Emerging investigator series: emerging biotechnologies in wastewater treatment: from biomolecular engineering to multiscale integration

Yixin Zhang† , Huan-Hsuan Hsu† , John J. Wheeler , Sishuang Tang and Xiaocheng Jiang *
Department of Biomedical Engineering, Tufts University, Medford, MA 02155, USA. E-mail: Xiaocheng.Jiang@tufts.edu

First published on 10th June 2020

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Water impact This frontier review discusses the recent progress in biocatalyst and bioreactor engineering to improve the efficiency and specificity of existing bioremediation technology, which is expected to open up new opportunities for many relevant applications from hazardous pollutant removal to valuable materials recovery from waste water.

1. Introduction

The generation of considerable amounts of wastewater owing to increased human activities, unsustainable agricultural practices and rapid industrialization exacerbates the water shortage and pollution problem in modern society.¹ Among various strategies used for restoring wastewaters through chemical (oxidation, reduction) and physical mechanisms (filtration, solidification and reverse osmosis),^2–4 biological treatment methods, which involve utilization of microorganisms to remediate pollutants in wastewater, have become one of the major approaches.^5,6 After billions of years of evolution, living microorganisms have developed various detoxifying mechanisms to maintain homeostasis and resistance to the contaminated environment by transforming organic or inorganic wastes into biologically degraded or less toxic forms.⁷ Therefore, biotreatment strategies are considered to be cost-effective and environmentally-friendly as they do not add additional toxic chemicals or secondary pollutants during purification.⁸

Biological treatments can be realized either independent of microbial metabolism, which is known as “biosorption”, or through metabolic activities, which is referred to as “biotransformation”.⁹ Based on physical/chemical interactions, microorganisms such as bacteria, algae and fungi have shown tremendous potential for remediating a wide range of pollutants, such as heavy metal ions (e.g. arsenic, cadmium, chromium, lead, etc.), organic micropollutants (e.g. ibuprofen, carbamazepine metoprolol, etc.) and industrial wastes (e.g. azo dye).^10–12 Although a broad range of treatments have been achieved based on biological activities, the biological process is usually slow, which could lead to accumulation of pollutants and failure to efficiently degrade some complex contaminants.¹³ And for some specific anthropogenic chemicals, including persistent organic contaminants, such as dichlorodiphenyltrichloroethane (DDT), 1,2,3-trichloropropane (TCP), and some polychlorinated biphenyls (PCB), natural strains have not evolved efficient catabolic traits to degrade these pollutants.^14,15 As a result, there is an ongoing interest to construct engineered strains that are capable of degrading various wastes in a fast and efficient manner in contaminated environments. Advantages such as small genome size, relative simplicity of the cell, short replication time, rapid evolution and adaption to the new environments make microorganisms especially favorable candidates to meet genetic engineering purposes.¹⁴ Meanwhile, the development of multi-omics technologies including genomics, transcriptomics, proteomics, metabolomics and fluxomics has enabled researchers to better understand and reprogram biological systems.¹⁶ So far, there are many successful examples of engineering microbes to enhance treatment efficiency which can be summarized in these aspects: expressing degrading genes for different targets in one single strain, modifying proteins/enzymes to increase affinity and specificity, constructing stable and efficient metabolic pathways, and regulating communications in coordinated bacterial networks.^8,17–19

Effective operation of biological treatment systems not only relies on highly active microorganisms but is also contingent upon microbial cell–contaminant interactions that happen inside the bioreactor. The efficient mass transport inside the bioreactor, by providing fresh gas and nutrients and exhausting purified water timely, will allow microbes to work in the most appropriate environment; therefore, ensure optimal treatment efficiencies. Conventionally, mass transport inside bioreactors is facilitated through optimizing flow mechanics such as continuously agitating microorganism/wastewater mixtures or choosing porous material to allow bacteria to anchor and grow. Nowadays, breakthroughs in materials and mechanical engineering offer extensive opportunities for engineering flow dynamics down to millimeter to sub-millimeter scale. Specifically, 3D printing technology stands out as it offers a unique platform to customize bioreactors by designing complex 3D structures with flexible material choices. Through rational design of the biotic–abiotic interface and spatial modulation of bio-physical and bio-chemical microenvironments, the microbial loading density, local mass transport, and overall bioactivity can be simultaneously optimized within the bioreactors. These superior characteristics open up new possibilities in engineering microbial communities for wastewater treatment.

Considering abundant progress in engineered biosystems for wastewater treatments, in this review, we will provide an overview of recent advances on engineering biological systems for higher treatment efficiency in two parts. In the first part, synthetic biology tools used to engineer two basic treatment functions of microorganisms – physical adsorption and catalytic transformation – will be discussed. The second part will focus on engineering bioreactors by exploring 3D printing technology. Two key parameters that have a large impact on the interactions between microbes and pollutants – structure and materials – will be analyzed. Lastly, a sustainable approach – microbial fuel cells (MFCs) will also be briefly covered. Especially, we identify municipal wastewater as the primary focus of this review, which is considered as the major point-source that can cause severe damages without proper treatments.²⁰ As municipal wastewater is rich in phosphorus and nitrogen related compounds (the general target of biological wastewater treatment), biotechnology is one of the most important strategies in the treatment of municipal wastewater.²¹ Furthermore, advanced biotechnologies also provide new possibility in removing emerging containments (e.g. metals, pharmaceuticals, synthetic organic pesticides, microplastic etc.)^22,23 that continuously challenging the conventional treatment technologies.

2. Biological machineries: overview, microbial consortia, and practical challenges

2.1 Biosorption based treatment–surface binding engineering

Microorganisms exhibit strong, non-specific metal adsorption abilities as enabled by their high surface area per unit weight and the prevalence of electronegative functional groups on cell surfaces such as hydroxyl groups, sulfhydryl groups, carboxyl in anionic groups, phosphate groups and nitrogen containing groups like the amino groups.^24,25 Good adsorption performances have been identified in gram positive bacteria (Bacillus, Corynebacterium, Streptomyces, Staphylococcus sp., etc.), Gram negative bacteria (Pseudomonas, Enterobacter, Aeromonas sp., etc.) and cyanobacterium (Anabaena sp., etc.).^26,27 Specifically, by expressing metal-binding proteins/peptides in the cytoplasm (intracellularly) or on the cell surface (extracellularly), followed by cell lysis to release metal ions or direct desorption, respectively (Fig. 1a),²⁸ some types of microorganisms have shown specific metal removal capability with superior selectivity and binding efficiency.^29,30 However, limited strain types, binding sites, and lack of specificity in the presence of other chemicals compromise their applicability as biosorbents.³¹ Therefore, there is a great need to engineer the metal binding proteins/peptides to be displayed on different cells with both enhanced affinity and specificity towards target metal ions. Currently, metallothioneins (MTs) and phytochelatins (PCs) are widely used to engineer microbial biosorbents for metal removal.^32–35 By using microbial cell surface display technology, these foreign proteins/peptides can be immobilized on different host cells by fusing with anchoring proteins (Fig. 1b). Among widely used host cells, E. coli, P. putida, and the yeast S. cerevisiae stand out for their well-studied genetic engineering paradigms. These microorganisms have been engineered to display MTs or PCs for binding a wide range of heavy metal ions, such as Cu²⁺, Cd²⁺, Hg²⁺, Pb²⁺ and Ni²⁺.^36–40 When high specificity for a target metal ion is required, metalloregulatory proteins can be also expressed due to the “metal sensing” capability by translating binding events into conformational changes.⁴¹ For example, by co-expressing Hg²⁺ transport system and MTs in E. coli, Deng et al. demonstrated efficient and specific removal of mercury (>90%) in the presence of other metal ions.⁴² Similarly, high and specific uptake for Ni²⁺ was also realized by using recombinant E. coli where both nickel transmembrane proteins and MTs were expressed.⁴³

