Elsevier

Chemosphere

Volume 259, November 2020, 127444
Chemosphere

An efficient oxic-anoxic process for treating low COD/N tropical wastewater: Startup, optimization and nitrifying community structure

https://doi.org/10.1016/j.chemosphere.2020.127444Get rights and content

Highlights

  • Low-DO OA reactor enhanced N removal in treating low COD/N tropical wastewater.

  • Post-anoxic denitrification phase reduced the effluent NO3–N to below 0.3 mg/L.

  • The recommended operating HRT and SRT are 16 h and 20 days, respectively.

  • Nitrospira dominated the nitrifying community at long SRT operation (20 days).

  • qPCR revealed potentially novel comammox affiliated with Nitrospira in the reactor.

Abstract

In this study, we assessed and optimized a low-dissolved-oxygen oxic-anoxic (low-DO OA) process to achieve a low-cost and sustainable solution for wastewater treatment systems in the developing tropical countries treating low chemical oxygen demand-to-nitrogen ratio (COD/N) wastewater. The low-DO OA process attained complete ammonia removal and the effluent nitrate nitrogen (NO3–N) was below 0.3 mg/L. The recommended hydraulic retention time and sludge retention time (SRT) were 16 h and 20 days, respectively. The 16S rRNA sequencing data revealed that long SRT (20 days) encouraged the growth of nitrite-oxidizing bacteria (NOB) affiliated with “Candidatus Nitrospira defluvii”. Comammox made up 10–20% of the Nitrospira community. NOB and comammox related to Nitrospira were enriched at long SRT (20 days) to achieve good low-DO nitrification performance. The low-DO OA process was efficient and has simpler design than conventional processes, which are keys for sustainable wastewater treatment systems in the developing countries treating low COD/N wastewater.

Introduction

The Sustainable Development Goal (SDG) 6.3 aimed to reduce water pollution by 2030 through proper wastewater management. Despite the ongoing improvement, United Nations (2019) estimated that 700 million people in the developing countries still lack access to proper wastewater treatment system. The discharge of inadequately-treated wastewater into environment is a main cause of nitrogen pollution in rivers (Department of Environment, 2017). To overcome the nitrogen pollution, many developing countries in the Southeast Asia introduced effluent discharge limits of ammoniacal nitrogen (NH4–N) and nitrate nitrogen (NO3–N) for wastewater treatment plants (WWTPs) (Department of Environment, 2009; Ministry of Natural Resources and Environment, 2010). Following the enforcement of increasingly stringent effluent discharge limits, WWTPs are required to incorporate biological nitrogen removal process into their design.

The most common biological nitrogen removal process adopted by WWTPs is pre-anoxic denitrification, such as anoxic-oxic (AO) and anaerobic-anoxic-oxic (A2O) processes (Wang et al., 2015). The effluent from conventional pre-anoxic denitrification processes often contains unremoved nitrate (NO3) produced from nitrification (Baeza et al., 2004). A strategy to improve the NO3 removal in the pre-anoxic denitrification process is to recycle NO3 in the effluent to the anoxic tank using a mixed liquor recycle (MLR) stream (Baeza et al., 2004). The MLR strategy is suitable for treating domestic wastewater typically low in chemical oxygen demand-to-nitrogen ratio (COD/N) because the COD in the raw wastewater is first utilized for denitrification before being consumed in the oxic stage. However, the MLR flow rate is 1.5–3.0 times of the influent flow rate, which results in higher energy usage and operating costs (Cheng et al., 2017; Shen et al., 2019). The high operating cost may not be sustainable for developing countries with limited resources. To promote a more sustainable wastewater treatment systems in the developing countries, United Nations World Water Assessment Programme (2017) encouraged the use of low-cost system with simple and efficient design.

Several studies showed that the post-anoxic denitrification process has a potential to achieve high NO3 removal efficiency in low COD/N wastewater (Zhao et al., 2018; How et al., 2019; Gao et al., 2020). Zhao et al. (2018) and Gao et al. (2020) reported 95%–98% total nitrogen (TN) removal efficiency for low COD/N wastewater (3.5–4.4) in an anaerobic-oxic-anoxic post-anoxic denitrification system. Operating low-dissolved-oxygen anoxic-oxic-anoxic (low-DO AOA) reactor also significantly enhanced the NO3 removal efficiency by fourfold in low COD/N tropical wastewater (How et al., 2019). The low-DO AOA process eliminated the need for energy-intensive MLR stream in conventional AO process and used low-DO condition to reduce the aeration energy. How et al. (2019) suggested that the low-DO AOA process may be simplified into a low-DO oxic-anoxic (OA) configuration as 89% of the NO3–N was removed in the post-anoxic stage. The simplified process design may reduce the capital and operating costs required to achieve a low-cost wastewater treatment system advocated for developing countries. Nevertheless, study on the critical design and operating parameters, such as hydraulic retention time (HRT) and sludge retention time (SRT), in OA reactor is still lacking to maximize the nitrogen removal capacity and efficiency.

