Encapsulation technology to improve biological resource recovery: recent advancements and research opportunities

Abstract

Encapsulation technology has been extensively investigated for various microbiological applications for decades. Combined with biological processes, encapsulated bacteria have great potential to efficiently and cost-effectively recover resources (e.g., hydrogen, methane, metals) from various waste streams, such as wastewater, metal wastes, and sludge. This review focuses on recent advances in four areas: encapsulation of specific bacteria or biocatalysts, effectiveness in separating the solids retention time from the hydraulic retention time, control of the encapsulant internal environment, and improved reusability and storability of encapsulated bacteria. In most cases, encapsulation technology enhanced biological resource recovery by facilitating stable high-rate operation; nevertheless, challenges remain that limit application. Consequently, this review points to three major research opportunities that, if addressed, could enable broader application of encapsulated bacteria for resource recovery: (1) encapsulant impacts on bacterial growth, leakage, and community changes with time, (2) use of encapsulant chemistry to control the encapsulant internal environment, and (3) modifications of the encapsulant matrix to improve the reusability and storage of bacteria. With the better control and predictability that should result, the use of encapsulation technology in biological processes could be further advanced as a reliable and efficient option for resource recovery.

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Water impact

Encapsulation technology provides a method of retaining particular microorganisms within a process, which has the potential to improve biological resource recovery. This Frontier Review discusses recent advances in encapsulation technology and explores research opportunities to enhance the control of encapsulant properties for better predictability and performance of biological resource recovery. Such insights should speed the implementation of encapsulation technology for resource recovery, enhancing sustainable waste management and resource use.

1. Introduction

To enhance sustainable management of waste streams, such as wastewater, metal wastes, and sludge, there has been a great deal of interest in low cost and efficient recovery of resources from wastes using biological processes.^1,2 Encapsulated bacteria have multiple advantages over suspended bacteria, making this technology particularly useful for biological resource recovery from wastes. In addition, treatment systems utilizing encapsulated bacteria exist at full scale, indicating that such systems can be cost effective.³

Cell encapsulation was first investigated in the 1930s,⁴ and since then various host matrices, including natural polymers, synthetic polymers, and inorganic materials, have been studied to encapsulate bacteria or other biocatalysts for applications in the biomedical, food, biofuel, and agricultural industries, as well as for wastewater and waste stream treatment.^5–9 The advantages of encapsulation for resource generation and waste treatment include the ability to encapsulate specific target bacteria, to separate solids retention time (SRT) and hydraulic retention time (HRT),¹⁰ to improve microbial resistance to environmental stresses (e.g., varying pH, high salinity),¹¹ to reach high microbial loading densities,¹⁰ and to enhance microbial activity.^12,13 These have been recognized since the 1970s, when researchers demonstrated the ability to continuously produce hydrogen using encapsulated Clostridium butyricum.¹⁴ Over the last 3 years, literature reviews on encapsulation technology have been published, specifically providing overviews on encapsulation host matrices (i.e., carrageenan, lignocellulosic material, and edible coatings for probiotic encapsulation),^15–17 novel encapsulation methods (i.e., electrospinning),¹⁸ and various applications of encapsulation technology (i.e., lead removal, dye removal, biohydrogen production, fermented food production, agricultural applications, biomedical applications, wastewater treatment).^{5–8,19–22} Nevertheless, a knowledge gap exists in the application of encapsulation technology for biological resource recovery. In this review, we focus on these advances, specifically addressing recent work on the (1) encapsulation of specific bacteria or biocatalysts, (2) controlled separation of SRT from HRT, (3) control of the encapsulant internal environment, and (4) improved reusability and storability, all in the context of enhanced resource recovery from waste streams (Table S1†). Future research opportunities and needs for more predictable and widespread application of biological encapsulation technology for resource recovery are also discussed.

