Elucidating performance failures in use of granular sludge for nutrient removal from domestic wastewater in a warm coastal climate region.

The effects of domestic wastewater and a coastal warm climate on granular sludge and biological nutrient removal were evaluated using a pilot-scale sequencing batch reactor (SBR). The reactor operation employed two different operational strategies (OS) based on up-flow feeding regimes, defined as fast (OS1, flow rate = 18.0 L min-1 and flow velocity = 22.0 m h-1) and slow (OS2, flow rate = 3.5 L min-1 and flow velocity = 4.3 m h-1). Under slow feeding, larger (OS1: 290 µm; OS2: 450 µm) and faster settling granules were obtained (OS1: 109; OS2: 74 mL g-1 TSS). The slow feeding regime was also advantageous for the hydrolysis of particulate organic matter (OS1: 1.3; OS2: 3.1 g CODtot g-1 VSS d-1) and for phosphorus removal (OS1: <33%; OS2: >97.5%). Neither strategy resulted in substantial biomass accumulation in the reactor (OS1: 0.7; OS2: 1.5 g VSS L-1), and high concentrations of nitrite were observed in the effluent (9-27 mg [Formula: see text] L-1). Ordinary heterotrophic organisms dominated the granular sludge developed under both feeding regimes (OS1: 30% of Thauera; OS2: 56% of Comamonas), while polyphosphate-accumulating organisms (PAOs) were only detected during OS2 (2.3-3.4% of total bacteria). A successful granular sludge process should be able to cope with high fluctuations in wastewater loads caused by rain events (82-182 mm month-1 in Florianopolis, Brazil). In order to achieve higher water quality, strategies identified for an efficient granular sludge SBR operation included (i) management of an anaerobic phase for PAO selection, and (ii) aeration control for successful nitrification/denitrification.

Effect of particulate organic substrate on aerobic granulation and operating conditions of sequencing batch reactors.

The formation and application of aerobic granules for the treatment of real wastewaters still remains challenging. The high fraction of particulate organic matter (XS) present in real wastewaters can affect the granulation process. The present study aims at understanding to what extent the presence of XS affects the granule formation and the quality of the treated effluent. A second objective was to evaluate how the operating conditions of an aerobic granular sludge (AGS) reactor must be adapted to overcome the effects of the presence of XS. Two reactors fed with synthetic wastewaters were operated in absence (R1) or presence (R2) of starch as proxy for XS. Different operating conditions were evaluated. Our results indicated that the presence of XS in the wastewater reduces the kinetic of granule formation. After 52 d of operation, the fraction of granules reached only 21% in R2, while in R1 this fraction was of 54%. The granules grown in presence of XS had irregular and filamentous outgrowths in the surface, which affected the settleability of the biomass and therefore the quality of the effluent. An extension of the anaerobic phase in R2 led to the formation of more compact granules with a better settling ability. A high fraction of granules was obtained in both reactors after an increase of the selection pressure for fast-settling biomass, but the quality of the effluent remained low. Operating the reactors in a simultaneous fill-and-draw mode at a low selection pressure for fast-settling biomass showed to be beneficial for substrate removal efficiency and for suppressing filamentous overgrowth. Average removal efficiencies for total COD, soluble COD, ammonium, and phosphate were 87 ± 4%, 95 ± 1%, 92 ± 10%, and 87 ± 12% for R1, and 72 ± 12%, 86 ± 5%, 71 ± 12%, and 77 ± 11% for R2, respectively. Overall our study demonstrates that the operating conditions of AGS reactors must be adapted according to the wastewater composition. When treating effluents that contain XS, the selection pressure should be significantly reduced.