Head required for backwashing a filter

[bepalo]

I am doing a preliminary design for an ion exchange system. I have data for the headloss (pressure drop) across the media in the operating direction but how much head would be required to backwash and expand the media to 50%? I would imagine that the ion exchange media would behave similar to a sand filter if anyone has experience with that and not IX systems.

The bed depth is 6 ft.
The tech data I have states 0.5 psi/ft @ 2.5 gpm/SF for in-service flow and 2.5 gpm/SF to get 50% expansion.

Thanks!
Ben

 

[bimr]

The headloss across the media is greater because the bed is compacted.

When you are backwashing, the bed is fluidized and there is little headloss. The headloss is simply the sum of the losses through the piping and internal distribution laterals. The head loss during backwash is probably in the 2-5 psi range.

A valve is typically used on the backwash discharge line in order to lower the pressure.

What is the concern regarding the backwash headloss?

 

[bepalo]

This is a proposed addition to an existing surface drinking water plant that occasionally experiences nitrate issues. Based on the relatively small (~1.5% of total sys. flow) volume required, I was considering diverting some water from the plant distribution pumps but that water would need de-chlorinated to prevent premature degradation of the resin.

As an alternative, diverting some of the water directly from the effluent of the IX back into the backwash/rinse/brine system is considered. This will put an additional load on the low-pressure supply pump for the IX system.

To size the pump, I was wondering how much headloss would encountered during a backwash. Would the underdrain would provide the majority of the resistance then?

 

[bimr]

That is correct.