Investigating the relationship between toxicity and organic sum-parameters in kraft mill effluents

• Concentration addition was assumed for organics & metals of kraft mill effluents.
• Subtracting the metal toxicity from the WET helps to isolate the organic toxicity.
• The growth of Pseudokirchneriella subcapitata is negatively associated with increasing COD.
• The reproduction of Ceriodaphnia dubia is negatively associated with increasing BOD.
• These relationships, albeit rough, could be valuable for effluent management practices.

Elaborate toxicity diagnostics, such as toxicity identification evaluation (TIE) and effects-directed analysis (EDA) have helped in identifying the causative agents of effluent wastewater toxicity. However, simpler means of relating ecotoxicological effects to effluent composition could be useful for effluent management practices when there is no scope for more complex procedures. The aim of this work was to investigate and isolate the relationship between biological responses and commonly measured organic sum-parameters, such as chemical- and biochemical oxygen demand (COD and BOD, respectively) in kraft mill effluents. In a top-down approach, the whole effluent toxicity (WET) of effluent samples was first determined from Pseudokirchneriella subcapitata and Ceriodaphnia dubia bioassays. The theoretical toxicity that could be attributed to the metal content was then estimated, via a combination of equilibrium chemical speciation- and metal toxicity modelling. By assuming concentration addition, the metal toxicity was subtracted from the WET, isolating the toxicity thought to be caused by the organics. Strong and significant correlations between the ‘corrected’ toxicity and organic sum-parameters were found for both species. The growth of P. subcapitata was negatively associated with increasing COD concentrations, whereas reproduction of C. dubia was negatively associated with increasing BOD concentrations. The linear relationships, along with robust estimations of their uncertainty bounds, can provide valuable, albeit rough, guidance for kraft mill effluent management practices.

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