Brownstock washing is a key unit operation, affecting the efficiencies of both bleaching and the recovery cycle. Washing becomes even more critical with system closure, where alkaline/acid bleaching filtrates are recycled to the brown fibre line. The operation of brownstock washing has to be optimized prior to closure, to prevent excessive carryover and significant increase in the consumption of bleaching chemicals. Several approaches can be used to improve brownstock washing and reduce the carryover of black liquor to the bleach plant. These include: increased shower flow, additional shower bars, closed screen rooms and addition of defoamer and washing aid chemicals. Reconfiguring existing stages to employ filtrate segregation may also provide incremental improvement in washing efficiency. Process modeling can be used to predict the results of washing optimization and various system closure scenarios, ahead of mill trials. Process control is another cost-effective approach for improving washer efficiency. Sufficient filtrate surge tank capacity is recommended to balance accumulations in pulp storage tanks, especially during production rate changes. Sensors are now available to measure the entrained air in pulp suspensions and, therefore, provide better control in washing operations*. Many older mills are using original washers, which tend to be overloaded. However, modifications made to original washing equipment can allow mills to run at increased production levels. The new generation of washing equipment provides improved efficiency, reduced operating and capital costs and minimal environmental impact. Some of the new alternatives to the rotary vacuum drum washer include the Drum Displacer^TM, the Pressure Diffuser, the Chemi-Washer, the Compaction Baffle Filter and the Displacement Press.  
*R. Thompson and T. Mahmood, Recent developments in mill implementations of water reduction strategies, in “Water Use Reduction in the Pulp and Paper Industry”, 2^nd edition, PAPRICAN, December 2001.