Grease trap

Manhole covers of a Grease Trap outside of a restaurant.

File:Precast Grease Interceptor.jpg

Precast Concrete Grease Interceptor.

Grease traps (also known as grease interceptors, grease recovery devices and grease converters) are plumbing devices designed to intercept most greases and solids before they enter a wastewater disposal system. Common wastewater contains small amounts of oils which enter into septic tanks and treatment facilities to form a floating scum layer. This scum layer is very slowly digested and broken down by microorganisms in the anaerobic digestion process. However, very large amounts of oil from food production in kitchens and restaurants can overwhelm the septic tank or treatment facility, causing a release of untreated sewage into the environment. Also, high viscosity fats and cooking greases such as lard solidify when cooled, and can combine with other disposed solids to form blockages in drain pipes.

Grease traps have been used since the Victorian days, although Nathaniel Whiting obtained the first patent for a modern day grease trap in the late 1800s. They are used to reduce the amount of fats, oils and greases (FOGs) that enter the main sewers. Effectively they are boxes within the drain run that flows between the sinks in a kitchen to the sewer system. They only have kitchen waste water flowing through them and are not served by any other drainage system such as toilets. They can be made from a number of different materials; e.g. stainless steel, plastics, concrete & cast iron. They range from 35 litre capacity to 45,000 litres and above capacity. They can be located above ground, below ground, inside the kitchen or outside the building.

Types

There are four primary types of devices. The most common are the types specified by ASME (American Society Of Mechanical Engineers),utilizing baffles, or a proprietary inlet diffuser. The most popular are small point of use units used under three compartment sinks within the kitchen. They restrict flow and remove 85-90% of the incoming FOG. There are also several manufacturers that have engineered outdoor grease interceptors to meet the requirements of the ASME Standards, and to take advantage of the improved performance of hydro-mechanical design. Grease trap sizing is based on the size of the 2 or 3 compartment sink, dishwasher, pot sinks, and mop sinks. The cumulative flow rates of the aforementioned devices, as well as overall grease retention capacity (typically in pounds or kilograms) are considered. Currently, ASME Standard (ASME A112.14.3) is being adopted by both of the National Model Plumbing Codes that cover most of the United States. This standard requires that grease interceptors remove a minimum of 90% of the incoming FOGS. It also requires that grease interceptors are third-party tested and certified to compliance with the standard. This third-party testing must be conducted by a recognized and approved third-party testing laboratory.

The second most common type of interceptor is the large in-ground tank, which is usually 750-2000 gallons. These units are constructed of concrete, fiberglass, or steel. By nature of their larger size, they have larger grease and solid storage capacities for high-flow applications such as a restaurant or grocery store. These units can be designed to remove up to 80% of the incoming FOG (according to studies contained within the WERF Report 2008). These types of interceptors do not utilize the hydro-mechanical separation principle described above, but instead rely on gravity. They are commonly called gravity interceptors. To separate FOGS they require a large volume of water and a specific (extended) retention time. There is no test standard but there is a design methodology set out by PDI and IAPMO.

A third system type, known as AGRUs (Automatic Grease removal Units) or GRDs (Grease Recovery Devices), remove the surface grease automatically when trapped. They are smaller because they can empty the grease/oil automatically as often as needed to keep up with grease retention in the cabinet. Grease removal is by hydrostatic pressure, motorized skimmer device, or sensor driven automatic draw-off system. They have an internal strainer basket for food particles, an internal heater element, and an external collector box for removal of grease/oil to recycling container. GRDs using hydrostatic pressure are a recent entrant into the marketplace and are distinct from those units using skimmer devices because they separate FOG continuously rather than relying on timers and skimmers. GRDs do not store brown grease therefore they are odor free. GRDs do not need to be pumped out by an outside pumping service. A good GRD will take over 99% of the grease out of the kitchen waste water. GRDs are also less expensive than the installation of a Type II in ground grease trap and are a good alternative to an expensive in ground installation. Their limitation is in the fact that they require external power and rely on working heaters and in the mechanical types, timers, motors or skimmers. Testing certification for GRD's in North America is ASME A112.14.4.

