FOG is an acronym for fats, oils and grease. FOGs create a significant problem for the wastewater treatment plant and the pumping stations, the wastewater collection system, and the local sanitary sewer user. FOGs discharged by residential, commercial, institutional and industrial users account for approximately 25% of all sanitary sewer blockages in the City of Moberly, with numbers similar around the country.
Fats, oils and grease are often washed into the residential plumbing system through the kitchen sink. Fats, oils and grease have poor solubility in water. This means they separate from the liquid. When FOGs are poured down the kitchen drain, they cool down, according to the surrounding water temperature, and then separate. FOGs stick to the inside of sewer pipes from the point of entry into the sewer system until it reaches its final destination, the wastewater treatment plant. As the FOG travels through the system, it builds up, one layer after another, on the inside of the pipes. This build up reduces the pipe’s carrying capacity and will, over time, block the entire pipe.
The lack of carrying capacity created by the fats, oils and grease discharged to the sewer system is most evident during heavy rain events when a substantial amount of runoff is entering the system in a short period of time. Residential sewer pipe, in the City of Moberly, is mainly eight (8) inches in diameter. The pipe is laid at a grade that allows for the wastewater to gravity flow at approximately 2.5 feet per second. This translates into around 400 gallons per minute carrying capacity or nearly 24,000 gallons per hour with a full pipe. A one-inch build up of FOGs will decrease the flow by 25%, resulting in a carrying capacity of 18,00 gallons per hour. A two-inch build up will reduce the carrying capacity by 75%, allowing only 6,000 gallons per hour.
It may seem impossible that a two-inch build up can rob the pipe of 75% of its capacity, but the two inches of build up is around the entire circumference of the pipe. The eight-inch pipe is now a four-inch pipe. (See illustration.) A pipe’s carrying capacity is calculated by using the area of the pipe multiplied by the velocity of the flow. The area of an eight-inch pipe is 50.24 square inches, while the area of a four-inch pipe is only 12.56 square inches. Even at this reduced capacity of 6,000 gallons per hour, the normal sanitary flow is easily handled, but problems can occur when the rains begin. The reduced carrying capacity of the pipe is not always able to handle the added flow produced by the stormwater runoff, and the excess flow can back up in basements and/or area manholes.
The Public Utilities Maintenance Department implemented a sewer-jetting program in 2004 to address this problem. Jetting a sewer involves sending a large amount of water, under a great deal of pressure, through the sewer line. This helps to clean some of the build up from the inside of the pipe. The accumulated FOGs must then be sucked out of the pump station wet wells with the Vactor truck as they create havoc with the level controls and pumping equipment. Maintenance superintendent, Tim Patrick reported, “The Department’s goal is to jet ever inch of sewer line in the City.” “And then we will start over and do it all again. It is an ongoing process that must be done on a continuing basis in order to produce results.” The program also involves a monthly jetting schedule in the identified problem areas of the City as well as any blockages as they arise.
The program has produced results. Sanitary sewer overflows and back-ups have declined significantly since the program began. The following are a few steps area residents can take to keep FOGs out of the sewer pipes and maintain a free flowing sewer system.
1. Never pour fats, oils or grease down the drain. No amount of hot water will keep it from sticking to the inside of the pipes.
2. Wipe out greasy pots and pans with a paper towel before washing. Even a small amount of FOG will become a thick layer of build up over time.
3. Be aware of the “hidden oils” such as salad dressings, cheese, cookies and pastries, and the sauces and gravies. Scrap plates and bowls or wipe with a paper towel for disposal in the trash, not down the sink.
Educate family, friends and neighbors. We are all served by the same system, and what is being put down the drain upstream may be clogging up your pipes.