Temporary Caustic Dosing causing High Iron

[tipp79]

A colleague has been working on a scheme whereby a lime dosing system at a water treatment works is being upgraded and a temporary caustic dosing kit has been installed while the lime system is out of action. It is a conventional treatment works where lime is dosed upstream with ferric for coagulation. Before this scheme, the iron in the final water was appprox 10 - 20 ug/l. When the caustic dosing system was first switched on, the final iron slowly started to increase but is now over 200 ug/l on occassion. The unusual thing is that the oagulation pH has remained steady. Other relevant info is that the raw water is not hard. Has anyone ever experienced anything like this before when swapping from lime to caustic dosing? Any thoughts on what the cause may be? Thanks for any advice!

 

[bimr]

Would expect that the iron is in particulate form and your coagulation procedures may have to be adjusted somewhat. You may expect more particulate when using caustic.

 

[tipp79]

Thanks bimr. Why would we expect more particulate iron breakthrough when using caustic? In what way would coagulation need adjusting? I believe jar tests have been carried out to confirm coagulation pH. I can't see why coagulation pH would change just because caustic is used instead of lime. Surely when the end result (coag pH) is still achieved then whether its achieved through lime dosing or caustic dosing doesn't matter? Or am I missing something fundamental? If it makes any difference, the ferric and lime are dosed into the same flash mixer and DAF is used for clarification followed by primary RGFs and then Manganese contactors for manganese removal. Thanks for your help!

 

[bimr]

There are slight differences when the two chemicals are applied.

Lime offers advantages when the precipitation of metals is the goal. Calcium salts are normally quite insoluble, and due to the fact that lime is divalent, sludge densities are normally much higher than those formed with caustic (NaOH). Caustic precipitates may have a gel-like consistency.

Lime also acts as a coagulant during the precipitation/settling process, whereas caustic does not.

Lime sludge also tends to settle better and and clarification basins operate at higher overflow rates.