Bigger Is Not Automatically Safer

EPA states both directions

Undersizing and oversizing grease control equipment are both bad. Undersizing can allow short-circuiting and grease discharge to the sewer system. Oversizing can allow hydrogen sulfide generation, corrosion and odors.

That is not a nuance. It is two different failure modes with two different consequences, and only one of them is widely understood.

Source: U.S. EPA, FOG Management and Control Program.

What too small does

Short-circuits. Flow arrives faster than the vessel can slow it, water crosses from inlet to outlet without ever settling, and grease that was never given time to rise leaves with it.

The symptom is a device that is nowhere near full and is still passing grease, which is exactly the situation people misread as needing more frequent pumping.

What too large does

Water sits. Long residence in a vessel with no oxygen turns it septic, which generates hydrogen sulfide. That is the rotten-egg smell that gets blamed on the device being dirty, and it is also corrosive — it attacks the vessel and the structure around it.

So a vessel chosen by picking the biggest one that fits can produce a chronic odor complaint that no amount of cleaning solves.

Which is why we will not size one

Sizing is a calculation on a specific installation: fixture count and volumes, drainage arrangement, flow rate, the loading the kitchen actually produces, and whatever method your control authority requires. It is a design question, and design questions are not answered by websites.

Your authority’s plan review is where it gets settled, and EPA lists plan review and design standards among the nine elements a FOG program contains.

What to have ready when it comes up

The existing vessel’s stated capacity, its type, what is connected to it, and what your service records show is coming out. Sizing arguments made from the loading history are far better than sizing arguments made from a table.

• Trap or interceptor
• The paperwork