Recovery of dissolved methane from anaerobic effluent using hollow fibre membrane contactors

Between 2010 and 2015, wastewater treatment plants around the world produced 38 million tons of methane per year on average (which was 11% of the global anthropogenic sources of methane). Methane is one of the greenhouse gases that has 25 times more potential than carbon dioxide to contribute to global warming. Nevertheless, methane can be utilised as a natural alternative for fossil fuels. Thus, one of the sustainable approaches for wastewater treatment is the use of anaerobic reactors to produce and utilise the methane. This approach is in line with the UN Sustainable Development Goals (SDGs) 6: Clean water and sanitation, SDG7: Access to affordable and clean energy, and SDG13: Climate action. However, part of the methane produced in the anaerobic treatment system that is present in the treated effluent in dissolved form will be released to the environment (this could be as high as 88% of the total methane produced by the treatment system). Therefore, recovery of dissolved methane has become part of the goal of wastewater treatment facilities to limit the release of methane and to optimise its use as an additional source of energy. Based on research findings, the technique with the highest potential for the recovery of dissolved methane is the hollow fibre membrane contactors (HFMCs). However, studies on HFMCs are limited to bench scale analysis (technological readiness level is at development stage). Some of the identified research gaps related to these studies are: (i) limited evaluation on long-term operations, and (ii) lack of numerical models for upscaled HFMCs. Those gaps limit the adoption of the HFMCs for dissolved methane recovery in large-scale applications, although there are commercial membrane contactors available. Therefore, the main aim of this research is to evaluate the effect of operating time on the performance of porous HFMC and dense HFMC in recovering dissolved methane from anaerobic effluent. This is achieved with the aid of a numerical model. The main research questions addressed are: (i) What are the theoretical dissolved methane removal efficiency and desorption flux of the porous and dense HFMCs? and (ii) How does the long-term operation affect the performance of the porous and dense HFMCs? To answer the first research question, a numerical model was developed based on an existing theoretical framework. The fit of the model, in terms of dissolved methane removal efficiency and methane desorption flux, was determined using the experimental data in the literature. It was found that a distribution factor, m, was needed to improve the fit of the model. The factor m was incorporated into the model through the mass transfer coefficient and calibrated using the literature data. The correlations of m were determined against the feed flow, mass transfer resistance, dissolved methane removal efficiency, and methane desorption flux to determine if m can be experimentally estimated. It was found that the m was linearly dependent on the average methane desorption flux, with R2 = 0.92 and R2 = 0.85 for porous and dense HFMC, respectively. Those linear relationships were validated using the experimental data from this research. These experiments were carried out as follows: (i) set-up the membrane filtration and HFMC systems, (ii) filter the anaerobic effluent using the membrane filtration system, (iii) operate the HFMCs using the filtered anaerobic effluent as its feed (dissolved with a gas mixture of 60% methane, rest with carbon dioxide), and (iv) measure the performance of the HFMCs under varying feed flow rates, with each flow rates introduced to the HFMCs for one hour (short-term operation). The validation results showed that the linear relationships found were able to estimate the optimised value of the factor m for the experimental data. Also, energy analysis of an upscaled porous HFMC was carried out using the validated numerical model. In the simulated upscaled HFMC, the net energy produced is around 0.34 MJ∙m⁻3 for a wastewater flow of 11 m3∙d⁻1 (considering 88% dissolved methane removal efficiency and 35% methane content in the gas recovered from porous HFMC). The other research question was addressed by carrying out the long-term operation of HFMCs as follows: (i) continuously feed the HFMCs with filtered anaerobic effluent without cleaning and without continuous dissolving of the gas mixture, (ii) operate the HFMCs as with short-term operation (discussed previously) after one month of feeding with filtered anaerobic effluent (without cleaning and without continuous dissolving), and (iii) measure the performance and mass transfer resistances of the HFMCs under varying feed flow rates (this represent the performance of HFMCs with long-term operation). The differences with the performance and mass transfer resistances of HFMCs between the short-term and long-term operations under varying feed flow rates were determined. These differences were utilised to derive the relevant relationships for estimating the performance of HFMCs at any operating time (from the commencement of the experiment) and different feed flow rates. These equations were incorporated into the numerical model, through the factor m, to evaluate the performance of the upscaled HFMCs at any operating time. At short-term operation, the performance of porous HFMC is better than dense HFMC (dissolved methane removal efficiency is 90% and 80% for porous and dense HFMC, respectively). However, if the operating time is more than two weeks, the removal efficiency of dense HFMC becomes higher than the porous HFMC since the performance of porous HFMC decreases continuously over time while the performance of dense HFMC stays relatively stable. It should be noted that the empirical relationships derived from this study are specific to the HFMC set-up and operating conditions used. The evaluation using other pilot-scale HFMC systems is highly relevant to improve the application of these relationships and to model them for large-scale wastewater treatment plants. Still the outcomes of these research provide additional knowledge with regards to the performance of HFMCs subjected to anaerobic feed and operated for a longer period of time, which will promote and improve the adoption of HFMCs at large-scale applications. This will lead to the enhanced sustainability and economic feasibility of anaerobic systems and their positive impact on the environment.