Two upflow anaerobic sludge blanket reactors were used to investigate the effects of carbon source and COD/N ratio on simultaneous denitrification and methanogenesis (SDM). One reactor (R-1) used sucrose as the carbon source, whereas another reactor (R-2) was fed with acetate. The formation of SDM granules occurred by increasing the nitrate concentration of wastewater and thus decreasing the COD/N ratio. The analysis of scanning electron microscopy showed that the R-1-granule-predominant microbial community was strikingly different with the stepwise increase of nitrate. Indeed, the gas production, soluble metabolites, granular-sludge formation, and nitrogen- and COD-removal efficiency for SDM in the two reactors are dependent on carbon source and COD/N ratio. The average nitrogen- and COD-removal rates for R1 reached 89.1% and 91.8%, respectively at a COD/N ratio higher than four, On the other hand, the average nitrogen- and COD-removal rates for R-2 reached 81.8% and 83.4%, respectively, with a COD/N ratio higher than 10. The results of soluble metabolites production in R-1 demonstrated that propionate and butyrate were utilized by the denitrifiers. Sucrose should be used as an electron donor at high nitrogen-loading rates. Moreover, if the dissimilatory-nitrate-reduction-to-ammonia process is suppressed, then the buffer-agent dosage could be decreased. (C) 2020 Elsevier Ltd. All rights reserved.