To avoid the interference of PAA or chlorine as a part of the organic matter in wash water, the chemical oxygen demand (COD) was only measured on the tap water samples according to the small-scale sealed-tube method (LCI 400; Hach Lange, Belgium). Turbidity was determined with a turbidimeter (HI98703; Hanna Instruments, Belgium); while chlorine (both free and total chlorine) and PAA were measured according to the N,N-diethyl-p phenylenediamine (DPD) colorimetric method and an adjusted DPD method, respectively. The pH of the water was measured by a pH-electrode.