Anammox endogenous metabolism during long-term starvation: Impacts of intermittent and persistent modes and phosphates
Introduction
Discovery of anaerobic ammonium oxidation (anammox) process has revolutionized the removal of nitrogen from ammonium rich residual streams. The anammox process is the oxidization of ammonium to molecular nitrogen in the absence of oxygen with nitrite as the electron acceptor. To date, more than one hundred full-scale anammox plants have been installed and operated for treatment of municipal and industrial wastewaters, such as anaerobic digestion reject water, tannery, food processing, semiconductor, fermentation, yeast, distillery, winery industries [1]. The anammox-based process has been recognized as efficient, cost-effective and low energy consumption alternative to the conventional nitrification and denitrification processes [2], [3], [4]. Application of anammox for the nitrogen removal from mainstream allows treatment scenarios with a net energy production for wastewater treatment plants (WWTP). Energy-positive wastewater treatment would increase societal sustainability, and anammox is a key process that facilitates this goal. With recent advances in mainstream treatment, a net energy-producing sewage treatment plant is nearing feasibility [5], [6], [7].
However, due to large fluctuations in the flow and composition of wastewaters, the microorganisms responsible for biological wastewater treatment plants are frequently exposed to long-term famine conditions (days and sometimes weeks) [8]. Starvation can affect anammox bacteria in underloaded bioreactors or during the storage of sludge. In terms of the slow growth of anammox bacteria, new bioreactors are typically inoculated with an enriched biomass from other wastewater treatment plants [9]. Starvation significantly reduces the amount and activity of active microorganisms, and risks degrading the capacity, efficiency and robustness of wastewater treatment systems [10]. Additionally, in order to secure enough seeding biomass, the preservation of anammox biomass could be one of possible solutions for rapid and successful start-up of anammox process. Although extensive studies have been conducted to optimize storage conditions in terms of low temperatures and cryoprotectants [11], [12], [13], [14], [15], there are no reports on the effect of starvation modes on the endogenous metabolism of anammox bacteria.
Phosphates are commonly found in municipal wastewaters due to their use in detergents. Additionally, wastewaters from the production of fertilizers are rich in phosphates. Regarding P recovery, energy-producing wastewater treatment plants increasingly rely on phosphorus removal using iron [16]. Generally, this procedure was behind nitrogen removal process, resulting in that considerable amounts of phosphates might be introduced to anammox-based reactors. Similarly, due to large fluctuations in the flow and composition of wastewaters, exposure to phosphates can happen to or be along with starved anammox bacteria in underloaded bioreactors. However, disagreements have been reported in the literature in terms of the effects of phosphates on the fresh anammox biomass [17], [18], [19]. Unfortunately, no studies have focused on the effects of phosphates on starved anammox bacteria or on the storage of sludge so far.
Therefore, the purpose of this study is to identify (i) the endogenous characteristics of anammox granules in response to different long-term anaerobic starvation modes; (ii) the exposure/pre-exposure effects of different levels of phosphates on starved anammox bacteria and (iii) the indicator role of extracellular polymeric substances (EPS, particularly proteins) under starvation conditions.
Section snippets
Origin of the biomass
Anammox biogranules were used in all batch experiments. Bacterial sludge was cultivated and maintained in a 2-L laboratory-scale up-flow anaerobic sludge blanket (UASB) reactor fed with a synthetic medium at a loading rate of 10 kgN L−1 d−1. The sludge was dominated by anammox bacteria of the genus Candidatus Kuenenia stuttgartiensis [2]. The average size of the granules was 2.6 ± 0.9 mm (calculated using ImageJ software). The volatile suspended solids (VSS) content of the biomass from a nursing
Biomass decay rate
In most anammox bioreactors with extremely high sludge retention times, the active biomass equilibrium concentration is strongly dependent on the endogenous decay coefficient [26]. The correct estimation of the decay constant is a key factor to properly modeling and understanding the anammox process and is helpful in the design and management of an anammox reactor. In this study, the endogenous decay coefficient of anammox granules based on decreasing VSS levels during a 30-d period of
Implications
Although different strategies were adopted depending on the starvation mode, anammox cells possess a better survival strategy via a relatively low maintenance energy requirement to ensure their survival in nutrient-limited systems. In addition to storage of sludge, critical situations (i.e., fluctuations in influent quality or mechanical failures) are inevitable in engineering and can cause persistent or intermittent starvation. Strategies must be followed to avoid such events and, in the case
Acknowledgements
The authors wish to thank the National Key Technologies R&D Program of China (No. 2012BAC13B02), the Natural Science Foundation of China (No. 51278162), the National Training Programs of Innovation and Entrepreneurship for Undergraduates (No. 201410346017), the Xinmiao Talent Program of Zhejiang Province (No. 2015R423074) and the Xingguang Program of Hangzhou Normal University for their partial support of this study.
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2022, Biochemical Engineering JournalCitation Excerpt :Meanwhile, the PN/PS ratio increased to 6.1 ± 0.4 and 5.8 ± 0.4, respectively. It has been reported that the readily biodegradable polysaccharide was likely to be utilized for maintenance during the starvation period [38]. As the reactivation proceeded, the EPS content increased to 46.0 ± 2.0 and 54.5 ± 2.7 mg·g-1VSS, respectively for R1 and R2.
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2021, International Biodeterioration and BiodegradationCitation Excerpt :During the storage stage, obvious black surfaces formed with the decrease of cytochrome content and the increase of sulfide production under anaerobic starvation (Tang et al., 2011). And the black covering on anammox granules had been often observed when the anammox process got disturbed (such as starvation or storage), and this outer layer was regarded as a protective surface to anammox bacteria from toxic oxygen and other harmful compounds (Shi et al., 2019; Wang and Gao, 2018; Zhang et al., 2015). It was obvious that parts of the granules were broken off, causing the rough surfaces and irregular shapes (Fig. S6b).