guide to grease interceptors revised draft1[1]12208 · guide to grease interceptors eliminating the...

Guide ToGrease Interceptors

Eliminating the Mystery

Design & OperationStandards

PDI- G101

A112.14.3

A112.14.4

Sizing & PlacementMaintenanceNew TechnologyLarge Outside InterceptorsCodes & RegulationsWaste Water Treatment

THE PLUMBING & DRAINAGE INSTITUTE800 Turnpike Street ~ Suite 300

North Andover, MA. 01845Phone: 1-800-589-8956 ~ Fax: 1-978-557-0721

Web: www.PDIonline.org ~ E-Mail: [email protected]

© 1998 The Plumbing & Drainage InstituteRevised 2008

Acknowledgments

The Plumbing & Drainage Institute wishes to thank its member companies named below,For their support and assistance in producing this very much needed Guide To Grease Interceptors:

Josam Company

Jay R. Smith Mfg. Co.

Wade Division –Tyler Pipe

Watts Regulator Company

Zurn Industries, Inc.

We particularly wish to thank Jon Wehrenberg whose work entitled Understanding GreaseSeparation and Recovery formed the basis for our Guide and who wrote the initial draft.Thanks to George Flegel for his efforts in the areas of technical accuracy and proof-readingwhich are much appreciated. Many thanks to Cathy Ackil who produced the outstandingIllustrations, and to Jerry McDanal, who contributed much effort to revising and fine-tuningthe text. Finally, thanks to Billy Smith, Jim Hadley, and Al Becker for checking the finished work.

Plumbing & Drainage InstituteWilliam C. WhiteheadJune 15, 1998

Guide To Grease Interceptors 1

GREASE INTERCEPTORS:ELIMINATING THE MYSTERY

In the late 1800’s Nathanial Whiting of California patented the passive gravityseparator, a device known as a grease trap.That device remains relatively unchangedtoday as a means of separating fats, oils andgreases from wastewater. Despite the fact thatthe technology works well and is little changedfrom its original design, grease is a problem inalmost every locale in the country.

When grease enters the waste stream itcreates a variety of problems and once fats,oils and greases have entered the wastestream they are rarely suitable for recyclingbecause of contamination from metals,chemicals and pathogens. Further, disposalmust be in accordance with local and federalrequirements. The acronym “FOG” is utilizedin most contemporary references to fats, oilsand greases because with the increased useof vegetable oils and fat substitutes, greasesnow consist of more than variousconfigurations of fats.

The first problem is not one which affectsthe waste water system; it is the loss of apotentially valuable resource. When recycledbefore being in a drain, FOG can be used in avariety of products such as soaps andcosmetics, fertilizer, lamp oil, animal feeds andmunitions. Aside from the loss of a valuablecommodity, when FOG enters the waste waterstream there is a large and unnecessaryeconomic loss due to additional problems.Grease can block pipes, can form aggregateswhich in turn can also cause blockages, andgrease encapsulated solids can increase thetime and cost of treating the wastes at wastewater treatment plants.

The problems are not limited to any specificsize wastewater collection system or treatmentfacility. Private systems such as septicsystems will fail and require costly repair orreplacement just as will large systems whichmight be found in cities such as Chicago,Phoenix, New York or Miami. Grease has beenknown to cause blockages due to occlusion inpipes many feet in diameter, and in the city ofChicago (as an example) millions of dollarshave already been spent replacing largesewers whose internal diameters can now bemeasured in inches due to solidified grease.

All of the problems are unnecessarybecause separating grease from waste wateris easily accomplished. Grease interceptorsfunction using gravity and coalescence as ameans of separation. Greases, fats and oils areabout 89 or 90% of the weight of water. Toseparate them from water an interceptorprovides a separation chamber which allowsFOG to rise to the surface. FOG free water thenexits from the separation chamber at the lowpoint farthest from the inlet end.

To simplify the influences which affectseparation one could say there are only threemajor factors which must be considered. Theyare the design of the interceptor, theinstallation of the interceptor, and themaintenance of the interceptor.

Guide To Grease Interceptor 2

INTERCEPTOR DESIGN

Because of the scope of problems relatingto FOG, there are a large number of productdesigns and offerings. Due to this fact it wouldseem difficult for one to choose an interceptorwhich would function as designed. Fortunatelythat is not the case. See Figures 1 and 2.

In the early 1940’s the United States government through the Army Corps ofEngineers, the Quartermaster General, the

Surgeon General, and the Research Committeeof the Plumbing and Drainage Manufacturer’s Association (now the Plumbing and DrainageInstitute), and others held a series ofconferences to develop a testing program toestablish a means of rating flows and capacitiesfor grease interceptors manufactured at thattime.

