Partial nitrification-anammox (PNA) treating sewage with intermittent aeration mode: Effect of influent C/N ratios
Graphical abstract
Introduction
The partial nitrification-anammox (PNA) process involves ammonium oxidizing bacteria (AOB) converting approximately half of the ammonium in wastewater to nitrite, followed by the remaining ammonium and nitrite being converted to N2 by anammox bacteria. PNA is considered a cost saving alternative to conventional biological nitrogen removal via heterotrophic denitrification [1], [2]. The PNA process consumes 60% less oxygen and 100% less organic carbon source, produces about 90% less sludge compared with nitrification/denitrification [2]. To date, PNA has mainly been used to treat high-strength wastewater with low concentrations of biodegradable organic substances with organic carbon to nitrogen (C/N) ratios lower than 0.5 [3], [4], [5]. Recently, the mainstream PNA has been studied in both lab-scale and pilot-scale [6], [7], [8], [9], and the feasibility of PNA for low ammonium sewage treatment has been verified in full-scale plants [10], [11], [12]. However, there are still several challenges for its application in sewage treatment, including anammox bacteria enrichment, seasonal temperature variation of municipal wastewater and strict discharge standards of effluent nitrogen concentration [13], [14], [15]. Among these challenges, nitrite oxidizing bacteria (NOB) suppression is a major challenge in mainstream PNA process due to the lack of the inhibition factors such as high free ammonium and free nitrite [13], [14], [16]. For NOB suppression in mainstream treatment, intermittent aeration has been considered as an effective strategy because during the transition from anoxic to aerobic environment, NOB activity is difficult to recover in a short time [14], [15], [17], [18].
The influent carbon source is proved to be another critical factor for PNA performance [13], [19]. The effect of chemical oxygen demand (COD) on the PNA process has two opposing aspects. On the one hand, the influent COD affects the nitrogen removal performance by shifting the microbial population. High COD concentration leads to growth of heterotrophic bacteria, resulting in suppression of AOB and anammox bacteria. Anammox activity becomes a limiting factor for PNA and the nitrogen removal efficiency decreases from 79% to 52% when the influent C/N ratio increases from 0.5 to 0.75 [20]. Both the AOB and anammox bacterial activity decrease at a C/N ratio of 2.7–2.9 [21]. Specially, it was reported that a high COD concentration would suppress anammox activity and a COD concentration up to 292 mg/L could completely inhibit anammox bacteria [22]. While in another aspect, acetate can be utilized by anammox bacteria to remove nitrate [19], [23].
On the other hand, PNA performance improves under appropriate influent COD concentrations since the enriched heterotrophic bacteria can denitrify the nitrate produced by anammox bacteria, thereby reducing the effluent nitrogen concentration [24]. For example, a high nitrogen removal efficiency of 84–95% was achieved at an influent C/N ratio of 1.2 with glucose as carbon source [25].
Furthermore, the changes in COD can affect the biomass concentration and sludge characteristics, which is important to the enrichment of anammox bacteria. Anammox bacteria are slow growing microorganisms with a doubling time of 11 days [26]. Hence, efficient biomass retention is important for the anammox process. The biomass with dense structure and good settleability, such as granular sludge with larger particle size, could provide a long biomass retention time and a favorable inside anoxic environment, promoting the growth of anammox bacteria [23], [27]. In addition, anammox granular sludge exhibits significant advantage over floc sludge in the aspect of tolerance to fluctuation in external conditions [24]. In PNA process, the anammox bacterial activity decreased when the average particle size decreased [19]. COD was found to affect extracellular polymeric substances (EPS) [28], which promote floc formation by facilitating the aggregation of cells and sludge particles [28], [29].
The effect of influent C/N ratios on the PNA process treating high-strength wastewater has been investigated [20], [30]. There are several studies about the effect of influent C/N ratios on PNA process treating low-strength synthetic wastewater [23], [31]. Few studies focus on the treatment of low-strength sewage [21], in which the organic components are more complex than in synthetic wastewater. Furthermore, the changes in sludge morphology, which greatly influences the enrichment and retention of slow growing anammox bacteria, are not well understood.
The objective of this study was to elucidate the effect of C/N ratios on the nitrogen removal performance and sludge morphology during the PNA process when treating low-strength sewage. A sequencing batch reactor (SBR) under intermittent aeration was found to facilitate the stability of the mainstream PNA process [15], [17]. Specifically, intermittent aeration with a long anoxic phase could successfully inhibit NOB and enrich anammox bacteria. In this study, an SBR was used for PNA in intermittent aeration mode. The sewage was pretreated in an aerobic reactor to adjust the C/N ratio, which was increased stepwise from 1.1 to 2.5. At the highest C/N ratio (2.5), the sewage was fed into the PNA reactor without pretreatment. The absolute abundances and relative abundances of functional bacteria were measured using the quantitative real-time PCR (qPCR) and Illumina high-throughput sequencing analysis, respectively. Changes in sludge morphology were investigated and correlated with the anammox bacterial population and PNA performance.
Section snippets
Influent and seed sludge
Sewage was collected from a residential area near the Beijing University of Technology (Beijing, China, coordinate: 39.88424° N, 116.48332° E) and used as the reactor influent. During the early experimental stage, the sewage was pretreated aerobically to remove biodegradable COD, resulting in an average C/N ratio of 1.1. After day 98, the influent C/N ratio was incrementally increased (1.5, 2.0) monthly by adjusting the pretreatment aeration time. After day 177, the SBR was fed with untreated
Effects of C/N ratio on nitrogen removal in the PNA system
The nitrogen removal performance in the PNA system treating sewage under different influent C/N ratios was investigated over 218-day operational period (Fig. 1, Table 3). When the influent C/N ratio was 1.1, the total nitrogen (TN) removal efficiency increased slowly but was maintained at lower than 50.0%. In contrast, the ammonium removal efficiency and effluent nitrate concentration increased to 71.9% and 20.8 mg-N/L (day 98), respectively (Fig. 1). The average Nitrate pro/TN rem in one cycle
Conclusions
The influent C/N ratio of a PNA reactor treating sewage was increased stepwise from 1.1 to 2.5 to investigate the effects of C/N ratios on reactor performance. Based on the results of our study we conclude the following:
- 1.
The TN removal efficiency of the PNA reactor increased from 30.8% to 77.3% when the influent C/N ratio increased from 1.1 to 2.5, and subsequently decreased when C/N ratio was higher than 2.5.
- 2.
Both anammox bacterial activity and abundance increased when the C/N ratio increased
Acknowledgments
This reserach was financially supported by the Nature Science Foundation of China [51608013, 21677005]; the 111 Project [005000522106]; and the Funding Projects of Beijing Municipal Commission of Education.
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