Toxicity to Microtox, micro-algae and duckweed of effluents from the Bai Bang paper company (BAPACO), a Vietnamese bleached kraft pulp and paper mill

Toxicity of the combined effluent as well as effluents from different production units of BAPACO, a Vietnamese bleached kraft integrated pulp and paper mill, was studied using three ecotoxicological tests: Microtox, the micro-alga, Selenastrum capricornutum, and the duckweed, Lemna aequinoctialis. Physico-chemical characteristics of the effluents were also analyzed. Due to unstable operating conditions of the mill, a number of samples taken at different monitoring periods from each effluent were tested, in order to get statistically reliable ranges and averages of toxicity characteristics. For the mill combined effluent, it was found that the micro-algae were the most sensitive followed by Microtox bacteria, while duckweed was not sensitive. Microtox tests showed that the bleaching filtrate from the chlorination stage (C-stage) was the most toxic among all effluents in the mill. The combined effluent from the bleaching plant contributed the largest TEF (Toxicity Emission Factor) to the toxicity balance of the whole BAPACO combined effluent. A less-than-additive mode of toxicity was obtained. In spite of high pollution loads, e.g. BOD, COD, SS, to the Red River, the toxicity of the effluent was considered to be low. However, during periods of low river water sublethal toxic effects on the biota may be expected.