Scary descriptions of “monster fatbergs” that have blocked sewer water in Baltimore and the U.K. make for effective newswriting. But clogged-artery metaphors and gross-out tactics aside, fat, oil and grease (FOG) do pose an expensive threat to sanitary sewer collection systems.

The monetary impact nationally is in the neighborhood of $20 billion a year, according to a U.S. Environmental Protection Agency (EPA) report, based on sanitary sewer overflows that result in beach closures, stream contaminations, cleanups and repairs. The true cost is likely much higher, because the EPA figure doesn’t factor in increased maintenance and corrosion, according to Plumbing and Drainage Institute Executive Director Max Weiss.

Corrosion, especially, is a growing concern. Because the chemical composition of FOG has changed in recent years — along with the cleaning and sanitizing agents developed to deal with those compounds — hydrolysis and beta oxidation rates are much higher now than they were even a few years ago, Weiss notes.

“And so, the hydrogen sulfide generation and the subsequent breakdown of hydrogen sulfide to elemental sulfuric acid is also at a much higher rate,” he says.

That means it’s not uncommon to see PH values below 4 in a gravity grease interceptor left unemptied after 30 days. Federal prohibition on discharge is at 5 PH, Weiss points out. Those acidic PH values cause real damage.

“There’s physical evidence of concrete interceptors showing rebar in less than a year, structural failures in a year or less, cast-iron embrittlement and failure in a year, and pump lift station damage in a year or less,” he says. “All of it’s related to extended storage periods. By extended, I mean anything beyond 30 days in grease interceptors. In those instances, a poorly maintained grease interceptor is more likely than not to create more damage as a result of corrosion than if there was no interceptor at all. And then, there’s been a historical fact that the larger the interceptor, the poorer the maintenance.”


Find the right size

Right-sizing interceptors is a particular frustration for plumbing engineers. Not because the science or math is new or complicated, but because there is no consistency between jurisdictions regarding which method is required for sizing. Often, the building owner also views the necessity as a hassle or afterthought.

“It’s all over the map,” says Silvano Ferrazzo, commercial development manager, site works, for Zurn. “If you go across the country, for a typical quick-serve restaurant — depending on which jurisdiction you’re in — you could have a burger joint that uses a 5,000-gallon interceptor or one that uses a 1,300-gallon interceptor. And they’re all acceptable depending on where you are in the country. Same restaurant, same food, same servings, same load on the interceptors, but it widely varies across the country. And that drives us and engineers crazy.”

Some jurisdictions require the engineer to use the Uniform Plumbing Code method of measuring each drain in the restaurant and its flow rate to determine the size of the interceptor. Others require a method which considers the type of food, the number of dishes, whether flatware is washed or thrown away and other restaurant-specific criteria.

Ferrazzo thinks the second method often oversizes interceptors. And then, there are locations that have their own method, or just require every project to use the same size. If Ferrazzo could choose, the world would follow the PDI guide for hydro-mechanical grease interceptors and the UPC guidelines for gravity grease interceptors. Plumbing engineers can start there, at least, although depending on which side of a county or state line they are standing on, even those best practices may cause them to run afoul of local ordinances.

“My advice is to work with the regulators and inspectors and try to understand why they’re requiring that larger unit, and try to convince them otherwise,” he says.

A convincing argument is important, because, as noted by Weiss, the right size is important. If it’s too big, there will be more buildup of hydrogen sulfide that combines with water and bacteria to cause sulfuric acid. Larger interceptors also encourage less frequent cleanings.

“If it’s oversized, that just multiplies,” Ferrazzo says. “If you have a right-sized interceptor, there’s going to be less bacteria build-up, probably slightly more frequent cleaning, and that eliminates a lot of the problems downstream.”

Undersizing causes problems, too.

“Then you’re not giving the grease enough time to settle and separate,” says Brennan Dougherty, national specifications manager at MIFAB. “I think the most common mistake, if anything, is you’re not accounting for all the fixtures that are running to that unit. Whereas you might have a floor drain that might be going to that unit and you didn’t account for that at all, or you might have a whole bay of sinks that might be going to it.”


Long-distance running

Recent commercial trends have pushed grease interceptors further from the source of FOG. This is not a great idea, experts say.

“Historically, they’ve had interceptors that are right there underneath the sink,” Dougherty says. “Whereas now we’re trying to put it way outside the footprint of the building, so it’s having to travel a good amount of run in that pipe, which ultimately results in clogs and having to call in that plumber to clean the line.”

One option is to run a heat tracer along the long pipe leading to the interceptor to prevent coagulation, though Dougherty says he has never worked on a project that specified that. It can add tens of thousands of dollars to a project, he says, and can be avoided by designs that place the interceptor closer to the source. Dougherty understands why a restaurant owner doesn’t want a grease pumping company to run dirty hoses through the restaurant to empty the interceptor, but that’s not the only option. The design could specify a nozzle on the outside of the building for the pumper.

Plumbing engineer Ethan Grossman of SmithGroup in Boston remembers one such job that called for an external grease interceptor some 300 feet from the source, which he knew was a bad idea.

“What we ended up with was a compromise,” Grossman says. “We had a grease interceptor for the kitchen, and then we had another one that we put inside the building as well, because there was no way we could fit it outside near the building. But that was the compromise, which was kind of overkill, but we ended up doing it anyway.

