Operation and Performance of a Sidestream Aerobic Granular Sludge Pilot Reactor for Mainstream Bioaugmentation

Abstract

A pilot sidestream sequencing batch reactor (SBR) was fed centrate diluted with secondary effluent to grow aerobic granular sludge enriched with phosphorus accumulating organisms (PAO) and ammonia oxidizing bacteria (AOB) for bioaugmentation of a BOD-only continuous-flow mainstream treatment system at low solids retention time (SRT) to achieve nitrogen and phosphorus removal. The SBR cycle consisted of anaerobic, aerobic, settling, decant, and idle phases. The settling time was 7 minutes and 50% of the volume was removed during effluent decanting. Acetate was fed in the anaerobic phase for uptake and conversion to storage products by PAOs. Under aeration, denitrifying PAOs (dPAOs) utilized stored carbon for NO3-/NO2- reduction within the anoxic granule core simultaneously with nitrification on outer granule layers. Alkalinity production during denitrification was sufficient to maintain reactor pH above 7.0 for uninhibited nitrification rates. The reactor was operated for about 10.5 months after a 95-day startup and 31-day stabilization period. Over the 10.5-month period the removal efficiencies for ammonia (NH3) and total inorganic nitrogen (TIN) averaged 90% and 85%, respectively, at NH3-N loading rates of 0.38 ± 0.07 g/L-d. Simultaneous nitrification and denitrification (SND) efficiency averaged 94% and was over 80% for each sampling day, with the exception of 3% of measurements. Periodic PAO and nitrifying inhibition occurred by unknown changes in the centrate or secondary effluent; however, granular integrity was maintained, and microbial activity rapidly recovered. Five stable performance periods totaling 107 days were identified during the 10.5-month period. The SND performance and required COD:NH3-N feed ratio (COD:N) was affected by aeration DO concentration, anaerobic acetate feed rate, and effluent NH3-N concentrations. The NH3-N removal and SND efficiencies of 96 ± 5% and 93 ± 4%, respectively were achieved at an NH3-N loading rate of 0.44 ± 0.02 g/L-d and COD:N feed ratio of 2.94 ± 0.19 g/g. The SRT, controlled by effluent solids and manual wasting, was 23 to 40 days during stable performance periods. MLSS concentration ranged from 11.3 to 15.0 g/L with over 90% granules based on 212 µm sieve size. The sludge settling velocity was over 9 m/hr. The weighted average granular size diameter, assuming spherical geometry, ranged from 1.0 to 1.5 mm during stable performance periods and the size distribution ranged from 0.212 to 3.0 mm. The average granular size and distribution provided more than sufficient surface area without limitation to nitrifier growth or nitrification rates. Specific nitrification rates ranged from 1.8 to 2.6 g NH3-N/g VSS-hr and were influenced by NH3-N loading rate, COD:N feed ratio, and bulk liquid DO and NH3-N concentrations. Shortcut nitrogen removal was achieved when there were low ratios of DO to NH3-N concentration in the bulk liquid. Maximum suppression was found at a DO:NH3-N ratio of 0.2 or less.