The edge of an infinity pool functions as a gigantic skimmer weir. A normal pool with wall skimmers only has 8" of skimmer weir (per skimmer). A vanishing edge pool may have 20-50 FEET of skimming weir.
When the pool is in the "spill-over" mode, all of the debris in the pool flows over the edge. This explains why the catch basin is always full of leaves, bugs and dirt.
Without a filter on the pump that returns water from the catch basin to the upper pool, this debris would be ground up and recirculated into the upper pool. Negative edge pools that do not have a filter(s) on the edge pump, turn cloudy the instant this debris is pumped back into the pool.
Besides, who wants to swim in a cloudy pool full of ground up bugs, worms, tree frogs and leaves?
Filtration of the pool can usually be performed with a small energy efficient pump. Usually a 1/2 to 1 horsepower pump is sufficient. Large volumes of moving water are not necessary. Water clarity is improved at slower velocities with longer filtration cycles (longer run times).
The pumps required to create the "flooded edge" effect are usually much larger than what is required to merely filter the pool. Pumps of 2 to 5 horsepower are not uncommon.
To design a vanishing edge pool, that combines the edge pump and primary filtration system into one system is foolhardy. Though $5,000 - $7,500 might be saved on the initial project cost, the expense of operating this larger pump to filter the pool will quickly exceed any initial cost savings.
The edge pump really only needs to operate for two hours daily, when the pool is in use or when the flooded edge effect is desired. Therefore excess energy will be consumed daily, to merely achieve the necessary filtration turnover rate.
Don't step over dollars to pick up pennies!
Paolo Benedetti - Aquatic Artist
"Creating water as art."™
Aquatic Technology Pool and Spa