Chemical Conditioning/Disinfection Post UF/MF Membrane

[tipp79]

I’m working on a project for a water company upgrading a UF/MF water treatment membrane treatment works. Currently hypo is dosed post membrane direct into the pipeline followed by pH correction through a limestone contactor.

As part of the upgrade, the hypo would be dosed into a static mixer which will result in a relatively high headloss. Therefore, we are considering changing the order of the chemicals and have the water flow through the limestone contactor then hypo dosing.

The spec seems to be written with disinfection followed by pH correction in mind. Can anyone foresee any issues with our proposals (obviously chlorine dose will need to be adjusted for higher pH)??

Thanks

 

[bimr]

Not sure why you need a static mixer or why you should have a relatively high headloss in the static mixer. The chlorine will be mixed in to the solution with a just few pipe elbows so the static mixer is not necessary. We normally just use a corporation stop. I do not see static mixers being used for this application unless the designer is a newbie.

In my experience, limestone contactors are relatively ineffective as they will operate successfully only within a very limited pH range. Seems kind of odd to be using an advanced treatment process such as membranes with a limestone contactor.

Organics will tend to grow an any water solution, so it is generally preferred to chlorinate as far upstream as possible. If you chlorinate downstream, you may consider want to consider adding provisions for periodic chlorination of the upstream equipment.

 

[tipp79]

Thanks bimr. The water authority's specification requires a static mixer and from previous discussions are reluctant to grant waivers from this. The site is fairly tight for size and it would be approx only 0.3 m from dose point to limestone contactor. Would this give sufficient mixing in your opinion?

I believe the reason the site has a limestone contactor is due to its size (10 - 12 m3/d) and remoteness- it's in a forest up a dirt track 20 miles from the nearest decent sized town!

Do you think the small distance from dose point to limestone contactor would mean organics could grow on the limestone? Historical data shows a very variable chlorine & inciences of bar tastes/odour but pH seems fairly stable.

Thanks again.

 

[bimr]

You probably are aware that the preferred chlorination method incorporates contact time. However, most people do not want to spend the money for that. Installing the chlorination as far upstream as possible before the user does add some contact time.

I am not aware of particular problems with organics in calcite filters. Having said that, it would not be a surprise to find organics in these filters because microorganisms are ubiquitous and organic growth may occurs in media filters.