Impact of nitrogen feeding regulation on polyhydroxyalkanoates production by mixed microbial cultures

PHA and EPS production from industrial wastewater by conventional activated sludge, membrane bioreactor and aerobic granular sludge technologies: A comprehensive comparison

The present study has focused on the mainstream integration of polyhydroxyalkanoate (PHA) production with industrial wastewater treatment by exploiting three different technologies all operating in sequencing batch reactors (SBR): conventional activated sludge (AS-SBR), membrane bioreactor (AS-MBR) and aerobic granular sludge (AGS). A full aerobic feast/famine strategy was adopted to obtain enrichment of biomass with PHA-storing bacteria. All the systems were operated at different organic loading (OLR) rate equal to 1-2-3 kgCOD/m3∙d in three respective experimental periods. The AS-MBR showed the better and stable carbon removal performance, whereas the effluent quality of the AS-SBR and AGS deteriorated at high OLR. Biomass enrichment with PHA-storing bacteria was successfully obtained in all the systems. The AS-MBR improved the PHA productivity with increasing OLR (max 35% w/w), whereas the AS-SBR reduced the PHA content (max 20% w/w) above an OLR threshold of 2 kgCOD/m3∙d. In contrast, in the AGS the increase of OLR resulted in a significant decrease in PHA productivity (max 14% w/w) and a concomitant increase of extracellular polymers (EPS) production (max 75% w/w). Results demonstrated that organic carbon was mainly driven towards the intracellular storage pathway in the AS-SBR (max yield 51%) and MBR (max yield 61%), whereas additional stressors in AGS (e.g., hydraulic selection pressure, shear forces) induced bacteria to channel the COD into extracellular storage compounds (max yield 50%) necessary to maintain the granule's structure. The results of the present study indicated that full-aerobic feast/famine strategy was more suitable for flocculent sludge-based technologies, although biofilm-like systems could open new scenarios for other biopolymers recovery (e.g., EPS). Moreover, the AS-MBR resulted the most suitable technology for the integration of PHA production in a mainstream industrial wastewater treatment plant, considering the greater process stability and the potential reclamation of the treated wastewater.

Combined recovery of polyhydroxyalkanoates and reclaimed water in the mainstream of a WWTP for agro-food industrial wastewater valorisation by membrane bioreactor technology

The present study investigated the combined production of reclaimed water for reuse purposes and polyhydroxyalkanoates (PHA) from an agro-food industrial wastewater. A pilot plant implementing a two-stage process for PHA production was studied. It consisted of a mainstream sequencing batch membrane bioreactor (SBMBR) in which selection of PHA-accumulating organisms and wastewater treatment were carried out in, and a side-stream fed-batch reactor (FBR) where the excess sludge from the SBMBR was used for PHA accumulation. The performance of the SBMBR was compared with that of a conventional sequencing batch reactor (SBR) treating the same wastewater under different food to microorganisms' ratios (F/M) ranging between 0.125 and 0.650 kgCOD kgTSS-3 d-1. The SBMBR enabled to obtain very high-quality effluent in compliance with the relevant national (Italy) and European regulations (Italian DM 185/03 and EU, 2020/741) in the field of wastewater reclamation, whereas the performances in the SBR collapsed at F/M higher than 0.50 kgCOD kgTSS-1d-1. A maximum intracellular storage of 45% (w/w) and a production yield of 0.63 gPHA L-1h-1 were achieved when the SBMBR system was operated with a F/M ratio close to 0.50 kgCOD kgTSS-1d-1. This resulted approximately 35% higher than those observed in the SBR, since the ultrafiltration membrane avoided the washout of dispersed and filamentous bacteria capable of storing PHA. Furthermore, while maximizing PHA productivity in conventional SBR systems led to process dysfunctions, in the SBMBR system it helped mitigate these issues by reducing membrane fouling behaviour. The results of this study supported the possibility to achieve combined recovery of reclaimed water and high-value added bioproducts using membrane technology, leading the way for agro-food industrial wastewater valorization in the frame of a circular economy model.

Unlocking growth potential in Halomonas bluephagenesis for enhanced PHA production with sulfate ions

The mutant strain Halomonas bluephagenesis (TDH4A1B5P) was found to produce PHA under low-salt, non-sterile conditions, but the yield was low. To improve the yield, different nitrogen sources were tested. It was discovered that urea was the most effective nitrogen source for promoting growth during the stable stage, while ammonium sulfate was used during the logarithmic stage. The growth time of H. bluephagenesis (TDH4A1B5P) and its PHA content were significantly prolonged by the presence of sulfate ions. After 64 hr in a 5-L bioreactor supplemented with sulfate ions, the dry cell weight (DCW) of H. bluephagenesis weighed 132 g/L and had a PHA content of 82%. To promote the growth and PHA accumulation of H. bluephagenesis (TDH4A1B5P), a feeding regimen supplemented with nitrogen sources and sulfate ions with ammonium sodium sulfate was established in this study. The DCW was 124 g/L, and the PHA content accounted for 82.3% (w/w) of the DCW, resulting in a PHA yield of 101 g/L in a 30-L bioreactor using the optimized culture strategy. In conclusion, stimulating H. bluephagenesis (TDH4A1B5P) to produce PHA is a feasible and suitable strategy for all H. bluephagenesis.

