On-site evaluation of the performance of a full-scale down-flow hanging sponge reactor as a post-treatment process of an up-flow anaerobic sludge blanket reactor for treating sewage in India

https://doi.org/10.1016/j.biortech.2015年07月01日5 Get rights and content

Highlights

  • •
    Process performance of a full-scale DHS reactor was evaluated over 1800 days.
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    The DHS reactor consistently produced effluent with a BOD of 6 mg L−1.
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    Sludge yield of the DHS was estimated to be 0.04 kg SS kg−1 COD removed.
  • •
    Power consumption of the UASB + DHS system was 0.82 kW h kg−1 BOD removed.
  • •
    UASB + DHS systems may be the most cost- and energy-efficient sewage treatment methods.

Abstract

A down-flow hanging sponge (DHS) reactor is a novel, unaerated, aerobic, biofilm reactor that is used to polish effluent received from an up-flow anaerobic sludge blanket (UASB) reactor for treating municipal sewage. A full-scale DHS reactor was constructed for post-treatment of a full-scale UASB reactor at a municipal sewage treatment plant in India. Performance of the DHS reactor was evaluated with respect to organic removal over 1800 days of continuous operation. The UASB + DHS system consistently produced effluent with chemical oxygen demand (COD), biochemical oxygen demand (BOD), and suspended solids (SS) values of 37, 6.0 and 19 mg L−1, on average, respectively. The sludge yield of the DHS reactor was estimated to be 0.04 kg SS kg−1 COD removed or 0.12 kg SS kg−1 BOD removed, which is considerably lower than other aerobic treatment methods that have been employed for polishing UASB effluent.

Introduction

Application of up-flow anaerobic sludge blanket (UASB) technology to treat municipal sewage is increasingly being regarded as a viable sewage treatment option in developing countries such as India, Brazil, Colombia and Egypt (Draaijer et al., 1992, Schellinkhout and Collazos, 1992, Schellinkhout, 1993). This is primarily due to the relatively low construction, operation and maintenance costs associated with these systems, as well as the relatively small physical footprint and low sludge production (Uemura and Harada, 2000). However, sewage treatment by a UASB reactor alone was insufficient for meeting the effluent discharge standards of any countries.
Based on this, the Indian government commissioned the installation of final polishing units (FPUs), for post-treatment of effluent at UASB-based sewage treatment plants (STPs) in India. Although each FPU was designed to have a hydraulic retention time (HRT) of 1 day, none of these UASB + FPU systems produced effluent that met the required discharge standards (Sato et al., 2006). Furthermore, the large land area required to install an FPU makes this solution impracticable in the metropolitan areas of developing countries, such as India, which are currently experiencing rapid urbanization. The need therefore exists to develop more efficient post-treatment technologies that have a smaller physical footprint.
In an attempt to develop such a post-treatment process, Harada and his colleagues have conducted extensive pioneering research in this area. The result of their efforts over the last 15 years is a new aerobic, post-treatment process referred to as a down-flow hanging sponge (DHS) reactor (Tandukar et al., 2007, Onodera et al., 2014). The unique characteristic of the DHS reactor is that the design employs a simple polyurethane sponge as a medium for retaining active biomass/sludge. The concept of the DHS is based on the conventional trickling filter (TF). The sponge module contains a series of sponge media in the column section of the reactor. As the UASB effluent, which is supplied to the top of the DHS reactor, trickles down through the sponge module, extensive contact occurs between the wastewater and the retained biomass. No external aeration is required for the operation of the DHS since the wastewater is naturally aerated as it flows through the DHS reactor.
Bench-scale experiments of the DHS process have been conducted for more than 15 years in Japan, and several different sponge modules have been developed and analyzed to date. Our activities related to the development of DHS technology prompted the Indian government to construct a full-scale DHS plant for post-treatment effluent processing at the Karnal 40 million liters per day (MLD) STP (1 MLD = 1000 m3 day−1).
This study examined the performance of the full-scale DHS reactor, which has been in operation for more than 1800 days. For comparison, the performance of an existing FPU was also evaluated. In addition, the feasibility of the DHS reactor as a post-treatment process for UASB-treated sewage was examined and compared to other aerobic processes, with particular emphasis on sludge production, land requirements and power consumption. Thus, this is an epoch-making report for the performance evaluation of the first full-scale plant with a DHS reactor in the world.

Section snippets

Karnal 40 MLD STP/existing full-scale UASB reactors

An on-site evaluation of the UASB + DHS system for the treatment of municipal sewage was conducted at the Karnal 40 MLD STP located in Karnal, Haryana State, India. The general layout of the Karnal 40 MLD STP, including the four full-scale UASB reactors, is shown in Fig. 1. All of the reactors were the same size, measuring 32 m (length) ×ばつ 24 m (width) ×ばつ 5.51 m (depth, effective water level: 5.01 m) with a design capacity of 10,000 m3 day−1 (equivalent to a population of 70,000 inhabitants per UASB

Comparison of the performance of the DHS reactor and existing FPU

To compare the performance of the proposed system, i.e., the UASB + DHS system, with the existing system, i.e., the UASB + FPU system, effluent samples were also collected from the FPU for analysis.
Fig. 3(A) shows the changes in unfiltered COD of the raw sewage, UASB, FPU and DHS effluent samples, while Fig. 3(B) shows the percentage COD removal by the UASB reactor (calculated based on COD values of raw sewage vs. COD values of UASB effluent), by the UASB + FPU system (raw sewage vs. FPU effluent)

Conclusions

The performance of a full-scale DHS reactor was evaluated over 1800 days of continuous operation at 1.5 h of HRT. The DHS reactor exhibited excellent performance for removal of organic and particulate matter, sludge production and energy consumption. In conclusion, it was verified that, of the various post-treatment systems that have been coupled with UASB reactors used to treat municipal sewage, the DHS reactor is capable of treating organic and particulate matter at much shorter HRT than any

Acknowledgements

This study was supported in part by Research Grants from the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Japanese Ministry of Land, Infrastructure, Transport and Tourism (MLIT), Japan Society for the Promotion of Science (JSPS) and the Science and Technology Research Partnership for Sustainable Development (SATREPS).

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