Removal of Solidified Carbon from Filtration System

[jforbes]

Hi, I have a question!

My company operates a groundwater treatment plant that utilizes a liquid phase granular activated carbon adsorption system. The problem is that the water being treated is hard and is reacting with the carbon causing it to solidify. We have plans for softening the water in the future, but for now, we need to remove the hardened carbon to replace it with new carbon.

We are going to do this by adding 20% HCl to the hardened carbon. Some preliminary experiments resulted in the generation of, what we believe is, CO2 gas. After the reaction is complete, the remaining carbon is brittle and easy to break-up. The carbon supplier can then suck out the carbon and replace it with fresh carbon.

I have been asked to create a procedure for this process. My question is, does anyone out there have any experience with this? Any ideas on where I could find a similar procedure? What are some other options that are available to loosen up the hardened carbon?

Any advice would be GREATLY appreciated!

Thanks,
jforbes

 

[johnd43]

My first reaction on reading your post was a slight to medium raising of the hairs on the back of my kneck . If this is a typical ground water plant, you will be filtering directly on line using pressure filters and your wording implies breaking up the concreted filter bed by hand.

With one eye on the discussion elsewhere in this Forum (prompted by a fatality in a Confined Space) and your mention the generation of CO2, we would all urge you to make absolutely sure that there is no residual reaction if you do have people entering the filter for mechanical break-up of the agglomerated carbon.

I hope you will have already considered that one but treating filters with 20% strength acid is not a typical WTW procedure and you need to consider side issues as well.

I suspect you will treat this as a one-off action and greatly overdose acid, despite the expense, to make sure you do the chemical conditioning job properly. This will leave you with a strongly acidic liquor to drain away or displace before/as you apply flushing water for the sake of subsequent safety.

You need to check you drain route very carefully for suitability to take this stuff. For instance, if there is any hypochlorite in your drain, you will get chlorine gas evolved. I am not a chemist, but this is a case where I would cross-check very carefully with such a specialist to make sure all possible interactions are foreseen and allowed for.

The Law of Unforeseen Consequences is only allowed in politics.

 

[jforbes]

Thank you for your suggestions and concerns. Maybe the wording of my original thread was misleading, so let me explain.

The path that the groundwater takes through our plant is as follows:
water is pumped from extraction wells to an equalization tank, from there is pumped through 2 bag filters in series, it then flows though an air stripper where most of the VOC's (the primary pollutants at the site) are removed. The water then flows through 2 more bag filters in series, then into the carbon adsorber (where the remaining VOC's are removed), and finally the treated water is discharged into the environment.

I got the impression from your post that you thought we were trying to break-up the stuff in the bag filters, but this is not the case. The carbon in the tank is in a granular form and when the hard water passes through it, it binds the grains of carbon together so we are left with one GIANT chunk of carbon.

Our idea was to slowly add (1-5 gallons at a time) the 20% HCl though the top of the tank (after the tank is filled with potable water), allow the reaction to take place and then drain the spent solution to the building sump where it will be treated with limestone (or something similar) to increase the pH to neutral before it is pumped back into the equalization tank. An operator would then stand at the top of the tank and use a blow gun, attached to an air compressor, to break up the brittle carbon which can then be shoveled/vacuumed out.

Again, thank you for your suggestions. Please feel free to pass along any more advice you might have. Sometimes talking things out really helps!

Thanks again,
jforbes

 

[johnd43]

Your description completely changes the image I had previously of your installation, as you never mentioned aeration, bag filters or the fact that you are discharging to nature. But hell, who needs facts? Your plans to soften should have given me a clue! Are there any other minor details still outstanding? For instance, do you dose any chemicals or is it entirely a passive process based on aeration and adsorption? Is it urban or a rural setting? How big is your contactor? I am interpreting that there is only one vessel, but what shape is it?

All this has not changed my view sufficiently to alter the context of my first suggestions although I had supposed you were supplying potable water to distribution and not, as I now understand, running some form of remediation project.

Whatever the application, your carbon bed is concreting up under a spontaneous precipitation reaction. Have you had any chemical analysis done to confirm the composition of the binding precipitate? If not, you should check this carefully to make sure that you understand exactly what the reactants are going to be and what the reaction by-products will be - particularly any gas emmissions if you have any neighbours to worry about. There is clearly serious contamination involved and you should not embark on any such procedure as an act of faith based on a guess, although your planning for solid residues sounds like a reasonable start.

If acid washing is confirmed as your way forward, then my reaction to this second post is to suggest that you adopt a much longer, less macho approach. Why fill it with potable water and then pour strong acid into the supernatant pool? The middle and bottom of the bed will never see the low pH water. I would use a weak acid in a number of flushing actions that each start with an empty filter. I descale the taps and fittings in my kitchen and bathroom with malt vinegar.

This is a very weak acid so you have to be patient, but it does the job and is much cheaper at the domestic level than proprietory products intended for the application. Best of all you can keep it in the cupboard without risking life and limb. You could not say the same about 20% acid.

As a further option, can you use your dump tank as a temporary sump from which you can pump and return your acidified wash, checking pH as it runs? In this case you could blow down spent liquor periodically and replace with fresh make-up solution. By establishing a continuous cycle instead of a series of batch processes, you would offset some of the time lost to using low strength reagents. Using this approach would allow you to employ an upward flow through the bed which would naturally tend to disturb the carbon material and may (when the acid has had full effect) expand the bed naturally and obviate the need for manual break-up.

On the matter of upflows, how did you beds get in such a parlous state in the first place. What wash cycle do you use? Do you air scour?

I still understand that your carbon contactor is a closed vessel so the cautions about confined spaces and the rest still stand. And the other cautions about noxious gas emissions merely change. I don't suppose you are chlorinating to discharge to nature, but I am still concerned that you might be releasing all sorts of nasties when you carry out your process.

After all this, probably the best council is to advise engaging a professional advisor to run a specialist subcontractor on your behalf.