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Review

Current Status and Challenges in the Commercial Production of Polyhydroxyalkanoate-Based Bioplastic: A Review

by
Shina Gautam
1,*,
Alok Gautam
2,
Juily Pawaday
2,
Rekha Karshanbhai Kanzariya
3 and
Zhitong Yao
4,*
1
Department of Chemical Engineering, School of Chemical Technology, Harcourt Butler Technical University, Kanpur 208002, Uttar Pradesh, India
2
Department of Chemical Engineering, Shroff S R Rotary Institute of Chemical Technology, UPL University of Sustainable Technology, Ankleshwar 393135, Gujarat, India
3
Department of Chemical Engineering, Government Engineering College, Bhuj 370001, Gujarat, India
4
College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China
*
Authors to whom correspondence should be addressed.
Processes 2024, 12(8), 1720; https://doi.org/10.3390/pr12081720
Submission received: 6 July 2024 / Revised: 10 August 2024 / Accepted: 12 August 2024 / Published: 15 August 2024
(This article belongs to the Topic The Electronic Waste (E-Waste) Management and Treatment)
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Abstract

The escalating worldwide concerns over the difficult degradation and pollution of plastic and its associated environmental and health risks have amplified the urgent need to develop biodegradable alternatives to traditional plastics. Polyhydroxyalkanoates (PHAs) have emerged as a promising class of biopolymers that offer a sustainable solution. Their commercial success in various applications has highlighted PHAs’ potential to mitigate environmental impact. Critical to the economic feasibility of PHA production is the optimization of downstream processing methods, crucial for scaling operations from pilot to industrial scales. This paper reviews two decades of pilot-scale studies on PHA extraction, emphasizing the advancements and challenges encountered. It also discusses chemical extraction methods applied across different feedstock and microbial strains, highlighting their role in enhancing efficiency and sustainability. This comprehensive review underscores the imperative for advancing PHA technologies, particularly in refining extraction techniques, to facilitate broader adoption in industries seeking environmentally friendly alternatives to conventional plastics.

1. Introduction

Petroleum’s primary non-fuel application is in the production of synthetic plastics, which has seen significant global growth. From 270 million metric tonnes in 2010, production rose to 368 million metric tonnes by 2019 [1]. However, due to the COVID-19 pandemic, production dipped slightly to 367 million metric tonnes in 2020 [2], though this decrease is expected to be temporary. Since 1950, global plastic production has increased annually by nearly 10%. Projections suggest that annual production could reach 1124 million metric tonnes by 2050, consuming approximately 20% of the world’s oil production. The size of the global market for plastics in electrical and electronic equipment was estimated to be USD 37.72 billion in 2023 and is projected to grow at a compound annual growth rate of 5.2% from 2024 to 2030 [3].
These production trends have significant environmental implications. It is estimated that from 2010 to 2025, approximately 100 million metric tonnes of plastic waste will have entered the oceans. While plastic recycling efforts have gained momentum over the past decade, only 9% of the 6.3 billion tonnes of plastic waste produced globally from the early 20th century to 2015 has been recycled, and less than 1% has undergone more than one recycling process to date. The electrical and electronic equipment (EEE) industry accounts for 5–7% of the total plastic demand and the resulting waste EEE contains 10–30% plastics [4,5].
The bioplastic sector is emerging as a promising alternative to conventional plastics, addressing environmental concerns. Currently, bioplastics constitute approximately 1% of the total annual plastic production of 335 million tonnes. According to recent data from leading organizations like European Bioplastics and Nova Research Institute, global bioplastic production capacity is projected to increase from 2.11 to 2.62 million tonnes by 2023 (Figure 1).
This review focuses on polyhydroxyalkanoates (PHAs), a type of biopolymer known for their customizable properties. Despite currently comprising only 1.4% of the bioplastic market, PHA production is expected to quadruple by 2025. PHAs can be tailored for specific applications by adjusting their composition. They exhibit excellent barrier properties comparable to PET and mechanical properties similar to LDPE. Notably, PHAs are non-sticky when melted, facilitating easy molding into various commercial products.
PHAs, or environmentally friendly linear polyester, can take the place of conventional plastic. PHAs are created by the microbial fermentation of a sugar-rich substrate in conditions where nutrients are scarce [6,7]. PHAs are generally categorized according to the number of carbon atoms linked to the monomer units or the kinds of monomer units present. For example, short-chain-length PHAs, known as scl-PHAs, are repeating units containing three to five carbon atoms; medium-chain-length PHAs, known as mcl-PHAs, are repeating units containing six to thirteen carbon atoms; and long-chain-length PHAs, known as lcl-PHAs, are repeating units containing more than thirteen carbon atoms. PHAs are categorized according to the kind of monomer units they contain, in addition to their carbon atom number. Homopolymers, such as Poly(3-hydroxybutyrate) (P(3HB)), Poly(4-hydroxybutyrate) (P(4HB)), Poly(3-hydroxyhexanoate) (P(3HHx)), Poly(3-hydroxyoctanoate) (P(3HO)), and Poly(3-hydroxyvalerate) (PHV), are PHAs that only contain one particular type of monomer unit. Heteropolymer PHAs, such as Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(P(3HB-co-3HV)) and Poly(3-hydroxyhexanoate-co-3-hydroxyoctanoate) (P(3HHx-co-3HO)), comprise many types of monomer units [7,8].
PHAs’ diverse applications include high-value medical products such as tissue implants, bone plates, cardiovascular patches, and surgical mesh; areas that are being actively researched. The key characteristics of PHAs, such as their semi-crystalline nature, brittleness, wide thermal processing window, and high softening temperature, make them versatile for different industrial uses. PHBV stands out as the most common commercial PHA, owing to its favorable properties. The growth and potential of PHAs underscore their role in addressing the environmental and performance challenges posed by conventional plastics. Over the past decade, significant efforts have been directed towards scaling up the production of PHAs from the laboratory to the pilot scale. This focus stems from achieving high PHA recovery rates observed at the laboratory level. Several industrial ventures, such as Genecis, Bio-plastech, Sirim, Nafigate, and Pha Builder, have established research facilities and offer consultancy services to support the setup of pilot plants for synthesizing this promising biopolymer. Table 1 provides an overview of existing commercial plants operating worldwide.
This review comprehensively discusses various downstream processes for extracting PHAs from biomass, with a specific emphasis on solvent extraction at the pilot scale and the management of waste streams. These processes have been under scrutiny for over two decades in order to assess their feasibility for large-scale industrial applications. Notably, approximately 40% of the production cost of biopolymers lies in the substrate used. Hence, there is a critical need to identify suitable substrates sourced from the waste streams of industrial processes or municipal wastewater for efficient biopolymer fermentation.
This review also provides insights into the types of fermenters employed, microbial strains utilized, and the polymers produced from different waste streams. This holistic approach aims to enhance the sustainability and economic viability of PHA production, leveraging waste materials as feedstocks while optimizing extraction techniques for broader industrial adoption. A review by Zytner et al., 2023 covers only the pre-treatments used for lignocellulosic biomass, using inexpensive lignocellulosic biomass as the feedstock and yielding PHA production [9]. A recent study by Liang et al., 2024 demonstrates PHA production from mixed-culture fermentation using various organic wastes as feedstocks and PHAs as the yields [10]. In another study, Zhang et al., 2024 focused on the optimization and scaling up of PHA production from activated sludge [11]. A study by Pesante and Frison, 2023 focused on the extraction of PHAs from biomass [12]. However, this review provides insights into all aspects of PHA production, starting from the worldwide production of PHAs, and analyzing various inexpensive pre-treatment methods in depth and the waste streams used as feedstock for PHA production enhancement. It also includes the comprehensive PHA production yields from various microorganisms and feedstocks. This review covers different novel methods used for PHA extraction. Furthermore, it covers the scale-up challenges of PHA processes. In the previous literature reviews, only a few specific points are covered; however, all aspects of PHA production is covered in this review.

