Nitrates in runoff from agricultural fertilizers have been identified as the main culprit for the dead zones – where massive algal blooms feeding off the nitrates starve areas of water for oxygen so they won’t support other marine life.
But nitrates are also a byproduct of the oil-refining process. So ExxonMobil’s Baton Rouge refinery invested in a new system that uses microscopic organisms to feed on nitrates in the refinery’s water treatment system, breaking them down into non-harmful components like nitrogen, oxygen and carbon.
“We coach the bugs to denitrify our wastewater,” says Robert Berg, state regulatory advisor for ExxonMobil, employing his own affectionate term for the microorganisms. “Bugs eat nitrates in nature, but we have put a process in place where we can make the bugs even more efficient at breaking down nitrates.”
To do that, the refinery uses a “tank-in-tank” system at two above-ground steel tanks that each hold approximately 7 million gallons of water. Within these giant tanks, there are smaller tanks that play an important role in tricking the microorganisms into eating more nitrites.
“Just like you and I want to eat dessert first, bugs want to eat organic material first and will ignore nitrogen compounds if they can,” says Berg. “Nitrates are like the Brussels sprouts of the meal.”
So one tank is aggressively aerated to encourage the breakdown of organic materials. The second tank is not aerated, which encourages the microorganisms to feed on nitrates.
“We cycle water around using the tank-in-tank design basically telling the bugs ‘you’re going to sit here until you eat your Brussels sprouts,’” continues Berg.
The new system is already paying off.
Between 2014, when it was installed, and 2015, nitrate emissions at the Baton Rouge refinery dropped by 80 percent, resulting in the Louisiana Department of Environmental Quality awarding the refinery its 2015 Environmental Leadership Award. The state agency recognized that the refinery’s new system means fewer nitrates make it into the Gulf of Mexico, and also shows that taking the lead to work with nature on a local level in Baton Rouge can help pay dividends for the health of the entire Gulf Coast.