Partial closure in modern bleaching sequences

Despite the progress being made in research aimed at achieving a zero-liquid-effluent mill, there is justification for investigating the more gradual approach of partial recirculation of bleaching effluents (partial closure). Eka Chemicals, in cooperation with the University of Idaho, is carryng out research using mill trials, laboratory testing, and computer simulations to evaluate combinations of bleaching stages, operating conditions, washing, and filtrate recycling and treatment. One idea being investigated for ECF bleaching is that of keeping the alkaline and acidic effluent streams separate and evaluating acid effluent treatment and alkaline filtrate recycling. The laboratory potential of this approach on conventional southern pine softwood kraft pulp, O 2 -predelignified Scandinavian pulp, and conventional and O 2 -predelignified eucalyptus pulp is reported in this paper. For fully bleached ECF southern pine kraft pulp, comparisons of the results of external effluent treatment (anaerobic + activated sludge) on the total effluents, and on the acidic effluent, show that the acidic effluent is more easily treatable in terms of AOX and COD. Under well-controlled laboratory conditions with efficient washing, partial closure is promising. The extent to which these results will be influenced by more realistic mill conditions (i.e., carryover) and the degree of compatibility of the alkaline effluent with the recovery system are being evaluated through extensive simulation studies and mill trials. Application : partial closure of alkaline effluents seems to offer a synergistic effect since it not only reduces the level of environmental parameters but also improves the biodegradability of residual effluent in an external treatment