[Nitro]

Chlorine Demand is the single most important measurement you can make, to determine the health of your tub water.

Any organic "stuff" (dead or alive) in your tub, Chlorine will kill and oxidize. The more "stuff" in your tub, the more Chlorine is needed. The question becomes, is there a measurement of how much "stuff" there is in your tub. Yes, Chlorine Demand (CD)! CD is basically how much Chlorine (sanitizer) your tub is using. This also applies to Bromine. CD will tell you how much Chlorine is being used, and therefore tell you how much "stuff" is in your tub.

Now that we know what CD is, how do we measure it? That's the easy part.

1st, Shock your tub to around 10ppm FC, and make note of where it's at.
2nd, Cover and let sit for 24hrs. DON'T use the tub!
3rd, The next day, test FC again.

Now, take the 2nd measurement and divide it by the 1st measurement. This will give you a percentage of how much Chlorine is left in your tub after one day. Obviously the higher percentage the better. 100% means your tub is not using ANY Chlorine (not likely). 0% means your tub used ALL the Chlorine you put in it (NOT GOOD).

To take it one step further you can calculate CD, which is One minus the Percentage you calculated. That will give you the percentage of Chlorine your tub is using. Obviously the LOWER the better. If it's 0%, that means your tub is using 0% of the Chlorine (again not likely). If it's 100%, that means your tub used 100% of the Chlorine you put in (again NOT GOOD).

Measuring CD is the easy part. Interpreting it is a little trickier. It's best to first measure CD on a fresh refill, BEFORE you use the tub. That way you have a good Baseline (BL). My BL is ~25%. That means, when I shock my tub to 10ppm FC, the next day it is ~7.5ppm. After you start using your tub, the CD will increase. The trick is to try to keep it as close to your BL as possible.

For example, let's say you use the tub (4 people for an hour). After you get out, you shock the tub to 10ppm. The next day you check FC and it's 2ppm. That means your CD is 80%. That's too high and means you didn't use enough Chlorine the night before. Not a problem, just shock to 10ppm again. The next day check your FC. It should be above 5ppm. If not, keep shocking until it is. If you're not using enough Chlorine, your CD will increase and be more difficult to get back down.

So what's a good CD then. Here's my recommendation.
~25% is Ideal. This should be your Baseline.
~50% is Ok. This is probably where most people are at.
~75% is Poor. This means "something" is using up chlorine too fast. You need to shock.
100% is Bad. This could mean you just had a Hot Tub Party, you've been using way too little Chlorine, OR you have "something" actively growing (a bug) in your tub. The former two mean you should start shocking. The latter means you need to decontaminate your tub.

NEVER let your CD get to 100%, or you'll have problems. Also, the older your water gets, the more difficult it will be to lower the CD. If you find it difficult to lower your CD, it could mean you're due for a water change. It could also mean your filter is dirty and needs to be cleaned. Make sure your keep you filter clean, or it will use up Chlorine. Lastly, MPS will help Chlorine oxidize waste, and therefore lower your CD. So if you're using MPS, you may want to wait a few days (week) before you measure CD. If you find it difficult to lower CD, try shocking with MPS.

Hope that helps you keep you water cleaner.

Happy Tubing!


UPDATE: Mike (aka mcw53) created a spreadsheet to represent Chlorine levels based on Chlorine Demand. Thanks Mike.

[cofive]

For example, let's say you use the tub (4 people for an hour). After you get out, you shock the tub to 10ppm. The next day you check FC and it's 2ppm. That means your CD is 80%. That's too high and means you didn't use enough Chlorine the night before. Not a problem, just shock to 10ppm again. The next day check your FC. It should be above 5ppm. If not, keep shocking until it is. If you're not using enough Chlorine, your CD will increase and be more difficult to get back down.

Nitro,
When you say "you didn't use enough Chlorine the night before", it is because the chlorine will act as an oxidizer first and santizer second? There appears to be a residual, which is what we've all been taught to maintain, but I'm guessing that the process of Chlorine working as a sanitizer, which in turn creates the chloramines, which are then oxidized by the remaining FC, depletes the levels.

Ken

[Nitro]

What I mean by"you didn't use enough", is exactly that. You didn't use enough Chlorine to oxidize all the bather waste, and sanitize all the bugs. During a soak, most of the Chlorine gets used up oxidizing waste (sweat, oil, skin, urine etc.). It will also sanitize (kill) new bugs (bacteria, viruses) brought into the tub by the bathers. To give you some perspective, Chlorine performs about 90% of its work oxidizing waste, and about 10% sanitizing.

