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SardMan
Associate

Join Date: Dec 2008
Posts: 31

PID Control for Wastewater Disinfection

04/15/2009 11:24 AM

I am working on a wastewater disinfection system, installing new metering pumps for dosage of sodium hypochlorite. I would like to replace the existing control system, which is a PID controller, with a new unit. I need to handle two analog signals (flow signal and Chlorine Residual ) and control the pumps with a 4..20 mA signal in three control modes:

1) flow paced

2) flow paced with residual trim

3) residual

I can't install a PLC.

I heard of compact controllers that offer more options than a mere linear response for the Proportional fraction of the PID loop (something like a six-point curve would be nice, the chlorine demand in my plant is not proportional to the flow on the whole span)

Also the time it takes for the chlorine residual probe to pick up a variation in dosage, which is proportional to the hydraulic detention time, can be tied to the flowmeter signal. The unit currently installed has this parameter as a constant.

Please, let me know if anybody is aware of any manufacturer that produces a controller that does all this things.

Thanks,

M.
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Pathfinder Tags: disinfection PID controller
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Mikerho
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Hobbies - Musician - Engineering Fields - Chemical Engineering - New Member Engineering Fields - Control Engineering - New Member Engineering Fields - Instrumentation Engineering - New Member

Join Date: Jan 2007
Location: Moses Lake, WA, USA, Thulcandra - The Silent Planet (C.S. Lewis)
Posts: 4216
Good Answers: 194
#1

Re: Instrument for PID control of Wastewater disinfection

04/15/2009 11:53 AM

Why can't you install a PLC?

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SardMan
Associate

Join Date: Dec 2008
Posts: 31
#2
In reply to #1

Re: Instrument for PID control of Wastewater disinfection

04/15/2009 12:05 PM

I have very little money for this project, the panel already exists, my background is Environmental Engineering and have no clue how to spec a PLC. Fair enough?
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Mikerho
Guru
Hobbies - Musician - Engineering Fields - Chemical Engineering - New Member Engineering Fields - Control Engineering - New Member Engineering Fields - Instrumentation Engineering - New Member

Join Date: Jan 2007
Location: Moses Lake, WA, USA, Thulcandra - The Silent Planet (C.S. Lewis)
Posts: 4216
Good Answers: 194
#9
In reply to #2

Re: Instrument for PID control of Wastewater disinfection

05/12/2009 9:54 PM

Hi SardMan,

How much is very little money? There are some relatively low-cost PLCs out there these days (panelless ones do exist), and I'm sure that we could help you spec one - it's not as complicated as it may seem.

Mike

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craigza
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South Africa - Member -

Join Date: Nov 2008
Location: Johannesburg
Posts: 295
Good Answers: 12
#3

Re: PID Control for Wastewater Disinfection

04/16/2009 1:34 AM

Hi there,

I have had a quick look at the SIPART DR24 - I believe that this standalone controller will give you all you want and more.

Regards,

Craig
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tjmurf
Participant

Join Date: May 2009
Posts: 3
#4

Re: PID Control for Wastewater Disinfection

05/12/2009 1:13 PM

SardMan,

I have a similar project. What did you use? any comments.

Thanks
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SardMan
Associate

Join Date: Dec 2008
Posts: 31
#5
In reply to #4

Re: PID Control for Wastewater Disinfection

05/12/2009 1:20 PM

I am probably going to try the siemens instrument. I will meet a rep in a few days and see what the unit can do. Our plants treat Combined Sewage, so the flow varies within a large range. Having the proportional time fixed at 120 seconds (what we usually set for average flow) does not allow for rapid changes during rain events.

I will keep you posted when we install the instruments.

M.
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tjmurf
Participant

Join Date: May 2009
Posts: 3
#6
In reply to #5

Re: PID Control for Wastewater Disinfection

05/12/2009 2:17 PM

What type of residual monitor are you using, DPD?
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SardMan
Associate

Join Date: Dec 2008
Posts: 31
#7
In reply to #6

Re: PID Control for Wastewater Disinfection

05/12/2009 2:32 PM

at the moment we have a Hach CL17, which uses a colorometric dpd method. There are two problems with this unit, there is a sampling line that gets fouling or clogges and the sampling time is about 2.5 minutes. If I endup installing the siemens controller, I want something with a deadband much smaller. I would like to install an in-situ probe (by Prominent) with a response of about 4 seconds.
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tjmurf
Participant

Join Date: May 2009
Posts: 3
#8
In reply to #7

Re: PID Control for Wastewater Disinfection

05/12/2009 2:47 PM

We are looking at the CL17. I was wondering myself about the clogging. I will need to install a separate pump for the sampler. The 2.5 minute response time is how long the sample needs to develop the color. Can you forward me any info on this other probe you are considering?

I'm still looking at using a PLC for control. The Siemens is not supported in this country. I was told to use the Model 353 which is, still by Siemens. Its an old Moore Products digital controller.
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Users who posted comments:

craigza (1); Mikerho (2); SardMan (3); tjmurf (3)

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