Another problem with activated carbon filter

[Nicksun]

Hi, Dear Engineers

Now we are confronted with a fine-particle runing out problem with our activated carbon filter(ACF) in our UPW plant.

The plant was started 2 months ago. We took SDI sampling outlet ACF recently and were suprised to find that a grayish color on the membrane. And we found similar color downstream ACF, even after RO.

We tested our AC, the specifications were listed below:

coconut shell type
Iodine value: more than 1000
Hardness: more than 95%
size: 14-40mesh US

all these data met our requirements.
And the tank design is no problem, we used for a lot projects.

we use downflow operating mode.

But the fine particle is still running out ACF.
Anybody can help?
thank a lot for your experience and suggestions.

Sunbird
from China, Shanghai.

 

[gacguy]

Hello Nicksun:

What is the viscosity of the liquid you are treating? What procedure did you use to add the carbon to the vessel? What prewetting procedure did you use? Did you backwash before treating?

gacguy

 

[Nicksun]

Hi, Gacguy

We use city water(in Shanghai,China)to produce Ultra Pure Water. We use jet flow to load carbon to the vessel.
As for the prewetting procedure, we have to rinsing the carbon just before it was brought to the vessel by jet flow.
We have backwashing and rinsing procedure before treating.

The proplem is that we have this fine particle even after the filter was long time in operation.
We use pressure drop to control backwashing. We use residuel Cl to control rinsing procedure.

and the carbon is acid washed.

hope i have answered your questions and thank you for your comments.

Nicksun

 

[gacguy]

Hello Nicksun,

What is the mesh size of your carbon? When you backwash what gallons per minute/ft2 of cross sectional area flow rate to you use? Is your operation continuous or intermittent? What is the temperature of the backwash water? How often do you backwash? Does the color occur right after backwash or later in the run? Or is it continuous?

Regards,

gacguy

 

[Nicksun]

Hi, Gacguy

I answer your above questions as following:
carbon size: 0.4mm--1.4mm
backwash flowrate: 30m/h
operation mode: continuous downflow
temperature: 25-40 deg C
Backwash starts when pressure drop larger than 1.0barg or operation time more than 5 days.
the color appears all the time in operation, that is what I wondered most.

thank you for your help.

Nicksun

 

[gacguy]

Hello Nicksun,

I'm still not sure what is causing the problem. Would you answer the folowing questions?

1. Is this a coal or wood based carbon?
2. Is the water entering the ACF clear, that is not gray?
3. What are the contaminates in the influent to the ACF?
4. What is the pH entering and leaving the ACF?
5. What is the Cl concentration entering and leaving the ACF?

Regards,

gacguy

 

[gacguy]

Hello again Nicksun,

I apologize for asking the question re the type of carbon since you already said it is coconut. Two more questions:

1.When you load using 'jet flow' is the jet motive force water or steam?
2. What is the support for the carbon? Gravel, perforated plate, screen etc. What size are the openings?

Regards,

gacguy

 

[Nicksun]

Hi, Gacguy

Thanks for your thoughtful ideas, i would like to answer your questions as follows:

1. inlet ACF water is municiple water complied with chinese drinking water quality requirements.
2. the main contaminant, to me, is TOC. That is also our purpose to use activated carbon after multimedia filter.
3. PH of inlet water is 6.5-8.
4. We monitored total chlorine outlet ACF, the readings fall in the range of 1.5--0.5ppm. By the way, we dose NaOCl while back washing the filter.
5. it is water jet flow.
6. we use perforated plate with water cap(??? a plastic cap screwed on the plate).

i want to say, the fine particles is from carbon with no doubt. So how they keep flowing out the filter? For me, i just suspected the quality of the carbon. But we also made a test, the result shows the carbon is in good condition.

and also, there would be nothing wrong with design and operating procedure.

So, what the hell is going on inside the ACF?

thanks and regards.

Nicksun

 

[gacguy]

Hello again Nicksun,

Are you still experiencing carbon fines?
I still don't have a definitive answer. What is the opening size in the caps. If they are slots they should be 0.010 inches(~50-60 US Mesh; 48-60 Tyler mesh) for the carbon size you have.

You indicated a chlorine concentration of 0.5-1.5 ppm in the outlet. This is unusual since I would expect the carbon to remove the chlorine unless you have a very high concentration in the influent. As you probably know chlorine reacts with the carbon molecules and the carbon eventually breaks down. When this occurs the effluent turns brown. However this usually occurs afte many gallons of water have been processed when the influent is in the range of 2-5 ppm.

 

[Nicksun]

Dear Gacguy,

Thank you very much for your thoughtful input.

Is it possible that the dosage of NaClO reduced the hardness of activated carbon? If so, this possibly can explain why caron running out after a month's operation.

i would like to rethink the design paramenters.

thank you.

Nicksun