A novel partial nitrification-synchronous anammox and endogenous partial denitrification (PN-SAEPD) process for advanced nitrogen removal from municipal wastewater at ambient temperatures
Graphical abstract
Introduction
Anaerobic ammonium oxidation (anammox), which anaerobically oxidizes ammonium to nitrogen gas with nitrite as an electron acceptor, has been widely considered a cost-efficient process for nitrogen removal from wastewater (Gu et al., 2018, Kuenen, 2008). Combined with partial nitrification (PN) (NH4+ → NO2−), anammox-based processes reduce the requirements for aeration by 60% and for organic carbon by 100% and decrease sludge production by 90% in theory (Cao et al., 2017). To date, the PN/anammox process has been successfully applied for removing high concentrations of ammonium from landfill leachate, anaerobic sludge digestion of reject liquor (Lackner et al., 2014), and other applications. This technology has also been proposed for mainstream treatment to achieve cost-effective and energy-efficient municipal wastewater treatment.
The stable nitrite production via PN is critical to the mainstream PN/anammox process. Many efforts have been made to achieve PN through appropriate regulation of low dissolved oxygen (DO) (Ma et al., 2009), intermittent aeration (Kornaros et al., 2010), aeration duration control (Yang et al., 2007), free nitrous acid (FNA) (Wang et al., 2014), hydroxylamine (NH2OH) addition (Xu et al., 2012), etc. These approaches aim to promote the ammonia oxidation rate to out-compete the nitrite oxidation rate and eliminate nitrite-oxidizing bacteria (NOB). However, due to the difficulty in the suppression of NOB at low ammonium strength and ambient temperatures, the mainstream PN/anammox process has still not been applied successfully in municipal wastewater treatment (Reino et al., 2018). The excess growth of NOB often leads to fluctuations in the nitrite to ammonium (NO2−/NH4+) ratio and results in low nitrogen removal efficiencies (approximately ≤70%) with high nitrate accumulation in the final effluent, especially at low temperatures (Cao et al., 2017). In addition, the organic matter present in the municipal wastewater can result in the competition for nitrite between heterotrophic bacteria and anammox bacteria, which has adverse effects on the sustainability of the anammox-based process (Cao et al., 2017).
Recently, it was reported that endogenous partial denitrification (EPD) driven by glycogen-accumulating organisms (GAOs) resulted in a stable production of nitrite for the anammox reaction (Ji et al., 2017, Ji et al., 2018). GAOs are able to fully absorb organic matter contained in the influent to form polyhydroxyalkanoates (PHAs) under anaerobic conditions and then use PHAs as carbon sources for reducing nitrate to nitrite under anoxic conditions (Rubio-Rincón et al., 2017, Ji et al., 2017). Previous studies have evaluated the feasibility of achieving mainstream anammox in combination with EPD and nitrification for treating simulated domestic sewage (Ji et al., 2018). However, very few studies were conducted on the EPD/anammox process for the nitrogen removal from actual domestic sewage (Wang et al., 2019). It should also be noted that nitrate as the electron acceptor in EPD is produced by the nitrifying pathway, which consumes more aeration energy than PN (Ma et al., 2017a). Additionally, nitrate as a by-product of the anammox reaction remains in the final effluent of the EPD combining with anammox system (Ji et al., 2018), which requires additional treatment.
EPD also provides an important opportunity for enhancing the nitrogen removal efficiency by integrating it and anammox in a single reactor; however, this remains unexplored to date. The synergy of EPD and anammox may enable significant savings due to the lower requirements for aeration and organic carbon when combined with PN. In such a process, nitrate produced by NOB and anammox bacteria can be reduced to nitrite for the anammox reaction and the excess nitrite can be further reduced to nitrogen gas. This could complement each other’s advantages to provide more flexibility regarding the NO2−/NH4+ ratio for the anammox reaction, potentially enhancing the robustness of the mainstream anammox system. Compared with traditional PN-anammox technologies, this technology also has a unique advantage to remove excess nitrate, significantly improving the nitrogen removal efficiency. Moreover, the low carbon requirements allow the system to be easily integrated with high-rate activated sludge technology (Ma et al., 2017a), thereby making more organic material be used for biogas production.
The objective of this study is to develop a novel two-sludge PN-synchronous anammox and EPD (PN-SAEPD) process for treating pre-treated domestic sewage with a low C/N ratio. The long-term performance of the PN-SAEPD system was evaluated for a period of 205 d as the ambient temperature decreased from 27.4 °C to 15.0 °C. The microbial community dynamics of the PN and SAEPD reactors were assessed using high-throughput sequencing analysis. The nitrogen removal pathways were explored by determining the nutrient concentration in a typical cycle of the PN-SAEPD system and the robustness of the SAEPD pathway was investigated by conducting batch tests under various process conditions. The results of this study provide a novel concept and technique for cost-effective and energy-efficient municipal wastewater treatment.
Section snippets
Reactor setup and operation
Two laboratory-scale sequencing batch reactors (SBRs) with a working volume of 10 L were employed for establishing the 2-sludge PN-SAEPD system (Fig. 1), whose ambient temperature decreased from 27.4 °C to 15.0 °C during the experiment (Table 1). The pre-treated domestic sewage was fed into the SAEPD-SBR to transform biodegradable carbon sources to PHAs under anaerobic conditions. Subsequently, the supernatant was fed into the PN-SBR to oxidize ammonium to nitrite or nitrate and was then fed
The performance of the PN-SBR
The PN-SBR provides nitrite for the SAEPD-SBR and it is inevitable to produce nitrate with a relative low nitrite accumulation ratio (NAR). The performance of the ammonium transformation in the PN-SBR is shown in Fig. 2. During Phase Ⅰ (Day 1–52), the aerobic reaction was stopped through real-time control when nitritation was complete, which was indicated by the change in the pH (Yang et al., 2007). In the first 30 d of operation, a NAR of 87.5% was achieved owing to the good performance of the
Conclusions
This study demonstrated the feasibility of a novel PN-SAEPD process for cost-effective and energy-efficient municipal wastewater treatment. The performance and microbial community of the PN-SAEPD system for treating pre-treated municipal wastewater, which was simulated with actual domestic sewage and the final effluent, were evaluated for over 205 d as the temperature decreased from 27.4 °C to 15.0 °C. The following conclusions were drawn:
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In this process, a high TIN removal efficiency of 91.2%
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 study was supported by Beijing Municipal Science & Technology Project (D171100001017002), 111 Project (D16003) and National Natural Science Foundation of China (21806006).
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