Production of polyhydroxyalkanoate (PHA) copolymer from food waste using mixed culture for carboxylate production and Pseudomonas putida for PHA synthesis

Production of polyhydroxyalkanoates (PHAs) with high concentration of carboxylate, that was accumulated from solid state fermentation (SSF) of food waste (FW), was tested using Pseudomonas putida strain KT2440. Mixed-culture SSF of FW supplied in a high concentration of carboxylate, which caused a high PHA production of 0.56 g PHA/g CDM under nutrients control. Interestingly, this high PHA fraction in CDM was almost constant at 0.55 g PHA/g CDM even under high nutrients concentration (25 mM NH₄⁺), probably due to high reducing power maintained by high carboxylate concentration. PHA characterization indicated that the dominant PHA building block produced was 3-hydroxybutyrate, followed by 3-hydroxy-2-methylvalerate and 3-hydroxyhenxanoate. Carboxylate profiles before and after PHA production suggested that acetate, butyrate, and propionate were the main precursors to PHA via several metabolic pathways. Our result support that mixed culture SSF of FW for high concentration carboxylate and P. putida for PHA production enables sustainable production of PHA in cost-effective manners.

Donnan Dialysis for Recovering Ammonium from Fermentation Solutions Rich in Volatile Fatty Acids

For the production of polyhydroxyalkanoates (PHA) using nitrogen-rich feedstocks (e.g., protein-rich resources), the typical strategy of restricting cell growth as a means to enhance overall PHA productivity by nitrogen limitation is not applicable. In this case, a possible alternative to remove the nitrogen excess (NH₄⁺/NH₃) is by applying membrane separation processes. In the present study, the use of Donnan dialysis to separate ammonium ions from volatile fatty acids present in the media for the production of PHA was evaluated. Synthetic and real feed solutions were used, applying NaCl and HCl receiver solutions separated by commercial cation-exchange membranes. For this specific purpose, Fumasep and Ralex membranes showed better performance than Ionsep. Sorption of ammonium ions occurred in the Ralex membrane, thus intensifying the ammonium extraction. The separation performances with NaCl and HCl as receiver solutions were similar, despite sorption occurring in the Ralex membrane more intensely in the presence of NaCl. Higher volumetric flow rates, NaCl receiver concentrations, and volume ratios of feed:receiver solutions enhanced the degree of ammonium recovery. The application of an external electric potential difference to the two-compartment system did not significantly enhance the rate of ammonium appearance in the receiver solution. The results obtained using a real ammonium-containing solution after fermentation of cheese whey showed that Donnan dialysis can be successfully applied for ammonium recovery from such solutions.

Process conditions affect properties and outcomes of polyhydroxyalkanoate accumulation in municipal activated sludge

The developments of mixed culture polyhydroxyalkanoate production has been directed to maximize the biomass PHA content with limited attention to polymer quality. Direct comparison of PHA accumulation literature is challenging, and even regularly contradicting in reported results, due to underlying differences that are not well expressed. A study was undertaken to systematically compare the commonly reported process conditions for PHA accumulation by full-scale municipal activated sludge. A biomass acclimation step combined with a pulse-wise feeding strategy resulted in maximum average PHA contents and product yields. pH control and active nitrification did not result in observable effects on the PHA productivity. Under these conditions a high molecular weight polymer (1536 ± 221 kDa) can be produced. Polymer extraction recoveries were influenced by the PHA molecular weight. A standard protocol for an activated sludge PHA accumulation test including downstream processing and standardized extraction has been developed and is available as supplementary material.

Advances and trends in microbial production of polyhydroxyalkanoates and their building blocks

With the rapid development of synthetic biology, a variety of biopolymers can be obtained by recombinant microorganisms. Polyhydroxyalkanoates (PHA) is one of the most popular one with promising material properties, such as biodegradability and biocompatibility against the petrol-based plastics. This study reviews the recent studies focusing on the microbial synthesis of PHA, including chassis engineering, pathways engineering for various substrates utilization and PHA monomer synthesis, and PHA synthase modification. In particular, advances in metabolic engineering of dominant workhorses, for example Halomonas, Ralstonia eutropha, Escherichia coli and Pseudomonas, with outstanding PHA accumulation capability, were summarized and discussed, providing a full landscape of diverse PHA biosynthesis. Meanwhile, we also introduced the recent efforts focusing on structural analysis and mutagenesis of PHA synthase, which significantly determines the polymerization activity of varied monomer structures and PHA molecular weight. Besides, perspectives and solutions were thus proposed for achieving scale-up PHA of low cost with customized material property in the coming future.

From Organic Wastes and Hydrocarbons Pollutants to Polyhydroxyalkanoates: Bioconversion by Terrestrial and Marine Bacteria

The use of fossil-based plastics has become unsustainable because of the polluting production processes, difficulties for waste management sectors, and high environmental impact. Polyhydroxyalkanoates (PHA) are bio-based biodegradable polymers derived from renewable resources and synthesized by bacteria as intracellular energy and carbon storage materials under nutrients or oxygen limitation and through the optimization of cultivation conditions with both pure and mixed culture systems. The PHA properties are affected by the same principles of oil-derived polyolefins, with a broad range of compositions, due to the incorporation of different monomers into the polymer matrix. As a consequence, the properties of such materials are represented by a broad range depending on tunable PHA composition. Producing waste-derived PHA is technically feasible with mixed microbial cultures (MMC), since no sterilization is required; this technology may represent a solution for waste treatment and valorization, and it has recently been developed at the pilot scale level with different process configurations where aerobic microorganisms are usually subjected to a dynamic feeding regime for their selection and to a high organic load for the intracellular accumulation of PHA. In this review, we report on studies on terrestrial and marine bacteria PHA-producers. The available knowledge on PHA production from the use of different kinds of organic wastes, and otherwise, petroleum-polluted natural matrices coupling bioremediation treatment has been explored. The advancements in these areas have been significant; they generally concern the terrestrial environment, where pilot and industrial processes are already established. Recently, marine bacteria have also offered interesting perspectives due to their advantageous effects on production practices, which they can relieve several constraints. Studies on the use of hydrocarbons as carbon sources offer evidence for the feasibility of the bioconversion of fossil-derived plastics into bioplastics.

