(a) Grease interceptors shall be designed to provide a minimum of 24 minutes of hydraulic retention time at peak wastewater flow from the facility. The peak wastewater flow shall be calculated using the Manning Formula which takes into account depth of flow in the pipe, slope of pipe, pipe diameter and pipe roughness. The Manning’s equation to calculate the wastewater flowrate for a pipe flowing full under a non-pressurized condition is:
Qfull = [(s)/n] x (D/16)8/3
where:
Qfull = full channel wastewater flowrate, cfs
s = pipe slope, ft/ft
n = pipe roughness coefficient, unitless = 0.012
D = pipe diameter, inches
(b) The adjusted wastewater flowrate for a pipe flowing partially full shall be calculated based upon the flow depth ratio in the pipe using:
Qadj = Qfull x Q/Qfull
where:
Qadj = adjusted wastewater flowrate, cfs
Qfull = full channel wastewater flowrate, cfs
Q/Qfull = flow ratio based on depth ratio in pipe, unitless
(c) To calculate the minimum grease interceptor volume with 24 minutes hydraulic retention, use the following equation;
V = Qadj x 60 sec/min x 24 minutes x 7.48 gal/cu.ft.
where:
V = grease interceptor volume, gallons
Qadj = wastewater flowrate, cfs
(d) The minimum grease interceptor size that shall be acceptable is 750 gallons. The maximum grease interceptor size shall be 1,500 gallons. If a larger volume tank is required to meet the minimum hydraulic retention time, multiple 1,500 gallon interceptors shall be constructed and plumbed in series.
(e) Only in specific ‘hardship’ cases will a FSE be permitted by the city to install an under-the-sink or in-floor, automatic or manual, grease trap. In addition to the sizing factors described above, additional factors to be taken into consideration by the city shall include building space constraints, site elevation constraints, and location of the city’s sewer main.
(Ord. of 11/7/11, No. 07-11)