Grease interceptors and water conservation

Nathaniel Whiting patented the first grease "trap" in California in 1884. Whiting’s design was basically the same then as today’s passive or gravity type grease interceptors.

Water temperature, water chemistry (detergents) and other factors can affect an interceptor’s performance. Sometimes the FOG globule can adhere to food particles that are heavier than water and sink to the bottom of the interceptor. Some interceptors utilize heaters to improve the separation by melting the grease and allowing it to rise. This is why the flow of waste through the interceptor should be slow enough to allow gravity separation to work to separate the FOG by allowing tiny globules to float to the surface.

As globules of grease or oil get larger or coalesce together, they rise at a greater rate. Whiting calling his invention a “trap” instead of an “interceptor” or “separator” led to much debate and confusion in the industry. Years later, it was determined that if a plumbing fixture has two traps, air would be caught or trapped between the two traps and cause resistance to flow as the air bubble in between the two traps would resist being pushed down to flow through the second trap. This is often referred to as air binding.

This piping configuration causes sluggish drain flows, so someone proposed a code change to prohibit double trapping of plumbing fixtures. Soon after the code change was published, code officials started enforcing the new code and turned down many installations that had a grease “trap” with a p-trap at the fixture. However, confusion with p-traps and issues with double trapping led to code officials not allowing p-traps on many fixtures served by grease interceptors. This led to sewer gas escaping into the building, and the traps were soon replaced on the fixtures. The industry standards have realized the confusion with the term “trap” and they now use the term “interceptor.”

I am a member of the ASME Standards A112 committees for Plumbing Materials and Equipment. I have served on the task force for three standards related to grease interceptors: A112.14.3 Grease interceptors, A112.14.4 Grease Removal Devices, and A112.14.6 FOG Disposal Systems or Grease Remediation Devices. The standard committees and model codes generally agree on terminology and refer to the devices as interceptors. This language was brought forward into the codes over the last decade.

I recently attended an IAPMO chapter meeting in Long Beach, California with members of the Unified Plumbing & Piping Association (UPPA) where the guest speaker gave a presentation on large, fiberglass grease interceptors that conform to the requirements of the California Plumbing Code.The speaker discussed some of the types of materials used for grease interceptors (steel, fiberglass and concrete). He also discussed problems he experienced associated with precast concrete septic tanks being used for grease interceptors. When food particles and grease decay in a grease interceptor, it creates hydrogen sulfide gas, which is corrosive to concrete and steel reinforcing bars.

There is a newer standard for concrete grease interceptors titled, ASTM C1613 - 10 Standard Specification for Precast Concrete Grease Interceptor Tanks, which covers design requirements, manufacturing practices and structural performance requirements for monolithic or sectional precast concrete grease interceptor tanks. This standard describes precast concrete tanks installed to separate fats, oils, grease, soap scum, and other asscoaited kitchen wastesIn the ASTM C1613 standard, structural design of grease interceptor tanks is required to be done by calculation or a performance requirement for loading. Structural design requirements for concrete grease interceptor tanks include concrete strength, reinforcing steel placement and openings. Physical design requirements of grease interceptor tanks include capacity; shape; compartments; inlet and outlet pipes; baffles and outlet devices; and top slab openings.

Testing only addresses concrete strength and water tightness using either vacuum testing or hydrostatic testing. There is apparently no grease removal efficiency test or requirement for coating of concrete to prevent corrosive effects of exposure to the environment within the tank.

The UPC 2013 and California Plumbing Code states:

“Where it is determined by the Authority Having Jurisdiction (AHJ) that waste pretreatment is required, an approved type of grease interceptor(s) in accordance with the provisions of this section shall be correctly sized and properly installed in grease waste line(s) leading from sinks and drains, such as floor drains, floor sinks, and other fixtures or equipment in serving establishments such as restaurants, cafes, lunch counters, cafeterias, bars and clubs, hotels, hospitals, sanitariums, factory or school kitchens, or other establishments where grease is introduced into the drainage or sewage system in quantities that can effect line stoppage or hinder sewage treatment or private sewage disposal. A combination of hydro-mechanical, gravity grease interceptors, and engineered systems shall be allowed in order to meet this code and other applicable requirements of the AHJ where space or existing physical constraints of existing buildings necessitate such installations. A grease interceptor shall not be required for individual dwelling units or for private living quarters. Water closets, urinals, and other plumbing fixtures conveying human waste shall not drain into or through the grease interceptor.”