Ammonia recovery from acidogenic fermentation effluents using a gas-permeable membrane contactor

A gas-permeable membrane (GPM) contactor was used to recover ammoniacal nitrogen from a synthetic and a biowaste fermentation broth under different pH (from 6 to 11) and temperatures (35 and 55 °C). Ammonia mass transfer constant (K_m) increased as pH and temperature increased. For synthetic broth, pH 10 provided the best results, when considering the K_m (9.2·10^-7 m·s^-1) and the reagents consumption (1.0 mol NaOH·mol^-1 TAN and 0.6 mol H₂SO₄·mol^-1 TAN). Biowaste fermentation generated a broth with a high concentration of ammoniacal nitrogen (4.9 g N·L^-1) and volatile fatty acids (VFA) (41.1 g COD·L^-1). Experiments using the biowaste broth showed a lower K_m (5.0·10^-7 m·s^-1 at pH 10) than the synthetic broth, related to the solution matrix and other species interference. VFAs were not detected in the trapping solution. Overall, these results show that GPM is a suitable technology to efficiently separate ammoniacal nitrogen and VFA from fermentation broths.

Effect of the organic loading rate on the PHA-storing microbiome in sequencing batch reactors operated with uncoupled carbon and nitrogen feeding

Over the last years, in a search for sustainable and biodegradable alternatives to petrol-based plastics, biotechnological applications turned to the potentialities of mixed microbial cultures (MMC) for producing polyhydroxyalkanoates (PHAs). Under a feast and famine regime, an uncoupled carbon (C) and nitrogen (N)-feeding strategy may be adopted by dosing the C-source at the beginning of the feast and the N-source at the beginning of the famine in order to stimulate a PHA storage response and microbial growth. Even though this strategy has been already successfully applied for the PHA production, very few information is to date available regarding the MMC operating in these systems and the influence of Organic Loading Rate (OLR) on their selection and enrichment. To fill the gap, this study investigated the effect of the OLR on the selection of PHA-accumulating microorganisms in a sequencing batch reactor (SBR) operated with an uncoupled C and N feeding strategy. The SBR cycle length was set at 12 h and four OLRs values (4.25, 8.50, 12.75 and 18 gCOD L^-1 d^-1) were tested by changing the concentration of the feeding solution, made of a synthetic mixture of acetic (85% of the overall COD) and propionic (15%) acids. The PHA-storage yield increased by increasing the OLR (up to 0.69 COD/COD at 12.75 gCOD L^-1 d^-1) but significantly decreased (0.27 COD/COD) at 18 gCOD L^-1 d^-1 concomitantly with a longer feast phase and a lower PHA content in the biomass at the end of the feast phase. The selective pressure induced by the applied OLRs strongly influenced the microbiome composition revealing a high content of putative PHA-storing bacteria, such as Rhodobacter, Thauera and Paracoccus, in the SBR operated at OLRs 4.25, 8.50 and 12.75 g COD L^-1 d^-1 (up to 97.4% of total reads) and a low content (5.4%) in the SBR at 18 g COD L^-1 d^-1where the predominance of genus Nitrinicola was instead observed.

Rapid quantification of intracellular polyhydroxyalkanoates via fluorescence techniques: A critical review

Polyhydroxyalkanoates (PHA) are promising bioplastics with excellent physicochemical properties and biodegradability, whereas PHA products suffer from high manufacturing costs. To reduce costs of PHA production, experiments with mixed microbial cultures and low-cost substrates have been conducted widely, where rapid and robust PHA quantification methods are necessary. Compared with traditional gas chromatography methods, PHA fluorescence quantification (PHA-FQ) methods may be quicker, safer and more suitable for modern experiments with high throughput requirements. However, practical applications of PHA-FQ methods are still limited. Therefore, this review provides a comprehensive understanding of PHA-FQ methods. Performance of PHA-staining fluorochromes, relevant spectral properties, and important staining procedures are summarized. Current developments of PHA-FQ protocols are critically reviewed. Main considerations needed to make PHA-FQ protocol reliable are comprehensively discussed. Finally, potential improvements in various aspects of PHA-FQ methods are highlighted. This review could help researchers develop more effective PHA-FQ methods and facilitate future experiments related to PHA.

The impact of carbon to nitrogen ratios and pH on the microbial prevalence and polyhydroxybutyrate production levels using a mixed microbial starter culture

Growth conditions have been frequently studied in optimizing polyhydroxybutyrate (PHB) production, while few studies were performed to unravel the dynamic mixed microbial consortia (MMCs) in the process. In this study, the relationship between growth conditions (C/N ratios and pH) and the corresponding key-microbes were identified and monitored during PHB accumulation. The highest PHB level (70 wt% of dry cell mass) was obtained at pH 9, C/N 40, and acetic acid 10 g/L. Linking the dominant genera with the highest point of PHB accumulation, Thauera was the most prevalent species in all MMCs of pH 9, except when a C/N ratio of 1 was applied. Notably, dominant bacteria shifted at pH 7 (C/N 10) from Thauera (0 h) to Paracoccus, and subsequently to Alcaligenes following the process of PHB accumulation and consumption. Further understanding of the relationship between the structure of the microbial community and the performance will be beneficial for regulating and obtaining high PHB accumulation within an MMC. Our study illustrates the impact of C/N ratios and pH on microbial prevalence and PHB production levels using a mixed microbial starter culture. This knowledge will broaden industrial perspectives for regulating high PHB production and timely harvesting.