2. Production of Biopolymers

The production of PHAs involves several distinct steps, encompassing fermentation, the separation of biomass from the broth, biomass drying, PHA extraction, PHA drying, and finally, packaging [13]. Compared to conventional plastic production methods, the PHA production process offers several advantages primarily rooted in its sustainability.
One significant advantage is its ability to utilize waste streams from various effluents as substrates, where the purity of the raw material is not a critical factor. This characteristic not only reduces reliance on virgin resources but also helps in managing and repurposing waste effectively. Moreover, most PHA production processes operate under moderate temperature and normal pressure conditions. This feature contributes to lower energy consumption and operational costs compared to the high-temperature and high-pressure requirements of conventional plastic manufacturing processes.
Overall, the sustainable nature of PHA production, its ability to utilize diverse waste streams as feedstocks, and its operation under moderate process conditions highlight its potential as an environmentally friendly alternative to conventional plastics.

PHA Feedstock

Initially, research into PHA production primarily focused on using plant-based biomass as a carbon source, which proved effective in achieving PHA accumulation. However, using food materials for this purpose can potentially disrupt the food supply chain. Additionally, the seasonal availability of plant-based biomass poses challenges for maintaining consistent PHA production throughout the year. Given that the conversion of biomass to PHAs requires significant capital investment, achieving year-round production is crucial for maximizing returns on this investment. To address these challenges, municipal or industrial waste streams have emerged as viable alternatives to conventional biomass sources for PHA production. These waste streams are a sustainable and carbon-rich source, significantly reducing the overall production costs of PHAs by 30–50% [14]. Furthermore, PHA yield can be increased by employing pre-treatment steps in these waste streams; the pre-treatment methods for cane molasses (CM), wastewater from industries, glycerol and fats, vegetable oils, and waste cooking oils are elaborated in the following section.
During the production of sugar, a large amount of sugar-rich byproducts are generated; one of them is known as cane molasses (CM) and can be used as a feedstock for PHA production. CM mainly comprises sucrose, inverted sugar, and nutrients; however, it is readily available at a low cost, which makes it a potentially inexpensive feedstock for PHA production [15]. In the literature, it was observed that bacteria were unable to assimilate sucrose; therefore, it was necessary to convert sucrose into its monomer such as glucose and fructose for bacterial utilization. In the literature, chemical and acid pre-treatments, microwave-assisted acid pre-treatments, hydrothermal pre-treatments, and hydrothermal acid pre-treatments have been used with sugar cane molasses (Figure 2). In the chemical pre-treatment methods, 1N KOH or H2SO4 solutions were treated with CM and incubated for 1 h at 90 °C. In acid pre-treatment methods, 1N/1.5N HCl or H2SO4 solutions were treated with CM and incubated for 1 h at 90 °C. In the microwave-assisted acid pre-treatments, a 1N H2SO4 solution was treated with CM and incubated in an 800 W microwave oven for 5 min. In the hydrothermal pre-treatment using an oven and oil bath, CM was transferred into an autoclave reactor and incubated for 1 h at 105 °C in a hot-air oven, and submerged into a paraffin oil bath and incubated for 1 h at 110 °C. In the hydrothermal acid pre-treatments using an oven and oil bath, CM plus H2SO4 was transferred into an autoclave reactor and incubated for 1 h at 105 °C in a hot-air oven, and submerged into a paraffin oil bath and incubated for 1 h at 110 °C [15,16,17,18].
A cheap carbon source that can be utilized as a substrate for the synthesis of PHAs is glycerol or crude glycerol. Generally, 10 tonnes of crude glycerol are created for every 100 tonnes of biodiesel produced. Methanol and glycerol are byproducts of the biodiesel synthesis process. If crude glycerol from biodiesel byproducts is utilized as a substrate or if PHA synthesis is combined with biodiesel production, the cost of PHA manufacturing can be cut significantly. Crude glycerol, however, contains contaminants such as detergent, methanol, salts, and mono-, di-, and triglycerides that can hinder the growth of biomass [19,20,21]. Treating inhibitors in crude glycerol with charcoal frequently suffices in this situation, as was previously shown when detoxifying acidically hydrolyzed lignocellulose mixtures that were utilized as substrates for PHA biosynthesis [22]. As a pre-treatment step for PHB formation, this can be inhibited by adding casein peptone or casamino acids to glycerol, which produced 32 g L−1 CDW and 27 g L−1 CDW with Methylobacteriumrhodesianum [23]. Various organic waste mixtures from different processes, including food processing, paper mills, the sugar industry, breweries, and municipal sites, have been used to carry out PHA biosynthesis. As covered in earlier sections, the fermentation process utilizing wastewater as the substrate is essentially the same. The biosynthesis of PHAs can involve three methods: the first is to convert organic carbon into volatile fatty acid (VFA)-rich substrates, the second is the enrichment of PHA-producing bacteria in SBR, and the third is to accumulate PHAs (Figure 3).
A significant component of fats, vegetable oils, and waste cooking oils is triacylglycerols (TAGs), which facilitate the generation of PHAs from these low-cost carbon sources. Three fatty acids are esterified and have a glycerol backbone to form TAG molecules. TAGs can be categorized as both saturated and unsaturated fats; at low temperatures, they stay solid, while at normal temperatures, they become liquid (oil) [24]. Compared to plant oils, fats are more difficult for bacteria to use for PHA production, and they are also linked to the development of foam during fermentation [25]. Antifoaming agents can be used to remedy this issue; however, it is uncertain how they affect bacterial development.
In a previous study that explored using organic waste as substrates for PHA accumulation, it was found that different waste materials demonstrated varying levels of PHA accumulation efficiency. For instance, rice mill effluent led to the accumulation of 85.93% PHAs, followed by fermented dairy waste (75–80%), biodiesel liquid waste (77%), and sawdust fractions (74.7%) [14]. This diversity highlights the potential of utilizing diverse waste streams effectively in PHA production, underscoring their suitability as sustainable and cost-effective alternatives to traditional biomass sources. In recent years, activated sludge derived from municipal and household waste streams was used at a pilot scale over fresh food and agricultural organic materials. Around 54% of pilot-scale studies have achieved PHA accumulation above 40%. Figure 4 summarizes the results of PHA recovery at the pilot scale from various feedstocks.
In many pilot plants, a key sustainable feature is the integration of PHA production with existing waste treatment facilities. This approach allows for the utilization of the current infrastructure with minimal process modifications, thereby reducing the initial investment costs while addressing waste disposal challenges. Several successful pilot-scale studies have demonstrated the integration of PHA synthesis with various wastewater treatment processes such as nitrification, phosphorous removal, and ammonia stripping from feed streams. For example, a pilot plant operated using cellulosic primary sludge as its feed integrated a nitrification step to oxidize ammonia to nitrite with an oxygen supply, followed by PHA accumulation that achieved 55% accumulation efficiency. Similarly, Colombo et al. (2017) incorporated an ammonia stripping section with KOH before PHA extraction from food waste, resulting in an average recovery of 223 ± 28 g PHA/kg of feed with a hydroxybutyrate/hydroxyvalerate ratio of 53:47 [26].
