backwash rates for Rapid gravity filter

[Rees101]

Does anyone have any info on calculating acceptable backwash rates for rapid gravity filters for potable water.
Any info would be appreciated.
Thanks in advance

 

[Ussuri]

My textbook gives the following information:

Firstly, air at 1.0 to 1.5m3 per minute per square metre of bed area for about 2-3 minutes;

Followed by, upward washwater at the rate of 5.0-6.5mm/s.

 

[Rees101]

Thanks for the reply,

I am interested in a water only backwash. I beleive for a water only wash the upward washwater rate will be higher than for one with a combined air and water?

Again thanks for any input

 

[bimr]

Minimum backwash rates for filters is 10 gpm per sq ft of bed area and higher rates are preferable. In the US, the typical backwash rates are 15 to 20 gpm per sq ft of bed area. Backwash should be carried out for 8 to 10 minutes.

 

[Zetaman]

Hello Rees101,
In answer to your question there are several factors to be considered :-

1. The quality of water going onto your filters
2. The filter run times

I have involvement with a treatment works that has a water only backwash (21gallons/min/ft2 - a strange way of working things out...I prefer to use a linear velocity, which makes readings easily comparable between different sites..but i digress).

I look for a minimum bed expansion of 10% (generally achievable with an upwash rate of 20 - 30m3/hr).

But the best way to assess what you need is to carry out a backwash curve. I look for a > 95% reduction of the initial washwater turbidity levels coming off the bed and a final washwater turbidity of <10NTU. This method avoids using excess power and water.

You should then be monitoring your filter ripening and ideally carry out core profiles at yearly intervals as a minimum.

Hope this helps.....Chris

 

[semo]

10 states standards call for 15 gpm/sf; however, the media size and specific gravity will have an affect on what backwash rate is actually needed.

We typically see from 10 gpm/sf to 15 gpm/sf. You don't want it too high or you wash your media away.

The main thing is to get adequate bed expansion. Typical bed expansion should be from 30 to 50%.