In addition to displaying existing proteins/peptides, protein engineering that enables novel functions and improved performances by random designs or directed evolution could also enrich metal-binding abilities of known proteins/peptides. Recently, Zhou et al. developed an engineered uranyl-binding protein that is thermally stable and offers superb affinity and selectivity for uranyl (Fig. 1c).⁴⁴ Incorporating multiple binding sites within one protein/peptide is also promising to enhance binding affinity that could target several different metal ions simultaneously. For example, Mauro et al. constructed multidomain polypeptides and expressed them in E. coli.⁴⁵ A 65-fold increased Cd²⁺ uptake ability was achieved with recombinant cells, showing the possibility of developing novel multifunctional peptides/proteins to detoxify a wide range of compounds.

Because the biosorption process is metabolism-independent, non-living biomass can be also used as sorbents for pollutant removal. Particularly, extracellular polymeric substance (EPS) secreted by microbial cells during biofilm formation has attracted considerable attention owing to the abundance of charged functional groups, such as carboxylic and phosphoryl groups on the EPS matrix.^46,47 These charged moieties could serve as natural binding sites to adsorb charged species including heavy metal ions. Considering its high surface area and sustainability, EPS may provide more efficient and cost-effective adsorption capabilities compared with the cell surface display strategy. By now, there has been numerous examples of using EPS as biosorbents for removing metal ions, such as Zn²⁺, Cu²⁺, Cr²⁺, Cd²⁺, Co²⁺ and Ni²⁺.^48–51 Instead of using the whole EPS matrix, which consists of polysaccharides, proteins, lipids and DNA, adsorption is also feasible by using single component which can be directly extracted and immobilized onto a supporting substrate. Alginate gel beads extracted from seaweed and brown algae, for example, have been used to effectively remove divalent metal ions Cu²⁺ from aqueous media. Another important EPS component frequently used is amyloid protein nanofibers produced by E. coli bacteria, which could be easily genetically engineered to endow a variety of functions including specific binding to metal ions (Fig. 2a).⁵² Recently, Courchesne et al. developed a filtration method to quickly purify engineered fibrous proteins from the E. coli biofilm matrix, which then self-aggregated onto membranes to form free-standing films by removing anchoring protein CsgB (Fig. 2b).⁵³ This scalable approach further facilitates applicability of recombinant amyloid proteins to treat various pollutants.

2.2 Biotransformation based treatment – metabolic engineering

2.3 Engineering consortia

Besides monocultures, microbial consortia that multiple microorganisms work coordinately have unique advantages, such as performing complex tasks that individual populations are otherwise incapable of doing, as well as having higher tolerance to changing environments.^88,89 Wastewater usually contains a variety of pollutants or certain pollutants with complicated chemical structures. As a result, it is hard to use a single strain to remove all the wastes simultaneously. Additionally, more efficient and effective treatment can be anticipated when co-metabolic activities within microbial consortia complement each other. Currently, by developing specific consortia, researchers have successfully degraded various wastes, such as phenol,⁹⁰ organic acid,⁹¹ nitrate and phosphate^92,93 and cellulose.¹⁹ One example is consortia between cyanobacteria/microalgae and bacteria.^94,95 Specifically, photosynthetic microorganisms provide oxygen, which is indispensable for pollutant-degrading heterotrophic bacteria. In return, carbon dioxide from bacterial mineralization completes the photosynthetic cycle. This symbiotic interaction can be regarded as an ideal self-sustainable system which is better than conventional engineering designs of adding oxygen. Researchers also found when biomass degrader-anaerobic fungi are co-cultured with methanogenic archaea, effectiveness of waste breakdown can be enhanced by boosting synergistic relationships between them.⁹⁶ For example, co-cultivation of fungi Neocallimastix strain N1 with Methanobacterium formicicum strains cause cellulose digestion rate increased and at the same time, fermentation products are also shifted from less-valued chemicals, lactate and ethanol to more valued fuel energy – methane.⁹⁷ Being inspired by the natural symbiotic strains, rationally choosing and controlling desired cell–cell communications among engineered microorganisms to form coordinated cellular network may provide a way to mitigate metabolic burden in microbial cells or degrade complex compounds that are hard to construct entire degradation pathways in one strain.

