~20% of DE Remains in Filter after Six Months?

[Exchemist]

This was my first season with a pool and my first closing. The pool came with the house -- it's about 11 x 24, in-ground, vinyl with a 24 sq. ft. DE filter and a 3/4 hp pump. I calculated the volume to be about 12,500 gal. When I started the pool in the spring, I had trouble getting the filter to work. A pool service person determined that the previous owner used a cellulose-based DE substitute. I added standard DE which causes all sorts of problems. The service person said he washed the grids which seemed to fix the problem.

As part of my continuing education, I decided to take apart the DE filter and clean the grids. Somewhere I read that doing this once a year is a Good Ideaâ„¢ -- recommended but not required. I got the filter apart with only minimal problems. What greeted me was a total surprise.

Even after a thorough backwashing, everything was caked with blue DE and what looked like sand. Over the swimming season I used nearly 25 lbs of DE and it was most definitely white when added. I estimate there was ~5 lbs of DE coating everything (~20% of what I added over the season) including several 10's of sq. inches which were at least a 1/2" thick. I hosed off the grids and used shop vac to clean the inside of the housing. I'm planning to have the grids acid washed over the winter.

First, is leaving 20% of the DE in the filter typical and normal? If so, washing the grids each year seems required. Also, why would the DE turn blue? I assume the sand was blow into the pool and collected in the filter when I vacuumed.

Any thoughts or comments would be appreciated.

 

[waste]

First, let me WELCOME you to TFP

Getting all the De out of a filter is impossible, unless you break it down and thoroughly hose the grids/ fingers. A complete backwashing (i.e. backwash/ rinse, backwash/ rinse ... until the water runs ~ clear on backwash) will still only remove ~ 80% of the DE. As such, you should only add 80% of the recommended De when recharging the unit (if the unit takes 6 lbs clean, only add 5 lbs after a backwash) You probably saw the result of adding the full amount of DE after a number of incomplete backwashes, a LOT of extra De in the filter - which prevents the unit from functioning correctly/ efficiently.

As for the 'sand' in it, I'd say that it's calcium which has leached out of the water (it's hard to say because you didn't post any chem numbers) and the blue could well be the result of copper introduced into the water (though there are other causes, it's usually the case), and can also be a sign that the pH was allowed to be way low for a while. As you inherited this pool, you can't tell us what's been done in the past , however - you can now take a proactive approach and rectify anything that was mishandled in the past

I recommend breaking the filter down every fall, when you close, to do a complete cleaning of the filter elements and check it for grid tears, and gasket or o-ring problems, this way you know you're all set for next spring. The other instance in which I recommend breaking it down is after battling algae - algae can 'crud-up' a DE filter very quickly!

Again, welcome :!: here, there are lots of great and knowledgeable folk here to help with any pool question you have

 

[Exchemist]

Thanks for the reply, info, and welcome. This board has been a great resource while I've been learning. I wish I would have started reading before I opened the pool last spring. Things would have been easier.

As it was my first year, the chemistry has not been very stable over the season. Because it's a vinyl pool without a heater, I didn't worry about Calcium until half way through the season. It's been under 170 ppm. From what I've read, this is very low, but fine for a vinyl pool. I've been struggling with pH and Alkalinity all season. pH was generally on the low side 7.0-7.2, and Alkalinity 140-160 ppm. I do have high CYA, about 80 ppm.

Until I read about CYA on this board, I was continuing to use the Trichlor pucks the previous owners left. I since switched to using Calcium Hypochlorite, because the Calcium was low and I had 20 lbs of it left by the previous owners. (I suppose they got a good deal on it.) Because of the high CYA, I've been keeping the chlorine between 8 and 10 ppm. I wouldn't be surprised if the previous owners also added copper. Does the blue DE mean that at least some of the copper was removed when I washed away the blue DE?

Plan for this closing was to replace about 40% of the pool water to get the CYA down, have the grids acid washed (who knows when they were last cleaned well), and get set up to use liquid chlorine next year. Replacing the water should also remove 40% of any remaining Copper. Anything else I should be worried about?

 

[waste]

It sounds like you've got the gist of it

Close your pool and wash the filter (I recommend letting it soak for an hour or so in either a dilute TSP or some automatic dishwasher detergent mix for an hour BEFORE doing the acid wash -- acid seems to coagulate the oils on the grids if it's not first removed ). Feel free to browse through the site over the winter to find lots of helpful ideas that will keep your pool 'trouble free' 8)

Have a good winter and I hope to see you here next spring :!: