Measuring Chlorine Residual in Iron Removal filter backwash

[jartgo]

Has anyone had experience with using a DPD colorimeter to measure chlorine residual in iron removal filter backwash in support of NPDES permit compliance? In the interim we're using dechlor tablets, but are in a pickle as to how to accurately measure the chlorine levels. The system doesn't want to purchase the more expensive meters as we are working to eliminate the discharge altoghether, but are a few months out in doing so.

Consider a meter such as this one by LaMotte ($350.00) in USAbluebook:

https://usabluebook-onramp.com/docs/images/604.pdf

This meter claims a range of 0 to 4.0 ppm with a resolution of 0.01mg/L (10ug/L). For my application, the limit is 28 ug/L. Obviously the backwash water will be loaded with iron, but my response would be that I could test the water once the backwash has turned clear. Any thoughts?

 

[bimr]

Compensation for Sample Color and Turbidity
One critical problem when applying colorimetric procedures
to treated wastewaters is interference from
turbidity and color in the water. For certain parameters,
a preliminary filtration can be performed to remove
particulate matter from the sample. The residual sample
color is “zeroed”at the measuring wavelength with the
color-measuring instrument........

Hach Company studies indicate if the filtration is
performed after the development of the colored product
(a post filtration), removal of interfering sample turbidity
can be accomplished without concern for chlorine loss.
The selection of the filter media is important because the
Würster dye product is a positively charged ion.Some
membrane filter compositions have a surface charge that
would exclude using them. The selection of filter
porosity also is critical in terms of adequate removal of
the particle sizes that could interfere at the absorption
wavelength.

In the ULR-DPD Total Chlorine procedure for treated
wastewater, sample turbidity is removed, using a syringe
filter apparatus with a special inert 3-micron filter.A preliminary filtration is performed on the sample to
zero the photometer. A second portion of sample is
reacted with the reagents and a filtration is performed on
the reacted sample. When the post filtration procedure is
used, the net absorbance is adequately corrected for
sample color and turbidity.

3e. Interferences in the Amperometric Methods
Standard Methods states the amperometric method “is
the method of choice because it is not subject to
interference from color, turbidity, iron, manganese or
nitrite nitrogen” (Ref. 3.21). In reality, several of these
factors do affect the determination of chlorine species
when using amperometric methods. A brief review of
some of the common sources of errors encountered with
real world samples follows:

http://www.hach.com/fmmimghach?/CODE:L70191473|1//true

 

[jartgo]

Thanks bimr, I've found several articles such as this from Hach Corporation. Any others have any real world experience? I was thinking that since the water would be running relatively clear after backwashing for several minutes, the color shouldn't effect it that much. Thoughts? This is a small plant (<100,000 gpd) and simplicity is paramount, if possible.

 

[bimr]

One would think that the backwash water would have the same chlorine concentration as the effluent from the filter assuming that you were not holding the backwash water in a sump.

 

[jartgo]

the backwash water is being dechlorinated with dechlor tablets.

 

[bimr]

Have you thought about using a product such as Vita-D-Chlor. There is not a problem with overfeeding.

 

[jartgo]

yeah, the vit-d-chlor (or something very similar) is what they're using. I'm sure it's working, but since they have an NPDES permit, they are required to report the chlorine residual after the vita-d-chlor tablets make contact. Finding an economical and simple method (<$1000, no training required) to measure that low level of chlorine is what I'm after.

Anywho, after some more research over the weekend, I've decided that amperometric titration is really the only way to do it, I'm just going to see what a lab would charge to test it for a few months. More hassle than I was hoping for, but with a limit of 28 ug/L, I don't think there's any other way.