2.4 Challenges in the real world

Although genetically-engineered biological machineries have demonstrated promising treatment outcomes attributable to their extraordinary capabilities in adsorption and/or catalysis, their operations are still limited to ideal laboratory environments.⁹⁸ There are two challenges remained to be solved before engineered microorganism can function in real-world scenario. The first issue is the robustness of engineered cells in complex polluted environments where pH, salinity, temperature, dissolved oxygen, redox potential, radioactivity, and overall cleanliness show large deviations compared with laboratory parameters. Furthermore, overexpression of genes in the plasmids of engineered bacteria can cause unnecessary burdens for the cells and slow down their growth and reproduction.^81,99 As a result, it's still difficult for engineered bacteria to compete against natural strains in real environments as they are vulnerable and easily degraded. Therefore, future researchers shall identify key physiological parameters that have large influence on in-site biotreatment effect and explore more advanced synthetic biology technologies to minimize side-effects after gene editing. A simple strategy is to use low, rather than high, copy number plasmid vectors.^100,101 Alternatively, avoiding the use of vectors by directly integrating foreign DNA into chromosomal DNA of host microorganisms could also eliminate unnecessary antibiotic resistance marker gene products.

Concerns of genetically engineered microbes also lie with their potential threats to the ecological system and long-term impact to human-beings. These concerns could be addressed by exploring biological containment strategies to eliminate gene leakage to the environment. One approach is to introduce suicide systems into engineered bacteria so that it dies after completing required tasks.¹⁰² More advanced genome-free gene editing technologies, such as CRISPR-Cas9, are also capable of creating marker-free engineered cells.^103,104 Furthermore, physical encapsulation strategies by rationally designing bioreactors as discussed below also deserve special attention as they are more accessible compared with complex biological isolation methods.

3. Bioreactors: overview, upgrades, and self-sustaining operation

3.1 Overview of bioreactors

Central to biological wastewater treatments are the physical (e.g. absorption) and chemical (e.g. nitrification, transformation etc.) interactions between microorganisms and contaminants, which determine the treatment efficiency of each bioreactor.¹⁰⁵ Hence, accelerating these interactions through promoting mass transfers inside the bioreactors can significantly improve the treatment efficiencies. Since microbial communities in the bioreactors are developed through different growth conditions from suspended-growth (i.e. activated sludge; with high microbial activity) to attached-growth (i.e. biofilm; with high microbial loading density) based on diverse treatment conditions (e.g. reactor size, contaminate concentration, etc.),¹⁰⁶ enhancing mass transfers in these bioreactors requires customized engineering approaches.

Conventionally, mass transfers inside these bioreactors are facilitated through optimizing their flow dynamics. In suspended-growth microbial communities, stirred tank bioreactors are the earliest and still the most common approach to promote mass transfer by continuously agitating microorganism/wastewater mixtures.¹⁰⁷ Besides, agitation can also improve the gas phase mass transfer in wastewater to increase oxygen content, which is favorable to the metabolism of aerobic microorganisms and leads to higher treatment efficiency.¹⁰⁷ Furthermore, in attached-growth microbial communities, mass transfer is commonly enhanced through increasing the interface area between wastewater and biofilm. For example, packed bed biofilm reactors have demonstrated outstanding treatment outcomes by utilizing specific support biocarriers such as silica granules, polymer beads, activated carbon particles, etc. to construct porous matrix for microorganisms to anchor and develop high surface area biofilms.^107–109 Due to their high biomass content, the treatment efficiency of packed bed reactors is usually superior to the stirring tank reactors. Nevertheless, the concentration gradients of waste contained in the packed bed reactors (waste concentration continuously decrease along wastewater flow as a result of biotreatment) can lead to hydraulic instability owing to uneven biofilm distribution and consequently, increasing the complexities in operations and maintenances. The developments and optimizations of these bioreactors have been systematically summarized in several review articles.^107,110

