I'm using total chlorine DPD reagent pillows with a hach colorimeter II and getting a pink color and high reading past two carbon tanks, the first of which was rebedded two months ago. Permeate water is 0.00. The readings past tank 1 and tank 2 are almost identical. When using the interference test, the results are very low.
Is this indicative of interference or is there a sure-fire way of knowing? By the way, DPD #4 tablets show a little color change but still less than 0.1mg/L and Serim strips also show less than 0.1mg/L.

I'm not familiar with the interference test. Are you using the Pocket Colorimeter by Hach? I recently had my first carbon tank rebedded, and I had a break through problem with it shortly afterwards. The valves on the head of the tank were not operating properly, and I installed portable tanks until a new tank is installed. A pink color in your samples is not encouraging. I would pursue this problem immediately. Based on what you state, pink samples post carbon 2 tank, treatments cannot be performed at your facility until the problem is resolved.

I experanced this issue before. What the problem was there where organics coming from the water treatment plant. Do you get your water from a lake or river? Call you local water treatment plant and they should be able to tell you if they are have this type of problem.

We have experienced a similar situation in several of our facilities. What we have tried with considerable success is lignite carbon in a tank upstream of the "regualar" carbon tanks. RO membranes reject chlorines and chloramines very poorly. If your RO product water is 0.0ppm and your reject water is very high then the RO is rejecting something. I've heard it called a "big ugly organic molecule". The istallation of a lignite tank really helped, and there is provision for it in AAMI: RD-62:2006; section 4.2.9. For survey pruposes we call it a lignite tank or "organic filter".

By interference, are you referring to chromium and manganese, which can give you a false positive result with DPD?

Be sure you have written policies and procedures in place.

Mark, we do show the same level in the reject water as past tank #1. How would these "organic" molecules affect the carbon? Would it keep it from being as capable of removing the chloramine?

jdbiomed, this system is offline. we have two seperate treatment systems.

Our experience has been that the lignite carbon does a better job in removng organic material pre carbon, allowing the "regular" carbon tanks to do their job more effectively. There has been some debate that the "regular" carbon tanks are working better simply because there is more carbon in-line. The lignite tanks tend to be somewhat smaller than the "regular" carbon tanks so I don't think it's just more carbon. We re-bed the lignite every 6-12 months and the anxiety of "positive" chlorine/chloramine tests is reduced or eliminated.

We have a mustard/brown build-up in piping throughout the multimedia and two series carbon tanks but little to none post water softener. We are considering a scavenger tank. Any thoughts on that?

I would suggest that you do a comprehensive profile on your feed water to determine the cause, i.e. TOCs or interference caused by manganese ect.

Try testing with DPD reagent from a different lot. We had some false positives. When we opened the packets you could see difference, the ones that gave a false positive looked grainy compared to the new ones from a different lot. it looked as if moisture had got in.

We are going to send samples to a lab and we tried several different lot #'s and different instruments to narrow down causes. We have replaced all carbon in tanks and will proceed with watchful eyes. Thanks for all input and future input.

I also opened up some reagent packets for the 10ML samples for the Pocket Colorimeter the other day and the powder in them had solidified like they had been expsoed to moisture. I am sorry that I did not get the lot number on them. The powder was lumpy and had already discolored pink to lavender when they were opened.

You may be reading bacteria if your water provider has switched to free chlorine for a short while. You can confirm this by filtering your sample, then retest with the DPD. If you get a negative result, then most likely bacteria is your culprit.

Manganese and related compounds such as potassium permanganate are chemicals which interfere with DPD chlorine tests. Manganese is amphoteric, meaning it can be present as either cation or anion compound which results in producing both positive and negative DPD test results. You can order the following from Hach to neutralize them: Potassium Iodie (hach catalog # 343-32) and Sodium Arsenite (hach catalog # 1047-32). You put 3 drops of Potassium Iodide to adjust pH to 6-7 than 3 drops of Sodium Arsenite. You do normal Chlorine testing using Hach meter get your results. Than using same sample you add the two mentioned above. Subtract results you get after putting in the two mentioned above from your origianl reading. (IE first test 0.14, second test using two chemicals 0.11, true acutal reading would be 0.03 Total Chlroine.