A method for determining the lignin or lignin by-product content of a process stream includes measuring the fluorescence parameter of a fluorescence spectra of the process stream, comparing the measured fluorescence parameter with predetermined a fluorescence parameter of lignin or lignin by-product reference samples, determining the amount of lignin or lignin by-product based on the comparison with the reference samples. Lignin or lignin by-products can then be removed from a process stream by adding a sufficient amount of a compound suitable for precipitating the lignin or lignin by-product to the process stream, and removing the precipitated lignin or lignin by-product from the process stream.

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