Enforced pretreatment ordinances are affecting the most commonly used pretreatment device - the grease separator.

Municipal sewer districts for many years would not enforce pretreatment ordinances in commercial kitchen applications, such as fast food franchises, full-menu restaurants and cafeterias. They instead stood on the sidelines and allowed plumbing inspection departments to interpret sewer-use regulations. But times are changing, and manpower has increased, resulting in the strict enforcement that industrial users have experienced for years. This will have an effect on the most commonly used pretreatment device in such applications, the grease separator, which includes outside grease traps/interceptors, inside manual interceptors and automatic grease removal devices.

While an undersized separator may adequately meet the local plumbing authority guidelines, it will not usually meet the pretreatment requirements if closely enforced. Increasingly stringent environmental regulations require that fat, oil, grease and suspended solids be reduced in the effluent prior to discharge. However, plumbing code authorities require the food grinder, and now even the dishwasher, to bypass the grease collection system and discharge directly to the sewer. There are simple techniques and equipment available to remedy this.

Sewer-use bylaws vary, but typically limit total fat, oil and grease emissions (of animal or vegetable origin) to 150 mg/l or less. Most separators cannot meet this effluent limit because of insufficient retention time (the time the wastewater spends in the separator). Stricter effluent requirements need a longer treatment time and thus a larger volume separator. Because large outside grease traps or interceptors usually provide proper retention time, this article will focus on the inside manual interceptors and the automatic grease removal devices.


It is difficult to establish an accurate gpm rating for grease separators. Nevertheless, such a rated capacity figure is required by the regulatory authorities as well as by those who specify such equipment. As is so often the case, the validity and usefulness of such a rated capacity will depend heavily upon the competence and integrity of the equipment manufacturer. There is a remarkable difference between manufacturers' design criteria in flow rate capacity. For example, comparable 35 gpm models range in static water level in gallons from 14.02 to 43.48. This disparity concerns code officials enforcing fat, oil and grease discharge limits.

An undersized separator can accommodate a flow rate far exceeding its gpm capacity, but it will not remove grease efficiently under such overload conditions. Unit retention time plus required effluent discharge limits are your prime factors in sizing. However, an inaccurate manufacturers' gpm rating will compromise this retention time.

Ideally all manufacturers would rate maximum gpm capacity by the static water level in the tank-a 35-gpm-rated separator would require at least 35 gallons static water level. This is the simplest method to assure an accurate gpm rating while ensuring minimum retention time, meaning at least one-minute treatment time. Certainly, in most cases, meeting the previously mentioned effluent limit would require more than the minimum treatment time. In field observations, a separator adjoining a three-compartment pot sink was the only type of installation able to meet the required effluent limit when using a one-minute retention time.

To realistically meet most effluent limits in commercial kitchen applications, five minutes retention time would be required, especially in a full-menu restaurant where supposedly all grease-producing discharges are connected to one grease separator. Mechanical and chemical emulsions, as well as vertical distance, will also have an effect upon the installation.

Mechanical emulsions are not a problem if the retention time is adequate, and increasing the volume of the separator will negate velocity differences in vertical distance. Chemical emulsions are typically produced by the automatic dishwashing operation and somewhat by the daily floor cleanup. There are floor cleaners available that allow contaminants to separate out rapidly, ensuring collection and the effluent quality. Usually grease and solids will accumulate at the pre-rinse station or cycle on large commercial dishwashers, and both will require connection to a grease separator. The harsh emulsifying detergents used in the cleaning cycle will break down any remaining grease into minute particles, rendering entrapment useless at this stage. If possible, connection should bypass cleaning cycle and permit its discharge directly to the sewer. But it is prudent to trap complete dishwasher if multiple discharges cannot be separated.

The necessity for longer treatment times was proven in a field test conducted 14 years ago by Lowe Engineering, using its Model 200automatic grease removal device. Installed in a full-menu restaurant, the 200-gpm rated model, with two integral screen baskets, had a static water capacity of 285 gallons. The kitchen's estimated peak discharge rate was 46 gpm with a vertical drop of 4.5 feet. The grease removal device provided a retention time of six minutes and 10 seconds.

