Zero Liquid Discharge

MacKay River’s Zero Liquid Discharge Process

At Suncor’s MacKay River in situ facility, the company is proud to demonstrate the only fully functional Zero Liquid Discharge (ZLD) system in the oil sands industry. ZLD has a number of benefits, such as recycling approximately 96% of the facility’s water.

Like most in-situ operators, MacKay River uses the steam-assisted gravity drainage process. Using a pair of wells, steam is injected underground to heat bitumen, which flows to a second well (the “producer”) and then to the surface. But that’s not all that comes up. In addition to bitumen, water with a high saline content also enters the producer well. This water, plus condensed steam from the injection well, also flow to the surface.

Water Recycled to Produce Injection Steam

Most in situ operators dispose of some saline water by pumping it into a disposal well underground, but MacKay River does not have a suitable disposal well nearby. So instead, the company devised ZLD. Through ZLD, produced water is treated on-site to remove salts and is then recycled to produce more injection steam. More than 90% of the injection steam required to run MacKay River is recycled continuously in this manner, and MacKay River needs to draw very little water from underground aquifers for its operation.

Combining the ZLD system with MacKay River’s low “steam-to-oil ratio” (meaning less water is required for steaming than at competing projects), results in the use of one sixth of a barrel of subsurface water for every barrel of heavy oil produced.

How Zero Liquid Discharge Works

MacKay River’s ZLD process works like this:

  • A portion of the “boiler blowdown,” the water stream with the highest salinity in the plant, is routed to the first of two evaporators, where it is concentrated from approximately 5% dissolved solids (mainly sodium chloride) to approximately 10% dissolved solids.
  • From there, the water flows to the second evaporator and the concentration is increased to approximately 25% dissolved solids.
  • After going through the evaporators, the briny liquid flows to a crystallizer where salt begins to precipitate from the brine.
  • When it reaches approximately 30% total solids, the resulting slurry is sent to a rotary drier where the remaining water is removed.
  • Out of this drier comes a dry “salt cake,” which is transferred to an on-site Class II landfill for final disposal. The salt cake is primarily sodium chloride, but also contains other water-soluble material, both inorganic and organic.
  • All of the evaporated water is condensed and returned to the steam plant for further recycling.

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