A pilot-scale study of a down-flow hanging sponge reactor as a post-treatment for domestic wastewater treatment system at short hydraulic retention times
Graphical abstract
Introduction
Rapid urbanization in Southeast Asian countries puts stress on water supply and sewage treatment. Some urban areas in Bangkok, Thailand, have observed the deterioration of surface water quality caused by urbanization. The National Environment Board in Thailand categorized surface water quality into five classes, from Class 1 (very good) to Class 5 (very poor). Most canals in Bangkok have poor water quality, exceeding the standard values for biochemical oxygen demand (BOD), ammonium nitrogen (NH4-N), and total coliform at 4.0 mg/L (Class 4), 0.5 mg/L (Class 2–4), and 2 × 104 MPN/100 mL (Class 3), respectively [1]. A clustering analysis of canal water quality based on the public data from 2005 to 2008 by the Department of Drainage and Sewerage-Bangkok Metropolitan Administration (DDS-BMA) summarized that the BOD, NH4-N, and total coliform of canals were 6.1–40.9 mg/L, 0.8–9.4 mg/L, and 1.5 × 106–6.4 × 109 MPN/100 mL, respectively [2]. The major reason for poor surface water quality is insufficient wastewater treatment facilities, so the direct discharge of domestic wastewater to nearby surface water is likely to happen. Only 45 % of generated wastewater is currently treated by eight large-scale centralized wastewater treatment plants (WWTPs) in Bangkok [3]. Considering the socio-economic impacts of installing sanitation systems, decentralized systems have several advantages compared to centralized systems, i.e., pipeline construction to collect wastewater, low installation and maintenance costs, and on-site reuse of treated wastewater [4]. Noticeably, the existing decentralized WWTPs employ lower treatment performance because the Building Effluent Standard [5] is not strict (i.e., BOD < 20 mg/L, TKN < 35 mg/L, no restriction for fecal contamination). In addition, some decentralized WWTPs have exceeded the Building Effluent Standard for nitrogen in our investigation (unpublished data). Therefore, the remaining nitrogen in the effluent of decentralized WWTPs should be removed to protect the water environment.
It should be noted that Bangkok's water demand will rapidly increase annually [6]. The direct reuse of effluent from WWTPs is a sustainable solution to water shortage; for example, 1.7 to 28.7 % (v/v) of treated effluent from eight centralized WWTPs in Bangkok has been utilized for road washing, green belt irrigation, temple cleaning, etc. [7]. The effluent of decentralized WWTPs can also be regarded as a water reuse source [8], [9], if the water quality of decentralized WWTPs' effluent achieves at least the criteria for full-scale WWTPs' effluent (SS < 30 mg/L, BOD < 20 mg/L, NH4-N < 5 mg/L, no restriction for fecal contamination). Recently, ISO provided “Guidelines for decentralized/on-site water reuse system” [4]. Although the disinfection process is recommended in these guidelines for safe water reuse, WWTPs in Bangkok do not have a disinfection process (e.g., chlorination). Therefore, an appropriate post-treatment technology that can reduce fecal contaminants is required to facilitate water reuse. In a densely populated city like Bangkok, where there is limited area for construction, the post-treatment process must be compact; namely, it should be able to operate under shorter hydraulic retention time (HRT) conditions.
In the past few decades, a down-flow hanging sponge (DHS) reactor has been developed as a post-treatment of anaerobically pretreated sewage and pre-settled sewage (100–300 mgBOD/L) treatment using several types of sponge media [10]. The DHS reactor was also applied for the treatment of low-strength sewage (around 20 mgBOD/L), achieving higher BOD removal (> 85 %) in the tropical region [11], [12]. The favorable points of the DHS reactor are energy-saving, high maintainability, and practical easy-contract installation [13], [14]. The DHS reactor also benefits from easy maintenance and operation [10], which is free of tricky issues like membrane fouling in membrane bioreactor processes or sludge bulking in activated sludge processes. Therefore, the DHS reactor may have good potential as the post-treatment technology for WWTPs' effluent.
Our previous study implemented a pilot-scale DHS reactor in Bangkok to treat raw domestic wastewater under HRT of 4 and 5 h. It was successfully applied as a decentralized treatment system [15]. This study is a follow-up investigation of this DHS reactor's treatment capacity to treat low-strength wastewater (i.e., simulated effluent from other treatment processes) under shorter HRT conditions (1−3 h) for over 1000 days. Furthermore, this study also provides new insights into the DHS reactor's removal efficiencies of fecal contaminants by monitoring the numbers of Escherichia coli as an indicator of fecal contaminants and the numbers of Arcobacter spp., which are potentially numerous pathogenic bacteria in domestic wastewater [16].
Section snippets
DHS reactor operation
The pilot-scale DHS reactor used in this study continuously employed the same reactor used in our previous study [15], but changed from raw domestic wastewater (RDW) treatment to the post-treatment in this study. Briefly, the DHS reactor was installed at a small-scale WWTP in the Bongai community of Bangkok to treat effluent from a conventional activated sludge process (AS) receiving RDW (separated sewer system, 400 m3/day). The HRTs of the aeration tank and sedimentation tank of the AS were
DHS reactor performance
Although the effluent quality of the small-scale conventional AS effluent of Bongai's WWTP (i.e., influent of the DHS reactor in phase 1) reached the criteria for full-scale WWTPs' effluent (SS < 30 mg/L, BOD < 20 mg/L), its ammonium nitrogen concentration did not meet the regulated value (NH4-N < 5 mgN/L; Table 1) [22].
In phase 1, the DHS reactor performed the post-treatment of Bongai's WWTP, and the remaining BOD and ammonium were further oxidized by the DHS reactor to meet the full-scale
Conclusions
In this study, a pilot-scale DHS was successfully demonstrated as a post-treatment of existing WWTP by treating several low-strength wastewaters under HRT of 13 h. The retained sludge activities could be compatible with the respective operational conditions. The DHS consistently produced superior water quality (TSS < 10 mg/L, BOD <10 mg/L, NH4-N < 5 mg/L, and E. coli < 103 CFU/mL), and Arcobacter spp., a potentially numerous and pathogenic bacterial group, was removed with > 2-log10 reduction.
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Kazuaki Syutsubo reports financial support was provided by National Institute for Environmental Studies.
Acknowledgements
We appreciate the Department of Drainage & Sewage, Bangkok Metropolitan Administration for permission to conduct the pilot-scale study. We thank J. Temeeyakul, M. Phumrakchart, M. Sadsara, C. Yensuang, N. Boonprom, R. Jantajamraspanya, T. Pattanasedtakarn of Kasetsart University, and Sakai K. of Nagaoka University of Technology, for operation and maintenance of the DHS reactor. We also thank Inoue Y., Ito S., Arima S. of our lab for analytical support. This study was financially supported by
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