Resource recovery from industrial wastewaters by hydrophobic membrane contactors: A review
Graphical abstract
Introduction
Globally, the industrial sector has experienced burgeoning growth as a result of rapid population increase and economic development. While this has resulted in poverty reduction [1], it has also brought about a wide range of problems, including environmental issues and resource depletion. Water pollution is of particular concern as many industries use an abundant and continuous supply of water, which generates a large volume of contaminated wastewater. Without proper treatment, the wastewater can pose adverse effects on the environment, causing ecosystem imbalance and human health risks.
Contaminants in wastewaters can be removed by either biological or physicochemical means. Biological treatment processes are the core technology being employed for the removal of biodegradable organic matters. Among these processes, aerobic waste treatment is the most widely used due to its effectiveness and ease of operation, although sludge disposal remains a problematic issue [2]. Anaerobic digestion is increasingly becoming more popular, given that it generates biogas as a by-product, which can be channeled back as an energy resource to help waste treatment plants to offset their energy expenditures. Physicochemical treatment methods, on the other hand, are usually applied as a tertiary treatment for resource recoveries from wastewaters. Some examples are membrane filtration processes that provide excellent quality effluents but require high maintenance cost, ion exchange processes that can concentrate all types of targeted contaminants but require a large volume of the ion exchange columns [3], and adsorption techniques that offer high selectivity and cost-effectiveness but produce massive amount of secondary wastes, and are low capacity and time-consuming [4,5].
Contaminants in wastewaters are often an underrated source of resources, which can contain dissolved biogas [6,7], and other valuable heavy metals including mercury [8,9], copper [[10], [11], [12], [13], [14], [15], [16]], palladium [[17], [18], [19]], lanthanide [20], lead [[21], [22], [23], [24], [25]], cadmium [26], cesium [[27], [28], [29]], and gold [30]. To date, these resources are mostly captured, removed, and disposed of. Nevertheless, in this age of depleting natural resources, it is imperative to recover these resources, and recycling them as value-added products instead. However, there remains an important challenge of demonstrating the economics of resource recovery – whether the value of the recovered products can offset the additional expenses incurred by the recovery processes. Hence, this drives a continuous development of new resource recovery technologies that require less energy input and low operating and investment costs.
One of the emerging technologies for resource recovery from wastewaters is the hydrophobic membrane contactors (HMCs) (Fig. 1). HMCs are membrane-based processes that involve the transport of substances from one phase to the other (on the opposite side of the membrane) without the dispersion of that phase into the other phase. The transferred substances are made to diffuse through the membrane pores by a driving force of either a concentration or vapor pressure gradient. In this process, the hydrophobic membranes are non-selective and serve only to act as a physical barrier between the two phases to facilitate mass transfer to occur. The separation selectivity, on the other hand, is provided by the affinity of the substance toward the receiving phase. The most common commercial hydrophobic porous membrane materials and their properties are shown in Table 1. Commercial membranes are usually hollow fibers packed into a single module. The enormous number of fibers that can be packed into the module leads to high packing density, which gives HMC a significant advantage of having a high interfacial contacting area as compared to other mass transfer equipment. This can result in volume reductions of 1.4–15 times in HMCs over conventional packed columns [34]. The other competitive advantages of HMCs over conventional mass transfer equipment, such as packed, spray and bubble column are: (1) more flexible operations due to no direct contact between the two phases, which allows mitigation of issues like emulsion, flooding, and foaming, (2) more straightforward scaling-up of the technology due to the modularity nature and the precise control over the interfacial surface area of the membrane modules, and (3) low solvents holdup which is particularly attractive for expensive solvents [35,36].
There are three types of operations in the HMCs, gas-liquid, liquid-gas, and liquid-liquid MCs, as shown in Fig. 2. The gas-liquid MCs (Fig. 2a) have been used extensively in gas absorption applications, such as CO2 capture, biogas, and natural gas purification [37]. For the liquid-gas operations (Fig. 2b), they are more common for the removals of volatile organic compounds from wastewaters [38], or the recovery of dissolved biogas from anaerobic effluents [7]. Lastly, for the liquid-liquid mode (Fig. 2c), it is widely used in wastewater treatment applications, such as the removals of phenol or heavy metal ions [39,40].
Due to the advantages and wide uses of HMCs, we put together the major applications of HMCs for industrial wastewater treatment. Unlike previous reviews on related topics [31,41], the implementation of HMCs for resource recoveries and recycling of species from wastewaters is the key highlight of this review. Also, membrane distillation (MD) and pervaporation processes that employ hydrophobic membranes are not covered in this review because the main target in these applications is water rather than the transferred species [42]. The process principles, significant operating parameters, and current technical challenges are given and discussed. For the applications of HMCs, we divide them into two topics, namely (1) gas transfer HMCs for ammonia nitrogen, methane, and cyanide recoveries, where the recovered species are transferred in gaseous forms and (2) liquid-liquid extraction HMCs for phenol and metal ions recoveries, where these species are removed from wastewaters by organic solvents. Last but not least, we outline the technical challenges to do with membrane wetting and fouling as well as their potential mitigation methods.
Section snippets
Applications of HMCs in resource recovery from wastewaters
Hydrophobic membrane pores allow passage of gaseous species and organic solvents but not water or aqueous solutions. Therefore, HMCs can be used to recover dissolved gases or volatile species from wastewaters in gas transfer operations. On the other hand, for another application, the membrane pores can be filled with an organic solvent, allowing HMCs to be used in liquid-liquid extraction operations. These applications of HMCs for resource recovery from wastewaters are presented in the
Membrane wetting
Membrane wetting is a phenomenon in which the membrane pores are partially or completely filled with a liquid, as illustrated in Fig. 9. For the gas transfer and liquid-liquid extraction HMCs, membrane wetting occurs when the wastewater penetrates the pores. This leads to a significant increase in the mass transfer resistance, potential damage to membrane morphology, and the deterioration of process performances in long-term operations [119]. There are several reports of the membrane wetting in
Conclusions and outlook
Conventionally, HMCs are used for gas stripping, but we have recently seen increasing use for resource recovery, owing to the fast rate of resource depletion worldwide. In this review, we have provided an overview of the major resources that are recovered by HMCs from industrial wastewaters. These include recovery of ammonia nitrogen, dissolved methane, cyanide, phenol, and heavy metal ions. The key principles and mass transfer characteristics across the membranes were discussed. Furthermore,
Declaration of Competing Interest
The authors declare no competing interests.
Acknowledgements
This work was supported by the Walailak University, Thailand, Grant No WU62226. The first author also acknowledge support from Biomass and Oil Palm Center of Excellence, Walailak University.
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