Long-term operation performance and variation of substrate tolerance ability in an anammox attached film expanded bed (AAFEB) reactor
Graphical abstract
Introduction
As a kind of chemoheterotrophic bacteria of the phylum Planctomycetes, the anaerobic ammonium oxidation (anammox) bacteria use nitrite as the electron acceptor and ammonium as the electron donor to allow for the removal of ammonium from wastewater in the absence of oxygen and organic matters (Strous et al., 1999). Compared with the traditional nitrification–denitrification process in the treatment of ammonium-rich waste water, the application potential of the anammox process has attracted more attention in recent years since the anaerobic and autotrophic nature of anammox bacteria permits significant reductions in energy consumption and organic carbon investments for biological nitrogen removal (Lackner et al., 2014). However, it is hard to maintain stable operation of an anammox reactor since the anammox bacteria are highly sensitive to environmental conditions. A variety of inhibitors, such as the substrate, organic matter, sulfide, salinity, and other nutrient salts commonly exist in practical applications (Jin et al., 2012), among which, the substrate with too high concentration is the more common and important factor.
Generally, for the substrate-based inhibition in anammox process, FA (free ammonia) and FNA (free nitrous acid) are considered as the most critical inhibitors. However, a wide range of toxic threshold concentrations for both ammonium and nitrite make it difficult to design and maintain the anammox process (Jin et al., 2012). For the two-stage anammox system, a mixture of ammonium and nitrite with a reported reaction ratio of 1.32 is supplied into the anammox reactor. In this kind of anammox reactor, FA and FNA synergistically inhibit the anammox activity when the pH is controlled in an optimum range. Thus, the actual influent substrate concentration (SNinf, calculated as the sum of actual influent NH4+-N and NO2−-N) can be regarded as the main factor that simplifying the control of a two-stage anammox reactor when the pH is strictly controlled in the optimum range.
In order to improve the treatment performance and operation stability, new types of anammox reactors were widely developed and applied recently, such as up-flow fixed-bed column reactor (Ali et al., 2015), three-bio-electrode reactor (Yin et al., 2015), and closed sponge-bed trickling filter, etc. (Guillen et al., 2015). The biomass retention ability and the performance stability were significantly enhanced for these reactors. However, problems of gas stagnate and substrate short-circuiting caused by sludge blocking in these kinds of fixed-bed reactors increase the difficulty of reactor maintenance. The attached film expanded bed reactor was developed for the treatment of high organic strength wastewater with efficient biomass retention ability and good fluidization performance in the anaerobic digestion process (Jewell et al., 1981). However, its application in anammox process was rarely reported. Therefore, the research on the anammox attached film expanded bed (AAFEB) reactor has great value to improve the performance and simplify the maintenance of an anammox-based reactor. Moreover, the dilution effect of the effluent recirculation system, which is used for maintaining fluidization condition, significantly eases the substrate inhibition and enhances the operation stability as well as maintaining the optimum pH in the reactor.
Although the application of the AAFEB reactor significantly dilutes the influent substrate concentration, further study is required to investigate the transient concentration shock on the reactor performance since the nitrogen concentration is too high in some kinds of waste water, such as sludge digestion liquid (1200–4000 mg L−1), landfill leachate (1400–2800 mg L−1), and monosodium glutamate wastewater (15,000–25,000 mg L−1) (Furukawa et al., 2009, Ganigue et al., 2007, Shen et al., 2012). The tolerance ability to substrate concentration is known to be closely related to the species of anammox bacteria, reactor type and operation conditions, including temperature, pH and HRT. The promotion of the tolerance ability to substrate concentration is significant for maintaining the stable operation of an anammox-based reactor. In reports focusing on biomass acclimation under environment stress, the ability to resist a certain environmental stress improved after a transient shock or a period of acclimation (Ma et al., 2012). On the other hand, there are few reports focused on the degeneration of the ability to resist high substrate concentrations.
In this study, an AAFEB reactor was operated for 300 days to study the effects of transient substrate shock and the following recovery processes. The modified Gompertz equation and the modified 2-P logistic model were used to evaluate the change in anammox sensitivity, nitrogen loading potential (NLP), lag time (λ) and inhibitory concentrations of 10%, 50% and 90% (IC10, 50, 90). The substrates/product conversion ratios were also used as characterization parameters to further evaluate the reactor operation stability.
Section snippets
The experiment set-up and origin of the biomass
As shown in Fig. 1, an AAFEB reactor with 5 L working volume was used in the experiment. The recirculation ratio was maintained at a constant value of 250% to dilute the substrate concentration and maintain a certain hydrodynamic shear force to make the biofilm stronger, porous and heterogeneous (Liu and Tay, 2002). The operation temperature was controlled around 35 °C by a water jacket. Synthetic wastewater was feed into the reactor by a peristaltic pump (on 1 min, off 1 min). Substrate was
Performance during the stable operation period (phase I and phase II)
The operation pH was maintained in the optimum range of 7.8–8.5 throughout the experiment (Fig. 2a). As shown in Fig. 2, the AAFEB reactor was stably operated with an HRT of 3 hours and a TN concentration of 600 mg NL−1 for 25 days in phase I. During this period, the SNinf was maintained at 160 ± 9 mg NL−1 (Fig. 2b). From the 26th day, the substrate concentration was increased to 1250 mg NL−1. Correspondingly, the SNinf increased to 297 ± 29 mg NL−1 as well as the nitrogen loading rate (NLR) increased from
Conclusions
The AAFEB reactor performed high NLP of 20.95 gN·(L·d)−1 in the stable operation phase. The tolerance ability to the inhibitors of SNinf, FA and FNA was significantly enhanced in the stable operation phases but weakened after the substrate shock. The SNinf was considered accurately for monitoring the operation of anammox reactor and should be controlled below 320 mg NL−1 with the pH in the optimum range of 7.8–8.5 to ensure the absolute stable operation. The successfully application of the AAFEB
Acknowledgement
The authors wish to thank the Japan Society for the Promotion of Science (JSPS) for the financial support of this study.
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