The efficiency of BOD removal in the biological treatment of kraft pulp mill wastewaters is usually very high while the removal of total COD is very depended on the wastewater treatment process (WWTP) because of high concentration on non-biodegradable or slowly biodegradable organic compounds such as lignin or lignin derivatives. Nowadays wastewaters from pulp mills are relatively often treated with various modifications of low loaded activated sludge process. It is vital to have large aeration basins with abundant of biomass when treating wastewaters containing lots of non- or slowly biodegradable organics, because the removal of these compounds is mainly based on adsorption to biosludge. The disadvantages of low loaded activated sludge process are high energy consumption and the release of nutrients as the biosludge is already digested aerobically during the process. Due to this, there is a great need for continuous monitoring of lignin or lignin derivatives concentrations in different parts of the WWTP to ensure sufficient and energy efficient removal of total COD. Earlier studies have shown that refractometer is a reliable measurement when measuring total dissolved solids (TDS) in the area of brown stock washing, where the concentrations of dissolved materials are very high. Latest studies have also shown that accuracy of refractometer measurement is adequate for measuring low TDS concentration from 0.5 to 0.1 % in wastewaters. The aim of this study was to define whether it would be possible to identify the concentration changes of lignin or lignin derivatives in the water by the use of an on-line refractometer and whether these results could be utilized for evaluating the performance of the WWTP. Firstly, it was studied in a laboratory scale how a new precision refractometer reacts to different lignin concentration changes in the water. Secondly, on-line refractometer was tested in a laboratory scale activated sludge process to see, whether the measurement could be utilized to monitor the removal of COD in different parts of aeration basin. The results indicate that new precision refractometer can be used for the detection of very small changes in TDS at low concentrations (about 50 ppm). Good correlations between refractive index and lignin concentrations were found in laboratory tests. Online measurement could be used to estimate the concentration of lignin or lignin derivatives in different waste streams of WWTP. Results from laboratory scale activated sludge process studies suggest that the measurement could be used within aeration basin too. With online refractometer it was possible to detect the removal of organic matter via biodegradation and adsorption of organic compounds in different parts of aeration. In addition, TDS concentrations in latter parts of aeration basin correlated fairly well with residual dissolved COD and dissolved organic carbon (DOC).
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