Variation in character and treatability of organics in river water: An assessment by HPAC and alum coagulation

The treatability of NOM present in surface waters in South Australia (River Murray) impacted by a La Niña climate cycle was investigated. Over extended dry and wet climate conditions, the water quality had changed markedly from low to high DOC (and turbidity), requiring reassessment and adjustment of treatment conditions of these waters for potable supply. Investigation was conducted on the character and treatability of NOM through fractionation using DAX-8 resin before and after coagulation with alum and HPAC (a novel composite polyaluminum chloride coagulant), by high performance size exclusion chromatography with fluorescence detection (HPSEC-Fl) and by Excitation Emission Matrix (EEM) spectroscopy. In contrast to corresponding earlier dry periods where the DOC ranged from 2.7 to 4.8 mg/L, the DOC in the study period (summer-autumn) ranged from 10.1 to 15.8 mg/L. HPSEC-fluorescence indicated consistency in the presence of humic-like compounds over a molecular weight range of about 400 and 3000 Da for samples collected, while the presence of protein like compounds varied between months. High molecular weight (>50 kDa) protein like compounds were able to be removed but another protein-like component (4-9 kDa) was found to be recalcitrant to removal by coagulation. Alum was found to have high efficiency for removal of very hydrophobic, sorbed DAX-8 fraction (SDF) compounds in these waters while HPAC demonstrated better removals of organics over a wider range of molecular weights, i.e. smaller MW compounds. However, the overall removals of DOC were found to be similar for the two coagulants.