Understanding Retention Time With “The Test”

The most important variable in separating FOG’s from water is time. Think about oil and vinegar salad dressing. Once you shake it and let it sit, it starts to separate. WSSC (Washington Suburban Sanitary Commission) did studies to determine what retention time was needed to adequately separate fats oils and grease from water. They used a variety of oils including actual commissary grease, and considered the addition of soap as an emulsifier. WSSC has adapted sizing criteria based on retention time. For new construction they require a 24 minute retention time. For existing and repair systems, a minimum of an 8 minute retention time is required. Their requirements are well founded, and to prove it to yourself, try this test.

• Use 2 clear jars with tight fitting lids
• Add 3/4 cup oil, 1 1/2 cups warm water and a few drops of dishwashing soap
• Prepare another jar without soap for comparison
• Shake jars for 30 seconds to simulate discharge from dish washing facilities
• Set jars on table and start a timer
• Watch and record separation at 1 min, 2 min, 8 min and 24 min.

What retention time do you think is adequate?
Do most restaurants use soap? What difference does it make?

The Town of Cary, North Carolina adopted code in 1999 requiring a minimum of 24 minute retention time and has seen dramatic decreases in Sanitary Sewer Overflows.

Before Agitation	After Agitation

	After 1 minute, the oil/water starts to separate, no visible separation in bottle with soap.
	After 2 minutes, more defined oil/water separation, very little separation in bottle with soap. At this point, high concentrations of oil would be passed into the waste stream.
	After 8 minutes, the bottle with soap have separated, although not as well as the oil/water bottle.
	After 24 minutes, most of the oil has separated in the bottle with soap.