Removing TDS + BOD from an industrial effluent: Order of Operations

[plantprowler]

Consider an industrial effluent which has dissolved solids as well as organics. This leads to a combined load of TDS (say, 15% dissolved solids) and COD (say, 40,000 ppm).

For concreteness, say, the solids are NaCl / Na2SO4 and the organics are innocuous and biodegradable, nothing toxic / refractory etc.

Does it make sense to remove the solids (using a multi effect evaporator) before the organics (using microbial digestion) or the other way around? Is there a certain sequence or does this depend on each facility.

 

[ashtree]

plantprowler

I am going to say that this decision would be very site and process specific.


15% dissolved solids is really high 150,000mg/l which i suspect is above saturation. At that level of salinity assuming that number is correct microbial digestion could be quite slow. There may be bugs that will work at that salinity but any of the normal bugs would not survive.

Most of the organics will of course carry through the MEE but some may go out with your waste stream. However i am not certain that you can do 15% solids with MEE.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

There would be no point in trying to remove the organics.

The salt concentration is so high that one would send all of the waste to an evaporator.

 

[plantprowler]

@ashtree

Thanks.

But 15% is not above saturation. For both NaCl & Na2SO4 the saturation concentration is in the 25%-29% range.

Can you elaborate on what you meant by "i am not certain that you can do 15% solids with MEE"? The MEE evaporates enough water to take 15% up to (say) 25% which is the saturation point. At which point the Salt separates out and is sent to a landfill.

Do I misread what you meant by "i am not certain that you can do 15% solids with MEE"?

 

[plantprowler]

There would be no point in trying to remove the organics.

The salt concentration is so high that one would send all of the waste to an evaporator.

Thanks @bimr.

But you still need biological treatment post the evaporator.

So what you are saying is Evaporator -> Bio makes more sense than Bio-> Evaporator?

Or are you saying there's a way to do this right using just an Evaporator alone?

 

[bimr]

What is the end product here? Will you be landfilling the product?

With so much salt content, it will be difficult to treat biologically. I would try to take the slurry out of the evaporator and dry the slurry with a crystalizer or some other drying process.

 

[plantprowler]

The end-product, rather the goal is to get rid of a effluent stream.

The evaporator produces solids. Those go to a landfill. Some organics go with the solids as adsorbed impurities. Good riddance.

Unfortunately some (volatile?) organics come with the evaporator condensate. The condensate is intended to be reused as process / utility water. Hence the desire to remove the pesky organics that came with the condensate. That's where the Bio treatment comes in.

Makes sense?

 

[bimr]

Tough application.

Is it possible to remove the organics from the condensate with carbon?

 

[plantprowler]

@bimr

Carbon would work. Fentons works as well.

But I think Biological Treatment works out cheaper than both.

Besides, we don't need polished condensate. Something like 150 ppm TDS and 100 ppm COD works ok as make up water for the utilities.

 

[ashtree]

plantprowler

Then you have answered your original question.
Remove the TDS first then deal with the organics, by whatever method you choose.


Regards
Ashtree
"Any water can be made potable if you filter it through enough money"