CWMFC is a novel technology that has been used for almost a decade for concurrent wastewater treatment and electricity generation in varying scopes of domestic, municipal, and industrial applications since its implementation in 2012. Its advantage of low-cost enhanced wastewater treatment and sustainable bioelectricity generation has gained considerable attention. Nevertheless, the overall efficiency of this novel technology is inclined by several operating factors and configuration strands, such as pH, sewage composition, organic loading, electrode material, filter media, electrogens, hydraulic retention time, and macrophytes. Here, we investigate the effect of the wetland plant component on the overall performance of CWMFCs. The macrophyte’s involvement in the oxygen input, nutrient uptake, and direct degradation of pollutants for the required treatment effect and bioelectricity production are discussed in more detail. The review identifies and compares planted and unplanted CWMFC with their efficiency on COD removal and electricity generation based on previous and recent studies.
Over the decades, many wastewater treatment technologies have been employed to address the wastewater environmental menace. Wastewater treatment technologies, which consist of trickling filters, activated sludge, reverse osmosis, and membrane filters, are currently being used to treat all types of organic and toxic wastewater from industrial and municipal sources. However, they are not very productive, with regards to the cost and energy demand required in their operation [1]. It is projected that USD 2 trillion will be required in the U.S.A over the next 20 years to construct, operate, and maintain wastewater and drinking water facilities [2]. In addition to the current annual costs of USD 25 billion, around USD 45 billion is expected for wastewater infrastructure upgrades, with over half of operating expenditures aimed at aeration of wastewater. Power production measured here only for aeration could provide much-needed energy in the U.S.A from industrial wastewater alone [3]. According to Gude (2015), some of these conventional wastewater treatment systems require 0.3–0.6 kW∙h∙m −3 for treatment, whereas inherent in the same wastewater is energy that is equivalent to 10 times that needed for treatment [4]. Hence, the concept of generating electrical energy from the inherent chemical energy (organic matter) in wastewater during the treatment process will help offset the financial burden of treatment and provide access to clean water throughout the world, which would be highly recognized as sustainable [5,6][5][6].
In 1911, Michael C. Potter experimented and put forward the first microbial electrochemical technology (MET) and bio-electrochemical system (BES), established as microbial fuel cells (MFC), as a sustainable biotechnology [7,8][7][8]. A microbial fuel cell is an innovative wastewater treatment technology that uses electrochemical active bacteria (EAB) as a biocatalyst to transform the chemical energy inherent in sewage directly into electrical production without any environmental footprint [1,8][1][8]. MFCs use wastewater as a feed substrate for EABs to produce bio-electricity, while concurrently treating waste [1]. According to, Singh et al. [1], MFC as a technology holds great potential for a clean and green energy environment.
Constructed wetlands (CWs), on the other hand, are bio-physically assembled systems designed and built to take advantage of natural processes and interactions between wetland flora, soils, and associated microbial species to help regenerate wastewater [9,10][9][10]. Wastewater from a wide variety of sources, such as municipal, agricultural, or industrial wastewater, are treated by CWs [11]. They are easy to maintain and operate and can remediate many of the persistent pollutants that occur in conventional wastewaters into harmless by-products [12]. As a result, they have emerged as a substitute to traditional intensified systems for wastewater treatment [13,14][13][14]. A decade ago, researchers discovered that the embedded redox gradients, which naturally exist in wetlands, are highly compatible with the settings in microbial fuel cells, i.e., anaerobic zone in the inner–lower region and aerobic region at the air–water interface [15]. This connection makes their incorporation very plausible by creating a synergy between these two technologies for enhanced wastewater regeneration and bioenergy generation [16].