Valorization of lipid-rich wastewaters: A theoretical analysis to tackle the competition between polyhydroxyalkanoate and triacylglyceride-storing populations

The lipid fraction of the effluents generated in several food-processing activities can be transformed into polyhydroxyalkanoates (PHAs) and triacylglycerides (TAGs), through open culture biotechnologies. Although competition between storing and non-storing populations in mixed microbial cultures (MMCs) has been widely studied, the right selective environment allowing for the robust enrichment of a community when different types of accumulators coexist is still not clear. In this research, comprehensive metabolic analyses of PHA and TAG synthesis and degradation, and concomitant respiration of external carbon, were used to understand and explain the changes observed in a laboratory-scale bioreactor fed with the lipid-rich fraction (mainly oleic acid) of a wastewater stream produced in the fish-canning industry. It was concluded that the mode of oxygen, carbon, and nitrogen supply determines the enrichment of the culture in specific populations, and hence the type of intracellular compounds preferentially accumulated. Coupled carbon and nitrogen feeding regime mainly selects for TAG producers whereas uncoupled feeding leads to PHA or TAG production function of the rate of carbon supply under specific aeration rates and feast and famine phases lengths.

The impact of biomass withdrawal strategy on the biomass selection and polyhydroxyalkanoates accumulation of mixed microbial cultures

The production of polyhydroxyalkanoates (PHA) by mixed microbial cultures (MMC) has been studied as an alternative to pure cultures in order to reduce the price of PHA through use of open systems and low-cost substrates, such as agro-industrial sub-products. However, the widespread applicability of this process depends on the optimization of operational factors impacting PHA productivity. This study addresses the impact of biomass withdrawal strategy on the performance of MMC selection reactors and consequently on biomass productivity and global PHA productivity. Two selection reactors were operated in parallel under similar conditions, except for the timing of biomass withdrawal, at the end of the famine phase (Reactor 1, R1) versus at the end of the feast phase (Reactor 2, R2) at an organic loading rate of 100 Cmmol.L^-1.d^-1 and solids retention time of 4 days. The biomass selected in both conditions had similar PHA storing capacity as shown by the similar yields of PHA per substrate obtained in the accumulation assays; however, R1 reached a higher biomass productivity (about 4-fold higher than R2). This study demonstrated that removing the excess biomass at the end of the famine phase resulted in a much higher global PHA productivity and that the key parameter affecting the global PHA productivity of the 2-stage system was the volumetric biomass productivity. Results obtained provide important insight into how MMC systems can be best operated to maximize PHA productivity.

Polyhydroxyalkanoates Production by Mixed Microbial Culture under High Salinity

The fishing industry produces vast amounts of saline organic side streams that require adequate treatment and disposal. The bioconversion of saline resources into value-added products, such as biodegradable polyhydroxyalkanoates (PHAs), has not yet been fully explored. This study investigated PHA production by mixed microbial cultures under 30 gNaCl/L, the highest NaCl concentration reported for the acclimatization of a PHA-accumulating mixed microbial culture (MMC). The operational conditions used during the culture-selection stage resulted in an enriched PHA-accumulating culture dominated by the Rhodobacteraceae family (95.2%) and capable of storing PHAs up to 84.1% wt. (volatile suspended solids (VSS) basis) for the highest organic loading rate (OLR) applied (120 Cmmol/(L.d)). This culture presented a higher preference for the consumption of valeric acid (0.23 ± 0.03 CmolHVal/(CmolX.h)), and the 3HV monomer polymerization (0.33 ± 0.04 CmmolHV/(CmmolX.h) was higher as well. As result, a P(3HB-co-3HV)) with high HV content (63% wt.) was produced in the accumulation tests conducted at higher OLRs and with 30 gNaCl/L. A global volumetric PHA productivity of 0.77 gPHA/(L.h) and a specific PHA productivity of 0.21 gPHA/(gX.h) were achieved. These results suggested the significant potential of the bioconversion of saline resources into value-added products, such as PHAs.

Insightful Advancement and Opportunities for Microbial Bioplastic Production

Impetuous urbanization and population growth are driving increased demand for plastics to formulate impeccable industrial and biomedical commodities. The everlasting nature and excruciating waste management of petroleum-based plastics have catered to numerous challenges for the environment. However, just implementing various end-of-life management techniques for assimilation and recycling plastics is not a comprehensive remedy; instead, the extensive reliance on finite resources needs to be reduced for sustainable production and plastic product utilization. Microorganisms, such as bacteria and algae, are explored substantially for their bioplastic production repertoire, thus replacing fossil-based plastics sooner or later. Nevertheless, the utilization of pure microbial cultures has led to various operational and economical complications, opening the ventures for the usage of mixed microbial cultures (MMCs) consisting of bacteria and algae for sustainable production of bioplastic. The current review is primarily focuses on elaborating the bioplastic production capabilities of different bacterial and algal strains, followed by discussing the quintessence of MMCs. The present state-of-the-art of bioplastic, different types of bacterial bioplastic, microalgal biocomposites, operational factors influencing the quality and quantity of bioplastic precursors, embracing the potential of bacteria-algae consortia, and the current global status quo of bioplastic production has been summarized extensively.

