The connection between aeration regimes and EPS composition in nitritation biofilm
Graphical abstract
Introduction
Partial nitritation, the process of oxidizing half of ammonium to nitrite by ammonium-oxidizing bacteria (AOB), is an important method to provide nitrite for anaerobic ammonia oxidation (anammox) (van der Star et al., 2007; Winkler and Straka, 2019). Robust partial nitritation is the key to efficient anammox technology and requires that nitrite oxidation by nitrite-oxidizing bacteria (NOB) be effectively inhibited (Ma et al., 2016; Cao et al., 2017; Zhang et al., 2019). Biofilm reactors maintain high biomass content, and have been increasingly applied for nitritation to prevent AOB being washed out because of the overgrowth of heterotrophs (Lee et al., 2004; Bassin et al., 2012; Kowalski et al., 2019a, Kowalski et al., 2019b).
Nitritation biofilm reactors usually have high ammonia removal rates (Aslan and Dahab, 2008; Wang et al., 2019) and high resistance to load shock and toxic compounds (di Biase et al., 2019), which makes them ideal for the treatment of N-rich wastewater. Z.J. Zhang et al. (2011) successfully performed partial nitrification of reject water in a swim-bed reactor with a maximum nitrogen load of 5.9 kg-N·m−3·d−1 (corresponding to an ammonia removal rate of 3.1 kg-N·m−3·d−1). Nitritation biofilm reactors have also been shown to perform well at low temperature. Indeed, stable partial nitritation has recently been achieved at room temperature (e.g. 10–20 °C) in some lab-scale biofilm reactors treating low strength wastewater (Kowalski et al., 2019a, Kowalski et al., 2019b; Deng et al., 2020; Yu et al., 2020). This technology supports the application of autotrophic nitrogen removal to municipal wastewater (Cao et al., 2017).
Extracellular polymer substances (EPS), the main component of biofilm (Flemming and Wingender, 2010), strongly influence the performance of biofilm reactors and have therefore been widely studied (Shi et al., 2017; Morgan-Sagastume, 2018). For instance, the composition of EPS in nitritation biofilm was evaluated using excitation–emission matrix spectroscopy (L. Zhang et al., 2011) and confocal laser scanning microscopy (Xu et al., 2015). Similar to other biofilms used for wastewater treatment, components such as proteins, polysaccharides, lipids, nucleic acids and humic substances have all been detected in nitritation biofilm, and proteins and polysaccharides were found to be the dominant components (Xu et al., 2015). Proteins are the main component of the hydrophobic fraction of EPS, and an increase in the proteins/polysaccharides ratio enhances microbial aggregation. Shao et al. (2019) found that the protein/polysaccharide ratio in nitritation biofilm was significantly lower than that in nitrification biofilm. Similar results have been observed by comparing AOB-enriched cultures with anammox bacteria-enriched cultures or activated sludge (Yin et al., 2015). This suggests that polysaccharides may contribute more to microbial aggregation in nitritation biofilm because polysaccharides have a lower critical gelling concentration than proteins (Seviour et al., 2009). The composition of EPS in nitritation biofilm varies depending on operational conditions (Sheng et al., 2010). For example, heterotrophic environments lead to lower protein/polysaccharide ratios in the EPS of nitritation biofilms compared with autotrophic environments (Xu et al., 2015). Studies on the factors influencing biofilm EPS composition were conducted to optimize the operating conditions of biofilm reactors and the process of biofilm formation (Tsuneda et al., 2001). Intermittent aeration mode is a promising strategy for inhibiting NOB, and has been successfully applied in the nitritation of certain types of wastewater (Ma et al., 2016). However, less attention has been paid to the variations in EPS composition that result from different aeration modes in nitritation biofilms.
In addition, biofilm detachment has been neglected in the study of EPS properties although detachment is an indispensable step in the biofilm life cycle (Garny et al., 2008). We therefore also investigated the differences in EPS composition between nitritation biofilm and detachment biomass to elucidate the connection between biofilm behavior and EPS composition in nitritation biofilm reactors.
In this study, two MBBRs for partial nitritation were run under continuous and intermittent aeration with increasing influent COD/N ratio, and the nitritation performance, microbial diversity, and biofilm characteristics were investigated. The aims of this study were: 1) to quantify the nitritation performance and variations in EPS composition in nitritation biofilms under continuous and intermittent aeration; 2) to elucidate the connection between biofilm EPS composition and aeration regimes. The results of this study provide useful guidance for robust partial nitritation in biofilm reactors.
Section snippets
Experimental MBBRs
Two cylindrical plexiglass MBBRs for partial nitritation with mechanical agitators and air-compressors were constructed. Each MBBR had an effective volume of 6 L, and was packed with approximately 400 carriers with a total surface area of 1.04 m2. The MBBRs were fed continuously with 0.6 L h−1 of medium, resulting in a hydraulic retention time (HRT) of 10 h. The temperature was maintained at 30 ± 1 °C using a bath thermostat, and the pH in the reactors was between 7.0 and 7.5. One of the MBBRs
Long-term partial nitritation performance
The partial nitritation performance in MBBRs treating synthetic wastewater was investigated over 113 days of operation (Fig. 1). The experimental period was divided into five stages with different influent COD/N ratios. Stages I, II, III, IV, and V corresponded to influent COD/N ratios of 0, 0.17, 0.33, 0.67 and 1.0, respectively.
As Fig. 1 shows, both MBBRcon. and MBBRint. achieved satisfactory partial nitritation performance when treating wastewater without organic carbon (Stage I). The NO2−/NH
Conclusions
- 1)
Although intermittent aeration more strongly inhibited nitrite oxidation compared with continuous aeration, the partial nitritation performance in an MBBR under intermittent aeration was worse than under continuous aeration. This is attributed to the lower ammonia oxidation activity and shorter aeration time under intermittent aeration.
- 2)
EPS in nitritation biofilm on carriers were of higher HS/PS ratio (0.52 ± 0.13 and 0.72 ± 0.16 for continuous and intermittent aeration, respectively) than that
Credit authorship contribution statement
Junjie Wang: Writing – original draft, Data curation. Zhuwu Jiang: Conceptualization, Methodology. Weigang Wang: Investigation, Validation. Han Wang: Methodology, Software. Yao Zhang: Investigation, Validation. Yayi Wang: Project administration, Funding acquisition, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (51778446 and 51978485).
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