Two fairly universal reasons why grease recovery can be a problem rather than a solution are improper sizing and improper maintenance. The first problem is easily solved by referring to Standard PDI-G101, published by the Plumbing & Drainage Institute. An easy but comprehensive method is included in Appendix A of this Standard, which can be viewed and downloaded from the PDI Web site at The basic method for sizing grease interceptors is shown in Table A1.3 of the standard.

For dishwashers, a separate grease interceptor is recommended for each commercial unit. The size of the interceptor is determined by the gpm discharge rate of the dishwasher as specified by the manufacturer. Select an interceptor whose flow rate is at least equal to the gpm discharge rate.

Improper maintenance is a more complex situation since even the best-designed interceptors, properly installed, will fail if they are not maintained. Precise requirements for maintenance are not possible to define since conditions at each installation vary. In general, however, maintenance must be performed before the grease in the wastewater downstream from the interceptor exceeds 100 ppm, or whatever the local requirement is. One hundred ppm is mentioned because that is the most common requirement in the U.S.

In simplest terms, an interceptor should cleaned before it reaches its rated capacity. For instance, a 25 gpm interceptor, which can hold 50 pounds of grease, should be cleaned before it reaches this point. A user may determine a cleaning schedule by measuring how much grease has been accumulated over a period of time. Grease weighs about 7 pounds per gallon. Therefore, if it is determined that a 25 gpm interceptor accumulates 7 gallons of grease in a week, then the interceptor should be cleaned no less than once a week.

The actual frequency of cleaning an interceptor will vary depending on a wide variety of factors. For example, the type of food served determines how much grease enters an interceptor. Also, an interceptor used for cleaning utensils or limited to serving trays in a restaurant where no food is actually prepared will accumulate a lot less grease than one used in a full-service restaurant, where all of the food preparation equipment and utensils as well as dishes are washed. Figure 1 has instructions on how to remove fats, oils and grease (FOG) from an interceptor.

The cleaning cycle on large capacity interceptors typically located outside the building is less easily determined. From information gathered from a variety of sources, the consensus appears to indicate the cleaning frequency for large interceptors is in the range of 2 to 4 weeks. The annual cost of regular cleaning is likely to average between $2800 and $4000 depending on the local market.

Regardless of what the cleaning cycle is determined to be, it has been shown by actual field experience that one of the biggest obstacles to regular maintenance has been the odors usually associated with interceptors. The easiest way to eliminate this problem is frequent cleaning. If cleaning the grease interceptor becomes part of a regular maintenance routine, it usually will only require about 15 minutes and there will be limited or no objectionable odors.

An alternative is the use of an interceptor that is considered to be a Grease Recovery Device (or Grease Removal Device). A GRD is an interceptor that has as an integral part of its design a means by which grease is removed. That is to say, the grease is removed without human intervention. A GRD can be one of two basic types: timer controlled (Figure 2), or sensor controlled (Figure 3).

Timer controlled devices typically utilize a disk or belt which passes through the grease (FOG) and a squeegee device to wipe the accumulated grease from the disk or belt into a drain trough where it travels to a grease receptacle. Other means of removing the grease include a pump or gravity flow activated by the timer. They are usually regulated by a 24-hour timer that is set upon installation. The timer will operate the grease removal system for a set time or times each day.

Sensor controlled devices have the ability to sense the presence of grease. By detecting grease and initiating the removal process only when necessary and as often as necessary, a GRD can always keep the retained grease below the rated capacity of the device. The sensor operated devices use valving and gravity or pump assisted grease removal. The previous two methods to avoid routine maintenance are certainly good and acceptable choices.

Some others are not and are to be avoided in conventional interceptors. The first is the use of chemicals, often touted as environmentally friendly enzymes or emulsifiers. These materials may even have names which imply their use is environmentally acceptable. The second is the use of "bacteria" or organisms designed to digest grease.

In the first category, the materials used work by changing the structure of grease from a hydrophobic material that is unlikely to mix freely with water (thus allowing separation to occur) to a hydrophilic micelle which mixes freely with water, thus inhibiting or preventing separation from occurring in the interceptor. The use of these additives only changes the structure of the grease for a limited period of time, and eventually the grease will revert back to its original form, usually downstream in the public wastewater collection system where it can cause major problems in the sewer system of the local municipality.

The second method, the use of bacteria (or bio-remediation), works. The concept of bio-remediation is sound: trap greases and digest them in the interceptor to convert the grease permanently into the by-products of digestion. This is exactly what happens in a sophisticated wastewater treatment plant (Figure 4). New York City has done extensive testing using microorganisms for remediation of sewer blockages. Their testing shows that the process has merit and they use bio-remediation in concert with mechanical sewer cleaning to take care of sewer blockages.

There is a misconception that has been almost universal around the country. It is that high effluent temperatures are bad for the process of grease interception. This is a concept that is not based on fact or science and simply is not accurate. The Plumbing & Drainage Institute has tested grease interceptors at a temperature of 180¯F with no significant degradation of test results. In commercial kitchens the grease-laden wastes are likely to have some solids present. These solids can form globules when combined with grease. As the temperature rises however, the grease will be more likely to separate freely from these solids and not form globules that may sink and ultimately be discharged from the interceptor, causing problems downstream.

Grease-laden solids passing through the interceptor create two problems. First, they tend to form balls or aggregates (grease can become very hard) posing a blockage problem in the wastewater collection system. Second, if these materials make it to the wastewater treatment plant without creating any blockages, they can make wastewater treatment more difficult, because degradation of grease consumes oxygen necessary for the digestion of the waste in the treatment plant and because grease decomposition is quite slow, it can pass through the plant. This increases the effort required to treat wastes and can cause violations of the plant's discharge permit.

Unfortunately, this misunderstanding of the effects of hot temperatures has in many instances prevented dishwashers from being connected to grease interceptors. The Plumbing & Drainage Institute Standard PDI-G101 has always recommended that waste from dishwashers pass through grease interceptors. The Environmental Protection Agency, in their document EPA 625/1-80-012 "Design Manual: Onsite Wastewater Treatment and Disposal Systems," is specific in recommending the use of hot water to enhance the retention of grease.

The problems relating to grease are easily addressed. To do so requires an understanding of the principles of separation and a willingness to do all that is necessary. Dealing with grease problems is not limited to restaurant owners; it is an issue a number of parties must share in resolving. Codes must be written or, more precisely, rewritten to be technically correct. Administrative authorities must make certain when they write or endorse codes that all of the issues have been correctly addressed.

Interceptors that have been properly designed and certified must be required and used. They must be installed as they were tested and were intended to be installed. The devices must be maintained according to the codes and the manufacturer's requirements. Remember: Proper maintenance of even the poorest interceptor will provide better results than the lack of maintenance on the best interceptor.