Macrophyte | Initial COD (mg/L) | COD Removal (%) | HRT (hr) | Max. Power | Author | ||
---|---|---|---|---|---|---|---|
Canna indica | 1500 | 74.9 | 96 | 15.7 mW∙m | −2 | [16] | |
Phragmites australis | 1058 | 76.5 | N. A | 9.4 mW∙m | −2 | [67] | [45] |
Ipomoea aquatica | 180 | 86 | 72 | 0.302 W∙m | −3 | [34] | [28] |
Phragmites australis | 250 | 80–100 | N. A | 0.15 mW∙m | −2 | [68] | [46] |
Ipomoea aquatica | 193–205 | 94.8 | 48 | 12.42 mW∙m | −2 | [48] | [43] |
Ipomoea aquatica | 300 | 72.5 | 72 | 0.852 W∙m | −3 | [69] | [47] |
Phragmites australis | 411–854 | 64 | N. A | 0.268 W∙m | −3 | [32] | [25] |
Typha latifolia | 314.8 | 100 | N. A | 6.12 mW∙m | −2 | [70] | [48] |
Phragmites australis | 583 | 64 | N. A | 0.276 W∙m | −3 | [33] | [26] |
Taifa latifolia | 624 | 99 | 24 | 93 mW∙m | −3 | [71] | [49] |
Phragmite australis | 323 | 60.6 | 62.4 | 131 mW∙m | −2 | [72] | [50] |
Elodea nuttallii | 643 | 97–98 | 24 | 184.75 mW∙m | −3 | [54] | [34] |
Canna indica | -- | 78.71 | 72 | 31.04 mW∙m | −3 | [73] | [51] |
Phragmites australis | 200 | 90.45 | 48 | 0.20 W∙m | −3 | [74] | [52] |
Phragmites australis | -- | 82 | 72 | 3714 mW∙m | −2 | [75 | [53] |
In a CWMFC configuration, the appropriate selection of macrophytes is crucial for the system’s success. The macrophyte component is one of the most conspicuous and versatile parts of the CWMFC bio-electrochemical system. The type of wetland plant installed in CWMFCs has some unique properties that make them play such an essential role in the contaminant removal processes and bioelectricity generation [68][46][54][55]. Therefore, a thorough selection of the type of macrophyte to be used is hugely imperative to the system’s success. This decision can either enhance or retard CWMFCs efficiency significantly. Hence, an appropriate selection of wetland plants must be based on some unique characteristics, such as:
Table 2.0: Characteristics of Macrophytes and their relevance in CWMFC.
Macrophyte properties |
Relevance in CWMFC |
rapid growth and high biomass production |
For winter insulation in cold and temperate regions, and particularly for the removal of nutrients by harvesting as nutrients are absorbed by macrophytes to build their biomass [55]. In addition, according to Yang et al. [56], species with high biomass production in CWMFC enhances the cell voltage and reduces the internal resistance of the system which often result in higher bioenergy production. |
good natural adaptation to the local climate |
Native species should be best preferred. According to Sierra (2017), CWMFC plants are selected based on the region's most common aquatic plants [27]. Oodally et al., (2019), concluded that native species are best preferred due to their local climate adaptability. In their experimentation, the most common aquatic plants in the region showed improved performance in CWMFC than exotic species [47].
|
good root development |
To provide a substrate for attached bacteria and oxygenation [55]. Also, the root development or maturity of the wetland plant affects oxygen release. In a sediment microbial fuel cell (SMFC) with wetland plant experiments conducted by Chen et al., (2012), their investigation has shown that young roots could excrete more oxygen than mature or aging species. Similarly, Stolzenberg et al. [57] also observed that plant species with good root development produced better oxygen which presented the highest voltage value than plants with smaller poor root systems. In addition, Mosqsud et al. operated a series of 6-CWMFC using Oriza sativa species. In their experimentation, they observed a reduction in power production as plants attained maturation. This was mainly because the maturation of the plant affected both oxygen release and exudate production. This signifies that the maturity of the root and its development is an essential factor in wetland plant selection [58].
