Sulfate is the most common form of sulfur in pulp and paper
processing wastewaters, partly due to the oxidation of other sulfur compounds.
Chemical pulping wastewaters also contain a considerable load of organic sulfur
compounds, for example, neutral sulfite semichemical (NSSC) spent liquor con-tains 12.0 g/l sulfonated lignin, 2.7 g/l SO
3
2–, and 0.1 g/l SO
4
2–
(Guiot et al., 1991).