Sludge retention time impacts on polyhydroxyalkanoate productivity in uncoupled storage/growth processes

The process involving mixed microbial cultures (MMCs) and waste-based substrates emerged as an alternative solution to reduce the market price of polyhydroxyalkanoates (PHAs). The selection of an efficient MMC that displays a significant PHA accumulation potential and a high growth rate is considered a key factor for the MMC PHA production feasibility. This study used a pilot plant to investigate the dynamics of growth vs storage in a mixed culture fed with fermented fruit waste under uncoupled carbon and nitrogen feeding. Varying sludge retention times (SRTs) (2 and 4 d) and organic loading rates (OLRs) (from 2.6 to 14.5 gCOD.L^-1.d^-1) were imposed for this purpose. Results showed that, regardless of the OLR imposed, cultures selected at lower SRT grew faster and more efficiently using stored PHA. However, they had inferior specific storage rates and accumulation capacity, resulting in lower PHA productivity. Additionally, the polymer storage yield was independent of the SRT, and was directly linked with the abundance of putative PHA-storers in the MMC. The high PHA productivity (4.6 ± 0.3 g.L^-1.d^-1) obtained for the culture selected at 4 d of SRT was 80% above that obtained for the lower SRT tested, underlining the importance of achieving a good balance between culture growth and accumulation capacity to increase the viability of the PHA-producing process from wastes.

Optimization of aerobic dynamic discharge process for very rapid enrichment of polyhydroxyalkanoates-accumulating bacteria from activated sludge

The aerobic dynamic discharge (ADD) process has the potential to reduce the enrichment period of polyhydroxyalkanoates (PHA)-accumulating bacteria in PHA production using mixed microbial cultures (MMCs). This study aimed to efficiently enrich PHA-accumulating bacteria from activated sludge within a fixed period of 2 d by optimizing operating conditions of the ADD process. Based on the results, enrichment with separate feeding of carbon and nutrients in the feast and famine phases, respectively, and a settling duration of 10 min after the feast phase in the sequencing batch cycle for 12 h was found to be optimal. The MMC enriched at optimum conditions could store as much as 68.4 wt% of PHA. Dechloromonas and Zoogloea were identified as potential PHA-accumulating bacteria responsible for enhancing PHA accumulation ability in the enriched MMC. The optimized ADD process will facilitate the consecutive use of daily generated waste activated sludge for PHA production.

Established and Emerging Producers of PHA: Redefining the Possibility

The polyhydroxyalkanoate was discovered almost around a century ago. Still, all the efforts to replace the traditional non-biodegradable plastic with much more environmentally friendly alternative are not enough. While the petroleum-based plastic is like a parasite, taking over the planet rapidly and without any feasible cure, its perennial presence has made the ocean a floating island of life-threatening debris and has flooded the landfills with toxic towering mountains. It demands for an immediate solution; most resembling answer would be the polyhydroxyalkanoates. The production cost is yet one of the significant challenges that various corporate is facing to replace the petroleum-based plastic. To deal with the economic constrain better strain, better practices, and a better market can be adopted for superior results. It demands for systems for polyhydroxyalkanoate production namely bacteria, yeast, microalgae, and transgenic plants. Solely strains affect more than 40% of overall production cost, playing a significant role in both upstream and downstream processes. The highly modifiable nature of the biopolymer provides the opportunity to replace the petroleum plastic in almost all sectors from food packaging to medical industry. The review will highlight the recent advancements and techno-economic analysis of current commercial models of polyhydroxyalkanoate production. Bio-compatibility and the biodegradability perks to be utilized highly efficient in the medical applications gives ample reason to tilt the scale in the favor of the polyhydroxyalkanoate as the new conventional and sustainable plastic.

Polyhydroxyalkanoate (PHA) production via resource recovery from industrial waste streams: A review of techniques and perspectives

The problem of waste generation in the form of wastewater and solid wastes has caused an urgent, yet persisting, global issue that calls for the development of sustainable treatment and resource recovery technologies. The production of value-added polyhydroxyalkanoates (PHAs) from industrial waste streams has attracted the attention of researchers and process industries because they could replace traditional plastics. PHAs are biopolymers with high degradability, with a variety of applications in the manufacturing sector (e.g. medical equipment, packaging). The aim of this review is to describe the techniques and industrial waste streams that are applied for PHA production. The different enrichment and accumulation techniques that employ mixed microbial communities and carbon recovery from industrial waste streams and various downstream processes were reviewed. PHA yields between 7.6 and 76 wt% were reported for pilot-scale PHA production; while, at the laboratory-scale, yields from PHA accumulation range between 8.6 and 56 wt%. The recent advances in the application of waste streams for PHA production could result in more widely spread PHA production at the industrial scale via its integration into biorefineries for co-generation of PHAs with other added-value products like biohydrogen and biogas.

Emergent Approaches to Efficient and Sustainable Polyhydroxyalkanoate Production

Petroleum-derived plastics dominate currently used plastic materials. These plastics are derived from finite fossil carbon sources and were not designed for recycling or biodegradation. With the ever-increasing quantities of plastic wastes entering landfills and polluting our environment, there is an urgent need for fundamental change. One component to that change is developing cost-effective plastics derived from readily renewable resources that offer chemical or biological recycling and can be designed to have properties that not only allow the replacement of current plastics but also offer new application opportunities. Polyhydroxyalkanoates (PHAs) remain a promising candidate for commodity bioplastic production, despite the many decades of efforts by academicians and industrial scientists that have not yet achieved that goal. This article focuses on defining obstacles and solutions to overcome cost-performance metrics that are not sufficiently competitive with current commodity thermoplastics. To that end, this review describes various process innovations that build on fed-batch and semi-continuous modes of operation as well as methods that lead to high cell density cultivations. Also, we discuss work to move from costly to lower cost substrates such as lignocellulose-derived hydrolysates, metabolic engineering of organisms that provide higher substrate conversion rates, the potential of halophiles to provide low-cost platforms in non-sterile environments for PHA formation, and work that uses mixed culture strategies to overcome obstacles of using waste substrates. We also describe historical problems and potential solutions to downstream processing for PHA isolation that, along with feedstock costs, have been an Achilles heel towards the realization of cost-efficient processes. Finally, future directions for efficient PHA production and relevant structural variations are discussed.