2. Recent progress in the use of encapsulation for resource recovery

2.1 Encapsulation of specific bacteria or biocatalysts

2.1.1 Enhanced biological processes in encapsulation matrices. The encapsulation of specific bacteria or biocatalysts has been used to recover various resources from waste streams, such as wastewater, metal wastes, and sludge.^23–30 Proteinase-secreting bacteria have been encapsulated in alginate to facilitate sludge disintegration.^12,31,32 In such cases, encapsulation of these bacteria improved microbial growth and hydrolytic potential compared with suspended bacteria, overcoming the rate-limiting step of anaerobic sludge digestion.¹² In a slightly different application, co-encapsulating multiple cellulase enzymes in poly(methacrylamide-co-acrylic acid) at optimized ratios reduced inhibition by the intermediate product cellobiose and facilitated hydrolysis of a cellulose feedstock, producing up to 89.1% glucose.³³ Biocatalysts, such as nano-metals and mushroom extracts, have also been co-encapsulated with anaerobic bacteria and lactic acid-generating bacteria to improve anaerobic digestion and bacterial survivability, respectively.^34,35 Ta et al. encapsulated hydrogenic and methanogenic bacteria in a combination of κ-carrageenan and gelatin (2%/2% w/w) for use in a single-stage dark fermentation process to generate hydrogen-enriched methane. Adjusting the biomass ratio of the two encapsulated bacteria allowed control of the hydrogen produced, with hydrogen and methane generated at 64.6 and 395 mL L⁻¹ d⁻¹, respectively.³⁶ Similarly, encapsulating an anaerobic community with 10% (w/w) Clostridium acetobutylicum in a matrix of poly(vinyl alcohol) (PVA) facilitated destruction of carbonaceous materials in the waste stream, enhancing methanogenesis and therefore increasing the methane content in the produced biogas by 14 to 30%.³⁷

Metal recovery from waste streams has also been facilitated by encapsulated bacteria, with several studies reporting a higher metal adsorption capacity with encapsulated bacteria than with free bacterial suspensions.^38–41 Encapsulation of genetically engineered bacteria with lanthanide-binding sites in a polyethylene glycol diacrylate hydrogel resulted in the adsorption of 97% pure rare earth elements (REE) from a waste of electronic leachate.²⁷ In this application, a two bed-volume increase in the break-through point for REE compared to non-REE impurities was also observed.²⁷ It should be noted, however, that in metal adsorption, comparable or even less adsorption capacity has been observed with encapsulated bacteria relative to suspended bacteria;^27,42 this could be attributed to the large quantity of non-adsorptive material (i.e., 87%) in typical encapsulation matrices.²⁷ Effective encapsulation of desirable bacteria and biocatalysts in various matrices leads to the improved recovery of resources, including biogas, glucose, and metals, from various waste streams, such as wastewater, metal wastes, and sludge. Nevertheless, studies have stopped short of truly optimizing these processes through either modeling or experimental approaches, with the relationship between bacterial loading density and activity yet to be fully established.

2.1.2 Microbial growth. Bacteria have been observed to grow differentially in encapsulants of varied material stiffness, with some studies showing that bacteria cease to grow when encapsulated and others showing that growth continues under these conditions. Anaerobic ammonia oxidizing (anammox) bacteria were observed to grow in a polyethylene glycol encapsulant over 70 days, increasing from (1.3 ± 3.99) × 10¹⁰ to (3.4 ± 0.30) × 10¹¹ 16S rRNA gene copies per gram of encapsulant.³ Sodium alginate and sodium silicate encapsulants allowed for cell growth rates comparable to free cell suspensions, with the slightly lower stiffness of the sodium alginate encapsulant (elastic modulus of 0.64 ± 0.17 kPa) enhancing bacteria growth compared with that of a combination of sodium trisilicate and chitosan (elastic modulus of 0.90 ± 0.19 kPa).⁴³ Homburg et al. also reported the growth of photosynthetic bacteria when encapsulated in silica and sodium alginate matrices, respectively.⁴³ Comparable photosynthetic activity with encapsulated cells and free cell suspensions, however, suggested that encapsulant breakage had occurred as a result of cell growth.⁴² Conversely, Branyik et al. observed no cell growth in a silica encapsulant, suggesting that the higher rigidity and small pore size of the matrix material limited cell growth.⁴⁴ Likewise, bacteria confined in a material of high stiffness, such as a combination of tetramethoxysilane, methyltrimethoxysilane and colloidal silica nanoparticles (elastic modulus >5 MPa), were observed to be incapable of dividing.⁴⁵ Others have seen similar patterns, in that encapsulants made from tetraethylorthosilicate with a moderate stiffness (elastic modulus ≤4.23 ± 0.72 kPa) still allowed cells to replicate,⁴⁶ whereas barium alginate encapsulants with a stiffness higher than 10 kPa suppressed cell growth.^5,47 Understanding the growth of encapsulated bacteria should enable improved resource recovery while minimizing expensive or repeated encapsulated bacterial inputs.

