Series Versus Parallel Installations

October 30, 2014

When you need more capacity in a grease interceptor than is available for a given size, you have to make a decision; move up to a larger size interceptor or add another (or more than) one of the same size.

When working with multiple interceptors the question is, how do you connect them together, series or parallel?

What's the difference you may ask?

You may not have actually asked that question, but somebody else must have, because why would I be writing a post to answer a question that nobody has ever asked?

No, I can't share the name of the person who did ask for legal reason's.  Very binding, rigid and inflexible legal reason's that I don't have time to go into here.

Suffice it to say that someone out there, someone you don't know, did indeed ask me the question; you'll just have to take my word for it.


Let me illustrate the differences and then discuss the advantages and disadvantages of series and parallel systems so you know and are comfortable with the proper application of either type.


Series installations connect the outlet of the first interceptor to the inlet of the second, and the outlet of the second to the inlet of the third, and so on.

This system forces the entering wastewater to pass through each interceptor in the series providing two significant benefits; better efficiency and more capacity.

The limitation of series installations is, the maximum flow rate of the first unit in the series (what it is certified to) becomes the maximum flow rate of the system.

When the flow rate of the system is 100 gpm or less, series is the most effective setup for both
hydromechanical and gravity style interceptors.

The systems in the graphic here are both series layouts.  So long as the flow moves from the first unit through the second, through the third, and so on, the system is set up correctly.


In a parallel system the interceptors are set up to each receive a portion of the flow from the facility; the incoming flow is intended to be distributed to each interceptor evenly.

This system is necessary when flow rate requirements exceed 100 gpm, because there are no interceptors currently on the market that are certified to a flow rate over 100 gpm.

Parallel setup's in gravity drainage systems are persnickety. 

By persnickety, of course, I mean that getting the flow to distribute evenly is like getting your daughter to date the guy you's probably not going to happen.

Having raised two daughters and having endured countless "boyfriends", I can confidently tell you that I have liked...lets see...divide by...carry the zero...

Well, I'm not sure I got the math exactly right, but I think the answer is less than or equal to one.

Anyways...getting a parallel system to flow evenly is similar. 

The challenge in a gravity drainage system is that water follows the path of least resistance. Trying to force the water to divide evenly into two or more paths with a fitting is next to impossible especially at low flow rates.

Our own in-house testing of parallel systems revealed that the minimum flow rate needed for an even distribution to multiple tanks (using a fitting) is 50 gpm.  When you consider that the average flow from a restaurant is closer to 5 gpm, you can begin to see the problem. 

The secret to a balanced parallel system is found in what Schier calls the Flow Splitter, which utilizes a static water line to evenly distribute flow across multiple tanks.

Problem solved.

Flow control devices

With hydromechanical grease interceptors, one question that comes up from time to time is whether a flow control device is required for each interceptor in series or parallel installations.

In series installations the flow control on the first interceptor acts as the flow control for the whole system. Therefore in series installations you would only install a flow control on the first interceptor in the series.  This is code compliant with both UPC and IPC.

The point in using a parallel system is that you need to meet a requirement for higher flow rates (over 100 gpm) and as such a flow control device would be required on each interceptor.

Now that you understand series and parallel systems and how they should be utilized, unlike candidates for your daughters boyfriend, you should find these installations to your liking.

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