Grease buildup on interior of sewer line

The Problem

Bacon frying, chickens roasting, ground beef and onions browning, are all signs and smells of a good restaurant. But just as our own arteries are at risk with a diet high in fat, sanitary sewer systems (SSS) are at risk for buildup and blockages with flows full of fats, oils and grease (FOG’s). Cleaning methods in food service facilities cause FOG’s to pass through sinks, dishwashers and drains, and travel downstream. If left unchecked, sewer blockages can occur causing major problems of raw sewage overflow. Each year, municipal sanitary sewers overflow on 40,000 occasions, dumping potentially deadly pathogens into the nations streets, waterways, and beach areas. The EPA projects combined sewer overflows discharge 1.2 trillion gallons of sewage and storm water a year, enough to keep Niagara Falls roaring for 18 days.

Current Practice

Most “authorities having jurisdiction” (AHJ’s) require some method to remove FOG’s from the waste stream. This is usually done with large, outside (usually concrete) interceptors, or smaller indoor grease traps. Large outdoor interceptors have long retention times and are maintained by pumping from a service provider. There is a great deal of variation in design by jurisdiction, and sizing criteria is variable as well. Pumpouts are usually scheduled on a regular basis, sometimes dictated by the local jurisdiction. Usually this pumpout creates a paper trail that the local enforcement personnel can inspect to insure compliance. Large outdoor traps are also manually inspected using a “sludge judge” to monitor build up of solids and grease.

Not all locations have the room for an outdoor interceptor. Existing establishments, and those in urban areas, cannot always excavate and bury a large tank. Indoor, under the sink style traps come in a variety of designs and need to be cleaned out regularly by the food service personnel. Many manufacturers recommend daily cleaning. The local jurisdictions are recognizing this is difficult for the food service provider to do, and even more difficult to monitor. Consequently, the sanitary sewer systems are at risk for blockages.

Flow Rates

Currently indoor traps are rated by flow rate, usually in gallons per minute. The physical size of these traps (in gallons of holding capacity) can be calculated based on their size, and retention time can be determined. Many manufacturers don’t list the actual liquid capacity, so you may have to calculate based on: length times width times height of liquid level (in feet) times 7.48 gallons per cu. ft. For instance; assume you have a grease trap rated at 25gpm with a liquid capacity of 50 gallons. The retention time would be 50g/25gpm = 2 minutes. Size and flow rates through many of these small traps is a concern. Most are rated at specific flow rates, and when you calculate the liquid volume of the trap, the retention time is less than 1 or 2 minutes. In comparing traps and interceptors it is very important to look at expected flow rates, and the liquid capacity of the vessel. Retention times can then be estimated, and compared.

Understanding Retention Time With “The Test”

The most important variable in separating FOG’s from water is time. Think about oil and vinegar salad dressing. Once you shake it and let it sit, it starts to separate. WSSC (Washington Suburban Sanitary Commission) did studies to determine what retention time was needed to adequately separate fats oils and grease from water. They used a variety of oils including actual commissary grease, and considered the addition of soap as an emulsifier. WSSC has adapted sizing criteria based on retention time. For new construction they require a 24 minute retention time. For existing and repair systems, a minimum of an 8 minute retention time is required. Their requirements are well founded, and to prove it to yourself, try this test.

• Use 2 clear jars with tight fitting lids
• Add 3/4 cup oil, 1 1/2 cups warm water and a few drops of dishwashing soap
• Prepare another jar without soap for comparison
• Shake jars for 30 seconds to simulate discharge from dish washing facilities
• Set jars on table and start a timer
• Watch and record separation at 1 min, 2 min, 8 min and 24 min.

What retention time do you think is adequate?
Do most restaurants use soap? What difference does it make?

The Town of Cary, North Carolina adopted code in 1999 requiring a minimum of 24 minute retention time and has seen dramatic decreases in Sanitary Sewer Overflows.

Current Problem

Until now, locations with space constraints were forced to use an indoor trap, or grease recovery device. Small indoor grease traps seldom have the size and retention times to adequately separate out FOG’s. Some have less than a 1 minute retention time. Most are rated using a standard that does not address the addition of soaps in the discharge stream. Soaps emulsify the oil/water solution, greatly affecting the separation time. This must be
taken into consideration.

The GreaseSolution™

While large outdoor interceptors generally solve the problem of inadequate retention time, they are not always feasible for existing or urban settings. The GreaseSolution™ tank is the first grease interceptor specifically designed for long retention times, considering space restrictions of urban areas. It can be sized from 60 to 1,000 gallons, and designed to fit into custom, hard to reach spaces. The GreaseSolution™ tank can handle a variety of flow rates and still maintain a long retention time. Check your local codes for sizing criteria, or call us for assistance. For most applications we recommend that a 75-gpm flow not be exceeded to maintain optimal separation.

Operation and maintenance of the GreaseSolution™ tank may be optimized with the addition of a GREASEwatch™ monitor. The patented system uses transducers to continuously monitor sludge, scum, liquid level and temperature in the tank. The data is stored in the control unit that can be programmed to indicate when the tank needs to be pumped. An audible alarm will sound, or it will notify the service provider directly via modem. Data stored on the control unit can be downloaded onto a computer, which will document when maintenance is performed and ensure prescribed grease and sludge limits are not exceeded. The data can also be transmitted to inspectors via modem, which allows regulators to reduce or eliminate onsite inspections.

The Future

Sometime in the not so distant future, all municipalities will require that grease generators be responsible for what they generate. The cost will not fall on the local authorities to maintain clogged pipes, and clean up overflows. Everyone will understand the importance of retention time, and interceptors will be sized accordingly. The technology now exists to monitor and maintain grease interceptors, both small and large, so the operator can minimize maintenance costs while assuring compliance of effluent quality. Municipality costs will reduce as information is relayed via modem, and sanitary sewer overflows caused by grease will be a thing of the past.