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Backwash NTU

Backwash NTU

Backwash NTU

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?

Thanks in advance

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

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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