From the efforts of the involved parties, andas a result of exhaustive laboratory testing bythe Iowa Institute of Hydraulic Research at theState University, a standard now known as PDI-G 101 was developed. Since the first issue ofthe PDI standard in 1949 it has been widelyrecognized, and it is included as the basictesting and rating requirement of MilitarySpecification MIL-T-18361

As a result of the existence of PDI-G 101 aproduct which is designed to that standard andis certified as having met that standard can beinstalled with the confidence that it will be anefficient separator at flow rates up to andincluding its rated flow and up to and includingits rated capacity for retained FOG.

The products which are certified to PDI-G101 are Hydro Mechanical Grease Interceptorsup to 100 GPM which are typically installed atthe fixture or the point of use. Since a PDIcertified interceptor is small, relatively speaking,i t accomplishes its separation efficiency bymeans of specially engineered internal bafflingarrangements used in conjunction with anexternal vented flow control device.

Using the principles of fluid mechanics, aPDI certified interceptor takes advantage of airentrained in the effluent by the vented flowcontrol device to accelerate separation. SeeFigure 3.

Guide To Grease Interceptor 3

FOG laden waste water passes through avented flow control device on its way into a PDIcertified interceptor. The flow control devicehas an internal orifice which limits the flow intothe interceptor to the interceptor’s rated capacity. As the effluent passes through theorifice, which is sized to compensate for theamount of head in the waste water collectionsystem, air is introduced through the vent(which is actually an air intake). The entrainedair remains with the effluent until it enters thegrease interceptor.

Upon entering the grease interceptor, theeffluent is directed through the separationchamber of the interceptor by means of asystem of baffles. The baffles serve tolengthen the flow path of the effluent toincrease the time of separation while providinga non-turbulent environment for separation totake place. The entrained air will separate fromthe effluent quickly. As it does so, itaccomplishes two things; First, the escaping airaccelerates the separation of FOG as it risesrapidly to the surface of the water in theseparation chamber. The rising air bubblesliterally pull the FOG globules to the top of thewater. Second, the air released then providesa small amount of positive pressure above thecontents of the separation chamber to regulatethe internal running water level of the greaseinterceptor.

Most manufacturers provide methods toregulate internal air pressure to prevent thecontents of the separation chamber from beingforced downward thus reducing theinterceptor’s capacity and efficiency. See Figure4. Furthermore, most codes contain languagerequiring a means of preventing theinterceptor from becoming air bound.Typically, that language will state: “Venting.Interceptors and separators shall be sodesigned that they will not become air boundwhen airtight covers are used”.

For the specifier or purchaser of a greaseinterceptor to be assured the product will

perform as intended it is only necessary toverify the product has been certified to aknown standard such as PDI- G101.

No discussion of the design of grease in-terceptors would be complete without coveringlarge Gravity Grease Interceptors which are


typically located outdoors. In certain areas ofthe country existing codes will allow or evenrequire an interceptor whose sole specificationis size. Unfortunately, even today, more than100 years since Nathanial Whiting patented thegrease interceptor there exists no greaseretention performance standard for or base ofaccumulated data on large capacityinterceptors. There is a high expectation thatlarge capacity interceptors will work, but there isno consensus standard or test data to stipulateor verify their grease removal performance.See Figures 5a and 5b.

Since remotely located outdoor (Gravity)interceptors must deal with conditions differentfrom point of use interceptors the designrequirements will vary. One must first definethe required retention time based upon themaximum anticipated rate of flow. This variesfrom city to city or region to region so it mustbe left up to the appropriate administrativeauthority to establish this requirement throughtesting based upon installation conditions.These requirements currently vary from simplestatements (in local codes) of minimumcapacity size (such as 750 gallons) to retentiontimes based upon flow rates (such as 30minutes) to formulas which make assumptionsabout the amount of water used per mealserved. The lack of uniformity in sizingrequirements for remotely installed interceptorsis indicative of the lack of consensus abouttheir performance.


A properly sized and designed greaseinterceptor may not work or may work lessefficiently if it is installed incorrectly. As basic asit seems, the interceptor must not be installedbackwards. This is mentioned since far toomany interceptors which are condemned for notworking have merely been installed backwards.The problems relating to installation, however,go beyond the obvious. Regardless of whetherthe interceptor is a certified Hydro MechanicalInterceptor or a large Gravity interceptor, one ofthe most important installation practices tofollow must be to locate the interceptor as nearas possible to the source of the FOG ladenwater. See Figures 6 and 7. As statedpreviously, this is important because every footof piping between the source of FOG ladenwaste water and the interceptor is unprotectedand is a potential maintenance problem.