Cell growth is also affected by the biocompatibility of the encapsulant material. Homburg et al. encapsulated microalgae into four types of host matrices; in these experiments a tetraethylorthosilicate and tetra(n-propylamino)silane matrix base resulted in the presence of chemical residuals in the encapsulant that reduced encapsulated microalgae growth rates by 30% and 23%, respectively.⁴³ The use of sodium trisilicate or sodium alginate matrices for encapsulation of the same microalgae did not result in the presence of inhibitory chemical residuals, indicating higher biocompatibility.⁴³ Encapsulation with an eye to encapsulant biocompatibility will be important for expanding use in resource recovery and maximizing performance.

Gene expression profiles are also affected by encapsulation, which could be used as an indicator of growth stage or cell health. For example, yeast encapsulated in a matrix of alginate fed with sufficient nutrients stopped dividing after 5 days but remained highly metabolically active over the 17 day experimental period, producing 5-fold greater quantities of ethanol per gram of cell biomass when compared to log-phase planktonic yeast. In these experiments, encapsulated yeast exhibited stress-induced gene expression, which influenced the cell cycle and growth in the encapsulant.¹³ Differing growth and activity patterns have been observed for multiple reasons when bacteria and other microorganisms are encapsulated; therefore, a mechanistic understanding of growth within a range of encapsulants, including the physical effect of encapsulant stiffness and the chemical effects of the encapsulant itself, is needed to facilitate more thoughtful application and predictive resource recovery using encapsulated bacteria.

2.1.3 Microbial community changes. Because bacteria grow differentially in an encapsulation matrix, studies have been conducted to investigate how encapsulated microbial communities change over time and under different operating conditions. These studies are important for advancing our understanding of how encapsulation might affect some community members but not others, providing better predictability in resource recovery. When a microbial community was encapsulated in a combination of PVA and alginate, Yang et al. observed a shift in the dominating microbial genus from Acidocella to Metallibacterium when the encapsulated community was subjected to starvation conditions. After the delivery of a carbon source resumed, the microbial community structure returned to a structure similar to that of the initial community, suggesting that a predictable growth response had occurred.⁴⁸ Similarly, when an activated sludge seed was encapsulated in PVA-sodium alginate beads, the microbial community shifted from a Gammaproteobacteria-dominant to a Betaproteobacteria-dominant community.⁴⁹ Suspended versus encapsulated activated sludge diverged into distinct communities of nitrogen cyclers consisting of ammonia and nitrite oxidizers, which was thought to be a result of varying ammonium and oxygen concentrations available to the bacteria in the encapsulated versus suspended systems.⁴⁹ Additional studies investigating the effluent from an anaerobic forward osmosis membrane bioreactor containing encapsulated anaerobic digester sludge in an alginate matrix reported changes in the microbial community structure at different HRTs and in the presence of different salt mixtures.⁵⁰ Sivagurunathan et al. also studied encapsulated communities via quantitative polymerase chain reaction and identified changes in the abundance of different bacteria at varying substrate concentrations.⁵¹ Because bacterial activity (e.g., enzymatic ethanol production from glucose,²⁸ metal biosorption²⁷) has been found to be loosely associated with bacterial viability, a comprehensive understanding of how microbial communities change when encapsulated is needed for predictive and optimized resource recovery. Furthermore, research on shifts in absolute microbial numbers and microbial activity is also needed to clarify when and why the trends observed in relative abundance and population numbers do in fact deviate from trends in activity and performance.

