H2O2 (Riedel-de Haën, Germany) at 35 wt.% was used as received and diluted directly into the reactor water. Its concentration was measured in a spectrophotometer (PG Instruments Ltd T-60-U) at 410 nm with the help of the yellow complex formed with Titanium(IV) oxysulphate and H2O2 in glass cuvettes with a 1 cm path length. The Titanium(IV) oxysulphate method is used for drinking water analysis and has a 0.1 mg/L detection limit (DEV). The signal was read after 5 min incubation time against an H2O2 standard curve measured at the same time along with samples and was linear in the 0–100 mg/L concentration range. Titanium(IV) oxysulphate solution (Riedel-de Haën, Germany) was used as received. H2O2 concentrations were measured in the CPC reactor experiments, in which enough sample volume was available for analysis. Additional H2O2 measurements have been performed in these small volumes with strips-tests (Peroxide Test Method: colorimetric with test strips 0.5–2–5–10–25 mg/L H2O2 Merckoquant® Merck, Germany). This technique is less exact than the Titanium(IV)-method but it was used as a qualitative measurement to follow the approximate concentration and presence or absence of H2O2 without changing the experimental volume.