The need for effective pretreatment of that wastewater is crucial in restaurants. Ineffective grease removal from these waste streams results in the discharge of large quantities of grease into public sanitary sewer systems. Solidified grease can develop into stoppages in the piping networks downstream and create costly and time-consuming problems at municipal wastewater treatment plants. Grease blockages have been known to cause considerable unnecessary health hazards when raw sewage backs up into residences or commercial establishments. By trapping fats, oils and greases (FOG) and solids normally found in food service establishments, a grease interceptor is the last line of defense against potential problems downstream. In order to allow for adequate separation of the FOG from the waste stream, all grease separation devices must be designed with the following criteria in mind:
Retention time
Greases and oils have a lower specific gravity than water, so when a grease-laden mixture is left undisturbed, they will float to the surface. As such, there must be sufficient time for this process to occur. Smaller devices should be carefully evaluated for retention time. Metcalf & Eddy, an internationally acclaimed company that delivers a full range of environmental services, recommends a minimum retention time of 30 minutes to adequately separate a grease-laden mixture.
Flow
The grease separation device must be sized and configured to allow for sufficient retention time, taking into account the flow rate of the influent. Furthermore, it must be configured to minimize turbulence and allow the suspended FOG to separate. This is especially important in high-flow situations, such as when draining a large sink.
Storage capacity
The device must be large enough to allow for sufficient storage of accumulated FOG between cleaning operations without affecting the flow characteristics through the unit.
Cleaning frequency
The grease separation device must be sized according to realistic cleaning frequencies. Under-the-sink grease traps must be cleaned out routinely, even daily. There is generally no assurance that restaurant personnel will maintain the unit properly, so routine cleaning performed by a third party will more likely result in proper maintenance and supporting documentation.
In addition to all of the previously mentioned design considerations, given that all restaurants use soap, it should be apparent that soaps effects on separation time should be considered in the design for grease separation devices. There are many variables that affect the separation rate of emulsified FOG. As such, consider that the separation time will be increased by soap in order to ensure an appropriate minimum retention time.
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