2.2 Separation of SRT from HRT

When bacteria are encapsulated, they are more easily physically retained, with the liquid flowing through the reactor while the encapsulated bacteria remain within the reactor. In this manner, encapsulation decouples SRT from HRT, which allows for high-rate operation and reduced process footprint. Indeed, in most cases, bacterial retention in encapsulants enhanced biological processes and improved resource recovery. Hydrogen, a clean energy resource with a high energy density,^52,53 has been generated from the treatment of wastewater and sludge via anaerobic digestion or photosynthetic processes. Hydrogen, however, is rapidly consumed by other bacteria, and hydrogen-producing bacteria are washed out at short HRTs.^54,55 Studies have shown, however, that if encapsulated, hydrogen-producing bacteria are effectively retained and can contribute to hydrogen production at low HRTs,^26,54,55 with up to 2.4 L H₂ per L d⁻¹ produced at an HRT as low as 45 min.⁵⁶ In another example, under psychrophilic conditions (i.e., 15 °C), anaerobic sludge retained as bio-plates of cellulose triacetate was observed to produce 0.22 L of methane per g of COD removed when treating low-strength wastewater; this occurred at a 16 hour HRT and was possible because of the high concentration and long retention time of the active bacteria present.¹⁰ In such applications, leakage from the encapsulant is an important factor in maintaining a particular target SRT for encapsulated bacteria. The low percentage of leakage in a system that used bacteria encapsulated in Protanal® LF 10/60™ alginate to generate ethanol likely contributed to higher ethanol yields via the maintenance of a high bacterial density and long SRT.²⁸ Even with leakage, resource recovery may not be negatively impacted if it is controlled in some manner. For example, in one study, active formation of biofilm from leaked bacteria created a dynamic membrane, further enhancing hydrogen production via the re-establishment of the previously encapsulated bacteria in a sessile form.⁵⁴ In other cases, however, leakage of bacteria from encapsulant matrices can cause excessive loss of bacteria. For example, leakage up to 33.2 ± 7.6% has been observed from various encapsulation matrices, including alginate, Protanal® alginate, modified glass fiber spheres (MGFS) coated with polysulfone or poly(vinylidene) fluoride, and a combination of carrageenan and gelatin. Such losses reduced bacterial retention, diminished substrate utilization, and increased competition between suspended and encapsulated bacteria, ultimately compromising resource recovery in all cases.^{11,28,36,41,56,57} It is clear that the retention of bacteria via encapsulation enables high-rate small-footprint resource recovery. Nevertheless, the leakage of bacteria from encapsulation matrices could affect the long-term utility of such systems and should be studied to gain a deeper understanding of why and how this phenomenon occurs.

2.3 Control of the encapsulant internal environment

2.3.1 Encapsulant protection from chemical inhibitors or toxicants. Encapsulation matrices alter and control the chemical environment within the matrix, potentially providing a more favorable niche for encapsulated bacteria. When used in biological processes, such as photosynthetic hydrogen production and anaerobic digestion, encapsulation matrices (e.g., glass fiber, alginate, and polydopamine) have been shown to protect bacteria against extreme pH.^11,29,58 Encapsulated photosynthetic bacteria in a matrix of modified glass fiber spheres displayed a higher activity than suspended bacteria at pH values ranging from 5–9.¹¹ These bacteria removed 72% of the COD supplied even at the most acidic pH of 5, while suspended bacteria only removed 44%.¹¹ Likewise, the covalent combination, and therefore encapsulation, of cellulase with a poly(methacrylamide-co-acrylic acid) improved the pH stability of the cellulase, resulting in stable catalytic activity >70% of optimum over a broad pH range (3–8).³³ The activity of suspended cellulase was significantly reduced at pH values of greater than 6, only retaining 40% of the original activity at pH of 8.³³ Even under strongly acidic (1 and 3) and alkaline pH values (10 and 12), encapsulated bacteria have been observed to maintain high phenol degradation efficiency (>95%), while the activity of suspended bacteria decreased.⁵⁹ In addition, after a strong acid shock (pH 0.5, 30 min), an encapsulated electroactive biofilm produced current at 0.20 ± 0.05 A m⁻², the activity a benefit of encapsulation in a stable polydopamine matrix that was able to form a hard shell around the biofilm. This current generation was 1900% higher than that of an unencapsulated, and therefore unprotected, control (0.01 ± 0.01 A m⁻²).²⁹

Encapsulation also ameliorates the inhibition of microbial processes by high salinity¹¹ and other inhibitory substances.^59–61 Indeed, encapsulated bacteria in a matrix of PVA-sodium alginate-kaolin were able to degrade phenol when amended at a concentration of 250 mg L⁻¹, a concentration that completely inhibited free bacterial suspensions.⁵⁹ Encapsulated bacteria in a PVA cryogel also retained methanogenic activity upon the addition of a number of xenobiotics, including ampicillin, kanamycin, benzylpenicillin, methiocarb, and chlorpyrifos, whereas a 1.3–2.2-fold decline in methanogenesis was observed with suspended bacteria under the same conditions.³⁷ Finally, a recent study reported that the copper inhibition of both biological hydrogen and methane production was significantly reduced when the microorganisms were encapsulated, resulting from copper complexation with the encapsulant, which reduced its bioavailability.⁶² Aided by a stable in-encapsulant microenvironment, more efficient resource recovery has been achieved by encapsulated bacteria compared to free bacterial suspensions, especially under conditions in which the encapsulant is able to moderate the in-encapsulant microenvironment or bind chemical inhibitors. As more is understood regarding how such microenvironments are controlled, even more predictable resource recovery using encapsulated systems should be possible, with the eventual design of specific encapsulation matrices to protect from anticipated chemical inhibitors.