Combining the advantages of suspended-growth and attached-growth bioreactors, the moving bed biofilm reactor (MBBR) was developed in Norway during the late 80s to early 90s. In MBBR, biofilms are grown on small biocarriers that can be suspended and flow within the water streams inside the reactors where agitation is applied to ensure uniform mass transfer. MBBR can advance the volumetric treatment capability in stirred tank bioreactors as the biofilm-covered biocarriers can largely increase in biomass loading. These “floating” biofilm can also eliminate the common drawback of packed bed reactors in complex operations caused by the uneven biofilm distributions and backwash requirements.^111,112 The designs of biocarriers are always essential because they determine the efficiencies of MBBRs. There are two key criteria in creating high performance biocarriers, namely: (i) high specific surface area; and (ii) strong bacterial–surface interactions since high specific surface area can increase the volumetric loads of biomass while strong bacterial–surface interactions can avoid the detachment of biofilms induced by external hydraulic shearing forces. For instance, Wang et al. utilized porous polyurethane particles as floating biocarriers to construct a bioreactor that is capable of treating organic contents with high loading rates.¹¹³ Additionally, biocarriers consisting of different materials such as zeolite,¹¹⁴ nylon,¹¹⁵ ceramics,¹¹⁶ porous glass,¹¹⁷ PVA,¹¹⁸ and polyacrylamide^119,120 were also explored by various research groups. These developments in synthetic biocarriers have been comprehensively reviewed by Bouabidi et al.¹²¹ and Biase et al.¹¹¹

Emerging developments in addictive manufacturing/3-D printing offer extensive opportunities to further advance the designs of biocarriers beyond the conventional approaches (e.g. molding, machining, etc.) by customizing biocarriers with complex 3-D structures and flexible material selections to maximize both surface area and bacterial–surface interactions. Moreover, water-based polymerization technology allows immobilizing bacteria inside polymer matrix to limit the mobility while maintaining their catalytic activity. These immobilized bacteria have shown their potentials in the degradation of various types of wastewater contaminants, especially in nutrients, with enhanced efficiency.¹²¹ Based on these achievements in bacterial immobilization, bioprinting tools are recently applied to assemble these immobilized bacteria into hierarchical filters with modulated mass transfer, bacterial concentration, and polymer binding strength, which provide a novel approach for effective water treatment with minimum environmental impacts. Other than these conventional biological treatments that rely on chemical-to-chemical transformation, microbial fuel cell is proposed as an attractive solution by using microbial communities to directly convert organic waste inside wastewater into electrical energy for sustainable wastewater treatments. In the following sections, the two frontier developments will be systematically summarized.

3.2 Emerging designs in functionally enhanced biocarriers

The unique abilities of 3-D printing technologies in engineering the morphologies and mass transfer dynamics of biocarriers have demonstrated boosted efficiencies in MBBR. For example, Dong et al. created a series of 3-D honeycomb spherical biocarriers consisting of pentahedrons and hexahedrons for COD and NH₃ removal (Fig. 5a).¹²² These customized hollow designs enable an extra mass transfer pathway from inside the biocarriers toward biofilm. Combining with the mass transfer at wastewater/biofilm interfaces, biofilms on the 3-D printed biocarriers present enhanced bioactivity, whereas their improvements in treatment efficiency were not clearly observed. Exploiting a similar strategy, Elliott et al. manufactured spherical biocarriers with gyroid-based hollow interspacing (Fig. 5b), which provides up to 4.5 times larger specific surface area (2309 m² m⁻³) than many commercially available biocarriers (∼500 m² m⁻³). This increase in specific surface area can directly benefit the waste (NH₃) removal rate 1.620 ppm per day as compared to commercially available biocarriers (0.710 ppm per day).¹²³ In summary, current studies provide promising results which demonstrate the unique advantages of 3-D printing in fabricating biocarriers to produce biofilms with high biomass loading and/or high bioactivities. However, future optimization in these 3-D printed biocarriers is necessary to achieve the elevation in MBBR efficiencies. From this standpoint, future research is suggested to devote into two key parameters other than the current focus on surface area, which can also dominate the performance of biocarriers, namely: (i) structure and hydrodynamics; and (ii) materials and surface modifications.

