Why d-limonene and other solvents may solubilize grease but are bad for wastewater treatment systems

Problem gravity main before and after using a maintenance program.

Grease builds up in lift stations, gravity mains, and headworks. In severe cases, you may be tempted to use d-limonene or a solvent products to remove hard grease deposits. While these solvents work very well to mobilize grease, they do not actually remove the grease - it just passes into the water phase as a grease/water emulsion. This emulsion is the problem. 

After using the solvent product to clean the lift-station, the grease moves through the force mains down to the wastewater treatment plant. Suddenly, WWTP operators receive a slug loading of grease/long chain fatty acids. Bacteria in the plant can degrade grease, but it takes time. Let's detail why:

1. Grease even in an emulsion does not immediately cross the bacterial cell wall for metabolism. Exocellular enzymes must first cut the fatty acids into smaller units that can cross the cell wall.
2. Not all bacteria can degrade long chain fatty acids - the most common pathway in wastewater is via Beta-Oxidation which shortens the fatty acids by 2 carbons each pass through the metabolic "wheel".
3. Slug loads of grease attach to the MLSS or biosolids via adsorption. This just means that small grease chunks are stuck in the biomass EPS. This makes the sludge less dense - which can cause floating MLSS and scum on the clarifiers.
4. If the grease continues to enter the system, you will favor hydrophobic organisms that attach to grease particles. This means Nocardia forms. They thrive on long chain fatty acids but are slow growing.

If solvent products are bad, what can you do to remove grease in collection systems? First, never let the problem get so severe that solvents are required. I have worked for over 20 years with various treatments that use organisms with grease degradation pathways dosed upstream of problem grease areas. This is a maintenance rather than instant solution. With biological treatment, the grease deposits tend to be lighter and easily removed with water jets. This is because the bacteria have started the degradation process and the grease/fatty acids are becoming more water soluble and shorter in length. This means less chance of upset down stream in the WWTP from either grease slugs or Nocardia.​