Excess chlorine demand in clear pool -- can't beat it

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bbezzant

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Jul 16, 2015
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St Louis, MO
Several weeks ago, began having difficulties sustaining appropriate FC levels (I target 5-7% of CYE). Weather here in St Louis was rainy, then hot, lots of swimmers-- I figured those factors were driving higher chlorine demand, so I turned up the SWG...eventually to 100% and then extended hours until it was running 24 hours. Still had trouble keeping FC in the pool. Except for FC, water balance was fine (see below). Water was and is perfectly clear.

Became concerned it was the SWG, had the tech run a diagnostic-- it seemed fine. I turned the SWG off and adopted manual dosing-- pool was consuming about 5 gallons of 10% chlorine/day to maintain a ~5.0 FC. Tried an OCLT, dropped from 7.0 to less than 2.0.

CYA is 70-80 (I'd say 70, pool store says 80), so I went with a target SLAM FC level of 32. I'm 5 days into the SLAM, about 100 gallons of 10% chlorine and a replacement brush with no reduction in chlorine demand. The OCL last night was about 8ppm (from 32ppm to 24ppm). CC throughout has been between 0 and .5 (using a 10ml sample, but it has generally been clear or faintly pink). Filter is clear, light niches are clear, skimmers are clear. Water is still perfetly clear.

The one thing is this: my phosphates are high: about 2800. I've been stalwart in my belief that the phosphates don't matter if I maintain sufficient FC...and that this situation will respond to the SLAM...but I'm starting to wonder if there's something else going on here.

Four questions:
i) Does it make sense to augment the SLAM with polyquat-60?
ii) Should I do a significant partial drain to reduce my CYA and phosphates?
iii) Should I pursue some other phosphate removal approach? (I don't know how I'd vacuum-to-waste without hiring a pool company)
iv) Any other ideas about this chlorine demand?


Pre-SLAM water balance
FC: dropped below 2.0 (we generally target 5-6)
CC: <0.5
Ph: 7.5
TA: 100
CH: 310
CYA: 80
Phosphates: 2800
 
While the high phosphate level would accelerate algae growth, their growth is ultimately limited by sunlight and temperature. I used to have 3000+ ppb phosphates in my pool years ago and there is no question that it makes a pool "reactive" if the FC/CYA level gets too low, but once the algae is killed the demand goes away. We have one forum member with 30,000 ppb who maintains their pool with chlorine alone.

So I could understand the increased demand from the items you mentioned, but your large OCLT losses at SLAM levels indicate there is something still consuming chlorine in the pool. You said you checked the skimmers, but did you check the pump basket as well? Were there any other chemicals added to the water during the time you saw the chlorine demand increase? When you say the filter is clear, did you look at or clean it or just note that the pressure didn't rise?

Phosphate reduction would only reduce the rate of algae growth (and other living organisms) and wouldn't solve a chemical problem, for example. You won't need to vacuum-to-waste with phosphate removers. Most work by getting caught in the filter -- some are added in the skimmer (e.g. PhosFree Extra Strength), others in the bulk pool water and may need use of a clarifier with them (e.g. Orenda PR-10,000). At $56.28 for 101.5 fluid ounces and needing roughly 294 fluid ounces to reduce from 2800 to 100 ppb phosphate in 34,000 gallons, that's $163 for the PhosFree Extra Strength. It only takes around 41 fluid ounces of the Orenda PR-10,000 to do the same reduction so around $51 but as I noted it will cloud the pool and you'd need to use around 7-14 ounces of a clarifier such as GLB Clear Blue or BioGuard Polysheen Blue.
 
I say stick with the SLAM. I just went through the same thing, days of pouring 10% into the pool to keep the FC at shock level. As much as I poured, it kept going down. Finally one day it broke - FC began holding and passed OCLT.
 
As Chem Geek stated algae growth rate is directly dependent on the amount of sunlight available. If you have a dark or opaque swimming pool cover, use it. It will reduce the amount of light and it will inhibit algae growth. This might help chlorine as the rate of new algae formation will be smaller. The cover will also protect chlorine during the day, reducing the rate at which chlorine is degraded by sunlight.
 
