Dissolved methane recovery from anaerobically treated wastewaters using solvent-based membrane contactor: An experimental and modelling study
Graphical abstract
Introduction
Anaerobic wastewater treatment has been receiving increasing attention due to its advantages over the aerobic treatment process, including lower energy consumption, reduced sludge production and generation of methane as a renewable energy source [1], [2], [3]. Moreover, the recent development of low-temperature anaerobic treatment processes is promising to eliminate the need for heating the wastewater (to ~35 °C), which is required for conventional anaerobic treatment [4], [5], and therefore to extend the implementation of anaerobic processes to geographical locations with temperate climates [6]. For instance, methane production by anaerobic treatment at low temperatures has been demonstrated through process optimization of upflow anaerobic sludge blanket reactors [7], expanded granular sludge blanket reactors [8], membrane bioreactors [9], [10], and through enhancement of microbial kinetics [11], [12].
In most anaerobic processes, methane is obtained as biogas in the head space of the reactor, a desorbed gas from the anaerobically treated effluent. However, biogas recovery from anaerobic effluents at low temperatures remains a challenge due to the increased solubility of methane at lower temperatures [13]. Additionally, methane is typically supersaturated in AnMBR effluents (saturation index 1–1.5) and in UASB effluents with saturation index up to 6.9 [14], [15]. As a result, dissolved methane can account for >45% of the methane produced at 30 °C [13] and consecutively increase to higher fractions at lower temperatures [16], [17], [18]. This dissolved methane can be released to the atmosphere upon discharge of the effluent to the environment. As a potent greenhouse gas, capturing the dissolved methane is crucial to mitigating global warming as well as harvesting it in the form of renewable energy from a waste source.
Conventional processes are available to achieve low concentration levels of dissolved methane, including biological oxidation [19], [20], [21] and gas stripping [15], [22]. In addition to high energy consumption or operation costs, however, these processes are generally intended for removal of the dissolved methane gas, not its recovery. Generation of electricity from dissolved methane using microbial fuel cells has also been demonstrated [23], but the energy conversion efficiency is relatively low. Conventional membrane contactors have recently been investigated for dissolved methane recovery, in which sweep-gas/vacuum is applied to extract methane from wastewaters across a hydrophobic membrane [24], [25], [26], [27], [28], [29]. The ease of operation and the capability of offering large membrane areas through hollow fiber modules render this approach highly effective for methane extraction. Nevertheless, the significant water evaporation along with methane extraction results in the presence of water vapor in high concentrations in the desorbed gas [30], requiring subsequent energy-intensive purification steps, such as dehydration, prior to applying the gas in the combined heat and power systems. Moreover, the loss of latent heat due to the water evaporation can substantially reduce the feed temperature, which leads to an increased methane solubility and therefore slows down the methane extraction.
We recently proposed a solvent-based membrane contactor (SMC) process for dissolved methane recovery, which utilizes the solubility difference of methane between a feed (i.e., methane-rich water) and a draw solution that has a high methane solubility, as the driving force for methane transport across an omniphobic microporous membrane [31]. Using this process, we demonstrated >90% recovery of methane from anaerobically treated wastewater with negligible membrane fouling [32] as well as minimal water transport into the draw solution, which can drastically reduce the energy consumption for subsequent purification steps (such as dehydration), compared to other membrane recovery technologies [31]. While the potential was demonstrated, a thorough understanding of performance limiting parameters is critical to identifying optimal working conditions of the SMC process and fully utilizing anaerobic wastewater treatment as an energy-producing process.
In this work, we conduct an experimental and modelling study to investigate the impact of operating and solution conditions, and membrane properties on methane recovery performance of the SMC process. We first develop a theoretical mass transfer model that accounts for gas solubility parameters in disparate solvents, hydrodynamics, and gas transport through the membrane. Subsequently, we employ commercial omniphobic membranes in SMC experiments and measure methane transfer rates from methane-saturated aqueous solutions as feed, to dodecane as the draw solution, at various temperatures, hydrodynamic conditions, and feed solution compositions. Validated by the experimental data, the mass transfer model is then utilized to explore the potential of the SMC process by identifying key parameters for highly effective methane recovery and a methane-rich biogas production.
Section snippets
Working principle
The working principle of the SMC process for dissolved methane (CH4) recovery is illustrated in Fig. 1. An omniphobic microporous membrane is employed to separate a CH4-rich aqueous feed solution (e.g., anaerobic effluent) and a nonpolar and non-volatile organic draw solution. When exposed to the feed and draw solutions, the membrane traps air bubbles in the pores due to its anti-wetting property against both aqueous and organic solutions [33], [34], [35]. The high solubility of CH4 (nonpolar)
Materials
A cylinder of compressed CH4 (99%) was purchased from Praxair (Vancouver, BC). Dodecane (99%) was supplied by Alfa Aesar (Tewksbury, US). All other chemicals were used as received from Sigma-Aldrich. A commercial omniphobic membrane, Versapor® 450R, from Pall Corporation was used in the SMC experiments. This membrane is made of an acrylic copolymer cast on a nonwoven nylon support [34]. The porous structure and wetting resistance of the membrane against both water and low-surface-tension
Effects of temperature on CH4 transfer and recovery
While the increased amount of dissolved CH4 in anaerobic effluent is generally regarded as a major hurdle for low temperature anaerobic treatment processes, ensuring a high recovery of the dissolved CH4 from these effluents can prevent CH4 discharge to the environment and enable energy recovery. To investigate temperature effects on CH4 recovery, we performed SMC experiments at 15 °C, 25 °C, and 35 °C, simulating anaerobic effluents produced at psychrophilic (less than 20 °C) and mesophilic
Conclusion
In this work, we conducted an experimental and modelling study for performance analysis of SMC process to examine the potential of the process for the recovery of dissolved CH4 from anaerobic effluents. We developed a mass transfer model that accounts for gas solubility parameters in disparate solvents, which successfully describes CH4 transfer and its enrichment from the low concentration (aqueous feed) to the high concentration phase (organic draw, dodecane). Using this model, validated with
CRediT authorship contribution statement
Abhishek Dutta: Data curation, Investigation, Methodology, Writing - original draft. Xuesong Li: Data curation, Investigation, Methodology, Writing - original draft. Jongho Lee: Project administration, Supervision, Funding acquisition, Writing - review & editing.
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
Funding for this project has been provided by the Governments of Canada and British Columbia through the Canadian Agricultural Partnership, a federal-provincial-territorial initiative (Project number: INV059). The program is delivered by the Investment Agriculture Foundation of BC. Opinions expressed in this document are those of the author and not necessarily those of the Governments of Canada and British Columbia or the Investment Agriculture Foundation of BC. The Governments of Canada and
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These authors contributed equally to this work.