Evaluation of alternative electron donors for denitrifying moving bed biofilm reactors (MBBRs)

The effectiveness of four different electron donors, specifically methanol, ethanol, glycerol, and sulfide (added as Na₂S), were evaluated in post-denitrifying bench-scale moving bed biofilm reactors (MBBRs). With the requirement for more wastewater treatment plants to reach effluent total nitrogen levels approaching 3 mg/L, alternative electron donors could promote more rapid MBBR startup/acclimation times and increased cold weather denitrification rates compared to methanol, which has been most commonly used for post-denitrification processes due to low cost and effectiveness. While the application of alternative substrates in suspended growth processes has been studied extensively, fixed film post denitrification processes have been designed to use primarily low yield substrates like methanol. Bench-scale MBBRs were operated continuously at 12°C, and performance was monitored by weekly sampling and insitu batch profile testing. Ethanol and glycerol, though visually exhibited much higher biofilm carrier biomass content, performed better than methanol in terms of removal rate (0.9 and 1.0 versus 0.6 g N/m²/day, respectively.) Maximum denitrification rate measurements from profile testing suggested that ethanol and glycerol (2.2 and 1.9 g N/m²/day, respectively) exhibited rates that were four times that of methanol (0.49 g N/m²/day.) Sulfide also performed much better than either of the other three electron donors with maximum rates at 3.6 g N/m²/day and with yield (COD/NO₃-N) that was similar to or slightly less than that of methanol.