The 2012 International Plumbing Code (IPC) Chapter 10, section 1003 states:

2012 IPC - 1003.1 Where required. Interceptors and separators shall be provided to prevent the discharge of oil, grease, sand and other substances harmful or hazardous to the public sewer, the private sewage system or the sewage treatment plant or processes.

2012 IPC - 1003.2 Approval. The size, type and location of each interceptor and separator shall be designed and installed in accordance with the manufacturer’s instructions and the requirements of this section based on the anticipated conditions of use. Wastes that do not require treatment or separation shall not be discharged into any interceptor or separator.

2012 IPC - 1003.3.1 Grease interceptors and automatic grease removal devices required. A grease interceptor or automatic grease removal device shall be required to receive the drainage from fixtures and equipment with grease-laden waste located in food preparation areas, such as in restaurants, hotel kitchens, hospitals, school kitchens, bars, factory cafeterias and clubs. Fixtures and equipment shall include pot sinks, pre-rinse sinks; soup kettles or similar devices; wok stations; floor drains or sinks into which kettles are drained; automatic hood wash units and dishwashers without pre-rinse sinks. Grease interceptors and automatic grease removal devices shall receive waste only from fixtures and equipment that allow fats, oils or grease to be discharged. Where lack of space or other constraints prevent the installation or replacement of a grease interceptor, one or more grease interceptors shall be permitted to be installed on or above the floor and upstream of an existing grease interceptor.

2012 IPC - 1003.3.2 Food waste grinders. Where food waste grinders connect to grease interceptors, a solids interceptor shall separate the discharge before connecting to the grease interceptor. Solids interceptors and grease interceptors shall be sized and rated for the discharge of the food waste grinder. Emulsifiers, chemicals, enzymes and bacteria shall not discharge into the food waste grinder. This section was modified in the 2015 IPC.

2012 IPC - 1003.3.3 Grease interceptors and automatic grease removal devices not required. A grease interceptor or automatic grease removal device shall not be required for individual dwelling units or private living quarters.

2012 IPC - 1003.3.4 Hydro-mechanical grease interceptors and automatic grease removal devices. Hydro-mechanical grease interceptors and automatic grease removal devices shall be sized in accordance with ASME A112.14.3 Appendix A, ASME 112.14.4, CSA B481.3 or PDI G101. Hydro-mechanical grease interceptors and automatic grease removal devices shall be designed and tested in accordance with ASME A112.14.3 Appendix A, ASME 112.14.4, CSA B481.1, PDI G101 or PDI G102. Hydro-mechanical grease interceptors and automatic grease removal devices shall be installed in accordance with manufacturer instructions. Where instructions are not provided, hydro-mechanical grease interceptors and grease removal devices shall be installed in compliance with ASME A112.14.3, ASME 112.14.4, CSA B481.3 or PDI G101. This section shall not apply to gravity grease interceptors.

2012 IPC - 1003.3.4.1 Grease interceptor capacity. Grease interceptors shall have the grease retention capacity indicated in Table 1003.3.4.1 for the flow-through rates indicated.

2012 IPC - 1003.3.4.2 Rate of flow controls. Grease interceptors shall be equipped with devices to control the rate of water flow so that the water flow does not exceed the rated flow. The flow-control device shall be vented and terminate not less than 6 inches (152 mm) above the flood rim level or be installed in accordance with the manufacturer’s instructions.

2012 IPC - 1003.3.5 Automatic grease removal devices. Where automatic grease removal devices are installed, such devices shall be located downstream of each fixture or multiple fixtures in accordance with the manufacturer’s instructions. The automatic grease removal device shall be sized to pretreat the measured or calculated flows for all connected fixtures or equipment. Ready access shall be provided for inspection and maintenance.

The California OSHPD states grease interceptors shall not be installed in food preparation areas of the kitchens. While this may be a requirement in California, interceptors are often located in the kitchen with local grease interceptors under sinks in other areas of the country with small hydro-mechanical grease removal devices. The California OSHPD also states that grease interceptors shall be installed outside of the kitchen area in locations affording ease of maintenance and servicing. Each fixture discharging into a grease interceptor should be individually trapped and vented in an approved manner.

