Enhanced wastewater treatment with high o-aminophenol concentration by two-stage MABR and its biodegradation mechanism
Graphical abstract
Introduction
Aminophenol is an important chemical intermediate in various industries, such as medicine synthesis, dye, printing, and photography. Unfortunately, considerable amounts of aminophenol wastewater are generated, which may result in serious environmental pollution and affect human health even at low concentration. For instance, aminophenol may cause allergic reactions on the eyes, skin, and respiratory system. Given its high toxicity and refractory, aminophenol has been assigned as a priority pollutant by the US and European Environmental Protection Agencies (Directive 2008/105/EC, 2008). Therefore, the effective removal of these organic compounds from aquatic environments has attracted public attention.
o-Aminophenol (OAP) is a typical aminophenol. Physicochemical methods, such as adsorption (Gupta et al., 2006) and advanced oxidation processes (Chen et al., 2013, Li et al., 2014), have been widely introduced to achieve rapid OAP removal from wastewater. However, these technologies require complicated processes and inevitably result in secondary pollution and high costs. Although OAP can be toxic and inhibit certain microbial consortium, multiple microorganisms can mineralize or convert the compound to byproducts with relatively low hazard or toxicity (Afzal Khan et al., 2006, Saccomanno et al., 2018). Thus, biological methods can treat this wastewater and are environmentally friendly and cost-effective. Membrane-aerated biofilm reactor (MABR) has been successfully applied to degrade toxic xenobiotics in industrial wastewater, such as pharmacy (Celik et al., 2018, Wei et al., 2012) and coking wastewater (Lan et al., 2018). In these systems, the membrane serves as both bubble-less gas provider and microorganism carrier. The former role leads to high oxygen utilization rate and reduces the operating costs and the emissions of volatile organic compounds (Syron et al., 2015); the latter facilitates abundant microorganisms to colonize on the rough membrane surface and then form counter diffusional biofilms (Kinh et al., 2017). These biofilms consists of a stratification community embedded within a tight self-produced matrix of EPS (Pellicer-Nacher et al., 2014), which may possess high resistance to toxicity and load shock. Meanwhile, nitrifying populations prefer to inhabit in regions close to the membrane surface that are characterized by high DO and low organic compound contents, while denitrifying populations are predominant in the biofilm regions far from the membrane that are characterized by low DO and high organic compound contents. Consequently, MABR can also simultaneously remove C and N pollutions (Kinh et al., 2017, Semmens et al., 2003).
MABR has been applied to achieve the effective disposal of p-nitrophenol wastewater (Mei et al., 2018), thereby suggesting its potential to biodegrade phenolic compounds. However, few focuses are available on its application for OAP wastewater treatment, especially at high concentrations. In most cases, the biodegradation of phenolic contaminates at high concentration requires anaerobic and aerobic bioprocesses. Anaerobic acidification can enhance phenolic biodegradability and reduce their toxicity to microorganisms, whereas aerobic oxidation considerably guarantees further mineralization (Huang et al., 2016, Mei et al., 2018). The biofilms in MABR possess anaerobic and aerobic zones, which provide evident ecological niches for anaerobe and aerobes, respectively (Nerenberg, 2016). Thus, MABR may be beneficial for OAP wastewater disposal. In addition, the complete degradation of this wastewater involves the advanced removal of N metabolites derived from aminophenol breakdown. Since nitrifiers and denitrifiers are easily poisoned and inhibited by phenolic compounds (Bajaj et al., 2010, Kim et al., 2006), a single-module MABR hardly achieves OAP and contained N removal. Two or more than two stages MABR might approach the efficient treatment for high-OAP-concentration wastewater. Takenaka et al., 1998, Takenaka et al., 2000, Chirino et al., 2013 reported the metabolic mechanism for OAP in a single bacterial strain, such as Pseudomonas sp. and Burkholderia xenovorans. In most cases, phenolics biodegradation in wastewater treatment is always involved in the presence of syntrophic bacteria (Jiang et al., 2016, Zhou et al., 2018). The phenolic degradation function in MABR can be speculated based on related bacterial community structures (Mei et al., 2018), but it is not sufficient to obtain the information on overall phenolic metabolism pathways. Metagenomic sequencing can be applied to explore and compare the presence of key microbial taxa and associated functional genes deeply, thereby intensively revealing OAP degradation mechanism in MABR.
In this work, a laboratory-scale two-stage MABR system was designed to strengthen the treatment of synthetic OAP wastewater with high concentrations. MABR-1 was designed to achieve OAP removal, while MABR-2 was used to perform advanced treatment for N pollution from MABR-1 effluent. The combination of 16S rDNA high-throughput and metagenomic sequencing was used to evaluate the biological process in biofilms. The objectives of this work were to i) evaluate the feasibility of treating wastewater with high OAP concentration through the MABR technology; ii) illustrate the effect of the increased OAP load on microbial structure and function; iii) clarify the OAP removal mechanism in biofilms.
Section snippets
Reactor configurations and experimental design
The applied setup in the study consisted of two separate bench-scale bioreactors named MABR-1 and MABR-2 with similar construction and identical hollow fiber membrane module (Fig. 1 and Table 1, provided by Hydroking Sci. & Tech. Ltd., Tianjin, China). Each reactor had an effective volume of 9 L, where the membrane module was submerged in the chamber. Air was pumped into the membrane by a gas compressor and controlled using a gauge and a flowmeter. Every bioreactor was operated with periodic
Two-stage MABR system performance
In MABR-1, the influent OAP concentration increased from the average of 203 mg/L in stage 1 to 1179 mg/L in stage 4, whereas the influent COD concentration increased from 367 mg/L to 2158 mg/L. As shown in Fig. 2b, the effluent OAP concentration was mostly maintained at less than 5.0 mg/L, thereby corresponding to the high OAP removal ratio of more than 99%. Meanwhile, the specific OAP removal rates (Fig. 2a) at the end of each stage were 7.0 (1-S1), 12.7 (1-S2), 13.7 (1-S3), and 17.6 g/m2·d
Conclusions
Two-stage MABRs process was developed to investigate OAP degradation efficiency and mechanism in wastewater. The first MABR achieved high phenolic removal rate and effective organic mineralization, while the second MABR performed advanced N removal from the effluent in the first MABR. We also further evaluated the specific degradation pathways through metagenome data and indicated that multiple species can contribute to the initial catabolism toward OAP through collaboration, which may ensure
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No.31700105), Major Scientific and Technological Special Project of Henan province (No. 181100211400-8-2), the Fundamental Research Funds for the Provincial Universities (No. 2017QNJH12), the Key Scientific Research Project of Universities in Henan Province (No. 17B610005) and the Doctoral Scientific Research Start-up Foundation from Henan University of Technology (No. 2016BS004).
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