I am routinely asked whether a dishwasher should be routed to a grease interceptor or directly to the sanitary sewer. One of the main concerns seems to be the presence of detergents in dishwasher effluent and how that might "scour" a grease interceptor. Let's review how emulsification works to clear up confusion over where the effluent from a dishwasher should be discharged.
Here is a jar with water and oil. Actually it has four different oils - canola, corn, vegetable and olive. This was part of a demonstration I presented at the Miami-Dade DERM round table meeting this past January on mechanical and chemical emulsification.
I used four different oils in the demonstration to illustrate that all types of oil are immiscible with water, which means that oil and water cannot form a homogeneous mixture when added together.
So, here we have oil and water sitting together in a jar without mixing together. The basic reason they are immiscible is that oil is oleophilic (has an affinity for oil) and hydrophobic (repels water). Water is hydrophilic (has an affinity for water) but oleophobic (repels oil). If you place a drop of water on a flat surface near another drop of water they will form a larger drop of water. But place a drop of oil next that drop of water and they will never mix together.
How do you get oil and water to mix together? The simple answer is that you have to add a shearing force. In the case of this demonstration I shook the jar violently for a few seconds.
Here is what the jar looked like after I added the shearing force. This is called a MECHANICAL emulsification. The problem with mechanical emulsification's is that they are not stable. In fact, this emulsion will separate substantially in just a minute or two.
Here is what the emulsion looked like in just one minute and thirty seconds. Why isn't all of the oil separated? The answer is that according to stokes law, a grease bubble with a specific gravity of 0.90 at room temperature (68 deg. F) with a size of 150 microns will take one minute and three seconds (0:1:03) to rise just three inches. The same bubble with a size of just 50 microns will take nine minutes and eighteen seconds (0:9:18) to rise the same distance. With sufficient time the entire volume of oil will settle at the top of the water.
Next, let us consider the effect of adding soap. First let's understand what soap is and how it works. Immiscible liquids have a surface tension that prevents them from mixing together without a shearing force. Soaps and detergents are called surface acting agents or surfactants because they reduce the surface tension between immiscible liquids. They are amphiphilic (having an affinity for both oil and water) because they have oleophilic heads and hydrophobic tails. When properly mixed with oil and water they create a CHEMICAL emulsion.
Now, If I pour soap directly into this jar of oil and water what do you suppose will happen? Do you suspect that the soap upon coming into contact with the oil and water will immediately create an emulsion?
As you can see in the picture at right, the soap (Dawn) that I poured into the jar went right through the oil layer without emulsifying the oil and water. Why do you suppose that is? Something is missing! We need a shearing force to create the emulsion. What is the shearing force that exists in this jar? Correct, there is none.
But, what if I shake the jar? Correct again, shaking the jar adds a shearing force creating a stable chemical emulsification.
Now, let us consider the effect of emulsions in a grease interceptor. If we poured soap directly into a grease interceptor, what would happen? What is the shearing force inside of a grease interceptor that would cause the emulsion many assume is happening? The answer depends upon the type of grease interceptor we are dealing with.
Hyrdomechanical grease interceptors (HGI) must create a laminar flow environment in order to cause efficient separation of free floating grease in the two-minute residence time that most devices are tested, rated and certified to. A laminar flow environment is turbulent free and flows in even layers. There is no shearing force in a laminar flow environment.
Detergents that are dissolved in a waste stream entering a HGI will either rest at the interface between the collected FOG and water or will be discharged in the effluent leaving the interceptor. Without a shearing force to create an emulsion between the collected grease and the water, the surfactant is impotent.
The same cannot be said for gravity grease interceptors (GGI). Studies have confirmed that at higher flow rates, these devices suffer from short-circuiting as a result of high-velocity zones and turbulence. Turbulence is a shearing force inside of GGIs. The standard design, while having a large volume of water, does not control the influent to remove turbulence and create laminar flow, which is why they short-circuit.
That is why I recommend that dishwashers be routed to certified HGIs but not to GGIs! I'll get to temperature in another blog.