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Plug Flow, Complete Mix Which Is Better?

10/6/2014

7 Comments

 
Picture
Wastewater treatments systems are usually thought of as a plug flow, complete mix, or some hybrid type reactor. While most people know the definition of the reactors, they do not know when one is preferred over another.

Plug Flow
Best pictured as a pipe where flow moves in one direction, a tracer entering a plug flow system exits completely at Volume/Flow. There is no dispersion or back-mixing to “equalize” or mix flows from multiple time periods. We often see plug flow reactor systems where influent has little variation and no toxicity/inhibition to the biomass. In practice we rarely see a true plug flow reactor as aeration systems create some degree of mixing/dispersion.

Complete Mix
A complete mix reactor (or CSTR) is often modeled by a beaker wither a single large impeller. In this ideal example, a tracer added to the inlet is immediately dispersed evenly throughout the reactor. The tracer will appear in the effluent as the tracer/volume concentrations.  A CSTR is often used in systems with influent variation and toxicity/inhibition issues caused by high concentrations of the inhibitory compounds. The immediate dispersal/dilution allows for microbial activity to commence as the inhibitory concentration levels are avoided.

Hybrid Flows
In practice we rarely see a true plug flow or complete mix reactor. We often have hydraulic patterns that are somewhere in between the two ideal regimes. For example a long rectangular basin with aeration (either surface of diffusers) often has significant dispersion and back mixing so we have a lag before tracer appears at the effluent, but it appears well before time = volume/flow.

What makes hybid systems interesting is as you increase the number of cells or individual areas modeled by CSTR flows (occurs near a surface aerator), the overall tracer response appears closer to the plug flow ideal where tracer reaches the effluent near the design residence time.

The hybrid flow is used often because it is a more “natural” flow pattern and provides benefits of both ideal flow systems. To determine a working systems’ flow pattern, you can look at spill response data or run a Tracer Study where a conservative tracer is added and monitored at the effluent.


7 Comments
Waste water engineer consultant Australia link
8/1/2017 01:19:18 am

I love the way you write and share your niche! Very interesting and different! Keep it coming!

Reply
Jeremiah
2/15/2018 05:24:38 am

I've been mulling over ways to characterize these hybrid reactors, to little avail. Best I've come up with to date (that I can remember) is something like "equivalent consecutive complete mix reactors", where an infinite number would yield a plug flow reactor model. So you could be talking to a comrade about a plant, and say "the aeration runs at about 25 ECCMR with the RAS at 0.8Q" and he'd be like, "oh, yeah, I've operated one of those." I don't know, I mean I think it's a good idea.

Reply
Erik Rumbaugh
2/15/2018 05:32:48 am

In running tracer studies, a number of calculations gives dispersion information on how many CSTRs in series. While index numbers are great, I usually like to see tracer flow information in a graphic format. As the number of CSTRs increase, the tracer pulse becomes more similar to a plug flow system. Comparing the tracer study plot versus ideal CSTR and Plug Flow systems gives good insight into real flow patters. (I really like the time lag before tracer addition and appearance at the outlet.)

Reply
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3/8/2021 01:59:10 am

Interesting post. I Have Been wondering about this issue, so thanks for posting. Pretty cool post.It 's really very nice and Useful post.Thanks

Reply
Victoria Addington link
12/26/2022 08:42:17 am

I was captured when you discussed that hybrid flow has a more “natural” flow pattern. My friend told me that their plant needs a wastewater design. I should advise him to hire a contractor with vast experience in the field.

Reply
Steve Smith link
1/13/2023 06:00:13 pm

I like how you mentioned that a one directional flow of water should be the proper function ofa pipe. My cousin informed me a couple of days ago that he was planning to gather details about a surface aerator treatment for the proper management of wastewater on their industrial farm. He asked if I had ideas on the best option for consultation. I'm thankful for this valuable article, I'll tell him it will be much better if he consults a trusted surface aerator service to gather facts about their services.

Reply
aquos r2 price in pakistan link
3/12/2023 04:28:47 am

Thanks for taking the time to discuss this, I feel strongly about it and love learning more on this topic. If possible, as you gain expertise, would you mind updating your blog with extra information? It is extremely helpful for me

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    Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater  facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.

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