Comparison of coagulation efficiency of aluminium and ferric-based coagulants as pre-treatment for UVC/H2O2 treatment of wastewater RO concentrate

Coagulation using two aluminium- (alum and aluminium chlorohydrate (ACH)) and two ferric-based coagulants (ferric chloride and ferric sulphate) was investigated as a pre-treatment for the UVC/H2O2 treatment of a high salinity municipal wastewater reverse osmosis concentrate (ROC). The ferric-based coagulants were generally better than alum, and ACH was the least efficient in removing dissolved organic carbon (DOC), colour, and A254 (and thus improving UV transmittance (UVT)). Ferric-based coagulants removed a greater proportion of most of the DOC fractions. However the reduction of DOC was comparable (46-49%) for alum and ferric chloride at a similar metal dosage (1 mM) for UV fluence of 32 × 103 mJ/cm2, whereas ferric sulphate performed significantly better (58%). A similar trend was observed for colour and A254 reduction, and UVT improvement. The biodegradability almost doubled for UV fluence of 32 × 103 mJ/cm2 without pre-treatment. The change in biodegradability of the pre-treated samples during UVC/H2O2 treatment was different for each coagulant, due to the difference in the content and type of organic matter removed. Ferric chloride was superior in terms of total DOC reduction and electrical energy dose (EED) for the treatment of the high salinity ROC to meet the target residual of 15 mg C/L.