In another study, a pilot plant that operated for 439 days successfully accumulated PHAs at a rate of 6.9–9.2% g PHA/g TSSs (total suspended solids) from activated sludge. This process also achieved effective phosphorous and nitrogen removal, with 83 ± 12% total nitrogen removal and 93 ± 9% phosphorous removal in the form of struvite precipitate through MgCl2 dosing and pH adjustment. This was followed by ammonia oxidation to nitrate or nitrite and PHA accumulation within a 12 h cycle. Various waste streams have been investigated as potential feedstocks for PHA accumulation in these studies, highlighting the versatility and feasibility of using different sources to produce PHAs sustainably. Table 2 summarizes the range of waste streams explored in the literature for PHA production. These integrated approaches not only enhance the sustainability of PHA production but also contribute to improving overall wastewater treatment efficiency and resource recovery [27,28].
The fermentation of food waste and sewage sludge using mixed microbial cultures (MMCs) at a pilot scale has been investigated, employing a sequencing batch reactor over a temperature range of 16–33 °C. This study revealed that the MMC-based fermentation process resulted in a relatively consistent PHA accumulation rate of 36–48% g PHA/g VSSs (volatile suspended solids), indicating that PHA production was not significantly affected by temperature variations within the range of 16–33 °C [26]. This suggests that MMC-based PHA fermentation is not strongly temperature-adaptive across this spectrum. Similar findings have been reported in studies using different bacterial strains and various feedstocks. For instance, a Bacillus megaterium strain cultured on agro-industrial waste showed increased biomass and PHA accumulation between 25 °C and 30 °C. However, a further increase in temperature of 5 °C led to a decline in fermentation performance, highlighting 30 °C as the optimum temperature for this strain and consistent with optimal conditions for PHA production [27]. Studies on MMCs derived from activated sludge have also investigated the effect of temperature on PHA production. The results indicated that while PHA production did not vary significantly between 18 °C, 28 °C, and 35 °C, the highest yield was consistently achieved at 23 °C, where PHA content reached 24% of the cell dry weight [56]. This temperature dependency underscores the importance of optimal temperature conditions for maximizing PHA production efficiency. Furthermore, studies have shown that microbial metabolism slows below 20 °C and above 35 °C, with significant inhibition observed at extremes such as 10 °C, where reduced NADH/NAD ratios can lead to decreased PHA production due to metabolic pathway disruptions and reduced redox balance maintenance [57]. This metabolic behavior aligns with the typical operational temperatures found in wastewater treatment plants, where microbial activity is most robust around 20 °C.
In summary, optimizing temperature conditions is crucial for maximizing PHA production efficiency in microbial fermentation processes. These findings emphasize the need to consider temperature as a critical factor in designing and operating PHA production systems using MMCs or specific bacterial strains, ensuring that conditions are conducive to achieving high PHA yields while maintaining microbial metabolic stability.
Recent studies have explored various methods for recovering PHAs from activated sludge and other sources at a pilot scale. In one study, PHA recovery rates of 51–63% were achieved through a NaCl pre-treatment step applied to activated sludge. Another investigation integrated PHA accumulation (17–22% recovery) with the production of other compounds like triacylglycerols and wax esters from activated sludge in a pilot plant operational for 32 days. At the Brussels North Wastewater Treatment Plant, a pilot process spanning 22 months demonstrated the successful integration of PHA production (0.39 gPHA/gVSS) with municipal wastewater and sludge management services, utilizing a well-designed reactor network [58]. Operations using activated sludge as a carbon source consistently showed stable processing throughout their operational periods. Sequential batch reactors have been widely employed in these studies for fermenting municipal solid waste, leveraging its high organic content to create favorable conditions for PHA-producing microorganisms. This approach not only optimizes the fermentation and accumulation stages but also aligns with sustainable recycling practices for municipal waste, addressing current disposal challenges. However, the downstream extraction of PHAs from accumulated biomass has received less attention at the pilot scale. Notable efforts include the PHARIO pilot facility which has been active since 2011, utilizing a 10 L PHA extraction process and employing butanol as a solvent. Other researchers have explored the production of ultra-high-molecular-weight PHAs at a pilot scale using mixed cultures of nitrogen-fixing bacteria, recovering the biopolymers through acetone pre-treatment followed by dichloromethane solvent extraction techniques [58,59,60].
A review of the literature reveals that the waste generated by different industries has a wide range of chemical compositions, which affects PHA biosynthesis and cell biomass. This may be caused by contaminants in the waste feedstock and changes in its chemical composition over time. Process standardization thus becomes difficult for large-scale applications. The utilization of waste streams by microorganisms can be enhanced through several means, such as the development of a suitable pre-treatment for waste feedstock preparation, optimization of process variables, nutrient supplementation, essential fermentation techniques, and genetic engineering tools. This study covered pre-treatment methods for preparing wastewater, other waste streams, crude glycerol, and waste feedstocks such as CM. The expansion of pilot-scale integrated biorefineries is required for the production of biofuels, bioenergy, and bio-chemicals. Technology integration can assist in directing research and innovation towards pertinent issues by addressing real-world issues and reaping the benefits of surprisingly useful and unique discoveries. The genetic technologies used for microbe engineering documented in the literature were also included in this study. An integrated biorefinery technique is needed to synthesize PHAs and a number of other beneficial bio-chemicals or biofuels. As an illustration, the production cost of biodiesel was lowered by producing hydrogen together with PHAs. To attain maximum effectiveness while minimizing the environmental impact, scientists ought to focus on methods that require smaller amounts of chemicals, energy, and waste production. PHA extraction from MMCs and pure cultures should be combined as well, since the latter are already subjected to industrial processing. To obtain a good estimate of the theoretical PHA production capacity, it is generally advisable to perform a global mass balance of the process and a thorough characterization of the PHAs produced. Understanding the distinct substrate uptake rates of biomass and cellular nutrient content would also help with the kinetics of the biocatalyst and substrate consumption for large-scale PHA synthesis [61,62,63,64,65].
While pilot-scale operations may not fully replicate industrial-scale challenges, they play a crucial role in developing preliminary equipment design, establishing mass and energy balance, and evaluating the economic impacts of various processing parameters. These insights are essential for advancing towards scalable and economically viable PHA production processes that can meet industrial demands sustainably. On the other hand, a challenge was encountered during the scaling up of processes based on waste streams, which resulted in lower PHA accumulation as compared to pure substrates. This happened due to the low concentration of carbon and the presence of microbial growth inhibitors, which affected the mechanism of PHA accumulation in wastewater. Some means of achieving this include ultrafiltration, charcoal bed filtration, and the de-methanolization of the substrate, which result in the effective removal of the inhibiting compounds [66]. Therefore, it is essential to quantify carbon and other nutrients present in waste streams. In another study, it was found that “gamma-positive bacteria” were the least tolerant to cadmium, which strongly interfered with the action of Azetobacter sp. and Micrococcus luteus and resulted in immobilization [49].

