As a utility, you may be aware of the amount of time spent cleaning conveyance lines in order to prevent Fats, Oils, and Greases (FOG) buildup. You may be aware that pumping stations can be clogged by FOG. You may realize that FOG build-up can lead to Sanitary Sewer Overflows (SSOs). You may also be aware that FOG increases treatment costs for your utility. As you begin to assess the costs and what potential impacts you can have to reduce FOG in the conveyance lines, the logical place to begin is to estimate the amount of FOG currently in the system.
To estimate FOG released into the system, you need to know (a) how many and what type of food service establishments (FSE) are connected to your system, (b) how many of these have a grease interceptor, and (c) how well their devices are maintained – or to put it another way – how many days do they operate with the grease interceptor already at full capacity.
The number of FSEs may be known from health department permits, and they can be further categorized as low producers (FSEs that serve food prepared offsite or that have no kitchen), medium producers (FSEs that serve food from a limited menu with a limited amount of onsite preparation), or high producers (FSEs that serve a full menu of food prepared onsite).
For illustration purposes, assume your utility serves 1,000 high producing FSEs. We’ll assume each restaurant serves 500 customers (meals) per day. The estimate of FOG produced per meal is 0.035 pounds. Thus, the FSE is producing 17.5 pounds of FOG per day, or 6,300 pounds per year.
Further, assume that their grease interceptor has a 70-pound capacity. That means it should be pumped out every four days if it’s 100% efficient. (70 pounds divided by 17.5 pounds/day.)
Instead of pumping every four days, let’s say the FSE’s interceptor is pumped monthly. That means the grease interceptor is operating at full capacity for 26 out of 30 days, letting 455 pounds of FOG bypass the collection system monthly. In a year, the FSE will have 840 lbs. of FOG pumped out and 5,460 lbs. of FOG bypassed to the conveyance system.
To compare the impact of properly maintained grease interceptors: estimate that 1,000 FSEs (a) properly maintain their grease interceptors, (b) maintain them like the example above, or (c) have no grease interceptor.
If properly maintained, the only FOG released into the conveyance system would be due to inefficiencies, which can be estimated to be a 10% loss (90% efficiency). In that case, 1,000 FSEs x (10% of 17.5 lbs/day) x 365 days/yr = 638 thousand lbs/yr FOG into the utility conveyance system.
If maintained as in the example above, 1,000 FSEs x 455 lbs/mo x 12 mo/yr = 5.4 million lbs/yr FOG into the utility conveyance system.
If no grease interceptor exists, 1,000 FSEs x 17.5 lbs/day x 365 days/yr = 6.3 million lbs/yr FOG into the utility conveyance system.
If the goal is to minimize FOG discharges to the collection system and to maximize FOG resource recovery, you can begin to see how a jurisdiction’s policy on the sizing, selection, installation, and maintenance requirements for grease interceptors is crucial!
