Decolourisation of Secondary Effluent by UV-mediated Processes

The brownish colour of a secondary-treated effluent from a local wastewater treatment plant was shown to be mainly due to a high content of humic acid-like material. The effluent was treated with four different UV-based treatments: UVC and VUV irradiation, with and without the addition of hydrogen peroxide. The efficiency of these treatments was characterised in terms of the change in colour, absorbance at 254nm (A254), dissolved organic carbon (DOC), fluorescence excitation-emission matrix (EEM) spectra and size exclusion chromatography. The colour was readily removed (>90%) with concomitant reductions in A254 and DOC, loss of fluorescence and formation of lower molecular weight (LMW) compounds. The biodegradability of the organics was significantly increased following all treatments due to the production of these LMW compounds. The treatments involving the generation of hydroxyl radicals from the photolysis of hydrogen peroxide performed more effectively than either UVC or VUV irradiation alone. The rate of loss of colour, DOC and fluorescence of fulvic acid-like compounds in all the systems tested fitted first order kinetics, while the rate of loss of A254 and fluorescence of humic acid-like matter fitted parallel first order kinetics for all but UVC treatment alone. The overall performance in terms of decolourisation and loss of DOC (and thus electrical energy per order, (EE/O)) decreased in the following order: UVC/H2O2 (32mg/L)>VUV/H2O2 (16mg/L)>UVC/H2O2 (16mg/L)>VUV>UVC.