When Active Chlorine kills bugs and oxidizes organic waste, it becomes Inactive Chloride (salt). However, when Chlorine reacts with Ammonia (common in bather waste) it forms Chloramines (Combined Chlorine). Chloramines have a bad odor that irritate the eyes and nose. It is the biggest reason most people are turned off by Chlorine (in spas and pools). Chloramines will eventually get oxidized, but is a slow process. The key is to use plenty of Chlorine, so CC (or CD) won't build up. If CC (or CD) does get built up, an MPS shock does a good job of lowering it. Also, MPS added before a high bather load soak will help keep CC/CD down.

[moonlight]

So, what do you do when you want to empty and refill your tub? My tub's instructions say to let the chlorine level get to 1ppm and then it can be used on landscaping. Would you agree with this? For the health of the tub and the plants?

[Nitro]

Moonlight,

If you let the FC drop to 1 ppm, it won't harm any landscaping. To give you some perspective, I drained super shocked water (100 ppm FC) into unwanted grass and weeds thinking it would kill them off. Not a chance. The weeds and grass are still there. However, I don't recommend doing that if you want to keep the landscaping.

I believe that once you pour Chlorinated water into DIRT, the FC will drop to zero instantly. Think about it, imagine how much micro (and macro) organisms, organic material and waste is contained in DIRT. It would take an awful lot of Chlorine to clean up just a small amount of it.

As far as the tub. As long as you have some (>0) FC in your tub, you're fine. The problem comes in when you let it drop to zero for any length of time (even hours). However, even if you did that the fact you're draining right away, nothing should be able to take hold of your tub. Just do a shock (10 ppm FC) every time you refill your tub.

[PaulR]

Some places restrict what you can do with pool/spa water. Where I am (Silicon Valley) the rules say I can backwash onto the ground, but draining has to be into the sewer (not the ground, and not the storm drain in the street).
--paulr

[Nitro]

I don't want to get anyone into trouble, so everyone sould check their local ordinances. However, I don't see how it's even possible to drain a hot tub or pool into a sewer. What do they expect you to do, run a hose inside your house into the a bathtub or toilet? For some people that would be almost impossible.

The Chlorine in pool water is completely harmless to the enviornment. It quickly turns into salt. A lot of cities actually ADD Chlorine to their water supply. However, there is the question of CYA. Maybe Chem Geek can comment on the effects that CYA and pool/hot tub water has in general on the enviornment.

[chem geek]

As was noted by Nitro, many municipal water supplies use or used to use chlorine, typically at 1 ppm FC or sometimes a bit more. Now, many of them are switching to using monochloramine, also at around 1 ppm CC, because it lasts longer in the pipes and doesn't form as many disinfection by-products which are a concern for drinking water. So clearly, dumping chlorinated water to the ground is not an issue and as Nitro points out, it will tend to get used up pretty quickly. The worst effect might be in (temporarily) killing off some useful bacteria (nitrogen-fixing, etc.) for legumes, but only if the chlorine made it down that far into the soil -- odds are it will get used up near the surface.

As for Cyanuric Acid (CYA), this is described here which is an environmental study on Trichlor, but they mostly use Cyanuric Acid since that is what Trichlor breaks down into (the chlorine gets used up rather quickly). The toxicity levels are much higher than the concentrations found in pools or spas that are well managed.

The other main components of water in the spa are calcium and bicarbonate and sodium chloride salt. If the salt were to accumulate, this could be a problem, but if you are watering regularly then this will get diluted.

There will be various disinfection by-products if you are using chlorine or bromine so dumping the water into ground that goes into an aquifer used for drinking water wouldn't be great, though in practical terms the amount is much smaller then such by-products created from normal watering using chlorinated drinking water.

Richard

[PaulR]

Re how to drain into a sewer, when I drained my pool I ran the hose to a cleanout on the outside of the house. The hot tub is awkwardly placed for that so I do run the hose into the house and into a shower stall.

I'm not about to debate the technical merits of the ordinances, I'm just sayin' there's rules out there. In the San Francisco Bay Area there seems to be a definite better-safe-than-sorry attitude about what runs into the Bay untreated (which is where most of the local groundwater goes, not into aquifers); if you want to go harrass the Santa Clara Valley Urban Runoff Pollution Prevention Program about their rules for pools, be my guest.
--paulr

[Nitro]

Re how to drain into a sewer, when I drained my pool I ran the hose to a cleanout on the outside of the house. The hot tub is awkwardly placed for that so I do run the hose into the house and into a shower stall.

My buddy's tub is about 100 ft down a hill. It would take a pretty good size pump to get the water up to the house.

I'm not about to debate the technical merits of the ordinances, I'm just sayin' there's rules out there.

My neither. I'm just trying to get the real facts about dumping pool water into the environment. Did I not also say, "I don't want to get anyone into trouble, so everyone sould check their local ordinances."

