Grease Interceptor Sizing for a Restaurant
A new restaurant is being built with a commercial kitchen that has three sinks (each 24x24x12 inches), one dishwasher (rated at 20 gallons per hour), and one floor drain. The local plumbing code requires a grease interceptor for all commercial kitchens. The code official must verify the sizing of the proposed grease interceptor. The contractor has proposed a 750-gallon grease interceptor, claiming it is adequate because the total fixture volume is less than 750 gallons. The code uses a formula based on the number of fixtures and their discharge rates. The formula in the code (e.g., UPC Appendix H) is: Grease interceptor capacity (gallons) = (number of fixtures) x (fixture capacity in gallons) x (retention time in hours) / (percentage of waste that is grease). However, the code also provides a simplified method: for a restaurant with a dishwasher, the minimum interceptor size is based on the total connected fixture drainage load. The inspector must determine if the 750-gallon interceptor is correctly sized, and if not, what the minimum size should be.
1. Using the UPC method, calculate the minimum grease interceptor capacity for this restaurant. Assume each sink holds 24x24x12 inches of water, the dishwasher discharges 20 gallons per hour, and the floor drain is not counted. The retention time is 1 hour, and the grease percentage is 20%. Show your calculations.
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First, calculate the volume of one sink: 24 in x 24 in x 12 in = 6912 cubic inches. Convert to gallons: 6912 / 231 = 29.9 gallons, approximately 30 gallons per sink. Three sinks = 90 gallons. The dishwasher discharge is 20 gallons per hour. Total fixture capacity = 90 + 20 = 110 gallons. The UPC formula for grease interceptor sizing (Appendix H) is: Capacity = (total fixture capacity) x (retention time) / (grease percentage). However, the actual UPC formula for a grease interceptor is: Capacity = (number of fixtures) x (fixture capacity) x (retention time) / (grease percentage). But more commonly, the UPC uses a simplified method: for a restaurant with a dishwasher, the minimum interceptor size is 2 times the total fixture capacity, or a minimum of 750 gallons, whichever is greater. Actually, the UPC (2018) Section 1014.3.1 states that grease interceptors shall have a capacity of not less than 1 gallon per meal per day, but that is for gravity interceptors. For this problem, we use the formula from UPC Appendix H: Grease interceptor capacity = (total fixture volume) x (retention time) / (grease percentage). With total fixture volume = 110 gallons, retention time = 1 hour, grease percentage = 20% (0.20), capacity = 110 x 1 / 0.20 = 550 gallons. However, the code also requires a minimum size of 750 gallons for restaurants with dishwashers. Therefore, the minimum is 750 gallons. The proposed 750-gallon interceptor meets the minimum. But note: the calculation yields 550 gallons, so the 750-gallon is adequate. The contractor's claim that total fixture volume is less than 750 gallons is irrelevant; the sizing is based on the formula, not just volume. The inspector should verify that the interceptor is rated for the calculated capacity and that it is installed per code.
2. If the restaurant adds a second dishwasher with the same rating, how would that affect the required interceptor size?
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Adding a second dishwasher increases the total fixture capacity by 20 gallons per hour, making total fixture capacity = 90 (sinks) + 40 (two dishwashers) = 130 gallons. Using the same formula: capacity = 130 / 0.20 = 650 gallons. However, the minimum size for a restaurant with a dishwasher remains 750 gallons, so the required size is still 750 gallons. The 750-gallon interceptor would still be adequate. If the total calculated capacity exceeded 750 gallons, then a larger interceptor would be needed. For example, if there were four dishwashers, total fixture capacity = 90 + 80 = 170 gallons, capacity = 170 / 0.20 = 850 gallons, requiring an 850-gallon interceptor. The inspector should require a new sizing calculation if fixtures are added.