Wastewater Treatment at Kraft Pulp Mills : Effect of temporary effluents and mitigation measures

Sammanfattning: Large amount of water is used in the pulp-and-paper industry. Consequently, large amount of wastewater is produced in the various production processes. In general, the pollutants of concern involve organic compounds, nutrients, including nitrogen and phosphorous, suspended solids, and chlorinated compounds. Temporary effluents from the production processes often have increased concentrations of these contaminants and there is a risk of overloading the wastewater treatment, which then further increases the risk of discharge of insufficiently treated water to receiving water bodies. The aim of this study was to investigate the effect these temporary, temporary effluents from Kraft pulp mill processes have on the wastewater treatment, with the objective to develop an action plan with measures to address and remedy the problems that occur (to handle them in the best way possible in the future). This study has been conducted through interviews to collect the experiences from personnel working in Kraft pulp mills located in Sweden.  The results of the study from the interviews showed a total of 16 different types of temporary effluents. The results also showed that the most common incidents occurring in the mills were incidents connected to planned or unplanned maintenance stops, and the most common temporary effluents experienced were liquors from the different process steps and chlorate mainly from the bleaching plant. All results are compiled, in both Swedish and English for easier use. The most important measure for mitigating the effect of the temporary effluents was identified to be optimum conditions for the favorable microorganisms in the biological wastewater treatment, such as neutral pH, mesophilic temperature range (20 - 40°C), and sufficient oxygen and essential nutrients for the microorganisms to grow and reduce the contaminants. However, it should be considered that the measures for the different types of effluents can vary significantly depending on the incidents and the cause of the effluents, as well as the type of wastewater treatment implemented, and some measures are specific for each particular mill.