Combined Confusion – Part 1

When troubleshooting pool chemistry issues related to fluctuations and inconsistencies, I often joke that the only way to keep pool chemistry in perfect balance is to not let anyone use it. When the heat is off and no bathers are present, pools almost take care of themselves. Trace amount of chlorine give very high ORP values (the measure of the quality of disinfection) and very little chlorine is consumed to maintain those levels. Not only is the chemistry stable but the water itself stays in impeccable form. Incredible clarity and no offending chlorine (or ‘pool’) smell are the two healthy water qualities I look for in a well maintained pool. Being around commercial swimming pools for 10+ years now I can tell a lot about a pool just from the smell and a quick look through a true depth of water (the length of the pool, not looking top down – hold a mirror on the pool wall and evaluate how well you can see the other side). I don’t need to do a water test or walk through the mechanical room to know if the pool water is being properly maintained or not. My conclusion is that it takes a trio of good equipment, good procedures and good operators to deliver healthy pool water.

In Alberta, a measurable residual of free chlorine is required at all times for a pool to be open for public use. Although chlorine is highly toxic, in small quantities it is a highly effective as a disinfectant. Disinfection is important to keep your bathers healthy and free from the pathogens that can potentially inhabit the water (the worst of which are those that are introduced via fecal matter, hence the precautionary actions taken in the event of a fecal release). As good as chlorine is at disinfection, it’s by no means perfect. Chlorine is not a desirable oxidizer. The oxidation of organic material in the water (urine, sweat, dead skin, hair, cosmetics etc.) results in a reaction that forms some type of combined chlorine. (WHO Chlorination Concepts Fact Sheet 2.17) To say combined chlorine is formed grossly oversimplifies the matter, combined chlorines can be classified as monochloramine, dichloramine and trichloramine (nitrogen trichloride). The type of combined chlorine formed is dependent on the ratio of free chlorine to ammonia. Simple monochloramines are often present in fill water as the preferred disinfectant for potable water distribution lines. Monochloramines don’t have the odour problems associate with dichloramine and nitrogen trichloride. In drinking water, the independent dosing of chlorine and ammonia is controlled to ensure correct rations are present so only monochloarmine is formed (EPA Guidance Manual – Alternative Disinfectant and Oxidants, 1999). Although monochloramine is combined chlorine, it’s not so bad. US EPA allows up to 4.0 mg/L of monochloramine (recommended is 1.5-2.5) for drinking water. Having monochloramine alone is beneficial in a drinking water distribution system for a couple reasons. One is that the residual lasts longer but it also is less reactive than free chlorine and does not as readily form trihalomethane (THM) due to reactions with organic material (WQA Chloramine Fact Sheet, 2014). THMs should be considered the real enemy – not necessarily ‘combined chlorines’. The requirement for quick kill times means we can’t use monochloramine as a disinfectant in pool water (free chlorine is 200x more effective). The US EPA also sets the limit for THM at 0.08 mg/l in drinking water, the limit is 0.10 mg/l in Canada. Unfortunatly, THM testing is difficult due to the volatility of the compound and requirement for high end test equipment so it’s not realistic to expect to test it on the pool deck with current water analysis technology.

To summarize the points made so far, ‘combined chlorine’ is not a specific enough test to tell us if anything is ‘good’ or ‘bad’. The other big take away is THMs are the real concern. THMs are formed from excessive chlorine levels reacting with oragnic material (bather waste), more chlorine = more THM (WQA Chloramine Fact Sheet, 2014). If we were to remove the monochlormaine from the picture, we would have a better idea of the undesirable byproducts that are in the water. Dichloramine and nitrogen trichloride are undesirable byproducts in that they cause greater “swimming pool”- type taste and odor problems when they exceed concentrations of 0.80 mg/L and 0.02 mg/L (respectively). (WQA Chloramine Fact Sheet, 2014).
The problem with our pool industry test kits is the failure to awknowledge these established facts.