Some studies of nitrifying community in the tropical biological nitrogen removal system have found that Nitrospira-related NOB was the dominant nitrifiers in their systems (Yang et al., 2016; Cao et al., 2018; How et al., 2019). How et al. (2019) reported that nitrite-oxidizing bacteria (NOB) that thrive in low-DO condition, “Candidatus Nitrospira defluvii”, coexisted with Nitrospira affiliated with complete ammonia oxidizers (comammox) (van Kessel et al., 2015). However, quantification methods based on alpha-subunit of the ammonia oxygenase gene (amoA) are warranted to confirm the presence of comammox (Koch et al., 2019). A clearer understanding of the nitrifying microorganisms in the tropical wastewater treatment systems is important to provide operating conditions favorable for their growth.

To address the research gaps on the optimum operating conditions and functional nitrifying microorganisms in the post-anoxic process under tropical settings, we established a low-DO OA process to examine the long-term nitrogen removal performance. We then studied the effect of HRT and SRT on the nitrogen removal performance to optimize these operating conditions. In addition, we performed 16S rRNA amplicon sequencing and quantitative polymerase chain reaction (qPCR) targeting comammox amoA to investigate the nitrifying community structure.

Section snippets

Sampling of seed sludge and wastewater

We sampled seed sludge and influent from a WWTP, henceforth referred to as WWTP A, in Kuala Lumpur, Malaysia. The seed sludge and influent were collected from the return activated sludge line and raw wastewater after preliminary treatment of WWTP A, respectively. The plant is operating with extended aeration (EA) system combined with a pre-anoxic tank, while the preliminary treatment in WWTP A includes bar screen and aerated grit chamber. The raw wastewater characteristics were: total COD

Nitrogen removal performance in OA-1 parent sequencing batch reactor

OA-1 was set up to establish a low-DO OA process to treat the low COD/N tropical wastewater. The representative detailed profiles showing the evolution of NH4–N, NO3–N and DOC in a SBR cycle in P1, P2 and P3 are given in Fig. 1. The long-term reactor data on TSS, VSS, COD, NH4–N and NO3–N are provided as Supplementary Material Fig. S1. In P1, we operated OA-1 in a 3-h oxic phase and 8-h anoxic phase. The oxic:anoxic ratio of 3:8 resulted in an oxic HRT of 4 h. The steady-state TSS and VSS after

Conclusions

We successfully established a low-DO OA process to achieve efficient nitrogen removal from low COD/N tropical wastewater. The low-DO OA process produced low effluent NH4–N and NO3–N. The recommended HRT and SRT for the low-DO OA process were 16 h and 20 days, respectively. Long SRT (20 days) contributed to a stable low-DO nitrification performance by enriching NOB related to Ca. N. defluvii and comammox Nitrospira (Ca. N. nitrosa). The low-DO OA process design was simpler and more

CRediT authorship contribution statement

Seow Wah How: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Tadashi Nittami: Resources, Writing - review & editing, Visualization. Gek Cheng Ngoh: Resources, Writing - review & editing. Thomas P. Curtis: Conceptualization, Resources, Writing - review & editing, Funding acquisition. Adeline Seak May Chua: Conceptualization, Writing - review & editing, Supervision, Project administration, Funding acquisition.

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.

Acknowledgement

This study was funded by The Royal Society (United Kingdom), Academy of Sciences Malaysia (Malaysia) and Malaysian Industry-Government Group for High Technology (Malaysia) under Newton Advanced Fellowship (NA150341/BH152910/IF008-2016). The project was also partially funded by Ministry of Education (Malaysia) Fundamental Research Grant Scheme (FP047-2017A). We would like to acknowledge Indah Water Konsortium Sdn. Bhd. for facilitating the sampling activities. We thank Dr. Amy Bell from

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