@ Janet, Dave: my bad! sorry! (thanks for the correction!)

@ Serapium: what actually happens when you circulate water is mind-numbingly complex, but roughly you can think of it like this:

UP: unfiltered water in pool
CP: clean filtered water in pool
UF: unfiltered water in pipes
CF: clean filtered water in pipes

1. Initially, the system pulls UP into the pipes, pushes UF through the filter and discharges CF into the pool.

2. But, now the pool contains UP + a small amount of CP. There's a lag in mixing -- how much lag depends on the way the pool returns and drains are arranged. But anyhow for the next few minutes, the system continues to pull ONLY UP from the pool, and return only CF to the pool.

3. But, then, the system begins to pull a mix of UP and CP into the system, so the filter NOW is filtering water that is mostly UP, but contains a little CP. CP has ALREADY been filtered, so that's pretty much wasted effort.

4. As time goes on, the system pulls an ever decreasing amount of UP into the pipes and more and more CP, wasting more and more of the filtration by filtering a mix of UF and CF.

So, even though a 100GPM system can filter a volume of water EQUIVALENT to the volume in the pool, in only 1.7 hours, it will take MUCH longer than -- probably at least 10 cycles before 99% of the water in the pool has been filtered 1 time.

For a typical pool with side inlets, a main drain, and skimmers and an 8 hour turnover rate, it will take around 3 days to filter 99% of the water. Commercial pools, with laminar flow floor inlets and primary circulation through skim gutters can accomplish the same task in just one day. And, I've actually worked with one pool with this design: it was MUCH faster to clean up, then pools with more typical piping systems.