2.3.2 Diffusivity. Particular encapsulants, chosen for their diffusion characteristics or altered to control diffusion into and within the matrix, enable control of the encapsulant internal environment. In a system used for anammox, the effective diffusion coefficients for ammonium and nitrite in PVA-alginate beads were three times higher than the ones in (unencapsulated) granular sludge, which contributed to high anammox activity for the encapsulated system.⁶³ To enhance substrate permeability, dopants such as powdered activated carbon (PAC) and polyethylene oxide (PEO) have been added into encapsulants to increase porosity. With PAC doped into PVA-alginate beads, the permeability of N-octanoyl-DL-homoserine lactone through the encapsulant beads increased 29% compared with PVA-alginate beads that were not doped with PAC.⁶⁴ Likewise, PEO dopants into alginate greatly increased the porosity of the encapsulant and as a result, increased the ethanol production from encapsulated bacteria in the first 24 hours of incubation.⁵⁷ Applying external polymer coatings such as polyethylenimine, polysulfone and poly(vinylidene) fluoride has been observed to change the permeability and decrease the diffusivity of dissolved organic compounds into/through encapsulants.^11,56 If the relationship between chemical alterations to encapsulants and their transport characteristics were better understood, however, the alteration of encapsulant structures could be made a priori to tune the diffusivity and selectivity of particular compounds into/within encapsulants.

2.3.3 Hydrophobicity/hydrophilicity/electrostatic interactions. To optimize bacterial activity within an encapsulation matrix, modification of encapsulation matrices to alter their hydrophobicity, hydrophilicity, or electrostatic properties is possible. For example, Sakkos et al. incorporated methyltrimethoxysilane into silica gel to increase the hydrophobicity of the silica gel encapsulant, increasing the adsorption of a mixture of aromatic compounds (e.g., naphthalene, biphenyl, dibenzothiophene) 7-fold.⁴⁵ In another study, the homogenous distribution of the hydrophobic groups at pore surfaces, along with the co-localization of encapsulated bacteria, were able to enhance the simultaneous adsorption and biodegradation of hydrophobic atrazine.⁶⁵ With respect to electrostatic properties, functionalizing encapsulants with negatively charged groups promoted the attraction of positively charged metal ions. Indeed, Çelik et al. enriched a p(3-methoxyprophyl) acrylamide p(MPA) gel with 2-akrylamido-2-methyl-1-propane sulfonic acid (AMPS), adding anionic groups as a result, which improved the retention potential of cadmium by approximately 21%.⁴⁰ Electrostatic repulsion of negatively charged compounds, for example dichromate, as well as the electrostatic attraction of positively charged compounds, such as ammonium, has also been observed with alginate encapsulants, resulting in changes to the internal anion or cation concentrations.⁶² Electrical double layers formed at encapsulant surfaces have also been found to repel salt in the bulk solution, improving the tolerance of encapsulated bacteria for high concentrations of salt in the bulk solution.¹¹ Tuning encapsulants is advantageous to either improve substrate availability or repel, complex, or adsorb inhibitory substances, and if used strategically with specific waste streams, this should be able to improve resource recovery.

2.4 Improved reusability and storage

Another benefit of encapsulation is the fact that it preserves the activity of immobilized bacteria across operational cycles and during storage, greatly improving the longevity of the systems and providing the opportunity for long-term storage and stable and predictable performance. More than 60% of the original microbial activity has been retained throughout at least 3 cycles of operation using encapsulated bacteria for a range of applications, including hydrogen production,^11,55 enzymatic glucose production,³³ fermentative ethanol production,²⁸ and metal absorption/desorption.^{27,39–41,66} Compared with suspended bacteria, encapsulation enables the facile separation of desirable and active bacteria from an aqueous solution, which has enabled reuse of encapsulated bacteria for metal absorption²⁷ and sludge disintegration.^12,31,32 Encapsulation also prolongs bacterial storage. For example, encapsulation increased bacterial density and stability, which resulted in stable microbial fuel cell performance without maintenance for up to 5 months.⁶⁷ One study showed that when stored frozen at −18 °C with periodic substrate addition, encapsulated anaerobic sludge bacteria were able to retain a high level of metabolic activity for 3 years. This could facilitate the transport of encapsulated bacteria and potentially extend the geographical range of biological resource recovery.³⁷ Encapsulants have been observed, however, to decompose rapidly (e.g., several days) under some conditions.^11,37 This suggests that more study is needed before potential benefits, such as microbial activity across multiple operational cycles and extended bacterial storage capacity, are fully and predictably realized for resource recovery from waste.

