Wastewater : Magnesium hydroxide vs. caustic 1

[seasar]

I'm looking into switching our effluent alkalinity adjustment from caustic (50% solution) to a magnesium hydroxide slurry. Everything I've read and the vendors I've talked to espouse the benefits...are there any downsides on the treatment side? I've got a good understanding of what I'll have to modify on the equipment.

The waste water to be treated is from a large brewery and we only treat for pH adjustment. The fatty acids cause the pH to drop below our limit by the time it makes it to the municipal treatment plant so we add caustic to raise the alkalinity..we then add CO2 to bring the pH back down to an acceptable upper limit.

Thanks!

 

[bimr]

There is no downside to using magnesium hydroxide other than it is more expensive that caustic.

Don't understand why you would be overfeeding caustic and then adjusting the pH back down with carbon dioxide. Why don't you just feed the correct dosage of the caustic in the first place and save the money spent on carbon dioxide?

 

[seasar]

Bimr...your questions seems to be the $64,000 one.
This is the system I've enherited and I am by no means a WW expert (that expert has since left). My understanding of the system is that the enough caustic must be added to counter the formation of volatile fatty acids that would otherwise lower the pH below our limit by the time it reached the municipal treatment plant. However, adding enough to accomplish this raises the pH above the limit. CO2 is added back as it lowers the pH without affecting alkalinity...and it is the increased alkalinity that inhibits the formation of the acids.

Also, at this point mag hydroxide appears to be much less expensive than caustic.

 

[bimr]

One would expect that your pH is measured at your plant discharge point to the municipal sewer, not at the municipal WWTP. If your pH leaving the plant is at a pH just below 9, then one would not expect the pH to drop below 6 by the time that the wastewater arrives at the WWTP.

Do you have a separate issue with FOG?

 

[seasar]

It usually leaves the plant around 9-9.5 and arrives at 6.5 to 7 and we do have it measured as it arrives at the treatment plant as we have our own sewer line. Without adding back in the CO2 it can approach or exceed 11. I don't fully understand how the current setpoints for pH (separate to adjust with Caustic and then with CO2) and I am reluctant to experiment with them without fully understanding the chemistry..lots of good research on this site and the internet to help though.

No issues with FOG, just limits on BOD and TSS.

 

[ruggerjvd]

I have designed several systems with MgOH2. A few things to consider:

MgOH has a very high specific gravity, and must be mixed continuously to keep solids in suspension. We used a 6500 gal tank to be able to take a full tank load. The mixing arrangement was not trivial. It needed a fabricated steel frame to support the mixer.

Provide flushing and carrier water in the pipe system. The slurry will tend to settle in the lines when turned off, and it may take a fairly high pressure rinse to get it moving again.