Interceptors and separators: keeping our sewer arteries unclogged
My wife makes me cook bacon outside during the warmer months so the smell of the cooked bacon doesn’t linger inside the house. Last year, I had to replace my propane grill due to years of use and the fact birds continually attempted to build nests in it. After much research, I decided to give one of these new infrared gas grills a try. If you are not familiar with these grills, they use specially designed emitter plates just below the grill grates to cook your food with high radiant heat instead of convection. One of their selling points is that all grease and juices are instantly vaporized when they hit the emitter plate, so you have no flare ups and additional taste is imparted to the meat you are cooking.
After using the grill for several months and verifying that there actually were no flare ups, I had a magical idea: what if I cooked bacon directly on the grill? Doing so on a standard gas grill would probably end up with the loss of body and facial hair and a call to the local fire department, but, in theory, the grease from the bacon should be vaporized and not cause an inferno. So, I tried it. And. It. Was. AWESOME! Not only did I not get a single flare up, but I cooked an entire package of bacon at one time and, because of the smoke from the vaporized grease, there was an extra smoky flavor to the bacon. Plus, there was no leftover grease for my wife to pour down our residential kitchen drain.
Although much of the fats, oils and greases created in residential kitchens are disposed of by draining to the sanitary drainage system, there is no Uniform Plumbing Code requirement for individual residential grease interceptors. If a grease interceptor were to be installed, human waste from the drainage system must bypass the interceptor. Adding this waste to the interceptor will render it useless by overfilling it with solids. In commercial applications, however, there are numerous instances where a FOG removal system is required and numerous provisions governing their installation and use.
Maintenance is the most difficult part of the FOG removal system. The installation of these systems is mandated by codes and it is relatively easy to check compliance. However, ensuring the maintenance is accomplished is another matter. Although their maintenance is mandated, it is difficult, if not impossible, to ensure that it is being done. A poorly maintained system is as useful as no system at all.
When maintaining these systems, two factors must be addressed — service interval and FOG disposal. If the service interval is too long, the FOG will either simply pass through the overloaded interceptor or begin to breakdown into other harmful compounds. Long-term storage of FOG in a large interceptor can create a dangerous condition. The creation of hydrogen sulfide is a byproduct of FOG decomposition and could lead to the corrosion of piping and the interceptor itself, not to mention that it is a lethal gas if it escapes the system.
The development of the grease removal device and the FOG disposal system is a result of this maintenance problem. They can at least eliminate some of the maintenance problems by removal or remediation of the FOG. However, these devices do not solve all the maintenance issues. The devices will still need to be cleaned periodically and solids that have entered the interceptor removed.
Food disposers and dishwashers
The only time a food waste disposer should discharge to a grease interceptor is when the grease interceptor is specifically designed to accept food waste. Otherwise, a food waste disposer should never discharge to a grease interceptor as such a discharge can cause the grease interceptor to fail. The solids from the disposal will become entrained in the interceptor and begin to fill it unless cleaned out. If a disposal is connected to the grease interceptor not designed to receive food waste, a solids interceptor must precede the grease interceptor. The entrance of soapy water from a dishwasher into the interceptor will also be harmful to the system. The chemicals in dishwater will adversely affect the function of the interceptor and must be kept from the system.
Hydromechanical grease interceptors
The hydromechanical grease interceptor uses the principles of fluid dynamics by taking advantage of air entrained in the effluent through the use of the vented flow control (see Figure A). The FOG-laden wastewater passes through the flow control on its way into the interceptor. As the effluent passes through the orifice of the flow control, air is introduced to the flow from the air vent. Upon entering the interceptor, the effluent is directed through the separation chamber. The entrained air will separate from the effluent quickly and then accomplish two tasks. The escaping air accelerates the separation of FOG as it rises rapidly to the surface of the water in the separation chamber. The rising air burbles literally pull the FOG globules to the top of the water. The released air then provides a small amount of positive pressure above the contents of the separation chamber to regulate the internal running water level of the grease interceptor, keeping it from becoming air bound.
A flow control shall be installed on each fixture discharging to the HGI. A single flow control is allowed for multiple fixtures as long as the resulting total flow from the fixtures meets the capacity requirements of Section 1014.2.1. There should also be no fixture vent between the flow control and the interceptor. This would allow too much air to be entrained into the flow.
The placement of the flow control is vitally important to the functioning of the interceptor. It should be placed as close as possible to the fixture. There should be as little vertical height as possible between the fixture outlet and the flow control. If the flow control is placed well below the fixture, the added head pressure above the flow control will render the flow control useless as it will increase the flow through the system (see Figure B).
When sizing for a hydromechanical grease interceptor using fixture capacity, determine the compartment sizes of the fixtures and calculate the volume in cubic inches (L x W x D). Divide the volume by 231 to calculate the volume of liquid in gallons. Per Example 1014.2.1, multiply the gallons by the fill factor .75. This fill factor takes into account that the fixture is normally filled to about 75 percent of capacity, with the other 25 percent being displaced by the items placed in the fixture for washing. Therefore, the actual drainage load is 75 percent of the fixture capacity.
Once the actual drainage load is calculated, determine the flow rate per minute using a one minute or two minute drainage period. Typically, sinks will drain within a one-minute period, which determines the drainage load in gallons per minute.
When would the two-minute drainage period be used? When the capacity of the fixtures (in gallons) discharging into the interceptor is 2½ times the flow rate capacity of the HGI, a two-minute drainage period would be used. The code allows the capacity of the fixtures to be 2½ times the flow rate of the HGI to give latitude to space constraints for an interceptor. A smaller interceptor may be used, but the consequence is a longer drainage period.
For example, a four-compartment sink is to discharge into an HGI. Each compartment size is 24” x 24” x 12”. Using the method described in the first paragraph, the actual drainage load is 89.8 gallons. Using the drainage period of one minute, an HGI equal to or greater than 89.8 gpm would be required (a typical size is 100 gpm). Suppose a 100 gpm interceptor would not fit due to space limitations. The code would allow a 50 gpm interceptor with the acceptance that the fixtures would take longer to drain. Instead of one minute, the drainage period would be extended to two minutes. Remember, the flow control device will not allow the discharge rate to exceed 50 gpm. The drainage load of 89.8 gallons is within 2½ times the flow rate of 50 gpm. If the capacity of the fixtures had exceeded 125 gallons, then a 50 gpm interceptor would not be permitted.
Remember, the HGI itself must be vented to keep it from becoming air bound (see Figure A).
Gravity grease interceptors
The gravity grease interceptor, as stated earlier, is a large volume tank with two sections. Its function is relatively simple. The effluent enters the tank through a baffled tee so as to not disturb the accumulated FOG floating at the top of the water level. The effluent is retained in the initial chamber until flow forces it into the next chamber and eventually out to the sewer. The FOG is separated by gravity – the FOG floats to the top of the water level. The tank is sized to allow enough retention time to permit the FOG to separate.
A sampling box is installed in the outflow line to enable maintenance personnel to check on the functioning of the interceptor. The following sections give guidance to the design, location, and installation of the gravity grease interceptor (see Figure C).
Managing the grease produced in commercial kitchens and other applications is a necessity in order to keep our plumbing and sewer arteries unclogged and working properly. Ensuring that the correct FOG removal system is in place and that it is sized appropriately accomplishes this task.
When it comes to bacon, I feel some moral obligation to ask you to please check with your doctor prior to consuming mass quantities. As delicious as it is, apparently it isn’t the most heart healthy food on the market. Also, although I managed not to kill myself grilling bacon directly on an infrared grill, I do not know that the manufacturer would recommend doing so.