Research article
Evaluation of the resilience of a full-scale down-flow hanging sponge reactor to long-term outages at a sewage treatment plant in India

https://doi.org/10.1016/j.jenvman.2016年06月05日8 Get rights and content

Highlights

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    A full-scale down-flow hanging sponge (DHS) reactor functions successfully in India.
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    We subjected this reactor to a 10-day outage to evaluate its resilience.
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    SS, COD, and NH4+ contents were good 90, 45, and 90 min after restart, respectively.
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    Only 0.1% of the total biomass held in the DHS reactor was eluted after restart.
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    This DHS can withstand outages and achieved steady state soon after restart.

Abstract

Resilience to process outages is an essential requirement for sustainable wastewater treatment systems in developing countries. In this study, we evaluated the ability of a full-scale down-flow hanging sponge (DHS) reactor to recover after a 10-day outage. The DHS tested in this study uses polyurethane sponge as packing material. This full-scale DHS reactor has been tested over a period of about 4 years in India with a flow rate of 500 m3/day. Water was not supplied to the DHS reactor that was subjected to the 10-day outage; however, the biomass did not dry out because the sponge was able to retain enough water. Soon after the reactor was restarted, a small quantity of biomass, amounting to only 0.1% of the total retained biomass, was eluted. The DHS effluent achieved satisfactory removal of suspended solids, chemical oxygen demand , and ammonium nitrogen within 90, 45, and 90 min, respectively. Conversely, fecal coliforms in the DHS effluent did not reach satisfactory levels within 540 min; instead, the normal levels of fecal coliforms were achieved within 3 days. Overall, the tests demonstrated that the DHS reactor was sufficiently robust to withstand long-term outages and achieved steady state soon after restart. This reinforces the suitability of this technology for developing countries.

Introduction

Sustainable operation of sewage treatment facilities is a challenge in developing countries, where budget shortages, a scarcity of land on which to construct facilities, untrained technical staff, unreliable facilities, and/or unstable electrical power supplies are commonplace. In recent years, upflow anaerobic sludge blankets (UASB) have been widely used for sewage treatment in developing countries because of their simple operation and maintenance, low running costs, and high efficiency (Gomec, 2010, Monroy et al., 2000, Sato et al., 2006). However, because it does not attain the discharge standards set by most developing countries, the effluent from the UASB process needs additional treatment to improve its quality (Foresti, 2002, Haandel and Lettinga, 1994). Several conventional biological processes, such as polishing ponds (stabilization ponds), activated sludge, and trickling filters, have been applied as post-treatments for UASB effluent (Chernicharo et al., 2015, Chong et al., 2012, Kassab et al., 2010). Although much effort has been devoted to developing these post-treatment processes, the performance and applicability of the majority have not been verified at the full-scale (Chan et al., 2009, Kassab et al., 2010, Khan et al., 2011).
In an attempt to advance this situation, a post-treatment process called the down-flow hanging sponge (DHS) has been implemented in a full-scale experiment in India (Okubo et al., 2015, Onodera et al., 2016). The DHS is recognized as a promising post-treatment process (Kassab et al., 2010). It is somewhat similar to the trickling filter system, but it uses a sponge as an alternative support material (Agrawal et al., 1997, Machdar et al., 1997, Onodera et al., 2013, Tawfik et al., 2006b). The quality of the effluent from the DHS process is comparable to that of the effluent produced by the activated sludge process (Tandukar et al., 2007). In addition, because backwashing is unnecessary, the operation and maintenance of the DHS are simpler than for the trickling filter process (Onodera et al., 2016, Tandukar et al., 2005). While macrofauna overgrazing on the biomass is a long-standing problem in trickling filter systems (Daigger and Boltz, 2011), the DHS is able to prevent snail overgrazing on the sludge retained inside the sponge media (Onodera et al., 2015b). During the pilot-scale experiment, a DHS reactor with a flow rate of 50 m3/day showed better treatment performance, and used approximately 75% less energy and produced 85% less excess sludge, than the conventional activated sludge process (Tanaka et al., 2012). A full-scale experiment of the DHS reactor with a flow rate of 500 m3/day has been successfully run for more 1800 days at a sewage treatment site in India. This experiment confirms that, because of its simple operation and maintenance, low energy requirements, and low production of excess sludge, the DHS is a practical and suitable technology for developing countries (Okubo et al., 2015, Onodera et al., 2016).
To guarantee successful operation over the long-term in developing countries, the system must be sufficiently robust to cope with process outages. Process outages are often caused by facility inspections, handling accidents, and power cuts, and may last for anything from between a few hours to a number of days. In our experience, the treatment facility is sometimes interrupted at the site. In such situations, the sewage treatment system is exposed to starvation conditions, and there is concern that these conditions can cause the process to collapse. For instance, studies have reported a serious decrease in the performance of the activated sludge process because of anaerobic conditions (Kim et al., 2007, Urbain et al., 1993). Even under aerobic conditions, the amount of biomass, sludge activity, and the number of active cells in activated sludge decreased during a starvation period (Oviedo et al., 2003). The main concern for the trickling filter process is biofilm dry-out or detachment. Therefore, the treatment facilities need to be sufficiently robust to survive after outages, and facilities need to be able to both avoid process collapse and reach steady state rapidly once restarted. To our knowledge, to date, no studies have evaluated the robustness of these secondary treatment procedures in municipal treatment systems in developing countries.
The aim of this study is to evaluate the robustness of a full-scale DHS reactor to a process outage. The DHS reactor was stopped for 10 days, during which it had no water supply or rainfall. Outages may have various detrimental impacts on the process performance; for example, the retained sludge may dry out and sludge may be lost from the DHS reactor after restart. Further, the biomass may be subjected to starvation and anaerobic conditions, and the amount and activity of the biomass may decrease, leading to inadequate performance when the system is restarted after the outage. Therefore, during this study, we attempted to calculate the water retention in the sponge media during the outage, the amount of sludge lost after the restart, and the time taken for recovery of the performance on removing suspended solids (SS), chemical oxygen demand (COD), ammonium nitrogen, and fecal coliform.