The false and wrongly oversimplified chlorine test procedure is very familiar to pool operators:

Total Chlorine = Free Chlorine + Combined Chlorine

As stated previously, the combined chlorine that consists of monochloramine, dichlormaine and nitrogren trichloride is not much use to us. Let’s call this example A; If you have a monochloramine resuidaul of 3.0 mg/l but your dichloramine and nitrogen trichloride reading of 0.0 mg/l, then I would say you have really good water despite a ‘combined chlorine’ reading of 3.0. Example B is less desirable; monochloramine is 0.1 mg/l and 1.0 dichloramine, 0.5 nitrogen trichloride. Although the traditional ‘combined chlorine’ reading is 1.6 mg/l, these measurements are much more problematic because of how much worse dichloramine/nitrogen trichloride are at low concentrations. Example B is what happens during breakpoint chlorination, you have less of the ‘not bad’ monochloramine but the others (plus the THM levels) actually increase significantly.

Unfortunately, there is one more problem with combined chlorine testing. The test starts off good, DPD free chlorine reagent (usually a 2-part reagent) reacts specifically with HOCl which is free chlorine. The next step uses DPD3 (potassium iodide). DPD3 reacts with all oxidizers in the water. If you only had chlorine then the assumption that all oxidizers means total chlorine is correct, but the reality is to effectively run a pool you need a non-chlorine oxidizer. Oxone and Chlorine Dioxide are the most common, and are usually present in the water to help reduce the aforementioned undesirable chlorine reactions with organic material. So DPD3 will test positive for our helpful oxidizers, this further limits our ‘combined chlorine’ test accuracy.

If we were to advance our water test equipment and methods out of the 1970s, we would have to change that equation to:

Total Oxidizer = Free Chlorne + Monochlormaine + Combined Chlorine* + Oxidizer

*This new combined chlorine would be a more accurate depiction of ‘undesirable’ byproducts in the water (dichloramine and nitrogen trichloride) with a recommended value being less than 0.1 mg/l.
In pool water treatment we don’t have the luxury of controlling the amount of ammonia in the water. The outdated theory of ‘breakpoint chlorination’ is based on driving the oxidation process past the point of nitrogen trichloide formation. Hydraulics are a limit that prevent the uniform breakpoint occurring at the same time throughout the basin, but the real problem is that breakpoint increases levels of dichloramine/nitrogen trichloride and THM.

Traditional breakpoint chlorination theory for combined chlorine removal is outdated and inefficient. The problem is the result of undesirable oxidation caused by too much chlorine in the presence of high levels of organic waste, adding more chlorine to fix this is simply throwing more fuel on the fire and accelerating the reactions taking place – not preventing them. To further advance this comparison to a fire (which is also a form of oxidation), we can agree there are 2 ways to put out a fire. On one hand you can help the fire by feeding it so it consumes everything until there is no fuel left and it burns itself off, this is the approach of ‘breakpoint chlorination’ theory. This theory pre-dates non-chlorine oxidizers that have been commercially available for +30 years now. While breakpoint may appear to lower the ‘combined chlorine’ test result, in reality there is less monochloramine but more of the other combined chlorine and THMs. Switzerland and Germany both set limits on THM levels at 0.02 mg/l and to achieve this, they have to set strict limits on free chlorine as well (maximum 0.6 mg/l. Running at higher chlorine levels consistently leads to increased THM formation. It’s not realistic to expect to run under 0.6 mg/l unless you have a filtration system to back it up. Traditional shallow, spherical north american sand filters and pre-coat style ‘DE’ filters would not pass the performance criteria in these countries with strict recreational water quality standards. They rely on true depth filtration (multi-layer deep bed sand filters with optimized flocculation/coagulation) in order to get away with running lower chlorine levels.

You need to decide if your goal is to either feed the fire or prevent it altogether, you will want to consider treatment options that remove organic material from the water and prevent the undesirable reaction from occurring in the first place. The most efficient treatment choices will be those that improve filtration and enhance oxidation without creating more chlorine demand.

Keep watching for future posts in this series, including a detailed examination of available supplemental treatment options and a more in-depth look at combined chlorine testing.