Grease interceptors are also required to be maintained in efficient operating condition by periodic removal of the accumulated grease and other material. No such collected grease shall be introduced into drainage piping or a public or private sewer. Where the AHJ determines that a grease interceptor is not being properly cleaned or maintained, the AHJ has authority to mandate installation of additional equipment or devices and a maintenance program.

Food Waste Disposal Units and Dishwashers have been the subject of much debate and advertising campaigns with misinformation about the amount of grease a pre-wash sinks. The 2013 California Plumbing Code states, "unless specifically required or permitted by the AHJ, no food waste disposal unit or dishwasher shall be connected to or discharge into a grease interceptor." This makes sense, because food waste disposers grind up food and send it down the drain where it could plug up a grease interceptor quickly. Unfortunately, a food waste grinder is typically installed on a pre-wash sink in a commercial kitchen and a significant portion of the waste from a food waste grinder has food particles and grease with it, making for cement-like mixture when the grease congeals down the drain line, and often leading to drain line blockages, sanitary drain backups and sewer line overflows.

Requirements for solids interceptors to remove solids from the waste stream are being removed and replaced with language allowing the waste discharge from food waste grinders to bypass grease interceptors. These code change proposals to allow food waste disposer to bypass grease interceptors are sponsored by food waste grinder manufacturers because food waste grinders do not work well with grease interceptors and solids interceptors.

Many green kitchen designs have been removing the food waste grinders altogether and providing scrap food waste bins next to the pre-wash sinks. Eliminating food waste grinders and their electrical connections reduces first costs and saves the environment. If the food waste grinder is eliminated from the design, then the pre-wash spray, where a significant portion of the grease waste is generated, can route through the grease interceptor.

Placing food scrap bins at the pre-wash sink allows food waste to be scraped into the bins instead of being ground up by a food waste grinder and sent down the drain line. With the advent of lower drainage flow rates and water conservation measures, there will not be enough water to transport the food waste particles and grease waste down the drain and we could see an increase in drain line stoppages as the lower flow rate fixtures become mandated.

I would like to see a moratorium on legislation related to reduction in fixture flow rates or water use reduction, until research is done to determine what the minimum flow rates are for each pipe size and pipe slope to properly transport the waste solids and grease down the drains. In California, in April, Governor Brown announced mandatory water use reductions because of the limited snow pack in the mountains. They have anticipated lower water levels in the rivers supplying the reservoirs from melting snow. I drove around the Los Angeles area recently and there seemed to be little compliance with the restrictions.

Before the Uniform Green Technical committee meetings in Ontario, California, I talked to a few people and many commented on seeing lawn sprinklers flowing at all times of the day. There was also discussion of the agricultural uses of the water in California where growers in the valley are planting crops in desert areas like walnuts and other crops that require large amounts of irrigation water (20 gallons per tree per day). The decision to grow these crops in a desert is creating additional strain on already limited water resources.

I believe the expansion of irrigated crops and construction development in arid, desert areas should be restricted or limited and the water rates should be raised for everyone including agricultural growers to drive conservation. It is wasteful to have irrigation systems spraying water onto a strip of green in the roadway. This merely provides a free car wash, and the grass will need to mowed and blown off because it is being watered so often. That is not green design with respect to water conservation, though it is green design for the grass. Raising the water rates for all will drive conservation efforts without mandating it will allow fixtures to flow enough water to move the solids down the drain.

In the 2013 California Plumbing Code (CPC) section 1014.2 Hydro-mechanical Grease Interceptors states:

2013 CPC - 1014.2 Hydro-mechanical Grease Interceptors

Plumbing fixtures or equipment connected to a Type A and B hydro-mechanical grease interceptor shall discharge through an approved type of vented flow control installed in a readily accessible and visible location. Flow control devices shall be designed and installed so that the total flow through such device or devices shall at no time be greater than the rated flow of the connected grease interceptor. No flow control device having adjustable or removable parts shall be approved. The vented flow control device shall be located such that no system vent shall be between the flow control and the grease interceptor inlet. The vent or air inlet of the flow control device shall connect with the sanitary drainage vent system, as elsewhere required by this code, or shall terminate through the roof of the building, and shall not terminate to the free atmosphere inside the building. Exception: Listed grease interceptors with integral flow controls or restricting devices shall be installed in an accessible location in accordance with manufacturer instructions.

