Resource Centre | Grease Trap Resources
The physical size of an interceptor is the single most important factor that determines grease interceptor efficiency. An interceptor is essentially a retention chamber that allows the flow rate/velocity in the drainage system to be reduced sufficiently so that fats, oils and greases (FOG) and solids are permitted to float or settle and be retained.
Separation efficiency depends on the specific gravity (weight) of the waste, which can vary depending on: use of detergents (emulsifiers), water temperature, waste consistency (i.e. types and amount of solids - FOG/solids adhesion, etc.), and types of FOG etc.
Interceptor design factors such as baffles, shape and flow dynamics also have an impact on interceptor efficiency. Experience has shown that in order to affectively separate FOG, the interceptor should provide the equivalent of a minimum 30 minutes of retention time. Greater retention time should be used when difficult to separate waste conditions exist.
In addition to the volumetric capacity required for retention time, the size of the interceptor must also take into consideration waste holding capacity between pump-outs. As the interceptor accumulates waste the volume dedicated to retention capacity is reduced and retention time begins to diminish (negatively affecting the separation efficiency).
Therefore, in addition to the volume (size) required for retention time, space must also be provided to hold the captured waste between pump-outs. The amount of waste holding capacity directly affects pump-out frequency - the greater the holding capacity the longer between pump-outs.
Pump-out frequency should not be designed for greater that 8 week intervals because of adverse affects associated with the severe decomposition of the waste over long periods of time (for more information see Frequency of Cleaning).
Determining waste holding capacity is the most complex aspect of interceptor sizing. To be precise, it requires: accurate knowledge of waste concentrations in the discharge wastewater, the total wastewater discharge for the desired pump-out interval, and the efficiency of the interceptor. A rough "rule of thumb" is to add 25% of the retention time capacity for waste holding.
Two grease interceptor sizing calculations are:
1) the maximum wastewater flow over peak 30 minute period of operations X 1.25; or,
2) 20 x the maximum flow rate of the fixture(s) the interceptor is servicing (20 minute retenton time) + desired retention capacity for waste holding
Our expert technical staff is always available to provide design assistance at no charge. Be sure to instruct your designers/architects to consult with us before finalizing your plans. A small amount of additional planning at the design stage can save you significant maintenance costs for years to come.