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# Backwash NTU

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## Backwash NTU

(OP)
Hey guys,

I'm working on the design of an equalization basin for recycling backwashed water (this includes filter to waste as well) to the headworks of a traditional water treatment plant. Public health requires the recycled water to be no more than 2 ntu when sent back to the headworks.

I need to determine the detention time of the eq tank in order to avoid under sizing it...

What kind of correlations between turbidity and settling rates can I assume?

### RE: Backwash NTU

The typical clarifier has a guarantee of 10 mg/L effluent, probably should do 3-5 mg/L if operated correctly.

If you have a 10 MGD plant, the total solids is 10 mgd * 10 mg/L * 8.34 = 834 lbs/ day.

The typical filter wastes 5% of the throughput. For the 10 mgd plant, that would be 0.5 mgd per day.

That would give you a backwash solution turbidity of 200 mg/L.

The backwash storage tank is usually sized for 1-2 backwash volumes.

### RE: Backwash NTU

(OP)
Thanks for the feedback. I'm also wondering if the settling rate for backwash particles is known would this be enough to justify a detention time.

For example, if my average backwash turbidity was 20 ntu with a settling velocity estimated to be 0.25 ft/hour could i then say that four hours after a backwash the top foot of the equalization tank would be below 2 ntu?

Is there a rule of thumb for this?

### RE: Backwash NTU

Have never seen anything like that. Since, the standard clarifier has less than an hour of retention, you would expect that most solids should drop out in approximately 1 hour.

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