Liquidator: Metering bleach during day or night time

[Water_man]

As a new Liquidator user I’m concerned about the trade-off of using it during day time while the pump is working.

In what follows “FC” means hypochlorous acid, the active ingredient in bleach.
When FC is introduced into the pool during day time it has three possible pathways:
Associating with CYA, decomposing by UV, and “doing its job” – oxidizing the “bad stuff”.

The rate of UV burn-out doesn’t depend on the concentration of FC. At any given moment, the same percentage of FC present will be decomposed by UV. However, the rate of association to CYA depends on the FC concentration. The more FC you have, the quicker it will react with CYA.
Since the association to CYA is not instantaneous, and since UV decomposition competes with it, adding FC by a continuous flow of a small dosage during day time is not efficient and it burns FC. On the other hand, you want to have your pool pump working during day time, because this is when you have bather load and this is when you need to add FC in order to compensate for its loss.

If you meter the bleach into the pool during night time, you’d expect that by sunrise most of the FC has already associated with CYA, and therefore this is a more efficient way of adding bleach. On the other hand, if you stop metering FC during day time, your FC level will drop. I guess this shouldn’t be an issue if you start the day with the “target level” of FC and end up at night with the minimum level. The only trade off is running the pump 24/7 and remembering to turn the flow control valve on and off.
Any comments?

 

[chem geek]

This is a topic that I believe belongs in The Deep End due to its technical nature (mods, if you'd like to move it, that would be fine).

You are right that the loss of FC due to breakdown from sunlight is proportional to FC ALL ELSE CONSTANT, though technically it's proportional to the hypochlorous acid and hypochlorite ion concentrations (which are proportional to the FC/CYA ratio) with the latter breaking down faster than the former (so higher pH results in faster breakdown from sunlight). Though the breakdown from sunlight without any CYA is faster than one would like, it isn't "fast" in that it takes around 30 minutes for half of the chlorine to break down (at a pH of 7.5 with half hypochlorous acid and half hypochlorite ion).

The reaction of chlorine with CYA is VERY fast -- they combine essentially right away. Even chlorine releasing from CYA is pretty fast with half releasing every 4 seconds from the dominant species (HClCY-).

So it is incorrect to say that the time of chlorine/CYA association or disassociation is a factor. For all practical purposes, at a given FC and CYA level, the hypochlorous acid and hypochlorite ion concentrations are constant and do not drop appreciably in the presence of sunlight right away -- it's a relatively slow process that can easily be "matched" by feeding additional chlorine into the pool. That is, the hypochlorous acid and hypochlorite ion concentrations are essentially proportional to the FC/CYA ratio even in the presence of sunlight so would drop slowly only as overall FC slowly drops.

To maintain a constant FC level, you want to meter in chlorine at roughly the same rate on average that it is getting consumed. So you want to meter in more during the day due to chlorine breakdown from sunlight. Technically, you'd meter in more as the sun approached high noon (ignoring daylight savings time) and you'd meter in more as one approached the summer solstice. If there are times of higher bather load, you would want to meter in more during such times. Since the amount of chlorine loss overnight is generally low, you would meter the smallest amount during that time.

So generally speaking you want to meter in more during the day and don't need to meter in very much chlorine at night. So running the pump during the day and not at night makes sense from a chlorine delivery point of view in order to maintain a roughly constant FC level. From a circulation point of view, however, it is better to run the pump in times spread throughout the day and night so that there are no localized "dead spots" of chlorine loss due to lack of circulation. So a decent compromise might be to mostly run the pump during the day, but to also run it for an hour or two in the middle of the night.

Richard

 

[revstriker]

Fascinating! As a new LQ user, I have the same questions; what are the best times to run the LQ, and would I need to adjust the flow based on bather load, exposed sunlight, etc.

 

[Water_man]

Thank you, Richard, for another thorough response. I still have one unanswered issue .
You said:

The reaction of chlorine with CYA is VERY fast -- they combine essentially right away. Even chlorine releasing from CYA is pretty fast with half releasing every 4 seconds from the dominant species (HClCY-).

The reaction between chlorine, or better say hydrochlorous acid (HA), which constitutes most of the chlorine, and CYA is very fast, provided the two species meet each other. Under lab conditions the solution is constantly mixed, however in the pool most of the water body is away from the returns and the rate determining step is diffusion and Brownian motion, because there’s no effective stirring.

The Liquidator increased the FC in my pool by 3 ppm during 9 hours of night operation. Since I know my flow rate (48 gpm), the calculated level of FC feeding the pool is about 2.6 ppm. This low concentration goes through an enormous dilution in the pool.
So I wonder, wouldn't the HA molecule decompose before it has a chance to bump into a CYA molecule? Granted, once they meet the association is almost instantaneous. I'm sure you can dig some diffusion kinetics time constants

So far from the theoretical point of view. From the practical point of view: isn't it true that many experienced pool owners in the forums say that they add bleach during night time? If they were not concerned about the two competing pathways, namely degradation by UV and assciation with CYA, why would they do it at night time?

 

[JasonLion]

When adding chlorine via a Liquidator, the bleach is introduced to relatively large quantities of water in the pipe and passes through areas of turbulent flow, which mix everything throughly, before being exposed to sunlight. It will be completely reacted with the CYA before it even leaves the pipe.

A more important factor is that FC must be raised comparatively high if the pump is only running at night. Say we are aiming to have FC at 3 minimum. If chlorine is only added at night, we will need FC at around 6 first thing in the morning in order to have FC at 3 in the evening. Since FC loss is proportional to the FC level, holding the CYA level and sunlight flux constant, more FC will be lost than if FC is at a steady 3, by continuous addition throughout the day.

 

[chem geek]

As for why chlorine is usually added at night, it's because if you were only going to add it ONCE a day, then adding it at night means that you will have a higher chlorine level for an extended period of time at night and then it will drop during the day. If you were instead to add it in the morning, then the chlorine level would drop during the day and the entire night would have a fairly low level of chlorine. Given the choice and the risk of having the chlorine get too low, it's much better to add it at night since if you misestimate or have an unusually high chlorine demand during the day, then at least the low chlorine level will only persist until you next add chlorine that night; otherwise, having a low level all night long until the next morning could give algae a chance to grow.

Richard