The fourth type, Grease Converters, trap grease in a tank similar to a grease interceptor, which is then dosed with a culture of microorganisms to digest the grease, converting it to water soluble, bio-degradable products which are safely discharged into the drainage system. When used in conjunction with an automatic dosing system, Grease Converters require minimal maintenance, as the grease doesn't require removal, storage or disposal. Grease Converters are unproven technology in the eyes of certified plumbing organizations. While these work in theory the actual dwell time in a typical small interceptor ( less than 100 gpm) is insufficient to maintain a stable environment for the reaction to occur meaning the "benevolent mix" is quickly discharged to the drain line and sewer where the effects are lost. Also the bacteria is highly sensitive to temperature and needs a stable environment that is neither too hot or too cold, so once out of the "grease converter tank" the activity is either reduced or terminated. Testing Certification for Grease Converters ( Reducers) is ASME A112.14.6.

Uses

Restaurant and foodservice kitchens produce a lot of waste grease which is present in the drain lines from the various sinks dishwashers and cooking equipment such as combi ovens and commercial woks. If not removed the grease will congeal within the sewer and cause blockages and back-ups.

In the United States, sewers back up annually “an estimated 400,000 times, and municipal sewer overflows on 40,000 occasions”. The EPA has determined that sewer pipe blockages are the leading cause of sewer overflows, and grease is the primary cause of sewer blockages. Even if accumulated FOG does not escalate into blockages and sanitary sewer overflows, it can disrupt wastewater utility operations and increase operations and maintenance requirements”

For these reasons, depending on the country, nearly all municipalities require commercial kitchen operations to fit some kind of interceptor device to collect the grease before it enters the sewer. Additionally where FOG is a concern in the local wastewater collection system communities have set up inspection programs to ensure that these grease traps and/or interceptors are being maintained on a routine basis.

Method of operation

The traditional means of achieving this is with the passive grease trap (interceptor). The first patent was lodged by Nathaniel Whiting of California in the late 1800s. The design remains essentially unchanged. The current industry standard for passive grease interceptors is ASME A112.14.3, (or PDI-GD101).

A grease recovery device (GRD) is a recent development which aims to separate out the grease and water and collect the grease for recycling.Recovered grease from a GRD is referred to as yellow grease. The recovered grease is typically added to the waste vegetable oil (wvo) bin where spent vegetable oil is put from the deep-fryer. The current industry standard for GRD’s is ASME A112.14.4. however some GRDs have been successfully tested using EN1825 procedures in Europe.

Both traditional traps and GRD’s use the same physics which is that grease and oil are lighter than water and will rise to the top when the mix is allowed to stand for a time. They both feature a tank with an inverted weir at the outlet in order to allow water out but not grease. A traditional trap is designed to hold the grease within its tank constantly reducing its working volume and hence its ability to allow the required dwell time of 27 seconds or more for the grease/water separation to occur. A GRD on the other hand, continuously removes grease into a separate containment unit.

The design codes for traditional traps allow for an average efficiency of as little as 85% between cleanouts for the trap to be considered adequate. This means that on average 15% of the grease in the waste water is entering the sewer line. Eventually, even with a robust cleanout regime, it is likely that there will be a sewer backup.

A traditional grease trap is not a food disposal unit. Unfinished food must be scraped into the garbage or food recycling bin. Milkshakes, gravy, sauces and food solids must be scraped off the dishes before they enter the sink or dishwasher. This does not always occur in the busy restaurant environment thus the traditional grease trap is often filled with solids which turn into sludge and dissolve into BODs/CODs. The sludge fills the trap up and reduces its efficiency thereby allowing grease to escape into the sanitary sewer system. The sludge also creates a very unpleasant odor which can often back up into the kitchen. Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) is the amount of oxygen consumed in a oxidation processes once a contaminant or chemical compound is added to a water body. BODs/CODs are considered contaminate waste water which makes it more expensive for pumping companies to dispose of.

