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Managing Zero Chlorine Levels.


robquick

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Hi,

Generally I'm very disciplined at managing my Chlorine levels to ensure that they never fall to zero however, every so often they may drop to zero for 12 or 24 hours. I always ensure I increase the levels back to 5 ppm and maintain this for about three days before I use the tub again, a couple of questions.

Provided your Chlorine levels have not been at zero for more than approx 24 hours, what action should you take to ensure the water is safe ?

If the Chlorine demand is not greater than usual after a stint of zero Chlorine does this suggest the water is ok ?

Any guidance appreciated.

Rob.

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Bacteria can double in population every 15-60 minutes so it doesn't take too long for biofilms to form and they are chlorine-resistant. A few hours should be OK, but after 8 hours you might have problems. If you don't see an increase in chlorine demand or cloudy water or slimy spa surfaces, then you may be lucky, most likely due to the water being poor in algae nutrients or having something else helping to inhibit bacteria growth -- for example, 50 ppm Borates have some minor moderating effect.

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It takes 60 minutes for a 3-log reduction (99.9% kill) of Legionella at room temperature from 0.1 ppm FC chlorine with no CYA so in a spa with hotter water and higher active chlorine levels it's probably around 5 times faster. Certainly, if you have chlorine in the water, then within an hour the bacteria should be largely killed and certainly not in large numbers IF they have not already formed biofilms.

The risk for Legionella is real and has been reported on this forum in this post. However, that situation had repeated use without proper chlorination for extended periods of time.

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So, hypothetically if you left the spa for a long enough period of time for Biofilm to form and you did have Legionella present even if you added significant levels of Chlorine it wouldn't kill it ??

What would the signs be of the Chlorine not killing it? Would it be a dramatic increase in Chlorine demand ? I appreciate this is extreme and very unlikely however, I'm interested in understanding further.

Thanks.

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Yes you are correct that if you have an environment where bacteria can grow, so no chlorine and plenty of nutrients and no biofilm inhibitors (strong surfactants), then bacteria can form biofilms and once they are established, normal chlorine levels will not remove them. If there are significant biofilms, then chlorine demand will be increased. This is most often seen in some commercial/public pools where they can get significant biofilm in their sand filters resulting in channeling and high chlorine demand. We've also seen this in residential spas, though for newer spas there can be high chlorine demand from greases and oils leftover from the manufacturing process, though wet testing sometimes results in biofilms in pipes.

This paper shows that biofilms can be kept in check (i.e. prevented from being established) if chlorine levels are maintained where plaster surfaces had only 3.72 cfu/cm2 and sand filters had 2.34 cfu/g and this is from continual introduction of massive numbers of bacteria representing extraordinarily high bather-load where the control with no chlorine had 1.26x106 cfu/cm2 on the plaster coupons and 1.10x109 cfu/g in the sand filter.

So if you leave a spa with no chlorine for an extended period of time, then it is safest (especially if you notice higher than normal chlorine demand) to do a decontamination procedure using a spa flush product (e.g. Spa System Flush or Ahh-Some or Aquafinesse Spa Clean Tablet) and can follow that with a super-chlorination and water change.

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Only if the ozone left a residual in the water. Bacteria tend to adhere to spa surfaces and do not necessarily circulate so would not get exposed to ozone in the ozonator. This is why secondary disinfection systems (e.g. UV or ozone) are not primary ones and a residual of chlorine is still required. For spas, ozonators do tend to put some ozone into the water which is why they are usually turned off if you go into a spa and turn on jets, but they are not running all the time, so in theory they might help, but there have been plenty of reports of spas "going south" with cloudy water or slimy walls when the chlorine went too low in spite of having ozonators. Whether that was due to the ozonator being too weak or not running long enough, I don't know.

If you are worried about letting the chlorine get too low, then why don't you use an automated chlorine dosing system such as the ControlOMatic Technichlor?

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