If algae and silt is vac'd to filter, will it end up back in the pool?

[gregch]

Long story, but we had a "pool guy" open up this year as he was coming out here anyway and I thought it'd save me some work. It didn't.

I've now got the pool clean and water clear and sparkly. After shocking, the chlorine level fell back to 2.0ppm and for a day or two I had free chlorine of 2.0ppm and total chlorine of 2.0/2.1ppm. Fine. In previous seasons, that's been about where I've been able to maintain consistent chlorine level. Unfortunately, this time after the first day or two that dropped to 1.0ppm and 1.2ppm despite adding chlorine. I shocked again, and again the same story, below 1ppm and falling. To me that suggests there's still some organic stuff in the pool using up the chlorine. But how?

I have a "suspicion" that the pool guy vac'd all the debris - including really fine silt and stuff that settles on the floor after the chlorine shock's done it's thing - into the filter, rather than to waste as I would do. That's backed up by the fact the water level didn't change and wasn't topped up, and by the fact that when I did a backwash the water in the filter was dark brown and foamy. I've now backwashed 3 times, and although the water's now clear, there's a lot of floating crud and bugs etc in the filter. I'm hoping repeated backwashing over time might clean it up a little.

Is it *possible*, if super-fine silt is vac'd into the filter, that the filter is just slowly releasing some of that fine organic silt - too fine to see, but enough to use up the chlorine - back into the pool? Is that even a thing??

 

[Texas Splash]

There's a couple key notes to your situation:
1 - If someone isn't able to actually perform the SLAM Process, the algae most likely is never being killed or eliminated.
2 - When a filter gets overwhelmed with organic material, some of it will just blow back into the pool. The filter can only grab so much junk, and that junk is supposed to be dead algae.

In short, a pool service won't perform a SLAM Process for the customer. They don't have the time or in some cases the experience. A periodic increase the FC level (shock as you call it) will not remedy the situation.

Also remember to refer to the FC/CYA Levels to determine the proper FC level to maintain based on your current CYA level.

Hope that helps.

 

[sande005]

In my bad old non-TFP days I had an undersized sand filter. After l dumped in a lot of Cl to "shock" it, I was left with lots of dead algae. While I tried to vac to waste, that resulted in the pool being too low long before I was done vacuuming. So I would also vac to filter.
The prime sign I had pushed it too far? - Dirt blowing out of the returns back into the pool. So it is very possible it is releasing some back in the pool.
With a properly sized filter, a good backwashing would eliminate the problem. Perhaps a "deep clean" (see Pool School) if it has been a couple of seasons or more (or really bad conditions) since it was done would be advantageous.

The point of the Cl is to kill the living things - mostly algae, but bacteria, etc. also. With nothing to eat the organics, it is "sanitary". How clear or cloudy then is a factor of the efficiency of the filtering. So a poorly performing filter may be recycling "dirt" (organics) into the pool. But CL has been treating it while it was in the filter too, so releasing it back into the pool would have no additional effect on CL levels. If CL is not high enough/long enough to kill all the algae, that population explosion will be what uses up the CL.

 

[Mdragger88]

I too suspect you still have an algae problem and not really a filter/mechanical problem.
An Algae problem will overwhelm any filter as it proliferates quickly.
As mentioned above you need to do the SLAM Process to truly eliminate algae & prove it is gone.
A proper test kit is required to accomplish this
see—>Test Kits Compared

2ppm fc is the bare minimum the pool should ever be even if there is no cya & generally too low of a maintenance level for most. If cya is involved maintenance fc levels should be higher as listed on this chart as anytime the fc falls below minimum algae & nasties may have a field day.

How are you currently testing the water?
Can you share all your latest test results?
Fc
Cc
Ph
Ta
Ch
Cya

 

[gregch]

Thanks for all the excellent advice, as ever this place proves to be a great resource.

Because impatience, I'd already gone ahead and added some more chlorine yesterday, latest test results as folllows:

FC 2.6
TC 2.9 (so I guess CC 0.3?)
pH 7.4
TA 86
CyA 19

It proved impossible to get hold of any of the good test kits (TF-100, etc) here in the UK, so am using a Lovibond photometer one.

The filter was replaced last year, it's plenty big enough for the pool (I was careful to spec a bigger filter because I'd been told it works better that way).

Reassuring that the chlorine should eventually kill off any nasties in the filter, which I guess makes sense. I'll also keep backwashing it (although topping up the pool each time is a pain and will I guess ultimately mess up the water balance) and hopefully it comes clean eventually.

I'll also have a look at the deep clean process; although the filter and media was new last season and I'd looked after it well.

I also need to just face up to doing the SLAM process properly; seemingly no-one uses liquid bleach here though so everything's a little different!

 

[Mdragger88]

Here are some methods for getting the tests we recommend outside of the us.
The fas/dpd test & the cya test are the most crucial to performing the SLAM Process

How to Buy Palintest​

• SP 315C (similar to K-2005) AND/+ SP 300 FAS-DPD chlorine test
• UK: Tel: +44 (0) 191 491 0808 ext. 208 Email: [email protected] or camlabworld.com or pool-chem.co.uk

Pool Test Kits - Further Reading

www.troublefreepool.com

 

[Mdragger88]

You need to be able to accurately measure fc at slam fc levels for your cya- not sure of the limitations of your particular lovibond model.
also with less than 30ppm cya you will also find yourself fighting the sun more along with algae consuming fc. But you don’t want much more than that or the necessary fc levels needed increase.

 

[gregch]

Well, yes, you're absolutely correct the Lovibond Scuba doesn't read Chlorine beyond 6ppm, so no good for shock/slam accuracy.

Problem is, the first of those Palintest kits mentioned is what I used before, and I found the Scuba more reliable and if course easier to read accurately; and the SP300 kit (FAS/DPD) doesn't seem to be available any more (I think I spoke previously to someone at Palintest who more or less said that, because they're moving more toward photometers) but I'll reach out to them again with the info you provided.

Thanks again for the help!

 

[sande005]

It might be worth it to test current Cl levels. Then with a new sample dilute 50/50 with distilled water, add the DPD and run the test again to see if you measure 1/2 the previous. It may not be all that precise, but might get you good enough to know when you are above SLAM target, or have dropped below. No guess on whether dilution would work at all as you climb the CYA scale above 30, though.

 

[gregch]

It might be worth it to test current Cl levels. Then with a new sample dilute 50/50 with distilled water, add the DPD and run the test again to see if you measure 1/2 the previous. It may not be all that precise, but might get you good enough to know when you are above SLAM target, or have dropped below. No guess on whether dilution would work at all as you climb the CYA scale above 30, though.

Good tip, thanks!