A second reason for locating the interceptornear the fixture: FOG separates best when theeffluent is relatively hot.

While the laws of physics dictate that FOGseparates from water at a slower rate astemperatures increase, in these applicationsthe separation rates at room temperature andat elevated temperatures (testing has beendone up to 200 degrees Fº) are so close that theother benefits outweigh the slight improvementin separation rate. For example, in waste water,particularly the FOG laden waste water fromcommercial kitchens, it is likely there will besolids present. These solids and the FOG aremore likely to form a globule, the specific gravityof which exceeds that of FOG alone. As theeffluent temperature rises however, the FOG willbe more likely to separate freely from thosesolids.

Keeping the FOG from coalescing on thesolids is important because the resultant

INSTALLATION


material may sink, and ultimately be dischargedfrom the interceptor. If on the other hand, theFOG is free to separate from the solids in thewaste water due to the higher temperatures,which tend to make the FOG less viscous, theFOG is more likely to be retained in theinterceptor.

FOG laden solids passing through theinterceptor create two problems. First, they tendto form balls or aggregates (grease can becomevery hard) posing a blockage problem in thewaste water collection system. Second, if thesematerials do make it to the waste watertreatment plant without creating any blockages,they can make waste water treatment muchmore difficult since degradation of FOGconsumes oxygen necessary for the digestionof the waste in the treatment plant and becauseFOG decomposition is quite slow, it can passthrough the plant. This increases the effortrequired to treat wastes and can causeviolations of the plant’s discharge permit.

Unfortunately many of the codes inexistence around the country fail to recognizethe benefits of hot water in the FOG laden wastestream and require oversized GravityInterceptors to allow the waste water to cool.PDI has done extensive testing on the affect ofhot water on separation and can support throughdata the fact that hot water has little effect onseparation efficiency. The EnvironmentalProtection Agency, in their document EPA625/1-80-012 (Design Manual: OnsiteWastewater Treatment and Disposal Systems)is specific in recommending the use of hotwater and proximity to the source to enhanceretention of FOG.

When discussing the location as a factor ininstallations, it should also be pointed out thatin addition to proximity to the fixture, theinterceptor should be located so thatmaintenance can be easily performed.

Although this recommendation also seems soobvious as to not need discussion, someinterceptors have been installed under sinkswithout clearance for removal of the cover.Some interceptors have been placed in the floorand tiled over; some have been located so thatthey are literally hidden from view; and somelarge outdoor interceptors have actually beenpaved over. The placement should allow thecover to be visible and easily removable forcleaning, and clearances should be such thatthe internal baffling can be serviced. With thecover removed, all wetted surfaces should bevisible. This is necessary not only for access toclean the interceptor, but also to have thecapability to easily inspect the interior forpotential problems such as damaged bafflesand blocked air relief bypasses.

The flow control fitting furnished with PDIcertified interceptors must be installed in thewaste line ahead of the interceptor. It should belocated beyond the last connection from thefixture and as close as possible to theunderside of the lowest fixture to minimize theeffects of head pressure. When the wastes oftwo or more sinks or fixtures are combined tobe served by one interceptor, a single flowcontrol fitting may be used. Any flow controlfitting installation not in conformance with theserecommendations requires manufacturerconsultation.

The air intake for the flow control mayterminate under the sink drain board as high aspossible above the flood level of the sink inorder to prevent overflow. It may also terminatein a return bend at the same height outside thebuilding. When the fixture is individually trappedand back vented, the air intake may intersectthe vent stack. All installation recommendationsare subject to the approval of the local plumbingcode authority. See Figure 8.


One of the most controversial issues re-lating to Installation is: what fixtures or sourcesmust be part of the FOG interceptor system?All drain-borne FOG is a problem and if theproblem is going to be solved all sources ofFOG must pass through the grease interceptor.There is little controversy about connecting potsinks. There is some controversy aboutconnecting dishwashers. There are somequestions relating to floor drains, but dischargefrom food grinders (or garbage disposals) isalmost universally required to bypass thegrease interceptor or to have the pulverizedsolids removed from the waste stream before itenters the interceptor.

The food grinder (and the associated pre-rinse station at the dishwasher) is one of thesingle greatest sources of FOG. Yet despitethat fact, most codes forbid food grinderdischarge from passing through a greaseinterceptor. Technologically there is no reasonfor the waste stream to bypass the greaseinterceptor if the solids have been removed. SeeFigure 9.