A fattening factor to quantify the accumulation ability of microorganisms under N-starvation

Many microorganisms can accumulate biomass in the form of lipids and polysaccharides, which can be used for biofuels, bioplastics, food and feed. Some innovative bioprocesses exploit the competitive advantage provided by such accumulation ability, mainly under N-starvation, to select high-accumulating strains against biological contaminants, by using uncoupled nutrient feeding. However, there is no general and easily comparable parameter available to compare biomass accumulation ability among different microbial strains, which could measure the competitive advantage. Here, a parameter termed "fattening factor" (η_x) is described to quantify such strain-specific biomass accumulation ability in bacteria, yeasts and microalgae. This parameter measures how many fold a microbial population can increase its biomass just as the result of accumulation. It is derived from considerations about the main metabolic aspects of cells' response to N-starvation, which induces variations in cell cycle, biomass production and biochemical composition. The fattening factor described here should be easily estimatable in N-starvation for every culturable microbial strain, by measuring the amount of accumulated biomass.

Cultivation processes to select microorganisms with high accumulation ability

The microbial ability to accumulate biomolecules is fundamental for different biotechnological applications aiming at the production of biofuels, food and bioplastics. However, high accumulation is a selective advantage only under certain stressful conditions, such as nutrient depletion, characterized by lower growth rate. Conventional bioprocesses maintain an optimal and stable environment for large part of the cultivation, that doesn't reward cells for their accumulation ability, raising the risk of selection of contaminant strains with higher growth rate, but lower accumulation of products. Here in this work the physiological responses of different microorganisms (microalgae, bacteria, yeasts) under N-starvation and energy starvation are reviewed, with the aim to furnish relevant insights exploitable to develop tailored bioprocesses to select specific strains for their higher accumulation ability. Microorganism responses to starvation are reviewed focusing on cell cycle, biomass production and variations in biochemical composition. Then, the work describes different innovative bioprocess configurations exploiting uncoupled nutrient feeding strategies (feast-famine), tailored to maintain a selective pressure to reward the strains with higher accumulation ability in mixed microbial populations. Finally, the main models developed in recent studies to describe and predict microbial growth and intracellular accumulation upon N-starvation and feast-famine conditions have been reviewed.

A novel strategy for triacylglycerides and polyhydroxyalkanoates production using waste lipids

Lipids are one of the main components of the organic matter present in the effluents of the food-processing industry. These waste streams can be biotransformed into valuable triacylglycerides (TAGs) and polyhydroxyalkanoates (PHAs), precursors of biofuels and biomaterials alternative to petroleum-based products. These compounds are yielded by mixed microbial cultures, and considering that both TAG and PHA accumulators may coexist within the community, it seems crucial to define those operational strategies that might control the selection of the dominant metabolic pathways (TAG or PHA accumulation). In this work, residual fish-canning oil was used as a carbon source in a two-stage process (culture selection and intracellular compounds accumulation) in which the substrate was simultaneously hydrolyzed in these two stages without the need for a previous fermentation unit. It was pretended to maximize preferential TAG or PHA storage in the accumulation reactor by the imposition of certain selective pressures in the enrichment one. Uncoupling C and N feedings and limiting nitrogen availability in the medium, allowed to maximize PHA production (82.3 wt% of PHAs, 0.80 Cmmol_PHA/Cmmol_S). Besides, when low pH in the famine phase was considered as additional selective pressure, it was possible to shift the ratio TAG:PHA from 4:96 obtaining 43.0 wt% of TAGs (0.67 Cmmol_TAG/Cmmol_S). Therefore, this novel and simplified process demonstrated versatility and efficiency in the storage of TAGs and PHAs from a unique residual feedstock and using an open culture proving that product selection can be harnessed if choosing the right operational conditions in the enrichment stage.

Waste to bioplastics: How close are we to sustainable polyhydroxyalkanoates production?

Increased awareness of environmental sustainability with associated strict environmental regulations has incentivized the pursuit of novel materials to replace conventional petroleum-derived plastics. Polyhydroxyalkanoates (PHAs) are appealing intracellular biopolymers and have drawn significant attention as a viable alternative to petrochemical based plastics not only due to their comparable physiochemical properties but also, their outstanding characteristics such as biodegradability and biocompatibility. This review provides a comprehensive overview of the recent developments on the involved PHA producer microorganisms, production process from different waste streams by both pure and mixed microbial cultures (MMCs). Bio-based PHA production, particularly using cheap carbon sources with MMCs, is getting more attention. The main bottlenecks are the low production yield and the inconsistency of the biopolymers. Bioaugmentation and metabolic engineering together with cost effective downstream processing are promising approaches to overcome the hurdles of commercial PHA production from waste streams.