2013 CPC - 1014.2.1 Capacity. The total capacity in gallons (gal) (L) of fixtures discharging into a hydro-mechanical grease interceptor shall not exceed two and one-half times the certified gallon per minute (gpm) (L/s) flow rate of the interceptor in accordance with Table 1014.2.1. For the purpose of this section, the term “fixture” shall mean and include each plumbing fixture, appliance, apparatus or other equipment required to be connected to or discharged into a grease interceptor by a provision of this section.


1. For interceptor sizing by fixture capacity, see the example below.

2. 1⁄4 inch slope per foot (20.8 mm/m) based on Manning’s formula with friction factor N = .012.

The sizing requirement used in the CPC and the UPC is for full pipe flow, and sanitary drain pipes are typically sized based on half-full flow. The full flow rate comes from when a lever waste is opened on a three-compartment sink; the fixture branch will be flooded up to the flow control device, but not downstream of the flow control device.Hydro-mechanical grease interceptors are required to have a flow control device to control the flow rate through the interceptor to an acceptable flow rate.

2013 CPC - 1014.2.2 Vent. A vent shall be installed downstream of Hydro-mechanical grease interceptors in accordance with the requirements of this code.

2013 CPC - 1014.3 Gravity Grease Interceptors. Required gravity grease interceptors shall comply with the provisions of Section 1014.3.1 through Section 1014.3.7.

2013 CPC - 1014.3.1 General. The provisions of this section shall apply to the design, construction, installation, and testing of commercial kitchen gravity grease interceptors.

2013 CPC - 1014.3.2 Waste Discharge Requirements. Waste discharge in establishments from fixtures and equipment which contain grease, including but not limited to, scullery sinks, pot and pan sinks, dishwashers, soup kettles, and floor drains located in areas where grease-containing materials exist, shall be permitted to be drained into the sanitary waste through the interceptor where approved by the AHJ.

2013 CPC - 1014.3.2.1 Toilets and Urinals. Toilets, urinals, and other similar fixtures shall not drain through the interceptor.

2013 CPC - 1014.3.2.2 Inlet Pipe. Waste shall enter the interceptor through the inlet pipe.

2013 CPC - 1014.3.3 Design. Gravity interceptors shall be constructed in accordance with the applicable standard in Table 1401.1 or the design approved by the AHJ.

2013 CPC - 1014.3.4 Location. Each grease interceptor shall be so installed and connected that it shall be easily accessible for inspection, cleaning, and removal of the intercepted grease. A gravity grease interceptor in accordance with IAPMO Z1001, shall not be installed in a building where food is handled. Location of the grease interceptor shall meet the approval of the AHJ.

2013 CPC - 1014.3.4.1 Interceptors. Interceptors shall be placed as close as practical to the fixtures they serve.

This proximity requirement is to minimize the congealing of the grease within the piping before the greasy waste gets to the interceptor.

2013 CPC - 1014.3.4.2 Business Establishment. Each business establishment for which a gravity grease interceptor is required shall have an interceptor which shall serve that establishment unless otherwise approved by the AHJ.

2013 CPC - 1014.3.4.3 Access. Each gravity grease interceptor shall be located so as to be readily accessible to the equipment required for maintenance.

2013 CPC - 1014.3.5 Construction Requirements. Gravity grease interceptors shall be designed to remove grease from effluent and shall be sized in accordance with this section.

Gravity grease interceptors shall also be designed to retain grease until accumulations can be removed by pumping the interceptor. It is recommended that a sample box be located at the outlet end of gravity grease interceptors so that the AHJ can periodically sample effluent quality.

2013 CPC - 1014.3.6 Sizing Criteria. The volume of the interceptor shall be determined by using Table 1014.3.6. Where drainage fixture units (DFUs) are not known, the interceptor shall be sized based on the maximum DFUs allowed for the pipe size connected to the inlet of the interceptor. Refer to Table 703.2, Drainage Piping, Horizontal.


Ron George, CPD, is president of Plumb-Tech Design & Consulting Services LLC. Visit

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