i have had a few cheap backyard frame pools, and pretty much the "filtration systems" they come with are only good for people that never get in there pool dirty, have no friends, no kids, no dogs, nothing but time to wash or replace the filter constantly, always keep the pool covered, you get the idea.
my pool is a 16' round 42" deep. think it holds about 4000 gallons. basicly between all the people, and our 2 filthy dogs, the little cartridge filter was not cutting it. it hangs in there for the 1st couple weeks then the water just gets too nasty. even though i come home and sray it off every day and change it once and a while. the filtration setup that comes on the pool should probly only come on somthing with a few hundred gallons. whatever, i think it sucks. in the picture obviously im also dealing with some algae.
Step 1: The Basics
why do you want a sand filter? because sand is actually a really good filter, thats why most larger pools and aquatic facilitys and even your nicer backyard above ground pools use them.
how do they work? well its actually really simple. a "normal" sand filter is a sealed container half full of sand, water is forced though the sand by the pump. ya its that simple.
how exactly? a pump forces the water into the top of the container, usually through somthing that spreads it even so its not just a jet of water blasting a tunnel in the sand, you want a nice even sand bed the water has to go through. in the bottom there is somthing that usually looks like a a bunch of little pipes with slits in them the sand cant fit through that connect to a big pipe, that is the outlet where clean water comes out.
Step 2: Parts
i didnt really document this very well but its pretty simple.
because i dont have any big pump, or larger container with a water tight lid, i used a plastic 55 gallon drum i got free.
i re-used the little pump that came with the pool to get the water into the top of the barrel, i drilled and taped the barrel with 1" NPT pipe thread and used cheap grey pvc barbed fittings to connect the hoses.
the lower white hose that used to return to the pool now connects to my "spray bar" in the other picture, this evenly spreads the dirty water over the sand. this is made of 1" PVC with holes drilled in it, i put a union on it so it can be easily removed to ad/remove sand etc.
the clear re-enforced hose comming out the top is just an over flow of unfiltered water back into the skimmer/pool.. more on this later
Step 3: Rest of the Parts
now you want somthing that will stop the sand from comming out and only let clean water through. for this i used 1 1/4 pvc with lots of slits i put in it using my miter saw, then i covered each bar in cheap lawnscaping fabric and held it on with zip ties, this isnt on there yet in the picture.
i re-used the pump from last years pool for the return pump pumping the filtered water back into the pool.
as you can see i didnt use a whole lot of sand, about 4 bags of play sand, just enough so there a few inches of sand above the suction bars (there called latterals in a real filter), i started to fill it more but it seemed to slow the flow down too much (weak pumps) ended up using 4 x $3.50 bags of play sand.
the overflow: this is also why there is an overflow on the otherside... like all half assed backyard built sht theres usually some kind of design flaw, this one was the fact that i used the same crappy little pump on the in and out side of the filter, the pump on the skimmer pushing water into the filter has much less resistance and therfor can flow more water, filling the barrel a slightly faster then the lower one can drain it and push it uphill into the pool, against the water pressure of the water already in the pool.
the simple solution would be a higher flowing pump on the return side to the pool, with a valve if needed for adjustment so it didnt empty the barrel too fast either. i dont really have the spare $$ for this project right now , maybe i can get a bigger pump later.
option two would be to put the barrel up a few feet on somthing and let gravity assist.
Step 4: Hows Did It Do?
how did it work? well the water was green and cloudy and you couldnt really see the bottom of the pool, and there was a slimly layer (probly from the damn dogs) and it kind of just smelled no matter how much chlorine was in it. within a few hours it was noticeably less green, and you could see the bottom again (see picture above) within a couple of days the water actually looked pretty good the bottom of the pool just needs a vac. I did ad some algaecide but i never had it work this well, or much at all, so i give credit to the filter.
keep in mind i have the flow restricted * on the in side via the over flow wasting unfiltered water back to the pool and not many holes drilled in my "spray bar" because the water does not come out of the pump returning it to the pool very fast. better pumps would really kick some ass.
I had to backwash the filter once or twice after a couple days, just like a real sand filter you would reverse flow through the filter washing the sand and whatever has settled on top. DO not pump that back into your pool obviously, i just disconnected the hose from the IN side of the filter and let the filter spill the dirty water out and flipped the pump from the OUT side of the filter around so it was pushing back into the sand. it looked NASTY like the container from an outhouse.
a normal sand filter would have a multiport valve that makes back-washing easy. its kind of a hassle on my version.
you could even make one of these with a 5 gallon bucket, not sure how well it would work but it would probably work better then the filters that come in these !