Certified Pool Operator Question

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Ian.H

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In The Industry
Mar 3, 2009
9
Alicante - Spain
I am taking the CPO course later this month , so I just reviewed Certified Pool Operator (CPO) training -- What is not taught and had a quick read through the handbook, but there is one thing in particular that I just don’t get ?

Chem geek , has touched upon it in the OP , but if he or any one who has taken the CPO course could clarify , that would be great ,

On page 65 of the handbook it basically states that, the higher the TDS the more organic / nitrogenous contaminates are present , but what brings them to that conclusion ??

I can understand that , the more bather waste , the more chlorine is required, so that would leave a higher residual , which would read as TDS ( such as salt etc ) , but then it follows that the extra chlorine used would remove any such contaminates also, by way of “ breakpoint chlorination “ ?

Surely , it makes more sense to check such nitrogenous products by testing for FC and then TC than to be guided by TDS, or am I missing something obvious ?

Thanks
Ian
 
They assume that the increase in TDS, which is mostly the sodium chloride salt level, is due to frequent chlorination where chlorine becomes chloride (salt). For heavy bather-load pools such as the commercial and public pools mostly addressed by the CPO course, most chlorine usage is due to the bather load in order to oxidize the bather waste. This exceeds the loss due to sunlight that is the primary loss in residential pools. So the buildup of TDS in heavy bather load pools is associated with bather waste and therefore it is more likely for such pools to contain organics that are either not oxidized or are only partially oxidized.

The rough rule-of-thumb has the following categories for bather-load where I assume 4 grams of chlorine per bather-hour, 7 gallons per bather-hour water dilution, and that a hypochlorite source of chlorine is being used:

Bather Load . Gallons Per Bather . ppm FC for 8 bather-hours . TDS buildup per day . Dilution % per day
High .................. < 1000 ........................... > 8.5 ............................. > 14 .......................... > 5.6%
Medium ............. 1000 - 5000 .................... 1.7 - 8.5 ......................... 2.8 - 14 ...................... 1.1% - 5.6%
Low .................. > 5000 ............................ < 1.7 ............................. < 2.8 ......................... < 1.1%

Based on the dilution rate, the steady-state TDS would be around 250 ppm. The buildup of TDS in high bather-load pools usually indicates that they are not diluting at the recommended rate of 7 gallons per bather per day (assuming one hour per bather). In low bather-load pools the chlorine loss from sunlight tends to build up TDS even if one were to dilute the water from bather load.

Chlorine does not completely oxidize every chemical. It oxidizes urea somewhat slowly, for example, especially when there is no sunlight to create hydroxyl radicals from the breakdown of chlorine. While chlorourea will usually measure as CC, urea itself will not be measured in a chlorine test kit and the slow step for oxidation of urea is the initial chlorine attachment to the urea -- so most urea remains as urea in higher bather-load pools until it builds up to a level where chlorine oxidation of it equals its rate of introduction. This is one reason why such higher bather-load pools have higher CC and that such CC persists over days or a week or two even with no bather load.

So their statement most definitely does not apply to low bather-load pools such as most residential pools. There the buildup of salt from chlorine use does not correlate to a buildup of organics. In most residential pools the buildup of organics is minimal and can usually be ignored. In addition, in saltwater chlorine generator pools, the salt level is fairly constant regardless of bather load so the high and constant 3000 ppm TDS cannot be used as a proxy for organic buildup at all.
 
Morning Richard , thanks for the reply ,

So in a nut shell , they are making the point that high bather load pools , which obviously will have high bather waste , therefore high chlorine demand and usage , will contain organic contaminates , that are very slow to react with chlorine .
As these are so slow to react , then they will not show up or be measured by the regular CC test , so they are using the high TDS as a guide for them ?

If I have got that right , then it does make more sense , but still not sure its an accurate method to judge though !

Out of interest , approximately how long do these type of contaminates , such as urea etc , stay in the water before they do start to react with chlorine , and can then be measured by the CC test ?

Thanks
 
You've got it right and yes it's not the best way to judge. Besides, what do they do at that point? Presumably they expect one to dilute the water, but if one is doing that as they should be at 7 gallons per bather (especially if that were per bather-hour) then there would be no problem with buildup, but that's a lot of water replacement and as I noted it's often not done to that degree.

As described in this paper (available for free in this link), the urea concentrations in indoor commercial/public pools averaged 18 µM but was as high as 60 µM. When 50 µM chlorine was added to 50 µM urea (3 mg/L) only 20% of the urea was oxidized in 24 hours. Note that 50 µM chlorine is 3.5 ppm FC and that no CYA was used.

This paper describes how a steady-state buildup of Total Organic Carbon (TOC) including urea occurs after 200-500 hours of operation and that TOC was in the range of 6.5-28 mg/L. Note that they used simulated bather load and noted that at steady-state there was complete mineralization so oxidation of organic compounds, but they used a simulated body fluid analog so did not have dead skin cells and other organic material that would be removed by filtration/backwashing. Basically they are just saying that those compounds that are generally soluble in water and come from sweat and urine get completely oxidized eventually. The Body Fluid Analogue (BFA) they used had ammonia, urea, L-Histidine, hippuric acid, uric acid, citric acid, phosphate, and creatinine.

The higher the bather-load, the higher the steady-state buildup, and the higher the intermediate Combined Chlorine (CC). Blasting with more chlorine only partially alleviates the situation but is what some facilities do overnight. Unless there is churning of the water and good ventilation to remove volatile compounds, such shocking with chlorine just ends up producing more irritating chloramines especially nitrogen trichloride. So for high bather-load facilities it is better to use supplemental oxidation such as ozone or to do more water replacement. Removal of dead skin cells and other material including use of clarifiers such as 2-stage PRS can help remove organic precursors before they react with chlorine.

None of this is needed for residential pools since they are much lower bather load. Also, the above studies were for indoor pools. Outdoor pools have chlorine breakdown from sunlight which produces hydroxyl radicals that help oxidize organics faster. On the other hand, outdoor pools often have CYA in them and that significantly lowers the active chlorine concentration. Nevertheless, outdoor pools tend to not build up organics as much as indoor pools at the same bather loads.
 
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