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Modern pulp mills can produce biofuels and value-added bio-products, such as lignin products. However, these bio-products are often present in solutions that are too dilute to be separated in a cost-efficient manner, and concentration by filtration is difficult due to their high fouling properties. In this research, challenging liquor, tannin-containing spruce bark extract, was concentrated by forward osmosis using soda ash from the soda recovery boiler as a draw solution. Soda ash, which consists of about 90% sodium sulphate, could be considered as a cost-free draw solution, since part of it needs to be discharged daily in order to prevent enrichment of potassium and chloride in the chemical circulation of pulp mills. Draw properties of soda ash solution were compared with the properties of pure salts, namely sodium sulphate and sodium chloride. Soda ash generated the same osmotic pressure and similar flux than sodium sulphate, but was a weaker draw solution compared with sodium chloride. When the draw solution concentration was 20% at 40°C, the flux was 16 and 13 LMH with soda ash and sodium sulphate, respectively. Using FO, water recovery over 80% could be reached, which is double compared with water recovery obtained by nanofiltration. However, the salt concentration of the feed solution was increased due to the reverse salt flux of the draw solution, which may hinder the further processing of tannin-containing spruce bark extract.