•Influence of changes in initial process water on carbonization were evaluated.•Changes in initial water pH and organic content influenced carbonization kinetics.•High concentrations of CaCl2 influenced carbonization mechanisms.•Initial process water pH and organics imparted little influence on carbonization. Hydrothermal carbonization (HTC) is a thermal conversion process that has been shown to be environmentally and energetically advantageous for the conversion of wet feedstocks. Supplemental moisture, usually in the form of pure water, is added during carbonization to achieve feedstock submersion. To improve process sustainability, it is important to consider alternative supplemental moisture sources. Liquid waste streams may be ideal alternative liquid source candidates. Experiments were conducted to systematically evaluate how changes in pH, ionic strength, and organic carbon content of the initial process water influences cellulose carbonization. Results from the experiments conducted evaluating the influence of process water quality on carbonization indicate that changes in initial water quality do influence time-dependent carbonization product composition and yields. These results also suggest that using municipal and industrial wastewaters, with the exception of streams with high CaCl2 concentrations, may impart little influence on final carbonization products/yields.