Max. flow/pressure for upflow filter

[af1012]

I am working with a system where wastewater enters into the bottom of a tank filled with activated carbon/sand (upflow filter). The tank is 10 ft diameter, 20 ft high. wastewater inflow into the tank is about 200 gpm.

We are going to increase the flow of the wastewater over the next year up to 500 gpm. I am worried about suspending the carbon/sand and washing it out as we increase the flow/pressure. How can I estimate the max. inflow rate/pressure that the filter can handle before the media
is suspended and washed out?
Any help would be appreciated.

 

[epbpe]

The determination of loss of filter media involves consideration of the upflow rate in GPM/ft2, head differential (if any), freeboard, and the specific gravity and surface cohesion of the filter media. Consultation with the manufacturer of the filter media will provide the information you seek as all medias are designed and tested for a maximum value of cross-sectional flow rate. In your example, a 10' diameter filter basin has an area of 78.5 square feet; at a future filter rate of 500 GPM this will equate to a rate of 6.37 GPM/ft2 which is quite high for light carbon and sand medias not to mention the treatment process itself and loss of media is a definite possibility. One piece of information I don't have that would be needed to make a full judgment is the freeboard of the vessel.I still recommend checking with the filter media manufacturer and verifying the maximum rate you can use, epbpe.

 

[af1012]

epbpe-
There is 5 feet of water above the media before it spills off.

 

[BobPE]

I wouldn't think you will have much of a problem with upslow loading rates below 10 gpm/ft^2, but as epbpe said, discuss it with the manufacturer of the filter, not the media. They should know a good deal about your filter and will help you with the new cleaning rates too.

BobPE

 

[waterexpert]

Alternatively, consult Amirtharajah and Cleasby's paper on predicting filter bed expansion :

AMIRTHRAJAH,A AND CLEASBY,J.L. "Predicting Expansion of Filters During Backwash" J.AWWA Jan 1972

This will allow you to calculte how much the bed will expand at a given upflow rate.

 

[mur]

As Suggested by others above, consult the filter Manufacturer,particualy in regard the allowable loading on the holddown grid bars (if these are fitted)Do you have DP indication / instrumentation fitted to the units? this should be closely monitored during your trials.