A comparative study of effluent toxicity for three chlorine-bleached pulp and paper mills in Southeast Asia

Abstract

A study on toxicity discharges from three pulp and paper mills, two in Vietnam and one in Thailand, was conducted using the Microtox method, a rapid toxicity test with the marine bioluminescent bacterium, Photobacterium phosphoreum , as the test organism. Based on the EC 50 values after 5 and 15 min of exposure, i.e. 5 min EC 50 and 15 min EC 50 , respectively, the hypochlorite bleaching stage (H-H) effluent in the Thai mill was most toxic (EC 50 were about 0.1% of the undiluted effluent) but still comparable with corresponding data obtained for COGIDO (0.4–0.5%). In BAPACO, toxicity of the effluent from the hypochlorite stages (H1 and H2) was much lower with the EC 50 in the range of 38–83%. The chlorination (C) stage effluent in COGIDO with the 15 min EC 50 of 18% was less toxic while the 15 min EC 50 of this stage effluent was almost the same for both BAPACO and the Thai mill. The EC 50 of the alkali-extraction (E) stage effluent were almost in the same range in all three mills. The combined effluent from the bleaching process was the most toxic in the Thai mill (EC 50 =0.4%), second toxic in COGIDO (8–10%) and the least toxic in BAPACO (14–24%). TEF (toxicity emission factors) values of this effluent show the same ranking: highest for the Thai mill (15000) followed by COGIDO with much lower TEF (1200) and the least was BAPACO (800). A less-than-additive, i.e. antagonistic, mode of toxicity was found for the combined effluent from bleaching plants and for various other combined effluents in the three mills. The bleaching plant is the largest toxicity source in terms of TEF in BAPACO, but not in COGIDO where the discharge of black liquor is the largest contribution of toxicity. For this reason, the total toxicity discharged from COGIDO with TEF>6000 was much higher than BAPACO with TEF of 750. For the Thai mill, the TEF of the pond system effluent alone was already much higher than TEF of the other two mills. Good house-keeping and process control improvement may greatly reduce acute toxicity from the mills.