3. Upstream Processing of PHAs

The initial stage of PHA recovery from organic biomass involves microbial treatment through a fermentation process. In this process, biomass serves as a carbon substrate for specific microbial species, which synthesize PHA molecules intracellularly during fermentation. The selection of the appropriate microorganism for industrial PHA production depends on factors such as growth rate, polymer synthesis efficiency, PHA quality and quantity, and the maximum achievable polymer accumulation [1]. The microorganisms involved in PHA production are often substrate-specific, necessitating thorough research to select the strains suitable for the specific feedstock involved. Pure bacterial cultures typically require sterile conditions for optimal growth, which contributes significantly to the overall operational costs of the process [54,66,67,68].
The biosynthesis pathways for PHAs are complex and involve multiple enzymatic steps, as illustrated in Figure 5. Among various pure strains, Plasticicumulan sacidivorans has shown promising results at a pilot scale. For example, Tamis et al. operated pilot plants using wastewater from chocolate factories and the paper industry, employing a three-stage process comprising anaerobic fermentation, enrichment, and PHA accumulation [69,70,71]. Strict control over temperature and pH is crucial at each stage to maintain microbial activity and stability, adding to operational expenses. The enriched biomass accumulated approximately 0.7 ± 0.05 g PHA/g VSS from chocolate factory wastewater and 0.7–0.8 g PHA/g VSS from paper industry wastewater within a 4 h cycle time [43].
In contrast, a study focusing on PHA accumulation from municipal waste demonstrated that maintaining pH control during fermentation is not always necessary, achieving a maximum PHA content of 0.77 ± 0.18 g PHA/g VSS within 3 h, thus reducing economic constraints to some extent [47]. Further research has explored PHA production using wastewater from the food industry, employing various isolated and mixed microbial species. Different species and culture conditions resulted in varying concentrations of PHA copolymers. For instance, a culture of A. Ichthiosmia exhibited the highest accumulation of PHB at 84 mg/g, while B. cereus showed the highest PHBV accumulation at 69 mg/g. Mixed cultures of B. Pumilus and A. Ichthiosmia demonstrated the highest overall PHA accumulation, with 130 mg PHB/g and 100 mg PHV/g [58].
In summary, selecting and optimizing microbial strains for PHA production involves balancing factors like substrate specificity, growth conditions, and polymer synthesis capabilities. These factors play a critical role in achieving efficient and economically viable PHA production processes, contributing to the advancement of sustainable biopolymer technologies. The drawbacks of single microbial strains can be minimized at the industrial scale by incorporating mixed microbial cultures (MMCs), which can easily be derived from basic processing principles such as microbes encountered during secondary waste treatment. In addition to its ease in availability, MMCs have shown satisfactory PHA accumulation without the need for sterilization and highly specific conditions during the growth stage [31,43,65]. Among all the studies on municipal solid waste, the highest recovery was achieved using a pure microbial strain, resulting in 77% accumulation [72]. On the other side, individual industrial waste streams like paper mill wastewater showed promising results of 80% recovery, followed by chocolate factory waste (76%). Cellulosic primary sludge resulted in 87% recovery; this may be attributed to the fact that the elimination of secondary treatment on a large scale resulted in a low dilution of the process stream, which subsequently reduced the loss of PHAs produced by mixed microbial consortia (MMCs) through discarded supernatant streams. A summary of PHA production by different strains from different waste streams is included in Table 3.
Our review of pilot studies highlighted a significant trend where 48% of the plants utilized mixed microbial cultures (MMCs) of bacteria for PHA production, as opposed to pure strains (Table 4). This shift towards MMCs is based on the recognition that PHAs act as metabolic intermediates in microbial processes within wastewater treatment, making MMCs a natural fit for PHA production [97,98]. MMCs offer several advantages over pure strains, primarily in reducing the need for stringent sterile conditions during operation. This not only lowers energy requirements but also decreases processing time, contributing to operational efficiency. However, PHA production rates with MMCs often remain below 10 g/L due to challenges in achieving high cell densities. One strategy to address this limitation involves designing accumulation reactors that minimize cell mass loss during supernatant removal. This approach helps maintain high cell densities, thereby potentially increasing PHA production rates. Despite these advancements, there is ongoing exploration to develop approaches that bypass the substrate fermentation step altogether. A generalized production approach is also sought which can accommodate a wide range of feedstocks and microbial culture systems. This quest aims to streamline and optimize PHA production processes, enhancing their scalability, versatility, and economic viability in sustainable biopolymer production.

4. Downstream Processing of PHAs

PHAs undergo several critical stages in downstream processing. After the bioaccumulation and filtration of the washed substrate, the efficient recovery of the valuable biopolymer from the biomass becomes essential. The recovery stage aims to achieve high polymer recovery and purity while minimizing costs and operational time, which is particularly crucial for large-scale production (Figure 6).
Several factors influence the selection of a PHA recovery method:
Microbial strain: Different strains produce varying types of PHAs, influencing recovery method suitability.
Type of PHAs: The specific polymer type impacts the choice of recovery technique.
PHA load in biomass: Concentration affects recovery efficiency and economics.
Impact on PHA properties: Recovery methods should ideally preserve polymer properties.
Application of PHAs: The intended use dictates purity requirements and recovery considerations.
Downstream processing typically includes:
Pre-treatment: Biomass preparation (e.g., cell disruption, drying).
PHA extraction: Separation using solvent extraction, enzymatic digestion, or heat treatment.
Purification: Refinement via solvent precipitation, filtration, or chromatography.
Commonly cited recovery techniques include solvent extraction, enzymatic digestion, heat treatment, and non-solvent-based methods like supercritical fluid extraction or aqueous two-phase extraction.
Choosing the optimal recovery technique depends on the production scale, cost-effectiveness, and desired PHA purity. Efficient downstream processing is vital for the sustainability and economic feasibility of PHA production in the market.

4.1. Pre-Treatment

The primary objective of pre-treatment in PHA production is to weaken cell wall firmness, facilitating the subsequent extraction steps—biological, chemical, or physical—in order to release PHA granules efficiently [99]. Biomass drying is the most commonly employed pre-treatment method, although alternatives such as heat, alkali, freezing, and salt treatments have been explored to enhance cell disruption in PHA-enriched biomass. Heat pre-treatment, for example, denatures cell proteins and compromises cell membrane stability, thereby easing PHA release during extraction. Neves et al. conducted thermal pre-treatments at different temperatures, achieving a recovery of 93.2% P(3HB-co-3HV) with 94% purity using solvent extraction with chloroform. Similarly, the enzymatic extraction of PHB with 90% purity benefited from the prior heat pre-treatment of the biomass [35].
Chemical pre-treatments utilizing agents like NaCl and NaOH have also been effective in enhancing polymer recovery [100,101]. Sodium hypochlorite (NaOCl) treatment serves a dual purpose in PHA extraction—it disrupts cells and releases the biopolymer into the aqueous phase. However, it also induces significant PHA degradation, leading to molecular weight reduction. Combining NaOCl pre-treatment with solvent extraction helps mitigate this issue, reducing PHA degradation [102]. Other techniques such as ultrasonication and freezing also disrupt cell walls effectively but are limited in their industrial applications due to their operational complexity and high costs. In summary, pre-treatment plays a critical role in optimizing PHA recovery by preparing biomass for efficient extraction processes, balancing effectiveness with practical considerations for industrial-scale applications.
Pre-treatment processes are mostly developed for a particular microorganism, and they cannot be applied in a generalized way to all PHA-accumulating bacteria because they depend on the individual features of the respective bacterial strain [102]. Therefore, the incorporation of this step would require a detailed study of its effect on the type of culture involved.

