O3 supplementation on a side-stream, added to the return line before the belt filter (Ozonia CFS-14 2G generator, Degremont Technologies, Dübendorf,Switzerland; Mazzei GDT 60 g or 300 g O3 injectors, degas separators, and O3-destruct units, Bakersfield, CA, US, and online Sensorex 8000CD ORP probes)