“That idea that there has to be an external grease interceptor, but if it’s 300 feet away, why not just put it inside the building near the kitchen?” he continues. “I get there are pros and cons. Some people would say, ‘We want it outside because it’s easier to service.’ We could drive up a truck and pump it out. But the con is you’re going to have 300 feet of grease waste just trying to be conveyed through this pipe. It’s probably going to solidify before it even gets there.”

He says engineers need to remember to communicate.

“I think what would be helpful for young designers, and what’s been successful for me, is to tell people to try to be creative with their designs,” he says. “And talk to the department of public health, talk to the civil engineer, talk to the architect, and present different options. That will help the jobs go easier.”

Weiss agrees with Grossman’s decision not to place the interceptor 300 feet from the source.

“That installation would do more harm to the public collection system than if there was no interceptor at all,” Weiss says. “If you use a hydro-mechanical grease interceptor and place it close to the source, that is the most cost-effective and most pollution-preventative effective means available. And of course, everybody thinks that it belongs on the end of the pipe a long way away. Well, those kinds of paradigms and assumptions are extraordinarily injurious to the collection system.”



Weiss says there is a lack of education surrounding FOG prevention and grease interceptors, but no shortage of resources.

“The information is there,” he says. “I won’t say that it’s readily apparent, but for an inquiring mind, it’s there, publications are there. The evolution of fats, oils and greases is perhaps new, but the science which analyzes and quantifies that effect is not. There are practices and policies in place that are based on inadequate factual data, and it results in a paradigm or an incorrect conclusion that the interceptor should not be installed close to the source and that’s just completely wrong.

“It’s much easier administratively to sign off on a manifest for an outdoor tank than it is to visually and physically inspect and enforce maintenance on smaller interceptors. That’s just an administrative fact of life. And, unfortunately, policies gravitate toward the least expensive.”

Codes and standards state that interceptors should be placed as close as possible to the source, but that often gets lost by administrative bodies, especially ones operating in municipalities that view grease interception as an “unfortunate, distasteful and unwelcome obligation imposed by the pretreatment requirements and the Clean Water Act,” Weiss says.

“And they don’t receive the attention, the science, the funding and so on to be effectively administered,” he says. “You can’t blame the folks who are working there, because they’re fighting an uphill battle. They’re underfunded, underappreciated and undertrained. At the same time, they’re being assaulted from upstream, uncaring dischargers. I don’t envy them a bit, nor do I blame them. But it is a fact that pretreatment policies are based first on administrative convenience.”

Indeed, even a properly designed grease interception system is out of the specifying engineer’s hands once the project is over.

“Proper maintenance of these units can help protect a clean system and minimize chances of a blockage or ‘fatberg’ in the future,” Josam Director of Engineering Brian Tubaugh says. “While there have been advancements such as grease sensing/alarm devices and so-called automatic-type units using various methods to remove the grease layer from the interceptor, the fact is that all of them require some regular maintenance to ensure continuing performance.

“The most effective ‘innovation’ I have seen for plumbing engineers is not in product, but in practice,” he continues. “Specify grease interceptors that are tested and listed to the most demanding product performance standard in the industry - PDI G101 - and use proper sizing and unit location methods.”

The performance standard, sizing methods and unit location information can be found at


Markus Lenger, CEO and co-founder of FOG bioremediation technology company CleanBlu, has made two proposals on behalf of a task group in IAPMO’s WE-Stand technical committee. Both will be considered at a meeting this month in Ontario, California.

The first would prohibit wet food digesters from discharging to grease interceptors unless the grease interceptor is designed for that purpose. Wet digesters manage and dispose food scraps by using mechanical agitation and aerobic digestion, sometimes aided by the addition of enzymes or microorganisms and potable water, with the process residuals discharged to the sewer, Lenger wrote in the meeting monograph.

“There’s a great confusion out there with the food digesters, and very, very little guidelines,” Lenger said in an interview with pme. “We need to push that industry to start self-regulating and come up with something that a plumbing engineer can use. If you’re a plumbing engineer, you need to size this. You really wouldn’t know how to size a grease interceptor given the current form of this. You wouldn’t know what to do.”

He says digesters send more organic material to the grease interceptor than they are currently designed or sized for. When the excess organic material sits in the grease interceptor, it produces more odorous hydrogen sulfide, as well as hydrochloric acid, which causes corrosion in concrete grease interceptors, and in concrete sewer systems down the line.

“Before installing a wet digester, care should be taken to make sure grease interceptors are sized large enough to accommodate the additional organic loading and do not impede the grease interceptor performance,” he wrote in the monograph. “There is lack of data to prove there is no impact, there’s also lack of data to prove there is a significant impact. But we all can agree that a new organic source of unknown characteristics is introduced into a plumbing system and great confusion exists among regulators and customers about a potential impact to the plumbing system.”

The second proposal concerns the effect of high-temperature commercial dishwashers discharging into grease interceptors on the efficient separation of FOG waste. The task group recommends temperature limitations for wastes entering the grease interceptor. Lenger contends that high-temperature dishwashers prohibit FOG from dissolving in the effluent, which then allows the FOG to bypass the grease control devices. This proposal is expected to receive some opposition. When asked for his take on the relationship between temperature and grease separation, Weiss said all other variables equal, elevated temperature decreases time for vertical ascension of less dense material.