Grease Trap

The purpose of this equipment is to keep grease and oil from washed dishes from entering septic tanks and sewers. Grease can clog up pipes and infrastructure, requiring costly repairs, and makes the water more difficult to treat and sanitize. Because of the risks associated with letting grease enter the municipal sewer system, these are required by law in many jurisdictions. They are not an alternative to grease disposal systems and are not designed to collect spent grease from deep fryers. They are only meant to collect grease from wastewater. More

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Hobart GRSVESL-PLASML 2.5-Gallon Grease Disposal Vessel Plastic


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These units work by separating grease from wastewater and letting the water pass through. Water from sinks and dishwashers enters through the inlet and is slowed with a flow restrictor, usually passing through a series of baffles to help separate the grease from the water. Because grease is less dense than water, it floats to the top and gets trapped in the unit, while water flows freely through the outlet and into the sewer.

Location and Installation Considerations

These units are most often required to be used with pot washing sinks and dishwashers. They should be installed as close to the source of the water as possible so that pipes leading to the trap do not get clogged. There are options when it comes to installing these. They can be installed on the floor, partially recessed into the floor, or fully recessed with the top surface flush to the ground.

Since they must be accessible for emptying and servicing, overhead clearance is a concern, especially since many of them are installed in tight spaces like under sinks. Some require as little as five inches of clearance, but many can require upwards of two feet of clear space above so they can be properly maintained. Some municipalities require greater clearance. Be sure you understand the manufacturer’s guidelines and your local laws before determining the placement of your unit.

Sizing your Grease Trap

The first number to look at is the gallons per minute (GPM) rating of the unit. This is the number of gallons that can be effectively passed through the unit in a minute. Exceeding this limit means that grease will pass through the unit without getting filtered out, and the waste water may back up into the source.

For a dishwasher, you’ll need a trap with a GPM capacity equal to the washer’s operating capacity. Be aware that many dishwashers have a different GPM rating for their washing and rinsing cycles. Use the higher of the two numbers to determine the size you need.

To determine the right unit to attach to a pot sink, you must figure out the estimated GPM based on the dimensions for the sink. This involves four steps.

  1. Determine the capacity of your sink in cubic inches by multiplying the Length x Width x Depth of each section of the unit.
  2. Divide the overall capacity by 231 to convert to GPM
  3. Multiply the GPM by the number of sections your sink has
  4. Adjust for the contents of the sink, which will displace some of the liquid, by multiplying by 0.75.

This will give an estimated GPM of your sink, which you can use to size your grease interceptor.

Here is an example for a two-section sink in which each section measures 18-in. W x 18-in. L x 14-in. D:

  1. Determine the capacity of the sink: 18-in. x 18-in. x 14-in. = 4536 cu. in.
  2. Multiply by 2 for the two sections: 536 cu. in. x 2 = 9072 cu. in.
  3. Divide by 231 to convert to gallons per minute: 9072 cu. in. / 231 = 39.3 GPM
  4. Account for the contents of the sink by multiplying by 0.75: 0.75 x 39.3 = 29.5

Rounding up to 30, you’ll need a grease trap that’s rated for at least 30 GPM.

Cleaning Requirements

To keep it working efficiently, your unit should never be allowed to fill past 25 percent with grease. Most guidelines recommend cleaning at least once a month, more frequently if it is passing the 25 percent mark more often. Some jurisdictions will dictate the frequency at which the traps must be cleaned.