Nitrogen concentrations of effluent before and after treatment plant in two mills have been measured during five days time in a bleached kraft pulp mill and in a newsprint mill. In effluents before treatment the concentration of inorganic nitrogen was low but in the effluent of kraft pulping process, the main part is inorganic nitrogen. In effluent after treatment the concentration of inorganic nitrogen is low. After activated sludge treatment plant the concentration of dissolved organic nitrogen is about 0.6 mg/l and nitrogen in suspended solids determines fluctuation of nitrogen content in treated effluent. There is no need for biological nitrogen removal processes if the addition of nitrogen in the treatment is correct.
The potential danger of heat surface scaling during the evaporation of acidic bleach plant filtrates is discussed. The results of ECF and TCF filtrate evaporation tests are presented. The purity of condensates is analyzed and their potential use to replace fresh raw water in bleached pulp washing is evaluated.
No severe scaling was observed on the heat surfaces. The precipitation seems to happen for the most part in situ in the liquid phase. This means, however that the evaporator should be designed so that it tolerates suspended matter in the evaporated liquid. When the pH of the filtrate is properly adjusted the purity of the condensates meets, after slight stripping, the requirements set for the wash water of bleached pulp.
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