Influence of inoculum variation and nutrient availability on polyhydroxybutyrate production from activated sludge

Carbon recovery through polyhydroxybutyrate (PHB) production can create a value-added waste management system. Activated sludge as inoculum enables PHB production using cheap and renewable carbons source, bringing PHB at par to conventional plastics. The PHB accumulating potential of activated sludge needs to be improved to realize the objective. The interaction between the origin of activated sludge, petroleum refinery sludge and brewery sludge, and nitrogen availability was explored to effect culture enrichment, improve PHB accumulation, and polymer characteristics through aerobic dynamic feeding. Consequently, nitrogen excess and limitation enrichment of both sludges produced mix microbial culture with adequate PHB storage of 7.8 ± 0.05%, 14.4 ± 0.04%, 14.4 ± 0.04%, 13.4 ± 0.02% respectively. Batch accumulation revealed higher PHB accumulation of 76.1 ± 0.03% and 71.7 ± 0.05% under nitrogen limitation for PRS and BS enriched under nitrogen excess condition compared to any other combination. The higher decomposition temperature of 285 °C, 293 °C, and a lower melting point of 168 °C, 165 °C with a higher molecular weight of 4.3x10⁵g/mol and semi-crystalline arrangement indicates the potential applications for extracted PHB. PHB production enhanced under nitrogen limited conditions with culture enriched under nitrogen excess condition. However, similar PHB storage, physiochemical property, and overlapping microbial community show an insignificant effect of sludge origin on PHB production.

Recent developments in Polyhydroxyalkanoates (PHAs) production - A review

Polyhydroxyalkanoates (PHAs) are inevitably a key biopolymer that has the potential to replace the conventional petrochemical based plastics that pose jeopardy to the environment globally. Even then the reach of PHA in the common market is so restricted. The economy of PHA is such that, even after several attempts the overall production cost seems to be high and this very factor surpasses PHAs usage when compared to the conventional polymers. The major focus of the review relies on the synthesis of PHA from Mixed Microbial Cultures (MMCs), through a 3-stage process most probably utilizing feedstocks from waste streams or models that mimic them. Emphasis was given to the works carried out in the past decade and their coherence with each and every individual criteria (Aeration, Substrate and bioprocess parameters) such that to understand their effect in enhancing the overall production of PHA.

Recovery of polyhydroxyalkanoates (PHAs) from wastewater: A review

Polyhydroxyalkanoates (PHAs) are biopolyesters accumulated as carbon and energy storage materials under unbalanced growth conditions by various microorganisms. They are one of the most promising potential substitutes for conventional non-biodegradable plastics due to their similar physicochemical properties, but most important, its biodegradability. Production cost of PHAs is still a great barrier to extend its application at industrial scale. In order to reduce that cost, research is focusing on the use of several wastes as feedstock (such as agro-industrial and municipal organic waste and wastewater) in a platform based on mixed microbial cultures. This review provides a critical illustration of the state of the art of the most likely-to-be-scale-up PHA production processes using mixed microbial cultures platform and waste streams as feedstock, with a particular focus on both, upstream and downstream processes. Current pilot scale studies, future prospects, challenges and developments in the field are also highlighted.

High rate selection of PHA accumulating mixed cultures in sequencing batch reactors with uncoupled carbon and nitrogen feeding

The selection and enrichment of a mixed microbial culture (MMC) for polyhydroxyalkanoates (PHA) production is a well-known technology, typically carried out in sequencing batch reactors (SBR) operated under a feast-famine regime. With a nitrogen-deficient carbon source to be used as feedstock for PHA synthesis, a nutrient supply in the SBR is required for efficient microbial growth. In this study, an uncoupled carbon (C) and nitrogen (N) feeding strategy was adopted by dosing the C-source at the beginning of the feast and the N-source at the beginning of the famine, at a fixed C/N ratio of 33.4 g COD/g N and 12 h cycle length. The applied organic loading rate (OLR) was increased from 4.25 to 8.5 and finally to 12.725 g COD/L d. A more efficient selective pressure was maintained at lower and intermediate OLR, where the feast phase length was shorter (around 20 % of the whole cycle length). However, at the higher OLR investigated, the PHA content in the biomass reached a value of 0.53 g PHA/g VSS at the end of the feast phase, as a consequence of the increased C-source loaded per cycle. Moreover, 2nd stage PHA productivity was 2.4 g PHA/L d, 1.5 and 3.0-fold higher than those obtained at lower OLR. The results highlight the possibility of simplifying the process by withdrawing the biomass at the end of the feast phase directly to downstream processing, without a need for the intermediate accumulation step.

Ramakrishnan R, Venkateshaiah A, Krawczyk K, Padil VVT, Sobel B, Černík M. The Use of a Biopolymer Conjugate for an Eco-Friendly One-Pot Synthesis of Palladium-Platinum Alloys

Raising health and environmental concerns over the nanoparticles synthesized from hazardous chemicals have urged researchers to focus on safer, environmentally friendlier and cheaper alternatives as well as prompted the development of green synthesis. Apart from many advantages, green synthesis is often not selective enough (among other issues) to create shape-specific nanoparticle structures. Herein, we have used a biopolymer conjugate and Pd and Pt precursors to prepare sustainable bimetallic nanoparticles with various morphology types. The nanoparticles were synthesized by a novel green approach using a bio-conjugate of chitosan and polyhydroxybutyrate (Cs-PHB). The bio-conjugate plays the simultaneous roles of a reducing and a capping agent, which was confirmed by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and energy dispersive X-ray spectrometry (EDS) analysis, proving the presence of a Cs-PHB layer on the surface of the prepared nanoparticles. The EDS profile also revealed the elemental structure of these nanoparticles and confirmed the formation of a Pd/Pt alloy. TEM morphological analysis showed the formation of star-like, octahedron or decahedron Pd/Pt nanoparticles, depending on the synthesis conditions. The bimetallic Pd/Pt nanoparticles synthesized with various Pd/Pt molar ratios were successfully applied for the catalytic reduction of 4-nitrophenol to 4-aminophenol by borohydride. The calculated κc values (ratio of k_app to the concentration of the catalyst) revealed that the decahedron nanoparticles (size of 15 ± 4 nm), synthesized at the molar ratio of 2:1 (Pd/Pt), temperature of 130 °C, 10 g/L of Cs-PHB conjugate and time of 30 min, exhibited excellent catalytic activity compared to other bimetallic nanoparticles reported in the literature.

