Large pilot-scale submerged anaerobic membrane bioreactor for the treatment of municipal wastewater and biogas production at 25 °C
Introduction
As the most common type of wastewater sourcing from our daily life, municipal wastewater is characterized by its low organic strength and high organic particulate matter (Gouveia et al., 2015, Ozgun et al., 2013). The conventional activated sludge (CAS) process and its derivatives or upgrades are commonly used in the treatment of municipal wastewater and have been widely implemented all over the world (Gurung et al., 2016, Kong et al., 2016). Despite its reliable effluent quality and treatment capacity, the CAS process involves high energy consumption for aeration and a costly process for the minimization of excessive sludge (Martinez-Sosa et al., 2011). The more sustainable and cost-effective anaerobic digestion (AD) process has been widely applied in the treatment of wastewater and biomass for decades, with a high bio-energy recovery and low biomass yield among its advantages (Kong et al., 2018, Li et al., 2019).
Typically, AD process for the high-strength wastewater or solid wastes requires energy expenditure for the heating of the digester because the anaerobic digested sludge exhibits the highest methanogenic activity under either the mesophilic (35 °C) or thermophilic (55 °C) condition, and the biogas recovered from high-strength wastewater or wastes is sufficient for heating the digester (Arras et al., 2019, Kong et al., 2020). However, for one thing, since municipal wastewater is typified by its low COD concentration, only a small amount of biogas can be recovered from municipal wastewater, suggesting that the treatment of municipal wastewater should avoid reactor heating and operate under a natural ambient condition of 20 – 30 ℃. For another, the limited activity is considered sufficient for the treatment of low-strength municipal wastewater at low temperatures in temperate regions (Martinez-Sosa et al., 2011). However, the biggest challenge to the anaerobic treatment of municipal wastewater is associated with the low COD concentration in huge wastewater quantities: a short hydraulic retention time (HRT) (Farajzadehha et al., 2012). To handle millions of tons of municipal wastewater every day, wastewater treatment plants equipped with the CAS process require a short HRT. It should be noted that a very short HRT and sludge retention time (SRT) in the high-rate activated sludge (HRAS) systems usually results in a limited floc formation, causing sludge wash-out and insufficient removal of COD (Chuang et al., 2011, Majewsky et al., 2011). It is reasonable to assume the anaerobic treatment of municipal wastewater would be more difficult and the associated problems would be more serious because the degradation of high-strength organic matter in the AD process tends to require a long HRT and long SRT (Jahn et al., 2016, Zahedi et al., 2018). The anaerobic membrane bioreactor (AnMBR) has recently been considered as a viable alternative for the wastewater treatment (Kong et al., 2019b, Li et al., 2020). Compared with CAS process, wastewater treatment equipped with the AnMBR also has such economic advantages: 1) bio-energy recovered from wastewater helps lowering the expenditure of electricity; 2) the expenditure of constructing precipitation tank and land occupation can be completely saved (Shahid et al., 2020).
In recent year, there have been growing interest in applying AnMBR to the anaerobic treatment of low-strength municipal wastewater at low temperatures (Lim et al., 2019, Maleki et al., 2019, Schmitt et al., 2018, Yang et al., 2020). However, the majority of the studies were based on small lab-scale experiment with the feed of synthetic wastewater (Harb et al., 2015, Martin Vincent et al., 2018). The feasibility of AnMBR should be qualified by a large pilot-scale engineering demonstration and fed with real municipal wastewater (Damodara Kannan et al., 2020, Evans et al., 2019, Foglia et al., 2020, Robles et al., 2020). As yet, although there is a growing number of studies focusing on the treatment of municipal wastewater by AnMBR, only a few limited studies have been focused on the small pilot-scale AnMBR (Foglia et al., 2020, Giménez et al., 2011, Robles et al., 2013), and the operational volumes of the reactors have been small and the investigation has been limited to simple indicators even though some studies realized the operation at a temperature below 20 ℃ (Lim et al., 2019, Maleki, 2020, Mei et al., 2018).
