Deposition of Manganese on RGF Air Nozzles

[tipp79]

I’m currently investigating manganese deposition in the air nozzles of a RGF. The process was flocculation/coagulation with ferric and lime-poly-superpulsators-chlorine-contact tanks-MSP & Lime Dosing-RGFs-Final chemical Dosing-clear water tank with the RGFS backwashed using chlorinated, pH corrected final water. 3 years ago, the works was upgraded and manganese contactors added so the process is now was flocculation/coagulation with ferric and lime-poly-superpulsators-lime dosing-RGFs- lime&chlorine-manganese contactors-MSP&Lime-contact tanks-Final chem

Alternative causes of manganese precipitation as follows:

• Historically, pH may have been as high as 7 in the water to be filtered, which could have encouraged Mn precipitation. pH has been optimised at the 1st stage filter inlet & it’s now <6.5.
• Chlorine dosing – historically, chlorine was dosed at the 1st stage filter outlet which means the 1st stage filters were backwashed with chlorinated water, another potential cause of Mn precipitation. Apparently the dosing point has now been relocated so the 1st stage filter backwash is no longer chlorinated.
• Aeration of 1st stage RGF washwater – it’s possible that the configuration of the washwater tank inlet causes aeration of the washwater as it enters the tank. Andy H thinks this could be a major cause of Mn precipitation but if the pH is low enough and the water is unchlorinated, the rate of any precipitation due to this should be very slow.

The proposed solution is to install a new washwater main & pumping station to transfer clean washwater from the 2nd stage RGF washwater tank to the 1st stage RGF washwater tank, to provide a manganese-free backwash source. However there may be additional risks associated with using a manganese-free but chlorinated and high pH backwash source e.g. solubilisation of any coagulant carried over from clarification; creation of THMs due to reaction of chlorine with organics; precipitation of any soluble manganese in the filters.

Has anyone come across manganese precipitation on RGF filters as above and resolved the issue?

 

[QualityTime]

I was confused as to what the process train was and is now. A process flow diagram would be useful showing what the pH in each tank is to begin with. What pH are you driving the water from and to in the lime-poly-superpulsators? If you are driving it beyond 8....for sure you will precipitate the manganese and very quickly I might add. What is the contact time in the various tanks?

 

[bimr]

The pH must be above 9 to achieve a measurable degree of oxidation by air. It does not make sense that you are adding lime and your pH is low. Are you adding lime?

Chlorine oxidation is also difficult and requires a pH above 8.5 to achieve a measurable degree of oxidation.

It is likely that the manganese deposits are caused by manganese bacteria that are reducing the manganese. You can try to identify the bacteria with the BART test, but it may be difficult to identify the bacteria.

One would suspect that the lack of chlorination is allowing the bacteria to grow and that the use of chlorine has been limited to avoid the creation of THM's. That is a conundrum.

The solution is to periodically disinfect the filter.