Impact of impurities on vivianite crystallization for phosphate recovery from process water of hydrothermal carbonization of kitchen waste

•Acetate inhibited the formation of vivianite due to the oxidation of Fe(Ⅱ).•Protein, humic acid and carbonate reduced P removal and changed crystal shape.•Ca and Mg reduced the purity of vivianite rather than the removal of P.•Negative effects of furfural and SS on vivianite crystallization could be ignored.•Increasing Fe(Ⅱ) concentration effectively reduced the negative effect of impurities. Crystallization of vivianite is a sustainable way to recover phosphate from process water of kitchen waste hydrothermal carbonization (HTC). This study investigated impact of organic and inorganic impurities in HTC process water on vivianite crystallization and approaches to reduce their negative impact. Results showed that acetate, bovine serum albumin and humic acid reduced the phosphate removal and inhibited the vivianite crystallization. Ca and Mg triggered competing precipitations and reduced the vivianite purity while carbonate reduced the phosphate removal and the vivianite purity. In addition, negative effects of furfural and suspended solids on vivianite crystallization could be ignored. Increasing Fe(II)/P ratio rather than increasing pH minimized the negative effects of impurities on vivianite crystallization in real process water. The phosphate removal reached 98.2% when the Fe(II)/P ratio increased to 2 at pH 6. Therefore, it is technical-feasible to recover phosphate from HTC process water through vivianite crystallization.