backwash rates for Rapid gravity filter
backwash rates for Rapid gravity filter
(OP)
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
Any info would be appreciated.
Thanks in advance





RE: backwash rates for Rapid gravity filter
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.
RE: backwash rates for Rapid gravity filter
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
RE: backwash rates for Rapid gravity filter
RE: backwash rates for Rapid gravity filter
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
RE: backwash rates for Rapid gravity filter
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%.