turbidity rising after chlorination.

[kevinh20]

Hi we are treating our water with manganese greensand and anthracite filters with kmno4 being added first to react with iron and manganese in a retention chamber. Once the water is filtered it is entering the cistern at about 0.07 ntu so it's quite clean right before it hits the cistern we chlorinate it. when we check the ntu out of the cistern it is more like 0.16 ntu. So my question is what could be causing the ntu to rise after its been chlorinated then added to the cistern. We have a feeling it may be that we are adding to much kmno4 but we don't want to turn it down to much as we are trying to achieve a continuous regeneration of our filters. We are adding 6.8 l/h of kmno4 to the raw water which is coming in from the wells about about 511 l/m. and we keep our free chl about about 1.00 mg/l.. we test for iron and manganese at every backwash and its usually so low that it wont register on our meter but sometimes get results of about 0.02 mg/l for both fe and kmno4. any ideas would be greatly appreciated.

 

[bimr]

With some of the details regarding your system missing, it is hard to conclude anything.

You might investigate the manganese content. Manganese takes hours not minutes to precipitate from solution. One would suspect post precipitation of manganese.

 

[kevinh20]

yes i will have to check that, after the kmno4 is added to the raw it does sit in a retention chamber where it gets to sit and precipitate the water in the retention tuns brown of course but also there is a fairly distinct pink color from the kmno4.. I'm wondering if the pink should be much less obvious (a much lighter shade of pink) before it hits filters. I'm fairly new to the profession and the other guys usually just tweak the pumps and it's kind of trial by error. I'd like to be able to crunch some numbers and get there a little sooner.

 

[bimr]

One additional comment, the manganese oxidation also varies with pH. The reaction is quicker at a pH of 10 and slows as the pH drops.

The pink color is normal for KMO4.

 

[kevinh20]

Thanks for the advice guys, yes the pink is ok as a light pink but not so much when its half way to purple. I think I may have discovered what happened we have 2 supply pumps feeding the wtp, but one of them is malfunctioning and our flow rate dropped nearly 38% meanwhile the kmno4 pump has kept pumping at it's normal speed so for now until the pump is fixed we will have to reduce our kmno4 dosage my 38% hopefully this will fix our problem.. our ph is ok so what does happen when a high mg/l of kmno4 comes into contact with chl because right when this happened our chl numbers started to really rise as well.