Check everywhere. In the skimmers, in the filter, any thing that's in the pool. I notice you have a automatic cleaner. Be sure to take that puppy apart and get all the stuff out of it. Ladders, stairs and light niches seem to be the big hiding places this year.

One last thing, completely out in left field and for no reason other than I'm curious. Do you have some type of test kit that tests for ammonia? If not, I wouldn't sweat it. Just a theory I've been developing.
 
In the right conditions I've ran into ammonia where there is high phosphate. instead of feeding algae, it's feeding bacteria. I know, I know. CYA to ammonia is all the rage this year. It's just one of those weird things that is scratching at the back of my mind. It's from the time I spend fighting crazy things in my pond. I'm probably thinking Zebra just because I hear hoofbeats, but you never know.
 
Appreciate the input. SLAM is on-going: water still clear, CC: ~0, OCLT: >5.0

I can't think of anything else to clean! (lights, niches, auto-cleaner, baskets (skimmer and pump), skimmer housings, inlet fittings, filter housing, cartridges, chlorinator). Only thing I can think of, but haven't tried, is soaking the cartridges. When I squirted them off prior to the SLAM, there was no evidence of algal or bacterial growth.

Chemgeek: no new chemicals in the pool this year. During this season, the only things we've used are: pool opening CYA and chlorine shock, balancing doses of baking soda and calcium chloride, one dose of non-chlorine shock, periodic muriatic acid...and, recently, barrels and barrels of 10% chlorine.

Pabeader: I have a low-concentration ammonia test for an aquarium-- I'll test my ammonia and nitrates tonight (just for fun).

Other than expense (which, at this point, is subsidiary to the grind of CL dosing and brushing), any reason I shouldn't hit it with PhosFree?
 
Phosphate removers can cloud the water, but if you use PhosFree through the skimmer it's supposed to minimize that and it may have clarifier in it as well. So it should be just an expense issue.

I really doubt this chlorine demand is from anything related to phosphates since algae shouldn't grow at the high levels of active chlorine in a SLAM for sure. There are some chlorine-resistant bacteria (e.g. iron bacteria) but with the clear water and rather high chlorine demand even overnight this seems more like something chemical going on, but it's a mystery. There simply isn't enough phosphate in the water to directly have a chemical effect -- phosphates only indirectly affect chlorine demand via algae growth. So I fear you'll just be wasting your money, but of course that's up to you and it shouldn't hurt other than expense.
 

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The bacteria that initially breakdown CYA into ammonia are anaerobic but the issue is that most bacteria are killed by chlorine so while not impossible it is unlikely that what is causing this high chlorine demand is bacteria or algae growth in the bulk pool water. See this post where you can see that adding chlorine stopped the bacterial conversion but there was demand leftover from the partially degraded CYA as well as ammonia. If there were ammonia, then adding chlorine should show up CC, but that isn't being seen here. It could be partially degraded CYA, but that should show up as a drop in CYA level.

It might be something growing and/or seeping from an area not exposed to the chlorine in the water yet consuming chlorine. Or it could be something very resistant to chlorine, but having the water be very clear makes it more likely the demand is coming from some area not seen or that it is chemically based though a mystery as to where it is coming from.
 
To the OP -

How long ago did you use the non-chlorine shock?

You have to be careful with non-chlorine shock as it is MPS and can mess up your DPD-FAS testing by showing up as excess FC. Just wondering how long ago it was that you used it. If it's been a long time then you should be ok.


Sent from my iPhone using Tapatalk
 
Patrick_B said:
Richard, not arguing the point about what bacteria types use Cya, but where are anaerobes going to live in a fairly clean pool/filter system? Like one running in summer?
Click to expand...

They won't in a chlorinated pool. In a pool let go over the winter the chlorine goes to zero so bacteria can then grow. However, if one isn't careful during the summer, then the chlorine can also go to zero and bacteria grow and is what happened to me in It Can Happen to Anyone - Zero Chlorine, CYA-->Ammonia when I started my solar system during the spring. It doesn't take long for bacteria to grow in nutrient-rich conditions (at the time my pool had 3000+ ppb phosphates). Since the pool wasn't a swamp, the bacteria apparently don't require anaerobic conditions. Either that or something else is going on that we don't understand. I'm thinking that while anaerobic conditions may be preferred, they are not absolutely necessary for those bacteria.
 
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