A problem with a traditional trap is that it must be emptied either by scooping out or pumping all the contents and carting the effluent away to a specialist renderer or to a landfill. This is a very unpleasant undertaking and is, in consequence, often neglected, causing the same problems as if the trap were not there at all. When a trap is full of fat, oil, and grease , the grease has nowhere to go but down the drain and into the sanitary sewer or enter the septic system.

To try to maintain some degree of efficiency there has been a trend to specify larger and larger traps. Unfortunately, providing a large tank for the effluent to stand also means that food waste scraps also have time to settle to the bottom of the tank, further reducing the available volume and adding to the clean out problem. Also, rotting food contained within an interceptor breaks down producing toxic waste (i.e. sulfur gases) - hydrogen sulphide combines with the water present to create sulphuric acid. This attacks mild steel and concrete materials resulting in "rot out". A bigger interceptor is not a better interceptor. Because it will have been in the trap for some time, grease collected in this way will have been contaminated and is unsuitable for further use. This kind of grease is referred to as brown grease. The essential difference between a GRD and a traditional trap or interceptor is that the GRD constantly removes the captured grease into a separate container and thus maintains its efficiency.

Use of chemicals and emulsifying agents.

Grease trap chemicals and emulsifying agents liquify the grease and push the grease further downstream. The grease problem is fixed on a temporary basis leading to bigger grease clogs and blockages in the near future. The liquefied grease will end up in the sanitary sewer. This grease will harden causing clogs and blockages. The clogs and blockages will lead to sanitary sewer overflows. Grease Trap Ordinances and sewer bylaws will result in fines to the restaurant. New grease trap additives arrive on the market daily. Unfortunately, it is impossible to find a grease trap additive, chemical or natural, with third-party certified testing or recognized professional certifications. Frequently, grease trap additives do not perform to levels required for todays environmental standards. For this reason, their addition is prohibited by most municipalities.

In the UK certain grease trap manufacturers recommend the "combined approach" as described by London Grease Traps. This combination approach uses the mechanical efficiency of the grease trap to physically stop FOGs, along with the biological action of dosing through the direct application of biological agents into the grease trap. It can be highly effective as well as very simple to maintainain and cost effective.

Dosing systems must be heavily monitored to be effective. Much like caring for a swimming pool, a dosing systems chemicals must be actively monitored. Added chemicals to one grease trap will not be the same as adding the same amounts of chemicals to the next grease trap. Ineffective use of chemicals does not stop the restaurant from cleaning out or pumping the grease trap. It is difficult to find grease trap chemicals, bacteria, or enzymes that have had certified testing to prove their worth.

Grease Trap chemicals, bacteria, and enzymes are banned for grease trap use in most cities throughout the world due to the extreme costs to repair pumping stations and the removal of grease and grease blockages from the walls of sanitary sewers.

Disposal of grease

Grease that enters a GRD is called yellow grease. Yellow grease is recyclable for the future production of biodiesel, soaps, and cosmetics. Any grease that enters a grease trap or interceptor is called brown Grease. This recovered grease often requires further processing to be suitable for for recycling.

Do not be confused with the disposal of deep-fryer oil. Spent deep-fryer oil is typically dumped into a large barrel or container at the back of a restaurant supplied by waste grease removal companies. Restaurants and food service facilities are paid for the grease that is collected. This grease is dewatered and filtered. The final processed product is biodiesel fuel to name one of many products produced in the industry.

Other disposal solutions:

• Ingredients for paints
• Polymers
• Incinerator co-fuel
• Composts
• Digestion
• No. 6 Boiler Fuel Substitute
• Synthetic Fuels

See also

• American Society of Mechanical Engineers (ASME)
• Plumbing and Drainage institute (PDI)
• NERV (Reactor)
• WERF Report - Widely recognized as one of the best studies done on restaurant grease traps