What should I do if a large grease or oil spill hits my biological treatment unit?

Grease and Nocardia foam on oxidation ditch. (http://web.deu.edu.tr)

No matter how careful you operate the collection and pretreatment systems, the biological unit sometimes receives spills or shock-loads. Today, I am going into one of the most common spills - grease or oil (let's use FOG from now on for grease). What makes FOG so difficult is:

• Insoluble nature makes it difficult for bacteria compared to soluble organics.
  
• Takes time to degrade!
• Causes foaming on aeration basins
• Contributes to floating sludge on secondary clarifiers
• MLSS has oil droplets and excess water which makes for messy dewatering
• Fixed Film or mobilized beds can become coated with FOG - which can foul the media by "smothering" the biofilm
• Long term high FOG can encourage problem microbes such as Nocardia (causes thick greasy foam)

In this case, we will discuss response actions to a one time high FOG release into the system. In most cases we do the following:

1. Get ready to add anti-foam & secondary clarifier polymers.
2. Increase wasting rates to get rid of FOG saturated MLSS.
3. Try to maintain a Dissolved Oxygen (DO) above 2.0 mg/L to maximize biological activity.
4. Check upstream to find cause of FOG and try to make prevent further introduction of FOG.
5. With high wasting rates, it is a good idea to add bioaugmentation cultures to increase the number of active degrading microbes in the mixed liquor.
   
6. In fixed film systems adding bioaugmentation cultures can help re-establish the biofilm that was damaged by the FOG. It may be necessary to actually clean some of the media with surfactants if there is heavy grease buildup.