Last month we had a post on the microorganisms that are cleaning up our mess in the Gulf of Mexico. Today, Associated Press has another chapter in this story. See:
A recent report shows that oxygen levels have decreased by about 20% near the oil spill. What is the significance of this? Refer to our June 15 blog on Biochemical Oxygen Demand, or BOD. As microorganisms consume the crude oil “food,” they use up oxygen dissolved in the water. At best, water can hold about 10 mg/L of oxygen, so we don’t have much to start with. And a mile deep in the ocean, the levels are far lower, since this water is so far from the source of oxygen: the atmosphere.
The high BOD of the crude oil could have caused such a massive growth of bacteria that these low oxygen levels would be completely depleted — thus forming a “dead zone” where no non-microscopic life could survive. Instead, the relatively minor drop is great news! It tells us first that something is happening, in terms of the crude oil being consumed. Secondly, it tells us that this is happening at a sustainable rate — not a “boom and bust” that would have used up all the oxygen.
This balance between food and microorganisms is precisely what wastewater treatment Operators do for a living. Biological treatment is the most important part of modern wastewater treatment. Our Operators are essentially microorganism ranchers! They need to raise a “herd” to a size that matches the “range” — which is the food supply in the raw wastewater.
How did we get the right food to microorganism ratio in the Gulf? I’m sure luck and Nature had a big hand in it, because I doubt there are any textbook calculations that are in standard use for such situations.
But the tool used by the folks responding to the spill was a chemical dispersant. As good as the microorganisms are, they can only eat the outer, exposed surface of the crude oil. In the main spill, this would mean that only a tiny fraction of the oil could be consumed at one time — and the cleanup would take too long. The dispersant was used to break the large mass into smaller globules, thus exposing much more of the oil to the surface, and thus making more “food” available to our pals, the microorganisms.
If too much dispersant had been used, the food supply would have been too large, and oxygen depletion would have occurred. As it turns out, the folks responding appear to have added a nearly optimal amount of dispersant. In doing so, they have merely done once what wastewater Operators do every day: match food to microorganism levels.
Nice job, rookies!