MAINTENANCE

Even the best designed interceptors,properly installed will fail if they are notmaintained. The precise requirements formaintenance are not possible to define sinceconditions at each installation vary. In terms ofthe typical code, maintenance must beperformed before the grease in the waste waterdown stream from the interceptor exceedslocal limits.

While that is a simple statement to make, itis impossible for the user of a greaseinterceptor to determine when those limitshave been exceeded. The method fordetermining when an interceptor’s rated capacity has been reached is fairly simple if itis a PDI certified interceptor. A PDI certifiedinterceptor

has a rated retention capacity equal to twice itsflow rate expressed in pounds. For example, a35 GPM interceptor is rated to retain at least70 lbs. of grease. A user may determine acleaning schedule by measuring how muchgrease has been trapped over a period of time.

Grease will weigh about 7 pounds pergallon, and if it is determined that a 35 GPMinterceptor accumulates about 5 gallons ofgrease every 4 days it would be easily andcorrectly assumed that the interceptor must becleaned no less than once a week. In fact, if theuser must comply with a code which limitsgrease to 100 parts per million, cleaning wouldbe recommended every 2 or 3 days. Whencleaning is discussed, it should be understoodthat cleaning an interceptor should alwaysinclude the removal of grease from the top ofthe separation chamber as well as any solidswhich have accumulated along the bottom.See Figure 10.

The actual frequency of cleaning a certifiedinterceptor will vary depending upon a widevariety of factors; the type of food served willdetermine how much grease will enter theinterceptor. An interceptor used for cleaningutensils or limited to serving trays in arestaurant where no food is actually preparedis going to accumulate a lot less grease thanone used in a full service restaurant where allof the food preparation equipment and utensilsas well as dishes are washed. Another factoraffecting the cleaning cycle will be whether afood grinder is discharged into the interceptor,and whether the food specialty is high in FOG.

The allowable grease content in the waste


water will also determine the frequency ofcleaning. It should be noted that all PDIcertified interceptors will separate efficientlyenough to meet any grease limits (which mayrange from 50 parts per million up to as much as600 parts per million depending upon thejurisdiction). They may require cleaning whenas little as 25% of their rated capacity hasbeen reached depending upon the limitsestablished by the administrative authority. Thisstatement is based on an analysis by PDI ofaccumulated test data. That data wascollected at full rated flows, and does vary fromproduct to product.

The cleaning cycle on large capacityinterceptors is less easily determined.Anecdotal evidence gathered from a variety ofsources and communities indicates that theirsize is often interpreted as meaning lessfrequent cleaning is required, and to a degreethis may be true. From information gatheredfrom a variety of sources however, theconsensus appears to indicate the cleaningfrequency for large interceptors is in the rangeof 2 to 4 weeks. This amount of time is themaximum allowable for large interceptors to stillmeet the discharge limits on FOG. Due to thenature of the large interceptors, the user is notlikely to be the cleaner, and in some casesmay actually be prohibited from cleaning theinterceptor. Usually cleaning will be done by arenderer, a septic tank service, or a companywhich specializes in grease interceptorcleaning. The annual cost of regular cleaningis likely to average between $2, 800 and $4,000 depending again upon the discharge limitsand the local market costs. (January, 1998average cost)

Regardless of what the cleaning cycle isdetermined to be, it has been shown by actualfield experience that one of the biggestobstacles to regular maintenance has beenthe odors usually associated with interceptors.The easiest way to eliminate that

problem is frequent cleaning. If cleaning thegrease interceptor becomes a part of the dailyroutine it usually will only require about 15minutes and there will be limited or noobjectionable odors.

It has been determined that when foodgrinders are part of the waste system, and aproperly sized solids interceptor, cleaned daily,is located ahead of the grease interceptor, theodors normally associated with the greaseinterceptor are not present because the foodparticles which decay and cause odors neverreach the interceptor.

Use of the solids interceptor improves thegrease quality to extent that the recoveredgrease may be disposed of with the goldenfryer grease which is usually purchased by thelocal renderer. Now instead of paying fordisposal, the restaurant may be compensatedfor the grease, since it can be recycled into avariety of products.

When regular maintenance is notperformed the obvious result is a greaseinterceptor which becomes unable to separatethe FOG due to overloading, thus passingthese materials downstream. Unless i t isequipped with an electronic, sensor controlled,positive inlet closure valve to prevent suchoverloading, no grease interceptor wi l lotherwise automatically shut itself down toprevent overload discharge. Apart fromviolating codes or ruining the on-sitewastewater treatment system, sewer blockagesand the associated health risks are likely.Some FOG generators would rather do almostanything but clean a grease interceptor. FOGgenerators have several options, some ofwhich are acceptable alternatives, and someof which are possibly legal, but neverthelessunacceptable.

