The objective of this study was to investigate the dissolved solids buildup with increased effluent recycle in a paperboard mill. This was accomplished by studying Solvay Paperboard, a paperboard mill located in Solvay, NY, which conducted a water reduction program by effluent recycling and thereby reducing its fresh water consumption. Critical substances, those that were most detrimental for the process operation during effluent recycling, were identified and their buildup in the effluent and paperboard was investigated. To avoid the complications of the various internal loops in the process, a black box concept was adopted, in which the mill was considered as a black box and a mass balance was performed on each critical species entering the mill with the input streams and leaving the mill with the output streams. Solvay Paperboard underwent a water reduction program in a series of phases, mass balance models were developed for each phase to predict the solids buildup in the effluent and the paperboard taking into account the partitioning or retention of the critical species in the output streams during each phase.