|
High oxygen transfer capacity |
Oxygen transfer capacity from the roots creates an aerobic environment. Due to the great diversity of flora, different species have different radial oxygen loss (ROL) [19]. |
nutrient absorption capacity |
High nutrient absorption capacity helps in the effective removal of contaminants from the system. Species with high NAC use absorbed nutrients as a resource for their metabolism and growth [33][59]. |
adaptation and ease of propagation |
The ease in getting seedlings, seeds, or vegetative propagules must be well considered to ensure system sustainability. |
Good Rhizodeposition; release of carbon sources as rhizodeposits from plant roots. |
Rhizodeposition supports the growth and activities of microorganisms associated with bioelectricity production. |
C4 Plants |
The photosynthetic activity of plants is categorized into 3-phases: C3, C4, and CAM. In terms of oxygen production and CO2 fixation, plants in each category have different photosynthetic pathways. Plants in the group of C4 are those with advanced photosynthetic activity than plants in C3 and CAM groups. Consequently, because they have a higher conversion rate of solar energy into bioelectricity, it is suggested to integrate C4 plants [60]. |
Macrophyte properties |
Relevance in CWMFC |
rapid growth and high biomass production |
For winter insulation in cold and temperate regions, and particularly for the removal of nutrients by harvesting as nutrients are absorbed by macrophytes to build their biomass [46]. In addition, according to Yang et al. [80], species with high biomass production in CWMFC enhances the cell voltage and reduces the internal resistance of the system which often result in higher bioenergy production. |
good natural adaptation to the local climate |
Native species should be best preferred. According to Sierra (2017), CWMFC plants are selected based on the region's most common aquatic plants [27]. Oodally et al., (2019), concluded that native species are best preferred due to their local climate adaptability. In their experimentation, the most common aquatic plants in the region showed improved performance in CWMFC than exotic species [69].
|
good root development |
To provide a substrate for attached bacteria and oxygenation [46]. Also, the root development or maturity of the wetland plant affects oxygen release. In a sediment microbial fuel cell (SMFC) with wetland plant experiments conducted by Chen et al., (2012), their investigation has shown that young roots could excrete more oxygen than mature or aging species [82]. Similarly, Stolzenberg et al. [40] also observed that plant species with good root development produced better oxygen which presented the highest voltage value than plants with smaller poor root systems. In addition, Mosqsud et al.[83] operated a series of 6-CWMFC using Oriza sativa species. In their experimentation, they observed a reduction in power production as plants attained maturation. This was mainly because the maturation of the plant affected both oxygen release and exudate production. This signifies that the maturity of the root and its development is an essential factor in wetland plant selection [58].
|
High oxygen transfer capacity |
Oxygen transfer capacity from the roots creates an aerobic environment. Due to the great diversity of flora, different species have different radial oxygen loss (ROL) [27]. |
nutrient absorption capacity |
High nutrient absorption capacity helps in the effective removal of contaminants from the system. Species with high NAC use absorbed nutrients as a resource for their metabolism and growth [62][47]. |
adaptation and ease of propagation |
The ease in getting seedlings, seeds, or vegetative propagules must be well considered to ensure system sustainability. |
Good Rhizodeposition; release of carbon sources as rhizodeposits from plant roots. |
Rhizodeposition supports the growth and activities of microorganisms associated with bioelectricity production [84]. |
C4 Plants |
The photosynthetic activity of plants is categorized into 3-phases: C3, C4, and CAM. In terms of oxygen production and CO2 fixation, plants in each category have different photosynthetic pathways. Plants in the group of C4 are those with advanced photosynthetic activity than plants in C3 and CAM groups. Consequently, because they have a higher conversion rate of solar energy into bioelectricity, it is suggested to integrate C4 plants [19]. |
These factors should be primarily considered in the appropriate selection of macrophytes for CWMFC. Nevertheless, owing to the wide variety of aquatic flora, further investigation is needed to evaluate and select plant species with potentials for CWMFC for simultaneous wastewater regeneration and bioelectricity production [85].
Macrophytes, particularly emergent plants, can cause substantial water loss in CWMFC through evapotranspiration. As the volume of wastewater flowing through the system decreases due to water loss, the treatment efficiency in CWMFCs could be affected significantly when the evapotranspiration rate exceeds 2.5 mm/d [54][86][34]. Also, in the absence of light, plant cells and microorganism respiration will consume O2. Hence, the DO level in the reactor was reduced as DO consumption was more than production. The plant's photosynthesis and respiration altered the reactor's oxygen dynamics, ultimately leading to voltage fluctuations [52][61]. Therefore, macrophyte species that can help overcome this setback will be highly recommended.