Performance of high-loaded ANAMMOX UASB reactors containing granular sludge
Introduction
Anaerobic ammonium oxidation (ANAMMOX) is a promising biotechnology for the treatment of ammonium-rich wastewater (van der Star et al., 2007, Joss et al., 2009). Under anoxic conditions, the ANAMMOX bacteria accomplish autotrophic ammonium oxidation to dinitrogen gas employing nitrite as an electron acceptor (Strous et al., 1998). It offers several advantages over conventional nitrification-denitrification systems including higher nitrogen removal rate, lower operational cost and less space requirement (Jetten et al., 2005, van der Star et al., 2007, Joss et al., 2009). Combined with single reactor high activity ammonium removal over nitrite (SHARON) process in which half of ammonium is converted to nitrite, the first full-scale ANAMMOX process (70 m3) was applied to treat sludge dewatering effluents in Rotterdam, The Netherlands in 2002 (van Dongen et al., 2001, van der Star et al., 2007). It stably operated achieving nitrogen removal rate (NRR) up to 9.5 kg-N m−3 day−1 (van der Star et al., 2007).
High-rate is one of the prime objectives for ANAMMOX process. The NRR of conventional nitrogen removal biotechnologies was less than 0.5 kg-N m−3 day−1 (Jin et al., 2008); while for ANAMMOX process, it was higher than 5 kg-N m−3 day−1 as obtained by a number of researchers using different reactors such as upflow biofilter, upflow anaerobic sludge blanket (UASB) reactor and gas-lift reactor (Sliekers et al., 2003, Imajo et al., 2004, Isaka et al., 2007, van der Star et al., 2007, Tang et al., 2009a). To date, the highest NRR reported was 26.0 kg-N m−3 day−1 at hydraulic retention time (HRT) of 0.24 h (Tsushima et al., 2007). Previous works on anaerobic processes including anaerobic digestion (Thiele et al., 1990) and denitrifying process (Franco et al., 2006) attributed high volumetric removal rates to three main aspects. Firstly, the reactors should have high-quality sludge retention for sufficient biomass accumulation. Secondly, the microbial communities should aggregate as granular sludge or biofilms for optimum metabolic activity. Finally, the substrate requirements of ANAMMOX bacteria should be satisfied simultaneously avoiding substrate inhibition, especially nitrite inhibition (Strous et al., 1999, Isaka et al., 2007, Tsushima et al., 2007).
The granular sludge characterized by good settling property and high activity plays a pivotal role in the performance of high-rate bioreactors (Thiele et al., 1990, Franco et al., 2006, Zhang et al., 2008). The characteristics of granular sludge such as heterotrophic aerobic granules (Beun et al., 1999, Beun et al., 2002, Zheng and Yu, 2007, Adav et al., 2008), anaerobic granules (Hulshoff Pol et al., 2004, Show et al., 2004, Wu et al., 2009), hydrogen-producing granules (Mu and Yu, 2006, Zhang et al., 2008), denitrifying granules (Franco et al., 2006) and autotrophic nitrifying granules (Tsuneda et al., 2003, Liu et al., 2008, Belmonte et al., 2009) have been extensively studied. In case of ANAMMOX granules, the settling property, diameter distribution and substrate diffusion have been reported (Arrojo et al., 2006, Ni et al., 2009). The characteristics of carmine color of ANAMMOX granules and their associated extracellular polymers (ECPs) have also drawn considerable attention for the process optimization. The hydroxylamine oxidoreductase and hydrazine oxidoreductase are two important enzymes of the ANAMMOX pathway. Both of these enzymes are rich in heme c (Klotz et al., 2008, Schmid et al., 2008), which endows the granular sludge with the carmine color. The extracellular polymers are assumed to be a key factor in the formation of granular sludge, which can be secreted by ANAMMOX bacteria (Cirpus et al., 2006).
In the present study, two ANAMMOX UASB reactors were operated to investigate the performance of high-loaded reactors possessing carmine granular sludge.
Section snippets
Synthetic wastewater
Ammonium and nitrite were supplemented to mineral medium as required in the form of (NH4)2SO4 and NaNO2, respectively. The composition of the mineral medium was (g L−1 except for trace element solution) (Trigo et al., 2006): KH2PO4 0.01, CaCl2·2H2O 0.00565, MgSO4·7H2O 0.3, KHCO3 1.25, FeSO4 0.00625, EDTA 0.00625 and 1.25 mL L−1 of trace elements solution. The trace element solution contained (g L−1): EDTA 15, H3BO4 0.014, MnCl2·4H2O 0.99, CuSO4·5H2O 0.25, ZnSO4·7H2O 0.43, NiCl2·6H2O 0.19, NaSeO4
Volumetric capacity
Throughout the reactors’ operation, the influent nitrite concentration was maintained constant whereby the nitrogen loading rate (NLR) was progressively increased by shortening HRT. The nitrogen removal performance of R1 and R2 is depicted in Fig. 1A and B, respectively. During the first 250 days, the hydraulic loading rate (HLR) of R1 was increased from 3.5 L L−1 day−1 to 114.3–123.8 L L−1 day−1, that corresponded to HRT of 0.21–0.19 h; the NRR was enhanced to 72.5 ± 2.8 (70.3–78.5) kg-N m−3
Conclusions
A super high-rate performance with nitrogen removal rate of 74.3–76.7 kg-N m−3 day−1 was revealed for the lab-scale ANAMMOX UASB reactors, which was 3 times of the previously reported top value. The performance was stable until the HRT was shortened to 0.16–0.11 h with the hydraulic loading rate larger than 152.4–221.0 L L−1 day−1. The biomass concentrations in the reactors were 42.0–57.7 g-VSS L−1 with the specific ANAMMOX activity up to 5.6 kg-N kg-VSS−1 day−1, both of which were regarded as
Acknowledgements
Financial support of this work by the National High-tech Research and Development (R&D) Program of China (2009AA06Z311), the National Key Technologies R&D Program of China (2008BADC4B05) and the Natural Science Foundation of China (30770039) is gratefully acknowledged. We wish to thank the anonymous reviewers and editors for their valuable suggestions on revising and improving the work.
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