Colour decreased rapidly during all treatments (Fig. 1a).
The rate of reduction was lowest for irradiation with UVC alone; VUV irradiation
resulted in the next lowest decolourisation rate.
On addition of hydrogen peroxide (16 mg/L), the initial decolourisation rate increased with most colour being removed after 30 min, although there was no difference in the rate of colour reduction between the solutions irradiated with either UVC or VUV. The highest decolourisation rate was achieved with UVC/32 mg/L H2O2 where 99% of the
colour was removed after 120 min.
The reduction in A254 (which represents the conjugated bonds and aromaticity of the organic compounds) corresponded closely to the trends for decolourisation for the different treatments (Fig. 1b).
After 120 min of UVC irradiation, A254 had decreased by 55%; the addition of 32 mg/L H2O2 enhanced the reduction to 69%.
When hydrogen peroxide was added in the absence of irradiation, there was an initial increase in DOC but no change in colour or absorbance over the time tested (180 min) (results not shown).
The increase in DOC was attributed to the oxidation of fine particulate organic matter and colloidal material, resulting in an increase in dissolved organic matter [17].
Similar initial increases in DOC were observed on irradiation in the presence of hydrogen peroxide.
The high specific UV absorbance (SUVA) value of the secondary effluent indicated the presence of strongly hydrophobic and high molecular mass organic content of the water (Table 1).
Following the treatments, the SUVA values of the treated effluent were decreased in the same order as observed for colour and A254 (UVC (51%), VUV (73%), UVC/16 mg/L H2O2 and VUV/16 mg/L H2O2 (both 86%), and UVC/32 mg/L H2O2 (89%) after 120 min).