Activated Carbon Backwash Water 3

[RBAC]

This has probably been done before but I couldn't find anything.

I hope someone can help. I have a fairly remote WTW site which needs to be treated for pesticides.

The site has no discharge points and we would therefore need to tanker away any waste from GAC backwashing and regeneration.

There is a small river near to the site which has been investigated as a potential discharge point however I have now had several different opinions on the possibilty of discharging the Activated Carbon backwash water to this point.

What I am wondering is how difficult is it to settle out activated carbon backwash water, do you get carbon fines in the backwash water?

I know a lot of it will be site specific so I'm really after peoples experiences.

We would have a backwash volume of 75m3 for each backwash.

Can anyone offer any advice please?

 

[bimr]

It is simple to settle out the activated carbon fines. You also have to be careful with the backwashing step to avoid a rate too high that will cause the activated carbon to go to waste.

I don't understand why you are trying to backwash the activated carbon as the backwash water may contain some of the contaminants that you are trying to adsorb onto the carbon.

Usually, when you are using activated carbon for an application like you are describing, the vessel is almost completely filled with carbon and you will not be backwashing the activated carbon at all.

Activated carbon is typically sluiced out into a truck when you want to change-out the carbon. Alternatively, you can change-out the complete activated carbon vessel. The vessels can be transported while the vessels are full of activated carbon. It is also common to operate activated carbon vessels in series with working and polishing vessels.

If you have any filterable materials in your water stream, you probably should consider filtering prior to the carbon filters to prolong the carbon filter bed life.

 

[RBAC]

I thought it was standard practice to backwash GAC, it certainly is at our treatment works. So would you not recommend backwashing. If I were to try to settle out carbon fines how would I go about it?

Thank you for the response

 

[cub3bead]

Have you considered using a pre-coat type filter with powdered activated carbon as the pre-coat? The vessels would be much smaller (saves on Capex) and the absorption kinetics should be better due to the small particle size of the PAC. The WW produced during could be directed to a cone bottomed tank to concentrate the solids and thence to a small filter press for final dewatering. The clear water from the cone bottom tank and the press filtrate could be returned to the front of the process. The solids from the press would likely have to be handled as a hazardous waste but you sould have to do that in any case.

My 2¢ for what it is worth.

 

[bimr]

You need to provide more details on your application. Information such as the water flow, the specific contaminant, the concentration of the contaminant that you are trying to remove, your location, a water analysis and what you plan to use the water for would be quite helpful.

For a routine taste and odor activated carbon installation, backwashing would commonly be done once per week.

For a remediation application where you are trying to remove chemical contamination, backwashing may not be a routine operation. When you trying to remove a chemical contaminant, you are generally not allowed to discharge the contaminants with the backwash water.

If you have a small volume system, consider the use of serviced adsorption systems.

Carbon fine removal is simple. You need to collect the backwash volume in a holding tank and allow the fines to settle. The fines should settle out of the backwash water in an hour or so. This is commonly done in a conical bottom tank as cub3bead suggested. Alternately, you can sidestream filter out the fines (as cub3bead also suggested) and then discharge to waste the contents of the backwash holding tank.

 

[ch81pc]

We backwash our GAC contactors once a week, even with a two stage filtration up stream of the GAC we still get a build up of solids so the backwashing/airscour is used to remove the solids loading and so reduce the head loss through the filters. I am gussing your main problem with discharging your waste to the river would be concerns over turbidity (caried over from upstream processes or as a result of carbon attrition). We return our wash water to settlement tanks where we dose it with 0.25ppm of polyelectrolyte. The supernatant is then either returned to the head of the works or else back to the river.