Lime & Chlorine Dosing guidelines

[tipp79]

I'm working on a project where we'e moving an existing lime dose point downstream of a clean washwater tank (to prevent oxidation & precipitation of manganese in the tank).

Are there guidelines/recommendation for

(1) whether lime is dosed and pH is raised first or chlorine dosed first
(2) is there a recommendation for distances between dose points
(3) should there be separate static mixers (currently lime is dosed direct from flexible pipe w/o dosing lance)?

Thanks,

 

[bimr]

What is the treatment scheme?

 

[tipp79]

Its Lime&Ferric Odsing- Coagulation- Pulsators- Lime Dose to raise pH to 6-primary RGF-Lime Dose (currently upstream from the clean washwater tank)-Chlorine Dose for manganese removal-secondary manganese contactors-chlorine for disinfection.

Its the pre manganese contactors lime & chlorine dosing I'd like advice on.

Thanks,

James

 

[bimr]

In treating waters than contain manganese, it frequently occurs that the pH is not raised high enough, with the result that part of the manganese stays in solution. The pH will probably need to be in the pH range of 9-10 units for oxidation of manganese.

Although chlorine is somewhat more active than dissolved oxygen, the pH values required for complete oxidation are nearly as high as with air oxidation. You would need a pH of at least 10 to oxidize the manganese in 30 minutes.

Since the time for manganese oxidation is so long, it really does not matter whether you dose lime or chlorine first. The lime will raise the pH prior to the occurence of oxidation.

However, if you have 30+ minutes of retention in multiple tanks, then it would be more efficient to dose the lime in the first tank and chlorine in a second tank.

Don't think you need static mixers as long as you can achieve some mixing through elbows in the piping or open conduits that the water is flowing through.