The effects of carbon source and COD/N ratio on simultaneous denitrification and methanogenesis in an upflow anaerobic sludge blanket reactor

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 (R1) used sucrose as the carbon source, whereas another reactor (R2) 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 R1-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 R2 reached 81.8% and 83.4%, respectively, with a COD/N ratio higher than 10. The results of soluble metabolites production in R1 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.