Could I have a filter problem?

[stev32k]

For the last three years my pool has been clear as clean glass. Even at night with a 400 watt underwater light shinning you have to look closely to see any particles. Now the pool has clouded up. I can still see the main drain, but the water is turbid looking. I don't think it's an algae problem because the cloudiness is white there is no green or yellow that I can see in the water or on the walls or bottom. The water chemistry seems in line and I keep a close check. This morning's numbers are:
FC 5.5
CC 0
pH 7.5
TA 70
CH 50
CYA 40

The cloudiness started about two weeks ago and I initially thought the filter just needed back washing. The filter was very dirty and it took longer than usual to get the back wash clear. After three days the water was still turbid so I back washed again and the filter was not dirty at all. Next I tried adding 1 cup of filter aid (diatomaceous earth). The filter plugged off almost completely in about 1 hour. I back washed again and added 1/4 cup of filter aid and the filter plugged again in about an hour.

The filter is a Hayward S-244T sand filter that was installed in 1991. As far as I'm aware the sand has never been removed and the internals inspected. The filter has never operated the way I thought it should. When it is clean just after a back wash the pressure is a constant 24 psig. If it gets very dirty the pressure might get to 27 psig, but I normally back wash when the pressure gets to 25 or 26. I can't find the exact pump curve for my pump (Sta-Rite Dura-Glas, Model MPRA6F-148L, 1.5 H.P., single speed)any where, but the generic curves say the pump should put out a minimum of about 80' head or around 35 psig at dead head.

One explanation I can think of is that some of the laterals are plugged off. If one or more laterals were plugged or several are partly plugged that might account for a high initial discharge pressure. It might also explain the loss of filter efficiency because some portion of the filter bed above the plugged laterals would not be used. Another explanation might be the flow rate is too high for the filter. The filter name plate lists the maximum flow as 60 gpm, but since I don't have the pump curves and cannot measure the flow I don't really know if that is the problem or not. I don't have any valves on the discharge of the pump or filter and I don't want to throttle the pump suction valves so I'm kinda stuck. My next option is to take the filter apart, remove the sand and inspect the internals. But before I do that I wanted to get input from some of the knowledgeable people here - have I overlooked any thing and is there something else I can do before dismantling filter?

 

[Butterfly]

White cloudiness can be algae. You can do an OCLT to rule out (or in) organics: pool-school/overnight_fc_test

 

[stev32k]

That was one of the first things I did when the water turned turbid and I've repeated it once since then. There was no over night chlorine loss either time.

 

[duraleigh]

Since you have no overnight Chlorine loss, I would suspect the filter. Certainly, a 60 gpm filter is too small for your size pool and pump. I would take it apart and inspect but even if you find the issue, it will continue to be too small for that pool and that pump.

 

[Bama Rambler]

Have you tried adding a cup of DE in the filter and see if it helps? Did you put a sock over the return to make sure the filter isn't bypassing stuff back to the pool?

 

[stev32k]

Since you have no overnight Chlorine loss, I would suspect the filter. Certainly, a 60 gpm filter is too small for your size pool and pump. I would take it apart and inspect but even if you find the issue, it will continue to be too small for that pool and that pump.

I've known the filter was too small, but to add to it or enlarge it would be a larger project than I'm willing to undertake so I just run the pump 16 hours per day. It has done a good job for several years.

 

[stev32k]

I repeated the over night chlorine loss test and there was a 0.5 ppm difference. I've been running the filter 24 hours per day for several days now and the pool shows no sign of getting any cleaner. The color looks good but I can barely see the bottom drain. The particle size must be very small because nothing is settling out on the bottom.

BAMA: I did try DE - see my first post. How would I put a sock over the return?

 

[Butterfly]

Possible channelling? The sock over the return - you can use a skimmer sock and cord to tie it over the returns to catch any debris that could be coming thru back to the pool.

You might want to do a good cleaning as described here:
my-sand-is-channeled-how-to-fix-it-t7626.html

 

[stev32k]

OK, thanks I think I'll try the water hose in the filter trick and see what happens.

 

[stev32k]

I just spent 4 hours of my 4th of July cleaning the sand filter. Fully 50% (maybe more) of the bed was sealed off. There were layers of hard packed dirt/debris on one side that were so hard I had to use a mining nozzle to break them up. There was one area that took the mining nozzle about 10 minutes before it finally broke up. When it came apart air bubbles came to the surface. The overflow water first had the color and consistency of dark chocolate milk and then turned smoke white. There were at least two hard layers one on top of the other with a thin lens of sand between. No telling how many pounds of dirt were washed out, but the sand level was lowered about 2" when I finished and no sand was washed out.

I fully expected the discharge pressure to be lower after the cleaning, but it was not. It was 24 psig when I put it in service which is what it has always been after a back wash. That makes me think that plugged laterals may have caused the bed to seal over. So I guess I'm back to removing the sand and dismantling the internals.

 

[Butterfly]

Wow, sure wish you had taken a video of the process! And, you could show us what can be inside a sand filter

Hope you got it all squared away this afternoon. Please keep us updated.

 

[stev32k]

Here's an update. My theory now is that I really had two problems. The first is that I fed HTH though the skimmer for several years before I knew any better. The residue ended up clogging the filter and that was the main portion of the junk I washed out. Even though I thought I did a good job of back washing before I put it back in service there was enough left over to make, and keep, the water turbid.

The second problem is over running the filter. I finally received a copy of the pump curve from the manufacturer and if the curve is close to correct the flow into the filter has been about 30% over design. The design flow for the filter is 60 gpm, but the inlet pressure is nearly always 24 psig which equates to about 80+ gpm according to the curve. I contacted Hayward and asked them about the excessive flow rate and they said it would most likely cause channeling and loss of efficiency. That seems to be the case and is most likely the reason I could not get the water to clear up.

To try to correct the situation I added DE to the skimmer until pressure increased to 27 psig which should be about 60 gpm. The problem I got into then was that the pressure would not stay at 27 it rose to 32 psi in about 2 - 3 hours and shut-off head for the pump is about 34 psi. So I back washed added more DE and tried again - same results. I had to go though this about four times over the last 3 days. Today the filter has been running at a constant 27 psi since about 7 AM and the water is almost back to normal clarity.

The next step will be to install a valve on the discharge of the filter and throttle back the flow until I get a 27 psi inlet pressure. At the same time I'm going to put a pressure gauge on the discharge and maybe learn when back wash is needed based on the differential.