Elsevier

Separation and Purification Technology

Volume 151, 4 September 2015, Pages 309-317
Separation and Purification Technology

Anammox endogenous metabolism during long-term starvation: Impacts of intermittent and persistent modes and phosphates

https://doi.org/10.1016/j.seppur.2015.07.066Get rights and content

Highlights

  • The anammox endogenous metabolism involves maintenance and decay processes.

  • Intermittent starvation is more parlous due to bioactivity deterioration.

  • Different survival strategies are adopted depending on the starvation mode.

  • Extracellular protein-like components indicate intracellular metabolism.

  • Low levels of phosphates may be beneficial for resting cells.

Abstract

Starvation of anaerobic ammonium oxidation (anammox) bacteria can occur in underloaded bioreactors or during the storage of sludge. In this study, the impacts of intermittent and persistent resting modes and phosphates on the endogenous metabolism of anammox bacteria during long-term starvation were investigated. Batch experiments were performed to evaluate the response of anammox granules to different modes of starvation (persistent-resting and intermittent-resting cells) in the presence and absence of phosphates. The endogenous decay coefficient of anammox granules (based on the decreased biomass) over a 30-d persistent starvation period was 0.0062 d−1, while the intermittent starvation by means of providing substrates every 10 days accelerated the biomass decay approximately twice (0.0110 d−1). Moreover, the intermittent-resting cells displayed exacerbated decreases in specific anammox activity relative to the persistent-resting cells, whereas no significant difference was observed in the heme c contents. Regardless of the physiological status of the cells, the presence of low levels of phosphates may be beneficial; however, a high level of phosphates is not recommended. Although different metabolic strategies were adopted depending on the starvation mode, anammox bacteria possessed a better survival strategy via a relatively low maintenance energy requirement to ensure their survival in nutrient-limited systems. In addition, the variations in extracellular protein-like components were indicative of different intracellular metabolic regulation mechanisms. These results provide a better understanding of the endogenous metabolism of anammox bacteria.

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.

References (42)

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