The most common use of combined condensate is in brownstock washing, although some mills use condensate in bleach plants as well. The contaminated, or foul, condensate is either sewered, or steam stripped and re-used. Although heated water discharged from surface condensers can be used in other parts of the mill, a mill practicing water reduction will have excess hot water. The quality of condensate depends on the efficiency of the evaporators and the stripping systems. Mills use various methods to improve the quality of condensate, so that it can be used instead of water. For example, Irving Pulp & Paper in St. John, N.B., installed a reverse osmosis (RO) system in 1998 [1]. At that time, the mill had a pulp production of about 800 admt/d, and discharged untreated effluent. The mill has now added a moving bed bioreactor, which brings it into compliance with the regulatory limits on BOD, toxicity and suspended solids. The RO system treated about 3600 L/min of condensate from the evaporator’s fifth effect. Clean permeate (approximately 99% of the flow) was used as wash water on the second post-oxygen washer dilution conveyor. The installation of the RO system resulted in the removal of 88% BOD and 89% COD from the condensate, producing a permeate for use in post-oxygen washing with about 67 mg/L BOD and 115 mg/L COD.
A different approach was used at Stora Enso’s Skutskar kraft mill [2]. The mill embarked on an extensive modernization plan in 1995 to increase production (from about 1230 to 1570 admt/d), and to minimize environmental impact. The changes included a new falling-film evaporator line, with a stripper column to remove COD. The new evaporator line was designed with extended condensate segregation to improve the efficiency of condensate cleaning, and reduce COD emissions. An in-line condensate stripper and a new external stripper for foul condensate were, therefore, included in the plans. The treated condensate from the external stripper also went to the in-line stripper. The accept stream from the in-line stripper had a COD concentration of 400 to 700 mg/L. As a result of these changes, a COD reduction of 20% in mill effluent was achieved.
In closed bleach plants, the evaporator condensate, displaced by Eo filtrate, must displace fresh water to achieve water reduction. Possible uses could include a closed-water cooling system, wire cleaning showers on any caustic stage, and replacing fresh water in bleach plant showers. Södra Cell’s Värö kraft mill, with a bleaching sequence of Q(Paa)EopQ(PO), installed a system for stripping moderately contaminated condensate from the evaporation plant [3]. Together with a previously installed stripper for foul condensate, the mill had two separate stripping systems. The new stripper was integrated into a new evaporator train, and treated 200 m3/h of secondary condensate with a COD concentration of about 2000 mg/L. The system removed about 90% of COD, 95% of H2S and over 95% of DMS and DMDS, enabling the mill to use all the condensate in the bleach plant. Since the condensate does not contain any metals, the peroxide consumption decreased by about 30% and pulp strength increased considerably.
1. Dube, M., McLean, R., MacLatchy, D. and Savage, P., Reverse Osmosis Treatment, Effects on Effluent Quality, Pulp Pap. Can., 101(8), pp. 42-45, (2000).
2. Backman, K., Lindberg, H., Sjoberg, H., Recovery Modernization at Stora Enso’s Skutskar Kraft Mill, Tappi J., 83(7), p. 69, (2000).
3. Emilsson, K., Håkansson, M. Danielsson, G., Extended Stripping and Usage of Evaporator Condensate at Värö Mill, Sweden, 1997 Minimum Effluent Mills Symposium, Proceedings, San Francisco, CA., October 23-24, TAPPI Press, pp. 191-197, (1997).
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