In the San Francisco Bay Area there seems to be a definite better-safe-than-sorry attitude about what runs into the Bay untreated (which is where most of the local groundwater goes, not into aquifers); if you want to go harrass the Santa Clara Valley Urban Runoff Pollution Prevention Program about their rules for pools, be my guest.

No thanks!

I am curious to know what other cities/states have that ordinance besides CA.

[PaulR]

Sorry Nitro, the remarks about the rules were more addressed to chem geek than you!

I haven't tried to survey rules from lots of places, but when I was looking for the local rules I also came across some for Seattle (King County) WA, to wit:
- Can discharge to ground if chlorine < 3ppm; otherwise to sewer
- Do not discharge to ground if you used copper-based algaecide
- Discharge to ground must not cross property line or enter storm drain
- Something about DE I didn't really understand, looks like you must backwash to sewer
Re DE, I've also seen people mention a "settling tank" whatever that is.

But, this is straying kind of far from the original topic.
--paulr

[chem geek]

I live in the San Francisco Bay Area and you are right that one is not allowed to drain their pool into the street since, for cities near the Bay, storm drains flow into the bay. Instead, one is supposed to drain into the sewer and in fact the overflow from the pool is also supposed to drain into the sewer (the one that goes to sewage treatment -- not the storm drain).

I was responding to Nitro's question from the point of potential harm, not from ordinances.

It's interesting about the copper-based algaecide limiting where you can dump. That makes sense since you don't want that killing marine life. The same would probably be true for Borates if you use them though they are a weaker algaecide.

[Nitro]

I posted a spreadsheet in the first post created by Mike (aka mcw53) to show the effect of Chlorine Demand. Check it out.

[drewskie]

So when using the dichlor-bleacik method, it is not really necessary to shock the tub? Just keep the cd up? Is that correct?

[Nitro]

So when using the dichlor-bleacik method, it is not really necessary to shock the tub? Just keep the cd up? Is that correct?

Keep FC above zero at ALL times, and CD as low as possible. Do that (along with balancing your water) and you'll never have water issues.
I don't like to use the word shock. I shock the tub every time I use it. I'd rather just say add chlorine.

[NewB@Spa]

Nitro - I am leaning towards your method of maintenance when I refill (not certain when I will do that). I have had my new tub approx 2 months and I am now having issues with getting FC up. Water tested yesterday @ dealer was FC 0, TC 17, TA 90, CH 300, CYA 60 (although they said my MPS - EZ Spa could be making that inaccurate). So to get back on point here - if/when I do decontaminate/refill in your explanation of calculating CD you state to get FC to 10ppm - how do you do that? How much dichlor in a ~400 gal tub will that take? Of course that comes after balancing (PH, TA, CH) right? Also what type of tester do you recommend? I am leaning towards a ColorQ 7 (electronic), but everyone seems to use Taylor...

[Nitro]

Nitro - I am leaning towards your method of maintenance when I refill (not certain when I will do that). I have had my new tub approx 2 months and I am now having issues with getting FC up. Water tested yesterday @ dealer was FC 0, TC 17, TA 90, CH 300, CYA 60 (although they said my MPS - EZ Spa could be making that inaccurate). So to get back on point here - if/when I do decontaminate/refill in your explanation of calculating CD you state to get FC to 10ppm - how do you do that? How much dichlor in a ~400 gal tub will that take? Of course that comes after balancing (PH, TA, CH) right? Also what type of tester do you recommend? I am leaning towards a ColorQ 7 (electronic), but everyone seems to use Taylor...

Keep in mind if this is a new tub, you will want to change the water after a month or two anyway. After this first refill, the water should last ~6 months.

You can use the Pool Calculator to calculate how much chems you need based on tub size. It your case (400 gal), you should add an ounce of Dichlor. It doesn't have to be exact for calculating CD. Just measure it and make note.

I use, and recommend the Taylor K-2006 drop test kit. I don't have experience with electronic testers.

[chem geek]

The E-Z Spa has some sort of non-chlorine shock, either MPS or something else, and if you are using a lot then this will register as CC, though you are measuring quite a high value. You also are measuring zero FC and that is not good. As Nitro noted, with a brand new spa, you want to change the water earlier than otherwise. If you never decontaminated (superchlorinated) or used Spa System Flush, you might consider doing so. If you get into a standard program, such as with Dichlor-then-bleach, then don't use the E-Z Spa anymore. It just adds more than you need, makes things more complicated to figure out (due to false test readings), and is more costly.

The other possibility for the very high CC is that you have been using way too little oxidizer for the amount of soaking that you do. The rough rule of thumb (without an ozonator) is that every person-hour of soaking in a hot (100-104F) spa needs around 5 fluid ounces of 6% bleach or 3-1/2 teaspoons of Dichlor or 7 teaspoons of non-chlorine shock (MPS) to oxidize the bather waste.