Sulfur is an often-overlooked but key element in water resource recovery facilities (WRRFs). It undergoes biological and physicochemical conversions in the prevailing unit processes, while its emission as hydrogen sulfide (H2S) may cause odour nuisance, corrosion and have adverse effects on biogas production. Despite its importance, little information is available concerning the fate of sulfur in WRRFs, notably when it comes to full-scale and plant-wide characterization. This doctoral research work presents advancements in our understanding of characterization of total sulfur flow and sulfur transformation during wastewater treatment processes of municipal WRRFs. First, a comprehensive literature review was performed to identify potential sulfur transformations within individual unit processes of municipal WRRFs. Sulfur mass flows in a full-scale WRRF were quantified on a plant-wide level, assessing liquid, sludge and gas streams simultaneously. Besides showing the distribution of sulfur flows, this result specified key sulfur flows, process treatments, and enabled a comparison of process treatment units. Next, the transformations of sulfur species were studied in both water and sludge treatment lines. Particular attention was given to fate of sulfur species during anaerobic digestion process and the influence of thermal hydrolysis, as a sludge pretreatment process, on the fate of sulfur species during anaerobic digestion. Both full-scale and lab-scale data were used for this purpose. Overall, this work improves our understanding of sulfur transformation in WRRFs, and in particular highlights the important role of organic sulfur conversion during anaerobic digestion process, which has been widely over-looked in literature.