In this study, a large pilot-scale submerged AnMBR with an operational volume of 5.0 m3 (5000 L) was constructed in a local wastewater treatment plant to investigate and evaluate the long-term behavior and feasibility of the anaerobic treatment of real municipal wastewater: 1) With the bulking of effective volume, this reactor is the largest one-stage submerged AnMBR that has ever been reported, and the results obtained from this pilot-scale one are more convincing. 2) The long-term investigation of the AnMBR includes many indicators like large-size impurities, influent SS, mass balance, membrane permeating and online membrane cleaning. 3) This large pilot-scale AnMBR was a successful demonstration of high COD removal, stable biogas recovery and energy saving in municipal wastewater treatment, also providing reliable experience and guidance for the future engineering application of AnMBR.
We investigated the performance of the large pilot-scale AnMBR during 217 days of continuous operation for the anaerobic treatment of real municipal wastewater. The objectives of this study were: 1) to demonstrate the long-term performance of the large pilot-scale AnMBR fed with real municipal wastewater at an ambient temperature of 25 °C; 2) to investigate the feasibility and stability of the large pilot-scale AnMBR with the periodical shortening of HRT from 48 h to 6 h; 3) to evaluate the mass balance during the long-term operation of the large pilot-scale AnMBR; 4) to further explore the obstacles and challenges which emerge during the long-term operation.
Section snippets
Sampling method and procedure
Biogas production was recorded by a wet tip gas meter (Shinagawa, Japan) connected to the biogas outlet pipeline of the AnMBR (after the water seal tank and the desulfurization tank). The real gas temperature (°C) and atmosphere pressure (hPa) were recorded by a thermometer placed outside the AnMBR. The volume of biogas was standardized under the condition of 273.15 K and 1013.25 hPa for calculation. The components of the biogas including N2, CH4 and CO2, were detected by a gas chromatograph
Characteristics of raw municipal wastewater
It should be noted that municipal wastewater can be treated by the AnMBR only after the large-size particles are eliminated by the grille system (screen pore size of 1 cm) in the raw wastewater tank. These large-size impurities contain sands, rocks, hairs, plastics, and many other wastes. After being screened by the grille, the waste was abandoned outside the raw wastewater tank to avoid damaging the membrane module or placing an unnecessary burden on it, and was then collected for further
Conclusions
The large pilot-scale AnMBR realized a low HRT of 6 h at 25 °C, obtaining a COD removal efficiency over 90% and a BOD5 removal over 95% with a biogas yield of 0.25–0.27 L g−1 COD removed. 60%-64% of influent COD (>250 mg L-1) converted to methane and 20%-26% of that converted to MLSS (>80 mg L-1), while 10%-16% of influent nitrogen was taken by MLSS. The AnMBR realized stable membrane permeating at the HRTs of 24 h, 12 h and 8 h with a flux of 4.43–15.05 LHM and a △TMP variation of 0.9–12.7 kPa.
CRediT authorship contribution statement
Zhe Kong: Conceptualization, Methodology, Investigation, Data curation, Writing - original draft, Writing - review & editing. Jiang Wu: Investigation, Data curation. Chao Rong: Investigation, Data curation. Tianjie Wang: Investigation. Lu Li: Methodology, Data curation. Zibin Luo: Investigation. Jiayuan Ji: Methodology. Taira Hanaoka: Formal analysis, Investigation. Shinichi Sakemi: Investigation. Masami Ito: Investigation. Shigeki Kobayashi: Formal analysis. Masumi Kobayashi: Investigation. Yu
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.
Acknowledgements:
This work is a part of a national research project entitled innovative sewage treatment system for energy saving and energy production supported by the Low Carbon Technology Research and Development Program, the Ministry of the Environment, Japan.
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