Phosphorus recovery from municipal wastewater with improvement of denitrifying phosphorus uptake based on a novel AAO-SBSPR process

https://doi.org/10.1016/j.cej.2020.127907Get rights and content

Highlights

  • Successful operation of the AAO-SBSPR process was achieved using real wastewater.

  • About 60% of phosphorus (P) was recovered at the expense of lower P content.

  • Raising anoxic/aerobic tank volume ratio improved denitrifying P uptake.

  • Both population and diversity of PAOs increased in P recovery systems.

Abstract

Both synthetic and real municipal wastewaters were used to verify the applicability of a novel AAO-SBSPR (Anaerobic-Anoxic-Oxic/Sequencing Batch Sidestream Phosphorus Recovery) process developed for the phosphorus (P) recovery and nutrients removal. P recovery strategy based on P mass balance of the process was employed where sludge retention time (SRT) of the systems was extended. Meanwhile, the anoxic/aerobic volume ratio was increased to enhance the activity of denitrifying P uptake. The results show that up to 59.8% and 75.2% of P from real and synthetic wastewaters were continuously recovered at the SRT of 35 d, respectively. The lower P recovery efficiency of the system fed with real wastewater was due to its lower influent P load and P content in activated sludge after P recovery was decreased from 0.059 mgP/mgVSS to 0.033 mgP/mgVSS. The analysis of the kinetic and stoichiometric parameters suggested that activities of the polyphosphate accumulating organisms (PAOs) in P recovery systems were retained under high P recovery efficiency, and more glycogen was degraded to provide energy for acetate uptake. This resulted in the highest average TP removal efficiency of 94.0% for the AAO-SBSPR process fed with real wastewater. Moreover, the enhancement of denitrifying P uptake activity was observed in P recovery systems. It was further improved by increasing the volume of the anoxic tank, which resulted in better TN removal performance. Meanwhile, the diversity of the microbial community was increased as the system was changed from the AAO process to the AAO-SBSPR process, and the relative abundances of the key functional bacteria such as nitrifiers, denitrifiers, PAOs and DPAOs were increased which strengthened the N-related metabolic pathways of the system remarkably.

Introduction

Phosphorus (P) is an essential element to human society, which is widely applied in many industries, especially in agriculture. However, P is a non-renewable source and the scarcity problem of it will emerge within the foreseeable future due to the increase in global demand [1]. This will prompt the development of sustainable P recovery technologies to meet the increasing P demand. Municipal wastewater is one of the major sources of human society to discharge P to the natural water bodies, which on one hand may cause eutrophication problems and lead to a sharp decline in aquatic biodiversity; on the other hand, it is also a potential source for P recovery. It is reported that theoretically 15–20% of world demand for phosphate rock can be satisfied by recovering P from municipal wastewater alone [2].

To remove P in municipal wastewater, enhanced biological P removal (EBPR) process has been used in municipal wastewater treatment plants (WWTPs) without the need for chemical precipitants, and therefore it is regarded as a more efficient, economical, and sustainable P removal technique [3]. Besides, as the EBPR can enrich P in sludge, it provides feasibility for P recovery. Therefore, it is promising to combine the EBPR with P recovery, which turns the traditional biological nutrient removal processes into simultaneous nutrients removal and recovery processes [4]. In recent years, several P recovery processes based on the EBPR have been reported. Some of them used biofilm systems to recover P from wastewater and a repeated P release strategy was adopted to obtain highly P enriched supernatant [5], [6]. Meanwhile, long-term evaluation of the stability of the P recovery processes was also reported both in flocculent [7] and granular activated sludge systems [8], [9], and up to 60% of the influent P could be extracted without a deleterious effect on EBPR [7]. An average of 45% of the P in the influent was recovered as struvite precipitate as reported by Larriba et al. from a pilot-scale system [10]. However, several studies have observed the adverse impact of P recovery on the stable operation of the EBPR process caused by the polyphosphate (poly-P) reduction resulting from P recovery and upgrowth of glycogen accumulating organisms (GAOs) [11], [12], [13]. Therefore, a balance between P recovery and stability of the process is advocated, where specific operation strategies for the P recovery system are required [7], [12], [14]. For this purpose, a novel AAO-SBSPR (Anaerobic-anoxic-oxic/sequencing batch sidestream phosphorus recovery) process designed for simultaneous nutrient removal and P recovery from municipal wastewater was set up and an operation strategy based on the P mass balance of the system was proposed [15]. The operation strategy correlated the sludge retention time (SRT) with P recovery efficiency in order to mitigate the reduction of P content in activated sludge caused by P recovery. This strategy was evaluated based on the AAO-SBSPR process using synthetic municipal wastewater at different SRTs from 25 d 50 d. The results showed that this strategy can effectively reduce the impact of P recovery on P content in activated sludge and up to 65% of the influent P could be continuously recovered at the SRT of 35 d without compromising the P and nitrogen (N) removal efficiencies of AAO-SBSPR process [15]. Particularly, high P recovery efficiency and long SRT had a limited impact on the kinetic rates and population of PAOs in the AAO-SBSPR process, which indicated that it was a promising technology for recovering P from municipal wastewater [15]. However, there is still a need to further assess this strategy and performance of the AAO-SBSPR process using real wastewater. Real wastewater has more fluctuating characteristics and higher diversity of carbon substrates and sometimes may suffer from insufficient COD concentration, which can affect the contaminants removal efficiencies and microbial community of the AAO-SBSPR process. Particularly, complex substrates commonly present in real wastewater can not be directly utilized by polyphosphate accumulating organisms (PAOs), and this may also compromise P removal and recovery efficiency of the recovery system.