Section snippets

Operating conditions of the full-scale DHS reactor

The full-scale DHS reactor is in operation at the sewage treatment plant in Karnal, India. The plant consists of a UASB followed by a polishing pond. The hydraulic retention time (HRT) of the UASB reactor is approximately 13 h. Part of the UASB effluent is used as the influent for the DHS reactor. Detailed information about the sewage treatment facilities and operating conditions have been reported elsewhere (Okubo et al., 2015). A picture and a schematic diagram of the DHS reactor are provided

Process performance under steady state conditions

The full-scale DHS reactor had been in operation for about four years before the 10-day starvation experiment at the sewage treatment plant. Under normal operating conditions, the TSS, biological oxygen demand (BOD), COD, ammonium nitrogen, and fecal coliform contents in the effluent taken from before the clarifier in the DHS reactor were reported as 19 mg/L, 6 mg/L, 37 mg/L, 6 mg/L, and 1.57 ×ばつ 105 MPN/100 ml, respectively. The process performance has been reported elsewhere (Okubo et al., 2015

Conclusion

This study confirmed that long-term starvation had a negligible influence on the performance of the full-scale DHS reactor. The effluent quality recovered rapidly after the 10-day outage, within 90 min for TSS, 45 min for COD, 90 min for ammonium nitrogen, and within 3 days for fecal coliforms. Only 0.1% of the total biomass retained in the DHS reactor was lost when the system was restarted. Overall, the DHS is sufficiently robust to survive extended outages, and so can be considered as a

Acknowledgements

The authors would like to acknowledge support and assistance of various forms from the National River Conservation Directorate, Ministry of Environment, Forest and Climate Change, Indian Government, and the Water Supply and Sanitation Department of Haryana State. This study was partially supported by research grants from the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japanese Ministry of Land, Infrastructure, Transport and Tourism (MLIT), Japan Society for

References (31)

  • T. Onodera et al.

    Spatial changes in carbon and nitrogen stable isotope ratios of sludge and associated organisms in a biological sewage treatment system

    Water Res.

    (2015)
  • T. Onodera et al.

    Characterization of the retained sludge in a down-flow hanging sponge (DHS) reactor with emphasis on its low excess sludge production

    Bioresour. Technol.

    (2013)
  • T. Onodera et al.

    Long-term performance evaluation of down-flow hanging sponge reactor regarding nitrification in a full-scale experiment in India

    Bioresour. Technol.

    (2016)
  • T. Onodera et al.

    Development of a sixth-generation down-flow hanging sponge (DHS) reactor using rigid sponge media for post-treatment of UASB treating municipal sewage

    Bioresour. Technol.

    (2014)
  • N. Sato et al.

    Prospects for a self-sustainable sewage treatment system: a case study on full-scale UASB system in India’s Yamuna river basin

    J. Environ. Manag.

    (2006)

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      DHS reactors have been used for post-treatment of the anaerobic reactors that treat sewage and industrial wastewater (Fujihira et al., 2018; Onodera et al., 2013; Watari et al., 2015). DHS reactor's ease of maintenance and ecological sustainability have also been confirmed by long-term, full-scale sewage treatment experiments (Maharjan et al., 2018; Onodera et al., 2016). In both a full-scale sewage treatment experiment in India and a pilot-scale natural rubber (NR) wastewater treatment experiment in Vietnam, DHS reactors achieved higher effluent qualities with shorter hydraulic retention times (HRTs) than existing lagoon systems for anaerobic reactor post-treatment (Okubo et al., 2015; Watari et al., 2017a).

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