4.2. Extraction

4.2.1. Solvent Extraction

Solvent extraction is widely favored for PHA recovery due to its simplicity and the high purity of the polymer obtained [103]. Unlike other techniques, it does not necessarily require a high purity of the PHA-accumulated cell mass beforehand, which is advantageous. Solvents play a dual role in biomass: they first modify cell membrane permeability and then dissolve PHAs [41,68]. This method capitalizes on the fact that PHAs are insoluble in water but soluble in a limited number of solvents. Halogenated solvents such as chloroform and dichloromethane are commonly used for extracting short- and medium-chain-length PHAs, offering excellent extraction yields and product purity. However, non-halogenated solvents are also demonstrating potential in PHA extraction processes. For instance, PHB Industrial S/A (PHBISA) in São Paulo, Brazil, utilized fusel alcohols from distilled bioethanol production to isolate PHAs from waste streams generated during sugar and bioethanol production. This process enabled PHBISA to produce nearly 10,000 tonnes per annum of commercial PHAs [104,105]. Similarly, Tsinghua University in Beijing, China, achieved a 50% recovery of PHAs from glucose and lauric acid using ethyl acetate and hexane in collaboration with the Guangdong Jiangmen Center for Biotech Development [34]. In various studies summarized in Table 4, chloroform and dimethyl carbonate were frequently employed solvents for PHA extraction from substrates like activated sludge, consistently yielding high recovery rates of the biopolymer. Solvent extraction thus remains a pivotal method in the efficient isolation of PHAs, offering versatility and reliability in industrial applications.
Table 4. Summary of reported solvent extraction studies for PHAs.
Table 4. Summary of reported solvent extraction studies for PHAs.
Sr. No.FeedstockSolvent/Non-SolventBacterial StrainPHA CompoundPre-Treatment/Purification Stage% Recovery% PurityThermal DataMolecular Weight DataReference
1UnknownNaOH/EthanolCupriavidunecatorPHAPost-treatment: Freeze drying----[106]
2UnknownMethyl isobutyl ketone/HexaneRalstoniaeutrophaPHOPre-treatment: Freeze drying5599--[107]
Methyl ethyl ketone/Hexane95100
Butyl acetate/Hexane42100
Ethyl acetate/Hexane99100
3UnknownMethyl tert-butyl ether/EthanolPseudomonas putidaPHOPre-treatment: Freeze drying4--Mw = 155 kDa; PI = 1.8[108]
Ethyl acetate/Ethanol12Mw = 132 kDa; PI = 1.7
Acetone/EthanolPurification with acetone and activated charcoal13Mw = 138 kDa; PI = 1.7
Methylene chloride/Ethanol17Mw = 156 kDa; PI = 1.9
4UnknownAnisole/EthanolBurkholderiasacchariPHBPre-treatment: Freeze drying96.798.3-Mw = 6.8 × 105 Da; PI = 2.34[109]
Cyclohexanone/Methanol93.498.2Mw = 8 × 105 Da; PI = 1.46
Phenetole/Ethanol--Mw = 5.6 × 105 Da; PI = 2.24
5UnknownDMC/EthanolCupravidusnecatorPHBPre-treatment: Freeze drying-95TGA: Td = 280 °CGPC: Mw = 1 MDa; PI = 2.7[62]
6UnknownPropylene carbonate/AcetoneCupriavidusnecatorPHBPre-treatment: Thermal9584DSC: Xc% = 60; Tm = 175 °C; Tg = 4.9 °CMw = 740 kDa; PI = 3.1[99]
7BiodieselChloroform/EthanolPseudomonas citronellolisPHAPre-treatment: Freeze drying and ethanol wash26.6-DSC: Xc% = 10.4; Tm = 53.6 °C; Tg = −43.5 °CMw = 78 kDa; PI = 2.5[84]
8UnknownEthylene Carbonate/EthanolCupriavidusnecatorPHBPre-treatment with NaOCl98.698Xc% = 59.2; Tm = 176 °C; Tg = 4.8 °C-[110]
Methanol/Ethanol72.697-
Propanol/Ethanol28.4997-
Acetic acid/Ethanol36.7197-
DMSO/Ethanol60.695Xc% = 57.3; Tm = 176 °C; Tg = 5.1 °C
DMFO/Ethanol30.197-
Hexane/Ethanol2.883-
9UnknownChloroform/MethanolCupriavidusnecatorPHBPre-treatment: Freeze drying--Xc% = 0.52; Tm = 165 °C; Td = 310 °CMw = 283 kDa; PI = 2.9; [111]
NaOHPre-treatment: Freeze drying; Purification: Ethanol 7895Xc% = 0.6; Tm = 171 °C; Td = 247 °CMw = 837 kDa; PI = 1.61
NaOClPre-treatment: Freeze drying; Purification: Ethanol -98Xc% = 0.6; Tm = 169 °C; Td = 294 °CMw = 249 kDa; PI = 6.82
Dichloromethane/EthanolPre-treatment: Freeze drying and NaOCl digestion; Purification: Ethanol 9099Xc% = 0.6; Tm = 167 °C; Td = 296 °CMw = 361 kDa; PI = 2.61
Sulphuric acidPre-treatment: Freeze drying; Purification: NaOCl bleaching--Xc% = 0.84; Tm = 165 °C; Td = 303 °CMw = 321 kDa; PI = 7.29
10UnknownMethylene chloride/MethanolPseudomonas putidaPHA----Mw = 0.5 MDa[56]
Ethyl acetate/Methanol
Acetone/Methanol
11Unknownn-Hexane/MethanolPseudomonas putidaPHONA49100-Mw = 212 kDa; PI = 1.92 [112]
2-Propanol/Methanol1284Mw = 321 kDa; PI = 1.26
Dichloromethane/Methanol83100-
Ethyl acetate/Methanol7899-
THF/Methanol77100-
Acetone/Methanol77100-
12Green Grass JuiceChloroform/EthanolWautersiaeutrophaPHBPre-treatment: Freeze drying and ethanol wash.77.2-Xc% = 64.6; Tm = 180 °C; Tg = 6 °CMw = 432 kDa; PI = 4.02[113]
Silage Juice77.3-Xc% = 65.9; Tm = 180 °C; Tg = 6 °CMw = 434 kDa; PI = 4.01
13WheyChloroformHaloferaxmediterraneiPHBVPre-treatment: Freeze drying and ethanol wash72.8-Tm = 158.9 °C Td = 241 °C; Tg = 6 °CMw = 1057 kDa[114]
14GlucoseChloroform/EthanolCupriavidusnecatorPHBPre-treatment: Freeze drying and ethanol wash77-Xc% = 68; Tm = 178 °C; Tg = 6 °CMw = 665 kDa[101]
15Crude GlycerolChloroformHaloferaxmediterraneiPHBVPre-treatment: Freeze drying and ethanol wash75.4-Tm = 137.1 °C; Td = 285 °CMw = 391 kDa[115]
16Vegetable OilCyclohexanone/MethanolCupriavidusnecatorPHBPre-treatment: Freeze drying and acetone wash9999.5-Mw = 23 kDa[116]
Butyrolactone/Methanol4597.2-
17Unknown0.05M NaOHCupriavidusnecatorPHAPurification with ethanol96.996-Mw = 1.4 × 105 Da [106]
18UnknownChloroform/MethanolAlcaligeneseutrophusPHBPre-treatment with NaOCl9197Xc% = 65; Tm = 176 °CMw = 10 × 105 Da [60]
19UnknownSodium hypochloriteRalstoniaeutrophaPHBPre-treatment: freeze drying; Purification: isopropanol91.3296-Mw = 4.6–8.3 × 105 Da [117]
20SludgeDimethyl carbonate/EthanolCupriavidusnecatorPHBPre-treatment: Freeze drying8595Td = 280 °C Mw = 1 MDa[118]
Dichloromethane/Ethanol1794Td = 290 °CMw = 1.1 MDa
NH4Laurate10298Td = 264 °C Mw = 0.6 MDa
NH4OH7070Td = 272 °C Mw = 0.7 MDa
SDS9990Td = 271 °CMw = 1.2 MDa
21Corn OilAcetic acid/MethanolBurkholderiacepaciaPHBPre-treatment with SDS89.390.99Tm = 175 °C
Td = 280.7 °C		Mw = 8.31 × 105 Da[119]
Chloroform/Methanol10095.08Tm = 175.3 °C; Td = 291.3 °CMw = 8.97 × 105 Da
22SludgeButanolMMCPHBVPurification with acetone87NA--[40]
23SludgeButanolMMCPHBV----Mw = 11 × 105 Da; PI = 2.6
Chloroform/Hexane-Mw = 10.7 × 105 Da; PI = 2.6
24Activated sludgeChloroformMMCPHB-- --[120]
Food processing waste28.3
Jowar grain-based distillery spent water 34.7
Rice grain-based distillery spent water36.5
25Paper industry waste sludgeChloroform/MethanolMMCPHBV-39.6---[50]
26Paper industry waste sludgeDimethyl carbonateMMCPHBPurification with chloroform and activated carbon----[121]
27SludgeChloroformMMCPHBPre-treatment with acetone, ethanol, and NaOCl65.84---[59]
28Municipal wastewater sludgeChloroform/HexaneMMCPHA 58---[122]
29SludgeSDSMMCPHBPurification with NaOCl44.299.5--[13]
30Grain processing wasteChloroformMMCPHBPurification with hexane44-Tm = 100 °C-[123]
31Municipal wastewater sludgeChloroform/MethanolMMCPHBVSurfactant pre-treatment15-Tm = 152.3 °C; Td = 284 °CMw = 3.1 × 105 Da [124]
Acetone/Water-37-Tm = 165.5 °C; Td = 274 °CMw = 5.1 × 105 Da
32SludgeSDSMMCPHBV-32-Tm = 149.9 °CMw = 4.3 × 105 Da[125]
33SludgeDimethyl carbonateMMCPHA-96---[126]
34Food waste sludgeChloroformMMCPHBVNaOCl purification91100Tm,avg = 144 °C; Td = 252–278 °C Mw = 132 kDa[122]
35Oil mill wastewaterNH4 LaurateMMCPHBNaOCl pre-treatment and NH4OH and ethanol purification77100Td = 185 °C Mw = 0.918 × 105 Da [127]
PHBV7393Td = 205 °CMw = 1.438 × 105 Da
36SludgeChloroformMMCPHBPre-treatment with NaOCl44---[1]
37SludgeChloroform/MethanolMMCPHAPre-treatment with NaOCl and purification with activated carbon7.01---[67]
38Municipal wastewater sludgeDimethyl carbonateMMCPHBVPurification with 1-Butanol30.798.5--[128]
Chloroform3782.5
Dichloromethane3986.4
39Municipal
wastewater sludge		Dichloromethane/WaterMMCPHBVPre-treatment: Acetone wash30-Tm = 171 °C Mw = 3.16 × 106 Da; PI = 1.3[43]
40Crude GlycerolChloroform/Petroleum etherMMCPHBPre-treatment: Freeze-drying and acetone wash--Xc% = 66; Tm = 171 °C; Tg = 4.9 °CMw = 8.34–19.5 × 104 Da [107]
Solvent extraction is indeed valued for its ability to yield high-purity PHAs without degrading the polymer chain. However, this method has notable drawbacks. It requires large quantities of solvents, which increases production and operating costs. Additionally, recovering these solvents post extraction adds further to the overall expense. Handling solvent/non-solvent mixtures left over after PHA extraction poses challenges, complicating waste management processes. Moreover, the solvents used in extraction are often toxic and volatile, posing significant environmental and health risks [100,129]. These issues underscore the need for alternative extraction methods that are more sustainable and environmentally friendly, while still ensuring the high purity and yield of PHAs.

