurate with the unique requirements of the oil producing formations,” DEC said.
The seawater coming into the STP is screened to block debris, marine life and other materials, with that debris “diverted to a bin for removal to an upland site.”
The seawater then goes through 24 crossflow filters which remove the bulk of silt, sand and detritus, with seawater further treated with “two self-backwashing strainers (coarse filters) to remove silt, sand, and detritus” larger than 250 microns, DEC said.
The seawater is then heated to 15.5 degrees C to reduce viscosity and optimize treatment, with a portion of the heated water recycled through the intake bays to prevent freezing.
Particles greater than 50 microns are removed in a second set of fine filters, with reject water going to the outfall tank, and is further treated using seven ultrafiltration membrane membranes which remove silt and clays greater than 0.1 micron, with strainer and membrane backwash routed to the outfall tank.
Seawater is then injected with an inhibitor to prevent scale formation and with sodium bisulfate to remove chlorine prior to being treated in four sulfate removal unit membranes.
DEC said OSA requested authorization to discharge wastewater associated with the strainer/filter backwash, including water from backwashing the strainer/filters and neutralized water from periodic chemical cleaning of the membranes.
- KRISTEN NELSON