Enhancement of Denitrification in Upflow Sludge Bed Reactors with Sludge Wasting

From the performance data of the upflow sludge bed (USB) reactors (with sufficient carbon), the rate-limiting step in denitrification is nitrate reduction. Biological denitrification in the USB reactors (superficial velocity=0.5, 1.0, 2.0, and 4.0 m/h) can be greatly enhanced with sludge wasting from the bioreactor [i.e., maintain granular sludge retention time (GSRT) at 20 days], including high volumetric loading rates of up to 6.61 g ${\mathrm{NO}}_3^{-}$ –N/L day, high specific denitrification rates [arithmetic mean=0.31–0.42 g $\mathrm{N}{\mathrm{O}}_3^{-}$ –N/g volatile suspended solids (VSS) day], high denitrification efficiencies (97.6–97.8%), and relatively low washout rates of biomass granules (arithmetic mean $\omega$  =0.13–0.31 g VSS/L day). The biomass concentration, average granule size ( $d_p$ ), and microbial density of the USB reactors with sludge wasting were greater than those of the USB reactors without sludge wasting (i.e., the former grew more compact granules than the latter). From the granulation experiment, the granule size distribution and $d_p$ of the broken-up granules in the sludge-bed zone can restore to those of the original granules in one GSRT, implying that spontaneous flocculation of extra-cellular polymer of denitrifying-bacteria cells occurred in the USB reactor, which may also be accelerated by a rigorous backing-mixing effect of continuous production of biogas. Accordingly, the USB reactor with sludge wasting can be regarded as a promising alternative to treat high-strength nitrate wastewater.