Over a four-month period, effluent samples were taken five times and tabulated by the local code authorities. All five samples were below 100 mg/l in fat, oil and grease content (test data available upon request). Oddly enough, 12 years later, the owners replaced the Model 200 with a 50-gpm-rated unit (37 gallons static water capacity) from another manufacturer. So at times it will hydraulically "short circuit" causing scouring of the contents directly into the discharge line.

Historically, inside manual interceptors have had a larger volume than the comparable grease removal devices. This is necessary to provide a storage capacity for the accumulated contents. An effective interceptor must be large enough to accommodate significant fat, oil, grease and solids loading without affecting the separation efficiency. The buildup of floatables and solids within the interceptor diminishes the potential treatment area, causing a reduction in retention time. Sizing parameters must take into account the storage capacity required before maintenance, although the retention times would remain the same for both interceptors and automatics. In theory, contaminants should not be allowed to accumulate beyond 24 hours, but in the real world, enforcement of daily maintenance is impossible.

Outside grease traps

As stated earlier, retention time is usually not a problem in sizing large outside grease traps or interceptors because of the adequate storage capacity. Nevertheless, influents to these models typically contain extremely high organic loads including fat, oil, grease and dissolved food particles, as well as detergents and suspended solids. For this reason, it is imperative that they be properly maintained. The tank should be pumped at frequent intervals to minimize the impact of these materials on the treatment facility.

Maintenance frequency and end-users' out-of-sight-out-of-mind mentality seem to be the major headaches concerning pretreatment regulators. It is the responsibility of the regulators to respond accordingly and require maintenance schedules that will meet their effluent limits. Local waste disposal companies point out if they had easier access to the floatables and solids there could be an appreciable difference in removal rates. There are various manufactured units available that have easy access and, with a strict cleaning regimen, will meet most effluent requirements.

Catch 22?

While reviewing an upcoming cafeteria project in a small university, several self-defeating guidelines were encountered. Because new construction is the province of the building code authorities, the plumbing inspection department will be responsible for actual installation of any separator. According to the local plumbing regulations and the specifier, a 75-gpm rated separator will be sufficient for the kitchen's discharge. But the municipal sewer district requires a 100 mg/l fat, oil and grease effluent limit.

Prior calculations would suggest a separator with approximately 300 gallons static water capacity to meet the pretreatment guidelines. Incidentally, the dishwasher will bypass the separator and discharge directly to the sewer. This project clearly will not meet the sewer use requirements. After sampling, the customer will be surcharged by the local sewer authority or be required to replace the undersized separator. Pay now-or pay later.

Testing and rating procedures

Because performance parameters can change depending on the installation-and cannot be controlled by any written standard or certification test-their usefulness is and will be debatable. Nevertheless, there seems to be a consensus toward new standards and certification testing for separators rated up to 100 gpm.

No doubt there will be disagreement over general requirements, testing procedures and the proposed standards themselves. First, all certification test procedures are performed under controlled conditions. As a result they are free from contaminants and are neither mechanically nor chemically emulsified. However, in the real world, the influent will contain a wide variety of substances. Testing done under such ideal conditions are a good reference, but inadequate in real life applications.

Second, the performance guidelines considered passing for certification-in the several drafts currently written-would be unacceptable for most fat, oil and grease effluent limits.

Reconciling plumbing codes and pretreatment requirements

It seems unlikely that plumbing codes and pretreatment requirements can be reconciled while the food grinder and dishwasher continue to discharge directly to the sewer. Likewise, inadequate sizing concerns code officials who must enforce grease effluent limits, and proper retention time is the only correctable measure. Choosing a separator solely by the flow rate, especially while using manufacturers' selection charts, will result in an inefficient system.

It will not change overnight, but with increased sampling, surcharges and retrofits, all parties involved will eventually be compelled to administer environmental regulations uniformly.