4.2.2. Other Methods

Chemical agents such as surfactants and oxidants offer alternative methods for releasing PHA molecules from biomass without significant polymer degradation. These compounds work by digesting non-PHA cell material, solubilizing it, and thereby liberating PHA granules. Surfactants like sodium dodecyl sulfate (SDS) and ammonium laurate penetrate the cell membrane bilayer. Upon saturation, the additional surfactant breaks the membrane, forming micelles of the surfactant and membrane, thereby releasing PHAs into the solution [100,130,131]. However, a challenge with this method is the complete removal of the surfactant from the isolated polymer. Digestion with strong alkalis such as NaOH provides a green extraction route with minimal polymer degradation and high recovery rates [118,127]. Sodium hypochlorite, a strong oxidizing agent, selectively degrades non-PHA materials, leaving the PHAs intact and easily separable [37]. However, on a large scale, precautions are necessary due to the exothermic nature of biomass digestion with hypochlorite, requiring adequate cooling arrangements in the reaction vessel.
Researchers have also combined solvent extraction with digestion methods, achieving significant PHB recovery rates of up to 91% [103,132]. For instance, the chloroform–hypochlorite method involves mixing biomass with chloroform and sodium hypochlorite to create a three-phase mixture: a hypochlorite layer on top, a middle layer of non-PHA cell material, and a bottom layer of chloroform containing dissolved PHAs. The polymer is subsequently recovered by precipitation through the addition of a non-solvent [40]. These approaches highlight the diversity of techniques available for PHA recovery, each with its own advantages and challenges. The choice of method depends on factors such as efficiency, environmental impact, and scalability in industrial settings.
An alternative to solvent extraction for PHA recovery is enzyme digestion, which involves using specific enzymes to dissolve cell proteins, thereby releasing PHAs with minimal degradation. Proteolytic enzymes, in particular, exhibit high selectivity towards PHAs, ensuring efficient production with purity levels exceeding 93% [97]. Neves and Muller (2012) explored various enzymes for polymer recovery, successfully producing P(3HB) and PHBV with high purity, demonstrating the potential of enzymatic methods [108]. Despite these advantages, the high cost of enzymes and the complexity of the technique have limited its industrial adoption [35,99]. In contrast to chemical recovery methods, mechanical disruption techniques offer minimal environmental impact during downstream processing. Methods like bead mill disruption and high-pressure homogenization are notable examples. Bead mills utilize a vertical cylindrical grinding chamber with a rotor for agitation, applying solid shear to release proteins from cells. High-pressure homogenizers, on the other hand, use an air-driven pump to force a cell slurry through narrow slots under high pressure, employing liquid shear for disruption [101]. The efficiency of bead mills is independent of biomass concentration but can generate heat during operation. High-pressure homogenizers, however, face challenges in scaling up due to their dependence on biomass concentration [105,132]. These mechanical techniques can also be combined with chemical agents like surfactants to enhance PHA yield [133].
Supercritical fluid extraction represents another approach to downstream processing. Supercritical fluids, due to their low viscosity and high density, act as efficient solvents. Supercritical CO2, for instance, is suitable for solubilizing lipids and other hydrophobic components of bacterial cells, although it does not effectively dissolve PHA itself. Consequently, this method often follows a chemical solvent extraction stage to recover PHAs [62,134]. However, supercritical fluid extraction is associated with high maintenance and operating costs, limiting its widespread adoption. Each of these methods presents unique advantages and challenges, influencing their suitability for industrial-scale PHA production. The choice of technique depends on factors such as efficiency, cost-effectiveness, environmental impact, and scalability.
Dissolved air flotation and air classification are innovative methods employed in conjunction with various PHA recovery techniques to enhance yield. Dissolved air flotation, for example, can substitute traditional filtration or centrifugation processes. It operates based on interactions among particles, bubbles, and hydrodynamics, leading to the formation of aggregates that aid in PHA recovery [108]. Air classification, on the other hand, involves initially releasing PHA granules from cells through mechanical disruption. The resulting suspension is then dried and milled into a powder. This powder undergoes air classification to separate it into different particle fractions, followed by extraction and precipitation to recover the biopolymer [135].
Given the high solvent-to-biomass requirements of traditional methods, there is a growing interest in exploring non-halogenated solvents. Ethyl acetate, particularly under high-temperature conditions, has demonstrated significant promise, achieving recovery rates of up to 99% in certain studies [42,62]. These advancements highlight ongoing efforts to optimize PHA recovery processes, focusing on efficiency, sustainability, and cost-effectiveness across different industrial applications. Each method brings unique advantages and challenges, shaping their applicability in large-scale PHA production.

