beun et al 2002 - aerobic granulation.pdf

Water Research 36 (2002) 702–712 Aerobic granulation in a sequencing batch airlift reactor J.J. Beun, M.C.M. van Loosdrecht*, J.J. Heijnen Kluyverlaboratory for Biotechnolog y, Delft Uni versity of Technolog y, Julianal aan 67, 2628 BC Delft, The Netherland s Received 27 July 2000; received in revised form 15 March 2001; accepted 23 April 2001 Abstract Aer obi c granul ar sludge was cult ivated in an int ense ly mixed sequenc ing bat ch air lif t reactor (SBAR) . A COD loading of 2.5 kg Acetate-COD/(m 3 d) was applied. Granules developed in the reactor within one week after inoculation with suspended activated sludge from a conventional wastewater treatment plant. Selection of the dense granules from the biomass mixture occurs because of the differences in settling velocities between granules (fast settli ng biomass), and filament s and flocs (slow settl ing biomass) . At ‘stead y state’ the granules had an average diamet er of 2.5 mm, a biomas s dens ity of 60 g VSS/l of granules, and a set tli ng rate of >10 m/h. The biomass consis ted of bot h het erotro phic and nitrifying bacteria. The reactor was operated over a long period during which the granular sludge proved to remain stable. The performance of the intermittently fed SBAR was compared to that of the continuously fed biofilm airlift suspension reactor (BASR). The most importance difference was that the density of the granules in the SBAR was much higher than the density of the biofilms in the BASR. It is discussed that this could be due to the fact that the SBAR is intermittently fed, while the BASR is continuously fed. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Aerobic granules; Sequencing batch reactor; Airlift reactor; Biofilms 1. Introductio n Most wast ewater tr eatment systems have so me disadvantages such as a high surplus biomass produc- tion, low flexibility with respect to fluctuating loading rates, a large area requirement for reactors and clarifi ers and a rel ati vely low vol umet ric conver sion capacit y (0. 5–2 kg COD/ (m 3 d) for activa ted sludge or conv en- tional biofilm systems). For anaerobic processes much more efficient reactors have been develop ed (40 kg COD/ (m 3 d) for an UASB (upflow anaerobic sludge blanket) reactor [1]). In these reactors the biomass grows as well settli ng granules, which allows the accumul ation of high amounts of acti ve biomas s in the reac tor. Mor eove r clarifiers are not need ed beca use sludge separation is integrated in the UASB reactor itself. Despite the widespread use of UASB processes, the mechanism of biomass granul ati on is sti ll subject of dis cus sion. Our bas ic assumption is that, beca use of substrate diffusion limitations, bacteria prefer growing in suspension over growing in a floc, which has again prefer ence over growth in a biofilm or granule. Growing in suspension is the most favorable form because in a floc, and even more in a biofilm or granule, the bacteria experience diffusion limitations for all kind of compo- nents involve d. Growing as a floc, a biofilm, or a granule onl y occurs when the bacteri a are otherwise was hed out [2]. We con sider gr anular gr owth as a special case of biofilm growth. Granulation in low turbulent systems by acidifying bacteria, nitrifying bacteria [3] and denitrify- ing bacteria [4] has bee n observed. Granula ti on by aerobic heterotrophs has been observed in high turbulent syste ms [2, 5]. It has been hypo the siz ed that the structure of biofilms is the net result of biomass growth and detachment processes [6]. Growth of the biomass is mainly influenced by the substrate loading rate and the growth yiel d. Det achment in these high ly turbul ent *Corresponding author. Tel.: +31-15-2781618; fax: + 31-15- 2782355. E-mail address: mark.vanloosdr [email protected] l (M.C.M. van Loosdrecht). 0043-1 354/02 /$ - see front matte r r 2002 Elsevier Science Ltd. All rights reserved. PII: S00 43-135 4(0 1)0 025 0-0

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beun et al 2002 - aerobic granulation.pdf