Oh believe me this used to be very, very common...
As an ex cellar man, we used to (in all 10 or so pubs I worked in around central London) put all lager and bitter back in the barrels (2 buckets, one for bitter, one lager).

You'd filter both- larger was never a problem- degas the barrel using a grundy head with no pipes. But if bitter (real ale) was cloudy after a couple of filters, then chuck in a pint of tonic water.

The breweries knew exactly what was going on (they supplied the filter paper!)- this was with Bass and Scottish and Newcastle.
(Mr. Ploppy, Fri 8 Jul 2011, 15:22, 2 replies)

Oh, and when it was really too rank to put back in....
You'd chuck it into a Guinness barrel- get it near full of rancid off beer- chuck in a bottle of black food colouring and bingo, you've a dead Guinness barrel- dray would pick it up, one credit note, job done.
(Mr. Ploppy, Fri 8 Jul 2011, 15:25, closed)

Yeah
and lemonade works too.

Never ceased to amaze me when people said "Mmmm, nice pint that." when I'd served them a cloudy, rancid pint of recycled barley juice.
(Che Grimsdale Best before:, Wed 13 Jul 2011, 11:46, closed)