4.3. Purification

After the release of PHA granules from cells, a final purification step is often necessary to achieve polymers of sufficient purity, tailored to specific applications [62,121,136]. The choice of purification method depends on the recovery process employed and the intended use of the PHA. For applications such as medical devices, where very high purity and minimal endotoxin presence are critical, specific purification techniques are required. One approach involves pre-treatment with hypochlorite, which has been shown to reduce endotoxin content to some extent [18]. However, hydrogen peroxide treatment, although commonly used, has drawbacks such as instability in certain biomasses, the requirement of high operating temperatures, and potential reductions in polymer molecular weight, prompting the search for alternative methods [137]. Ozone has emerged as another potential purifying agent for PHAs, offering bleaching action, the removal of soluble impurities, and deodorization benefits [113]. Studies have explored post-extraction treatments with 1-butanol, which significantly increased purity from 91.2% to 98.0%, demonstrating its effectiveness as a purifying agent. Other agents like NH4OH solution and ethanol readily solubilize impurities but typically result in a modest 15% improvement in polymer purity at the cost of reduced recovery efficiency [138].
Current purification techniques are often energy-intensive, leading to polymer synthesis losses, and may involve agents that are not environmentally friendly. This highlights the need for future research focused on developing economically viable and technically feasible purification methods suitable for different grades of PHAs. Innovations in purification could potentially enhance efficiency, reduce environmental impact, and optimize the overall economics of PHA production.
In terms of efficiency, solvent extraction methods obtained 95–100% efficiency and 90–100% recovery and purity compared to surfactant methods, which achieved 85–95% efficiency and 60–98% recovery and purity; enzymatic cell disruption, which achieved 85–93% recovery and purity; supercritical fluid methods, which obtained 89% recovery; and ultrasonication methods, which obtained 96.5% purity and 80% recovery in the literature. In terms of the costs of surfactants, enzymatic cell disruption methods are far costlier due to the higher price of surfactants and enzymes, and these methods require a higher biomass-to-surfactant/enzyme ratio, which further increases the PHA production cost. However, mechanical disruption and supercritical fluids were found to be very difficult to scale up to a larger scale. The solvent extraction method is less costly and has higher efficiency for recovery, and purity, but the use of halogenated solvents creates an impact on the environment, as higher amounts of solvents are required for extraction processes if 100% recovery is not possible, whereas the usage of non-halogenated and green solvents can solve this problem. Ethyl acetate and dimethyl carbonate are examples of green solvents used for PHA extraction. The potential of non-halogenated solvents has also been explored in several studies. Ethyl acetate under high-temperature conditions achieved a recovery of up to 99%.

4.4. Life Cycle Assessment of PHA Production

Of greater prominence than PHAs’ ability to biodegrade is the fact that they are produced biologically by utilizing renewable resources. The manufacturing of PHAs using low-cost renewable and waste streams as carbon sources is explained in the previous sections. PHAs are intercellular product and must be released from bacterial cells through extraction. After the purification and molding process, PHAs can be used in various applications like packaging, implants, and medical fields. PHAs can be degraded into CO2 and CH4 in anaerobic conditions and CO2 and H2O in aerobic conditions. This CO2 and H2O can be utilized for the photosynthesis of plants and creating a carbon source which can be further utilized for PHA fermentation. Furthermore, PHAs are biocompatible, indicating that they do not harm living things in any other way. As this implies, PHA biosynthesis and biodegradation are entirely in line with the carbon cycle (as shown in Figure 7).

5. Conclusions

The escalating concerns over the difficulties of plastic degradation and pollution have amplified the urgency of developing bioplastic. PHAs have emerged as a promising class of biopolymers and their market is currently in its early stages of commercialization, presenting numerous unexplored research opportunities that attract global technologists and scientists. Transitioning PHA production from a pilot scale to an industrial scale poses several challenges, which primarily revolve around technical, economic, and operational aspects. The technical aspects include the parameters of the fermentation operation such as temperature, pH, and oxygenation, which are to be maintained during the operation as per laboratory experiments or pilot-scale trials. This review comprises the methods available for the possible extraction of PHAs from different substrates and the cultures available for their effective use at the pilot scale. This combination of information was scarce in the literature in our review. So far, reviews are available of laboratory-scale studies; however, the problems associated with the pilot scale are highlighted in the present work. Accordingly, there are huge objectives to be fulfilled for PHA production at the pilot scale.
One promising area of exploration involves utilizing activated sludge from waste treatment plants as a feedstock for PHA production. This approach not only addresses waste disposal issues but also transforms waste into a valuable, sustainable product. Compared to single-strain microbes, MMCs show potential for the economical and significant valorization of waste streams.
Despite these advancements, there remains a critical need to develop environmentally friendly and cost-effective downstream processing techniques for future aspects. High solvent-to-recovered-PHA ratios contribute to significant processing costs, underscoring the importance of optimizing extraction methods to enhance economic feasibility. In-depth knowledge of different solvents and their optimization is required for different waste streams as well as different substrates to overcome this obstacle for future work. Pilot-scale projects are pivotal in laying the groundwork for the future industrialization of the bio-based value chain. However, challenges such as high processing costs and the environmental concerns associated with current extraction methods have deterred widespread investment in PHA manufacturing. Addressing these challenges requires concerted efforts from the research community and supportive government policies aimed at boosting the marketability of PHA-based products.
In summary, while PHA technology holds promise for sustainable development, overcoming current technical and economic barriers is essential for its broader adoption and successful commercialization in various industries.

Author Contributions

A.G. and S.G. designed, supervised, and prepared the final draft of this study. J.P. prepared the first draft of the manuscript. R.K.K. prepared the second draft of the manuscript. Z.Y. and S.G. revised the manuscript. All authors read and approved its contents. All authors have read and agreed to the published version of the manuscript.

