The present study describes the precipitation of lignin by acidification of a black liquor (pH 12.56 and 12.44 g/l soluble lignin) produced by soda pulping of brewer’s spent grain. Sulfuric acid was added to the liquor to decrease the pH, forming a lignin-rich precipitate. Ten pH values (varying from 12.56 to 2.15) were studied. The lignin mass precipitated for each pH condition was determined, and the obtained liquors were evaluated regarding the color and concentration of soluble lignin. Some phenolic acids (vanillic, syringic, p-hydroxybenzoic, ferulic and p-coumaric) were quantified in the liquors to verify their removal profiles as a function of the pH alteration. Significant lignin precipitation was only observed at pH < 7.7. At pH 2.15 the concentration of soluble lignin was reduced to 2.31 g/l (removal of 81.43%) and the color of the liquor was strongly modified from dark brown to pale yellow. The phenolic compounds concentration was reduced in different proportions (from 74.4% to 32.1%), suggesting that each lignin derived compound is differently affected by the pH alteration.