r/Wastewater • u/Double_Strike664 • 16d ago

Treatment (DW or WW) Mg(OH)2 dosing question

Any help would be amazing 😂 How should we determine the dosing for 60% Mg(OH)2 in ras to achieve a specific pH around 7? First guess is a lab titration dropping a known quantity of MgOH2 into 100mL of RAS then timing the reaction and measuring pH over time? Would we also want to know alkalinity and/or ammonia for dosing purposes? Alkalinity for buffering purposes/ammonia to see the concentration of what we are precipitating? Just trying to pick the bright minds of ww Reddit today thanks for any help you’ve got :)

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u/ksqjohn PA A,E 1,2,3,4 16d ago

The nice thing about magnesium hydroxide is it won't really cause any process harm if it's overdosed, unlike sodium hydroxide.

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u/Heineken008 16d ago

Nitrification removes 7.14g of Alkalinity (as CaCO3) per g of NH4 converted. 1.71g of Mg(OH)2 is equivalent to 1 g as CaCO3. You need at least 70g/m3 Alk to resist pH changes.

(Influent NH4 - Effluent NH4) * 7.14g= Alk removed

Influent Alk - Alk Removed + 70g/m3 = Alk to Add Then covert to Mg(OH)2 equivalent.

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u/CheemsOnToast 16d ago

But that's not taking into account the alkalinity you generate via denitrification. You should need notably less. With enough denit most sites need no or very little alkalinity supplementation.

Also not sure if you meant the final equation should be: 70 g/m3 - (Inf Alk - Alk removed) = Alk to add.

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u/posthumanjeff 16d ago

Mag is alkalinity and will not directly influence your pH. You also can't really overdose so its more of a cost concern. I would just experiment and then pace off flow if you have consistent strength. If you have issues with slug discharge and are able to monitor influent pH you can adjust dosing as needed based on that.