Interactive effect of crude glycerin concentration and C:N ratio on polyhydroxyalkanoates accumulation by mixed microbial cultures modelled with Response Surface Methodology

Response Surface Methodology (RSM) was used to investigate how the crude glycerin concentration and the carbon to nitrogen (C:N) ratio in the culture medium affect four indicators of polyhydroxyalkanoates (PHAs) accumulation by mixed microbial cultures (MMC): the observed coefficient of active-biomass yield (Y_obs,BA), the observed coefficient of PHA yield (Y_obs,PHA), the PHA content in biomass (X_PHA) and the volumetric productivity (Pr_V). The C:N ratio had the largest effect on Y_obs,BA and Y_obs,PHA. When the C:N ratio was increased, Y_obs,BA decreased and Y_obs,PHA increased, regardless of the concentration of crude glycerin in the culture medium. The C:N ratio also had the largest effect on the PHA content, whereas volumetric productivity was strongly affected by both the C:N ratio and the crude glycerin concentration. The optimal conditions for PHA accumulation were a crude glycerin concentration of 8954 mg COD/L with a C:N ratio of 15.9 mg C/mg N-NH₄, which gave a Y_obs,BA of 0.29 mg COD_BA/mg COD, a Y_obs,PHA of 0.28 mg COD_PHA/mg COD, a X_PHA of 55.6% VSS and a Pr_V of 757.3 mg COD_PHA/L⋅d (550.0 mg PHA/L⋅d). The accumulated PHAs consisted mainly of 3-hydroxybutyrate. By using RSM, it was possible to predict crude glycerin concentrations and C:N ratios not tested here that will allow desirable values of PHA content in biomass or PHA productivity, which can be useful for designing PHA production with MMC.

Bacterial biopolymer (polyhydroxyalkanoate) production from low-cost sustainable sources

Twenty-six different bacterial strains were isolated from samples taken from different locations Dammam, Saudi Arabia, for screening of their polyhydroxyalkanoate (PHA) production capability. The initial screening was conducted by staining with Sudan Black B and Nile Red, followed by examination under fluorescence and electron microscopes to characterize PHA granule formation. The PHA-producing bacterial isolates were identified using 16S rRNA gene analyses; the most potent bacterial strain was identified as Pseudomonas sp. strain-P(16). The PHA production capability of this strain in the presence of different low-cost carbon sources, such as rice bran, dates, and soy molasses, was analyzed. PHA production in the presence of rice bran, dates, and soy molasses was 90.9%, 82.6%, and 91.6%, respectively.

Improving PHA production in a SBR of coupling PHA-storing microorganism enrichment and PHA accumulation by feed-on-demand control

With volatile fatty acids as substrates, the typical polyhydroxyalkanoates (PHAs) production by mixed culture always includes two steps: PHA-storing culture enrichment via aerobic dynamic feeding strategy and PHA accumulation under nutrient-limited condition. To simplify the PHA-production steps, the enrichment and accumulation step were coupled in a SBR. At start-up period, to investigate the effect of settling selection, one acetate-fed SBR was operated by settling selection-double growth limitation (SS-DGL) strategy, while the other was operated by DGL strategy. The results showed that the stable operation in SBR1 was obtained at about 21, 12 days faster than SBR2, implying the settling selection accelerated the start-up process. After omitting the settling selection under the stable operation, the SBR1 was run above 15 days. The results showed that the performance was not substantial altered. Therefore, the settling selection affected the start-up process but not the stable operation. At operational period, based on the sharp decreasing of oxygen uptake rate (OUR), the poly-β-hydroxybutyrate (PHB) content was improved 13%, from 70 to 83% by feed-on-demand control-double growth limitation (FD-DGL). And the harvested volumetric productivity was 5.0 gPHB/L/day, almost 1-folder improvement. That was to say, the PHB production in a SBR of coupling the enrichment and accumulation step was improved by feed-on-demand control. Meanwhile, the FD experiment can keep steady running for 10 SRTs. Therefore, the SS-DGL/FD-DGL strategy was a promising method for PHA production.

Recent Advances and Challenges towards Sustainable Polyhydroxyalkanoate (PHA) Production

Sustainable biofuels, biomaterials, and fine chemicals production is a critical matter that research teams around the globe are focusing on nowadays. Polyhydroxyalkanoates represent one of the biomaterials of the future due to their physicochemical properties, biodegradability, and biocompatibility. Designing efficient and economic bioprocesses, combined with the respective social and environmental benefits, has brought together scientists from different backgrounds highlighting the multidisciplinary character of such a venture. In the current review, challenges and opportunities regarding polyhydroxyalkanoate production are presented and discussed, covering key steps of their overall production process by applying pure and mixed culture biotechnology, from raw bioprocess development to downstream processing.