Higher Level of Cl, HOCL and OCL- , Stain/Iron, pH 7.3

[smallpooldad]

If one has iron issues, wind born dirt and iron, and pollen, but maintains a pH of 7.3, and sufficient sequestrant ppms, would it be better to maintain chlorine on the high side if the CyA, in a non-saltwater pool, is 50 ppm? Or even in a saltwater pool at a a CyA of 80 ppm.

The reason for asking this question is that at higher CL levels of say 8 or 7, in this theoretical example, the HOCL level would be higher than say at 5. I am lead to understand, higher HOCL levels oxidize considerably faster than the reserve OCL-, which oxidize at a far slower rate. Would this then oxidize pollutants entering the pool more efficiently than at the lower chlorine level of 5, for pools with wind born dirt and iron, and pollen issues? Thereby taking oxidized metals, and other pollutants, to the filter at a rate this faster acting HOCL can keep up with, thereby reducing potential staining issues; allowing these oxidized pollutants to be eventual removed during backwashing.

Or would the reverse be true, or would it make no difference?

If higher levels were better, would the higher reserve of OCL- also help to maintain control of contamination? As OCL- can keep the HOCL level at a higher constant, in that % HOCl (vs. Total Free Chlorine) does not change between FC 8 7, and 6, but at 5, and 4 ppm, drops from 1% to 0.9.

I realize that the consumption of chlorine might be increased somewhat, if the level is kept higher. That would be OK if overall stain control could be improved upon.

Thank you to those that might help

 

[JasonLion]

There is nothing to be gained here. Raising the FC level will use more chlorine, and will either have no effect on staining, or make staining more likely.

 

[chem geek]

In low bather-load outdoor residential pools, most chlorine is lost from breakdown from the UV in sunlight. Raising the FC level (at the same CYA level) just results in a higher chlorine loss so more chlorine needs to be added.

Most contaminants that get into a pool are undissolved particles that can be filtered out (dirt, pollen, even algae cells). Higher chlorine levels don't help with such circulation/filtration removal.

The only thing that is usefully sped up by higher chlorine levels (other than pathogen kill and algae inhibition rates) is oxidation of chemicals that chlorine can oxidize, but most of that is from bather waste and there isn't very much of that in low bather-load pools.

As for iron, that should be fairly quickly oxidized even at lower chlorine levels so there shouldn't be much benefit to raising them. I doubt that you have any significant ferrous iron (Fe²⁺) in your pool. It is converted to ferric ion (Fe³⁺) very quickly so it doesn't make any sense to have a higher concentration of chlorine for that purpose. The following is a quote from this paper with bold emphasis mine:

In natural waters, soluble iron and manganese usually exist in their divalent ferrous and manganous form, respectively (Stumm and Morgan, 1970; Sawyer and McCartly, 1978; Pouvreau, 1984). These species of iron and manganese lead to several disadvantageous results during drinking water treatment processes (i.e. metallic, astringent or medicinal taste problems, coloring of water, growths of certain microorganisms and pipe corrosion phenomena, etc.) (Wong, 1984). Fe(II) and Mn(II) oxidation to insoluble Fe(III) and Mn(III, IV) species followed by filtration processes represents the main iron and manganese removal method used during water treatment. Table 1 reports the apparent chlorination rate constants at pH 4 for Fe(II) and pH 8 for Mn(II). These results demonstrate a nearly instantaneous iron oxidation during chlorination at pH 4. At higher pH, a higher apparent rate constant can be expected for its reaction with chlorine due to the iron speciation in solution. Mainly Fe(II) hydroxy complexes which increase with increasing pH are quickly oxidized.

As for HOCl vs. OCl^-, though HOCl is a much stronger disinfectant, that is largely due to it being a neutral polar molecule that "looks like" water so permeates cell membranes to wreck havoc in pathogen and algae cells. Cell surfaces have a negative charge so OCl^- is generally repelled and ions generally enter cells through specific ion channels that are more regulated. However, for oxidation of chemicals, both HOCl and OCl^- participate in such reactions and in specific ways so some chemicals only react with one of the two. Having a pH near 7.5 gives a roughly equal balance between these, but having a pH of 7.3 is not a big deal in this regard.