One alternative is to engage the services ofa company which specializes in cleaning


interceptors. This is not an inexpensiveapproach, and in the case of large interceptorsis required. If the service is performed as oftenas necessary, it insures the interceptor willfunction as intended.

Another alternative is the use of aninterceptor that is considered to be a GreaseRecovery Device (or Grease Removal Device).A GRD is a Hydro Mechanical GreaseInterceptor which has as an integral part of itsdesign a means by which grease is removed.

A GRD will be one of two basic types:1. Timer controlled - See Figure 11.

2. Sensor controlled - See Figure 12.

Timer controlled devices typically utilize adisk or belt which passes through the FOGlayer and a squeegee device to wipe theaccumulated FOG from the disk or belt into adrain trough and into a FOG receptacle. Othermeans of removing the FOG include a pump orgravity flow activated by the timer. They areusually regulated by a 24 hour timer which is setupon installation. The timer will operate theFOG removal system for a set time or timeseach day.

Sensor controlled devices have the ability tosense the presence of FOG. By detecting FOGand initiating the removal process only whennecessary and as often as necessary, the GRDcan always keep the retained FOG below therated capacity of the device. The sensoroperated devices use valving and gravity orpump assisted FOG removal.

As FOG problems continue to be a factor, inmany jurisdictions the use of a GRD, is beingmandated. It must be noted that while a GRDeliminates the daily routine of greaseinterceptor cleaning, these devices do requireperiodic maintenance to remove trapped soliddebris, removal of scum and a check of systemoperation.

The previous two examples of methods toavoid routine maintenance are certainly goodand acceptable choices. Some others are notand are to be avoided in conventional greaseinterceptors. The first is the use of chemicals,often touted as environmentally friendlyenzymes or emulsifiers. These materials mayeven have names which imply their use isenvironmentally acceptable. The second is theuse of “bacteria” or organisms designed to digest wastes.


In the first category, the materials usedwork by changing the structure of FOG from ahydrophobic material that is unlikely to mixfreely with water (thus allowing separation toeasily occur) to a hydrophilic micelle whichmixes freely with water thus inhibiting orpreventing separation from occurring in theinterceptor. The use of these additives onlychanges the structure of the FOG for a limitedperiod of time, and eventually the FOG willrevert back to its original form, usuallydownstream in the public waste water col-lection system. While this practice, in conven-tional interceptors, works to pass the problemson to somebody else, the methods jurisdictionsuse today to detect FOG content in the effluentare sophisticated enough to accurately identifyany violator of the sewer codes.

The second method, the use of bacteria (orbio-remediation as it is called) works. Theconcept of bio-remediation is sound: trapgreases and digest them in the interceptor toconvert the grease permanently into the by-products of digestion. This is exactly whathappens in a sophisticated waste watertreatment plant. See Figure 13. Bio-remediation does not eliminate the need formonitoring the effluent quality, routinemaintenance to deal with undigestedmaterials, or inspections to insure allcomponents are clean and functioningproperly.

For an additive to have any positive effect, itmust be known to produce net reduction inweight and volume of the FOG either throughbiochemical or catalytic processes. Such -disposal methods require engineered devicesand professional administration.

When dealing with a conventional greaseinterceptor, the most practical and economicmaintenance practice is to regularly remove theFOG and dispose of it in accordance withapplicable solid and special waste disposalregulations.


S U M M A R Y

The problems relating to fats, oils andgreases (FOG) are easily addressed, to do sorequires an understanding of the principles ofseparation and a willingness to do all that isnecessary. Dealing with FOG problems is notlimited to restaurant owners; it is an issue anumber of parties must share in resolving.Codes must be written or, more precisely,rewritten to be technically correct.Administrative Authorities must make certainwhen they write and/or endorse codes that allof the issues have been correctly addressed.

Interceptors and FOG disposal systemswhich have been properly designed andcertified must be required and used. They mustbe installed as they were tested and wereintended to be installed. And last, but notleast, the devices must be maintainedaccording to the codes and the manufacturer’srequirements.

REMEMBER: Proper maintenance of even thepoorest interceptor wil l provide better resultsthan the lack of maintenance on the bestinterceptor.

guide to grease interceptors revised draft1[1]12208 · guide to grease interceptors eliminating the...

Documents