What is a FOG?
FOG: Fats, oils and grease
FOG stands for fats, oils and grease: the cooking fats and greases from restaurants, food processors and homes that solidify in sewer pipes, form blockages and cause sanitary sewer overflows. Utilities run FOG programs that require grease interceptors at food-service businesses, inspect them and licence haulers; at the plant, FOG receiving stations and grease-removal systems keep it out of the process, and a growing number of plants feed collected FOG into anaerobic digesters to boost biogas production. Klarifi tracks 221 headworks, screening and grease-removal projects in the US city capital plans it has extracted.
Why FOG matters to a utility
Grease cools and hardens in sewers, narrowing pipes until they block. The EPA attributes a large share of sanitary sewer overflows to grease, and every overflow is a permit violation and a cleanup cost. Collection-system crews spend a large part of their budget jetting grease out of lines, which is why FOG programs, grease interceptor rules and sewer rehabilitation sit together in capital plans.
FOG at the treatment plant
What reaches the plant is skimmed from primary clarifiers as scum or captured in grease traps at the headworks. Uncontrolled FOG fouls aeration diffusers, coats probes and reduces oxygen transfer, so headworks upgrades often include grease removal.
Collected FOG is also a resource: trap grease and food-processing waste have a high methane yield, and plants with anaerobic digesters accept hauled FOG at receiving stations to raise biogas output for heating and power. Klarifi lists 1,548 US plants with digesters or gas holders, the candidates for co-digestion.