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I took a look at some typical 50m (Olympic length) pool designs. For a 50m long (164.05’) x 25m wide(82’) x 4’-16’ pool, we are looking at about 862,000 gal plus 14,000 gal in surge capacity. Location of pump room and surge tank might vary the surge by a factor of 2-3; say about 900k gallons. The lap pool without diving might be about 300k gal.

Filtration lines tend to be sized with the pumping system to change the water through the filters 4x/day. say about once every 6 hrs give or take.

Piping of effluent and influent from filtration system to/from the pool tends to be 12” dia, dependent upon the size of inlets and drains in the pool.

All inlets and drains may not exceed 6fps in velocity or suction lock of 5ft in 100ft of line.

The main lines will flow about 2000 gpm, but the refill supply lines are likely 4” lines with likely recharge at between 200 to 400gpm.

Bottom line, even without short circuiting the flows, you have to drain about a million gallons at no more than about 2000 gpm and recharge at probably no more than 500 gpm using the existing plumbing.

It’s a public arena, so you probably can’t drain the fire mains by more than 500-1000 gpm without limiting the numbers of people in a gathering place to meet fire codes.

If you brought in fill and drain pumps, ran say 2000 gpm at night, using specific gravity pumps, you could drain in about 7.5 hrs and then refill it in another 7.5 hrs, but then you’d have to make sure that water was disinfected and treated and you have a water source available for a million gallons without risking other Fire Main reservoirs.

Most systems take about 3 days to drain and 3 days to recharge, or they focus on blending and chemical treatment to meet health standards.

In highly public water facilities a greater emphasis is usually placed on odor and appearance of the water. The chemistry can be daunting. For example, a trace of benzene in the water might make the water smell and taste infected, while the quantities are 3-5 orders of magnitude less than safe drinking levels and 2 orders of magnitude finer than most filtering processes.

FWIW, if the rivers and beaches are as dirty as advertised, they might not be tooled up with the trade in that industry. Many third world nations with high precipitation levels simply rely on the high water flow rates to keep things clean, and don’t bother treating it. Why treat it when the next rain washes it away???

It might be easier to blame the international visitors for littering their streets and jamming up the ecosystem, then clean it up afterwards at a slower pace by natural processes.