Purpose: The characteristics of organics in sulphite pulp mill effluent and in the receiving environment of effluent discharge were investigated to assess the basis for the persistence or attenuation of colour.
Methods: Characterization of organics was conducted through determination of SUVA, specific colour, and molecular weight distribution of organics using high performance size exclusion chromatography and by solid-state (13) C cross polarization (CP) NMR. The characteristics of organics from mill wastewater before and after secondary aerobic treatment, followed by lime treatment and from the receiving environment, an enclosed brackish lake were compared. Changes in the character of organics in lake water over a period of 14 years were studied in the context of changes in mill processing and climate impacts.
Results: High colour in mill effluent and in receiving waters correlated with high SUVA and specific colour levels, high molecular weight range and aromatic content. Conversely, lake waters with low colour had UV absorbing compounds of much lower molecular weight range and low relative abundance of aromatic compounds. Attenuation of colour and changes in the character of organics in the receiving environment coincided with increased concentrations of metal cations.
Conclusions: These increased concentrations appear to be due to the effects of climate change, lake management and their presence in mill effluent, with subsequent discharge to the lake. Attenuation of colour was found to be predominantly through removal of high molecular weight aromatic compounds where the removal processes could be through adsorption and co-precipitation with divalent metals, as well as through dilution processes.