In terms of encapsulated biocarriers, current development in natural and synthetic polymers such as polyvinyl alcohol (PVA),¹²⁴ polyurethane,¹²⁵ alginate,^119,120,126 chitosan,¹²⁷ agar,^119,120 and carrageenan¹²⁸ provide another material selection of biocarriers. Different from attached-growth versions where the biofilms cover on their surfaces, polymers allow microorganisms to be fully embedded “inside” the biocarriers, which offers a biologically relevant environment and effective mass transfer to preserve the normal functions of microorganisms. Additionally, the polymer matrix provides a physical boundary to minimize the microorganisms' exposure to toxic contents in wastewater. Generally, there are two types of polymer carriers, natural polymers and synthetic polymers. Comparing two materials, natural polymers provide good biologically relevant environments (e.g. suitable mechanical strength, sufficient mass transfer etc.), which can well preserve the viabilities of immobilized bacteria, whereas synthetic polymers have high mechanical strength and are more stable in wastewater compared with natural polymers. The selection between natural and synthetic materials should be determined by specific wastewater conditions. Among all materials, alginate is proposed to be one of the most used carriers for microorganism immobilization due to its water-based gelation process, mechanical & chemical stabilities, high porosity and superior biocompatibility, which can well preserve the activity of microorganism at both during and after immobilization.^121,129 For example, Ozer et al. synthesized P. boryanum contained alginate-based biocarriers for chromium(VI) (Cr⁶⁺) treatments, which are able to remove 97% of Cr⁶⁺ at 90 minutes after introduced into 100 mg L⁻¹ Cr⁶⁺ solutions.¹³⁰ Furthermore, attributed to the hydrogel boundary, environmental stresses such as toxicity of Cr⁶⁺ (up to 400 mg L⁻¹) and pH show negligible influence on bioactivities of embedded P. boryanum. PVA is another widely used material in bacteria immobilization owing to its low cost, simple handling process, low-toxicity properties, as well as its proper pore size for oxygen/waste organic matters diffusion.¹³¹ For instance, El-Naas et al. have utilized PVA immobilized P. putida for designing the treatment of contaminations from petroleum refineries in both lab and pilot plant scales.^132,133 The result indicated that these biocarriers can ensure certain level of activities even under highly toxic environments (i.e. high phenol concentration), which can achieve 96% reduction of COD and 100% reduction of phenol and cresols in the treatment of real petroleum refinery wastewater. In addition to alginate and PVA, various polymers are also fabricated for toxic contaminants (e.g. phenol, trichloroethane) removals such as B. cereus @ alginate,¹²⁶ methanogenic consortium @ agar,¹³⁴ and P. putida @ chitosan.¹²⁷ These developments of polymer immobilized bacteria in wastewater treatment are reviewed by Bouabidi et al. in details.¹²¹

While tremendous progress has been made through 3-D printing and hydrogel encapsulations, further enhancement of MBBR performance demands advanced structural and material designs of biocarriers that could transform the bio-integration to achieve higher treatment efficiency and longer-term stability. Potential opportunities include: (1) engineering the structures and materials of biocarriers to accelerate the interfacial mass transport; (2) effective immobilization/encapsulation of microorganisms to extend the lifetime; key parameters from biomaterials engineering perspective will be summarized in the next few sections.