Denitrifying P uptake relies on the fraction of denitrifying PAOs (DPAOs) which use nitrate and/or nitrite as electron acceptors for P uptake instead of oxygen [16]. Less chemical oxygen demand (COD) was required for the denitrifying P uptake compared with the separate N and P removal and reduction of 30% aeration as well as 50% sludge production in the EBPR process could be achieved [17]. In recent studies concerning the P recovery process, the occurrence of simultaneous N and P removals due to the denitrifying P uptake was reported. Among these studies, Shi et al. [18] proposed a novel P recovery process based on the A2N two-sludge system. Wong et al. [19] and Tian et al. [20] used the PAOs biofilm to extract P in wastewater. Zou and Wang [14] developed an EBPR-RP process for nutrients removal and P recovery. Meanwhile, a remarkable improvement of the denitrifying P uptake was also observed using the novel AAO-SBSPR process at longer SRT, as DPAOs have lower growth yield and long SRT is favored [15], [21]. Many factors can affect denitrifying P uptake, especially the characteristics of the influent wastewater, and therefore the improvement of denitrifying P uptake by AAO-SBSPR process needs to be further evaluated using real wastewater. Also, the denitrification rate of DPAOs is lower than the ordinary denitrifiers, and thus longer reaction time is usually preferable. As changing the anoxic/aerobic tank volume ratio can change the reaction time, and improvement of the denitrifying P uptake was observed by increasing the hydraulic retention time (HRT) of the anoxic tank [22], [23], it is of interest to investigate the influence of anoxic/aerobic tank volume ratio on the denitrifying P uptake in a P recovery system.

The structure of the microbial community and its link to the operation conditions should be explored to understand the mechanism of the P recovery process and maximize the efficiency, robustness, and stability of the system. Jin et al. [24] used the polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) method to investigate the variation of the microbial community in a DPR-Phostrip process and found that P recovery did not affect the diversity of the microbial community. Tian et al. [20] used the high-throughput sequencing technology to identify the variation of the microbial community and functional microbes of a modified biofilter harvesting P in wastewater, and noticeable variations in key functional bacteria were observed. As high-throughput sequencing can provide information about activated sludge community diversity, including the identification of uncultured taxa, and characterization of low-abundance but environmentally important populations [25], it has been widely used for microbial community studies in wastewater treatment systems. With the high-throughput sequencing technology, detailed information on the microbial community structure and composition of the P recovery systems based on the activated sludge processes could be provided.

In this study, the applicability of the lab-scale AAO-SBSPR process and the operation strategy based on the mass balance of the process were further evaluated using real wastewater. The performance of contaminants removal efficiencies and P recovery was compared with the one using synthetic wastewater. Meanwhile, the influence of P recovery on the denitrifying P uptake was investigated. In particular, the volume of the anoxic tank of the AAO-SBSPR system was extended to further enhance the simultaneous N and P removals. Moreover, the influence of P recovery on microbial community structure, composition, and metabolic pathways was assessed using high-throughput sequencing technology. This study can provide an in-depth understanding of the performance of AAO-SBSPR process, and raise the prospects of its application in the full-scale wastewater treatment plant.

Section snippets

Wastewater

Both synthetic wastewater and real wastewater were adopted in this study. The synthetic wastewater contained 150 mg/L sodium acetate, 50 mg/L glucose L, 130 mg/L peptone , 90 mg/L NH4Cl, 31 mg/L KH2PO4, 123 mg/L MgSO4·7H2O, 125 mg/L CaCl2 and 1.0 mL mineral medium solution. The composition of the mineral medium can be found in [26]. The pH value of the synthetic wastewater was maintained around 7–8 by adding NaHCO3. The real municipal wastewater was taken from the grit chamber in a full-scale

Performance of P recovery

Fig. 2 presents P masses in the influent, effluent, excess sludge as well as recovered P mass in R1 and R2. During the beginning period of R1, the P mass entered the system exceeded the P mass flowed out of the system through effluent and excess sludge discharge. The same phenomenon was observed in phase II of R2 when the external carbon substrate was added. This may indicate that excessive P was stored intracellularly during the growth of PAOs and kept in the systems, which caused the

Conclusions

The applicability of a novel AAO-SBSPR process designed for simultaneous P recovery and nutrients removal was verified using real municipal wastewater. Based on the P mass balance of the process, the SRT of the AAO-SBSPR process was extended to increase the potential of P recovery and reduce the impact of P recovery on the P content of the activated sludge. In this condition, the population of PAOs in the AAO-SBSPR process was increased compared with that in the AAO process, and a metabolic

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 Fundamental Research Funds for the Central Universities (No. 22120190202), Shanghai Science and Technology Committee (No. 18230712100) and Chinese Key Special Program on the S&T for the Pollution Control and Treatment of Water Bodies (No. 2015ZX07306001-03).

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