3. Research opportunities and needs

There are multiple research opportunities and needs in encapsulation technology for biological resource recovery from waste streams, such as wastewater, metal wastes, and sludge, that should result in more predictable and prevalent application. These opportunities follow from the recent research in this area, reviewed above, and focus on taking further advantage of the inherent benefits of encapsulation technology. These research needs are described below.

• Better understanding of how the encapsulant matrix impacts bacterial growth and leakage, and as a result, understand and predict community changes within the encapsulants with time.

As observed in prior research,^42,45,46 encapsulant stiffness impacts the activity and the genetic profile (e.g., stress response) of encapsulated bacteria,¹³ and different materials can limit bacterial leakage²⁸ or encourage bacterial growth.⁴³ Nevertheless, insufficient research has been performed to enable predictive application of different encapsulants to achieve a desired outcome. Such research should take advantage of advanced molecular techniques (e.g., DNA microarrays, deep sequencing, metagenomics, metatranscriptomics) to understand the interplay between encapsulant chemistry/structural characteristics (e.g., stiffness) and microbial growth, leakage, and activity. In particular, the ability of novel encapsulant structures (e.g., multi-layer, core/shell) or tightly controlled encapsulant pore sizes to control growth/leakage/activity should be studied.

• Determine how the encapsulant chemistry controls the encapsulant internal environment.

Modification of encapsulation matrices by tuning the encapsulant hydrophobicity, hydrophilicity, charge, and pore size should facilitate a priori control of the encapsulant internal environment. This should significantly improve resource recovery by enhancing substrate availability, decreasing pH and salinity impacts, and excluding inhibitory products or contaminants from the encapsulation matrix. For example, encapsulants could be modified to improve partitioning and availability of less soluble organic wastes (e.g., fat, oil, grease) that could serve as high energy substrates for energy recovery. Additionally, supplementation of trace metals and conductive materials into encapsulants could improve anaerobic processes and methane production, improving resource recovery and the stability of specific encapsulated bacteria or biocatalysts.

• Modify encapsulants to improve the reusability and storability of encapsulated bacteria.

Finally, though studies have demonstrated the longevity of some encapsulated cultures, the cost and predictability of encapsulation technology depends on a reusable system that is able to be stored and distributed reliably. Improved encapsulant rigidity via coating or doping encapsulants with particular materials (e.g., chitosan, alumina nanoparticles) has been observed.^28,46,49 Nevertheless, research is needed to further improve the integrity and structural stability of encapsulants long term so that the duration of efficient and reliable biological resource recovery can be extended. In addition, work should focus on evaluating additives that preserve low levels of microbial activity long-term, yet enable rapid increases in activity and possibly growth when the encapsulated bacteria are placed into a waste stream. This type of controlled response to different environments would facilitate both prolonged bacterial storage and rapid start-up, again enabling more cost-effective resource recovery from waste. Unique properties could also be imparted to improve the separation and recovery of encapsulated bacteria (e.g., magnetic properties), particularly for biological processes in which encapsulated bacteria have unique capabilities that are important to retain.

Encapsulation technology has a large number of advantages and has been applied successfully at full scale for nitrogen removal,³ indicating that it can be cost effective. Nevertheless, applying this type of system more broadly will depend on increasing our ability to predict and control performance while also retaining particular microbial activities of interest. Such improvements will provide a path to advance encapsulation technology as a reliable and efficient option for resource recovery from waste streams. In turn, as more research is performed, the operation space that is critical for cost effective application of encapsulation technology will be defined, with life cycle and technoeconomic analysis, as well as analysis of the most beneficial resources to recover based on value and life cycle impact.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the funding provided by the University of Minnesota through the MnDRIVE Initiative.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ew00750a

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