Simulering av COD-fraktioner i en aktivslamanläggning vid en sulfatmassafabrik
This thesis work was made for Södra Cell Mörrums bruk in Blekinge. The purpose of the work was to develop a computer model in the simulation program Extend for the removal and balance of organic oxygen consuming substances, measured as COD (Chemical Oxygen Demand), in the new biological treatment plant in the mill. The plant produces two different pulp products, TCF- (Totally Chlorine Free) pulp and ECF- (Elementary Chlorine Free) pulp, for sale. The different bleaching procedures affect the characteristic of the wastewater and thus the effectiveness of the biological treatment plant.In the model that was developed the COD in the wastewater was divided into five different COD-fractions: soluble easily degradable (SS), soluble hardly degradable (SR), soluble inert (SI), particulate inert (XI) and particulate biomass (XB). To create the model FlowMac? was used as a base. FlowMac? is a library module in Extend adapted for pulp and paper mills. Some of the blocks only needed a slight modification while others, for example the sedimentation basin and the bioblocks, had to be done from scratch. The calibration of the model was made from data from a period of eleven weeks and a validation was made from data that differed as much as possible from the calibration data. Then the model was used to find out in what way an increase in production to 480 000 annual tons should affect the discharge of organic oxygen consumption substances.Results from the model indicates that the goals for the total COD-discharge will be hard to achieve with an increase in production to 480 000 annual tons. Despite the fact that production of TCF-pulp generates more COD than ECF-production the total discharge of COD is lower at TCF-production. The reason is that easier degradable COD is generated in TCF-bleaching. One of the major problems is that the torrent of water that is bypassed the biological treatment plant contains a large amount of COD and the efficiency of the treatment plant must be very high if the goals for the total COD-discharge shouldn?t be exceeded.