3.3 Prospects of 3-D printed biocarriers

3.4 Sustainable approach – microbial fuel cell (MFC)

In biological wastewater treatment, energy consumption is always one of the major concerns that adversely impact the environment. In this regard, the microbial fuel cell (MFC) is proposed as an attractive solution by directly converting organic waste inside wastewater into electricity.^165,166 In MFC, bacteria on the anode metabolize organic matter and transport generated electrons through unique EET pathways. The produced protons flow through the semi-permeable membrane to the cathode, then combine with dissolved oxygen by either bio- or abio-catalyst-triggered oxygen reduction reactions (ORR) to form water. However, most studies indicated that electricity generation due to low EET efficiency remains one of the major challenges to be solved before MFCs could recover sufficient energy and truly reduce the environmental impacts of wastewater treatments.^62,167 To overcome this bottleneck, different designs of MFCs along with their working principles, electrodes and bacteria species have been extensively investigated in the last decades. Conventionally, power density in MFC is enhanced through increasing the surface area of anode to maximize the bacterial coverage. For this approach, carbon-based materials have been extensively studied to serve as the high performance MFC anode due to their high surface area. Additionally, their biocompatibility and conductivity can also facilitate the interactions between bacteria and the electrode. Recently, various carbon-based materials such as carbon cloth,¹⁶⁸ carbon brush,¹⁶⁹ carbon mesh,¹⁷⁰ carbon veil,¹⁷¹ carbon paper,¹⁷² graphite felt/rod/foam,¹⁷³ granular graphite,¹⁷⁴etc. have been tested in MFC. The achievements of these works are well-summarized by Zhou et al.¹⁷⁵ and Santoro et al.¹⁷⁶ On the cathode of MFC, various synthetic ORR catalysts can be applied to elevate the overall energy efficiency. Platinum (Pt) remains the most effective catalyst of ORR and is widely used in MFC for high energy productions.¹⁷⁷ However, the complex compositions in wastewater commonly challenge the long-term stability of Pt catalysts. Recently, many non-Pt catalysts such as gold,¹⁷⁸ palladium¹⁷⁹ and porphyrin-related compounds^180–182 have also been used to build high performance yet long-lasting MFCs. The current progress is reviewed by Wang et al.¹⁸³

Furthermore, nanomaterials that possess superb electrical properties and tunability present great potential to facilitate the EET at both bio–bio and bio-electrode interfaces to boost the power generation inside MFC. State-of-the-art developments in nanomaterial assisted MFC have been reviewed and discussed by Hsu et al.⁶² Lastly, by exploiting bioprinting (to accelerate the mass transfer) and nanomaterials (to facilitate the EET) to program the assembly of an S. oneidensis MR-1 community, Freyman et al. created a bio-abiotic integrated MFC anode that demonstrate over two times improvement in energy density as compared to its solid-state counterpart. This work provides significant insights in advancing energy generation of bioanode through engineering microbial communities, which can open up many new possibilities apart from conventional approaches that mainly focus on the materials of electrodes.¹⁸⁴

4. Conclusion

Microorganisms have shown tremendous potential in restoring contaminated wastewater inexpensively and sustainably. Recent progress in synthetic biology has provided reliable platforms to precisely engineer their structure and function, enhance adsorption and catalytic capability, and improve the efficiency and specificity for biotreatment. The advances in 3-D printing technology further promote microbial loading, mass transport and bioactivity through the development of novel biocarriers. Together these efforts are opening up new opportunities for many relevant applications from hazardous pollutant removal to valuable materials recovery and will benefit future research in minimizing potential environmental risks caused by genetically engineered microorganisms and their scale-up production in the real word.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge support from the National Science Foundation (NSF DMR-1652095).

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Footnote

† Y. Z. and H. H. contributed equally to this work.

This journal is © The Royal Society of Chemistry 2020