Funding

The authors would like sincerely thank the Department of Science and Technology, India, for funding the project (DST/TDT/WM/2019/064) under the Waste Management Technology scheme. This research was carried out under this project.

Data Availability Statement

The data are unavailable due to privacy or ethical restrictions.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Global production capacity of bioplastics.
Figure 1. Global production capacity of bioplastics.
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Figure 2. Several pre-treatment techniques applied to sugar cane molasses in order to increase PHA productivity.
Figure 2. Several pre-treatment techniques applied to sugar cane molasses in order to increase PHA productivity.
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Figure 3. PHA recovery from different feedstocks in pilot-scale PHA production from waste streams using MMCs.
Figure 3. PHA recovery from different feedstocks in pilot-scale PHA production from waste streams using MMCs.
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Figure 4. PHA recovery from different feedstocks in pilot-scale processing.
Figure 4. PHA recovery from different feedstocks in pilot-scale processing.
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Figure 5. PHA biosynthetic pathways.
Figure 5. PHA biosynthetic pathways.
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Figure 6. Downstream processing techniques.
Figure 6. Downstream processing techniques.
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Figure 7. Life cycle assessment of PHA production.
Figure 7. Life cycle assessment of PHA production.
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Table 1. PHA manufacturers available globally.
Table 1. PHA manufacturers available globally.
Sr. No.Company NameCountryYearPHA ProductCommercial NameDevelopment Stage
1GenecisCanada-PHBV-Research
2Danimer ScientificGeorgia2007mcl-PHANodax™Commercial
3Tainan Biological Materials Co. Ltd.China2000PHB, PHBVENMATCommercial
4Tianjin Green Bio Material Co.China2003P (3, 4HB)Sogreen®Commercial
5MetabolixMassachusetts1992PHBYield10 Bioscience-MIRELCommercial
6Shenzhen Ecomann Biotechnology Co Ltd.China2008PHBV, PHA+ Other Polymer BlendsEcomannCommercial
7KanekaJapan1948-AONILEXCommercial
8RWDC IndustriesSingapore -SolonCommercial
9Newlight Technologies, LLCUS2007PHBAirCarbonCommercial
10BiomerGermany1994PHBBiomer®Commercial
11BioplastechIreland-mcl-PHA-Research
12BASFUS- EcoflexCommercial
13Tepha Inc.Massachusetts1998P4HB, P(3HB-co-4HB)TephaFLEX®Commercial
14Poly FermCanada2015mcl-PHAVersaMerTMCommercial
15PHB Industrial S.A.Brazil2000PHB, PHBVBIOCYCLE®Commercial
16Full Cycle BioplasticsUS-PHBV-Commercial
17Cardia BioplasticsAustralia--CardiaCompostablesCommercial
18Blue PHAChina---Commercial
19Mango MaterialsUS-PHBYOPP PHAPilot
20SIRIM BioplasticsMalaysia2011Several types-Research
21GoPHANetherlands-Several types-Research
22PHA BuilderChina-Several types-Research
23COFCOChina-PHBCOFCO BiochemicalsCommercial
24NAFIGATE CorporationCzech2015PHB-Research
25Helian PolymersNetherlands-PHB, PHBVNPX BioBallsCommercial
26Terra VerdaeBioWorksIncCanada2009--Pilot
27Rodenburg BiopolymerNetherlands2000-Optinyl®Pilot
28BiomateraCanada1998PHA ResinsBiomateraCommercial
29MosantoJapan1996P(3HB-CO-3HV)Biopol-
30ZenecaUK1970P(3HB-CO-3HV)Biopol-
Table 2. PHA synthesis research conducted at pilot scale.
Table 2. PHA synthesis research conducted at pilot scale.
Sr. No.FeedCultureEquipmentPHA Accumulation/RecoveryPeriod of OperationReference
1Food waste and sewage sludgeMMCCSTR (380 L), coaxial centrifuge, SBR (100 L), FBR (70 L)48% g PHA/g VSS-[29]
2Activated sludgeMMCSBR-1(2 L), SBR-2(2 L)51–63%-[30]
3Wastewater from food industryE. coli, S. terrae, A. ichthiosmia, P. putida, B. pumilus, P. huttiensis, B. cereus, Y. frederiksenii.SBR (6 L), centrifuge, membrane filter (ceramic)Varied according to strain-[31]
4Activated sludgeBacterial consortium (S-150)Fermentation reactor (4000 L), membrane filter (ceramic), bioreactor (70 L)59.47% of dry cell weight-[32]
5Municipal solid waste and sewage sludgeMMCSBR (120 L), anaerobic CSTR (380 L), filter press (SS)46%2 months[33]
6Activated sludgeMMCSBR, fill-up reactor, decanter, bow sieve, dryer, centrifuge, oven17–22%32 days[34]
7Activated sludgeCupriavidus sp.Bioreactor (30 L), fermentation reactor (50 L), centrifuge28–63%-[35]
8Food industry effluentMMCSBR (460 L), mixing tank (500 L), bowl–scroll centrifuge, accumulation reactor (400 L)0.40 to 0.70 gPHA/gVSS1 year[36]
9Activated sludgeMMCFermentation reactor (1000 L), bowl–scroll centrifuge, micro-filter (60 μm mesh size), holding tank (500 L), SBR (500 L), accumulation reactor (550 L), settling tank (120 L), oven (Binder FP240)0.39 gPHA/gVSS22 months[37]
10Activated sludge-SBR-1 (500 L), SBR-2 (500 L), accumulation reactor (500 L), settling tank (120 L), biomass-thickening units, filter bed drum centrifuge, oven49%225 days[38]
11Cellulosic primary sludge-Rotating belt dynamic filter, fermentation unit (2600 L), ultrafiltration unit, nitration SBR (1100 L), selection SBR (2800 L), accumulation reactor (1000 L)55% of PHA (VSS basis)600 days[29]
12Food wasteMMCPercolation biocell reactor (100 L), percolate tank, centrifuge, ammonia stripper, SBR (800 mL), accumulation glass reactor (300 mL)223 ± 28 g kg−1 of feed30 days[39]
13Activated sludgeMMCBatch fermentation reactor (1200 L), FBR (500 L), solvent extractor (10 L)50%10 months[40]
14Activated sludgeMMCFermentation CSTR (200 L), enrichment SBR (400 L), FBR (180 L)--[41]
15Municipal solid waste and sewage sludgeMMCFermentation reactor (380 L), SBR (100 L), FBR (70–90 L)40–50%-[42]
Cellulosic primary sludge-44.0–13.0 and 56.0–87.0 w/w,
Fruit wasteUpflow sludge reactor (60 L), SBR (100 L), stirring tank reactor (60 L)70% of dry cell weight
16BiomassMMCSBR (1000 L), filter (polyamine monofilament cloth), solvent extractor18–30%-[43]
17Dairy wastewater-Anaerobic acidogenic reactor, upflow sludge reactor, PHA synthesis reactor, clarifier, centrifuge43% of dry weight-[38]
18Sugar processing wastewaterMMCTwo SBRs (4 L), accumulation reactor (500 L), FBR (400 L)60% g-PHA/g-VSS24 weeks[44]
19Municipal wastewater and sludgeMMCFermentation reactor (1000 L), bowl–scroll centrifuge, drum filter (mesh of 130–600 μm), feeding tank (500 L), SBR (500 L), accumulation reactor (500 L), sludge-thickening units, oven34% (g PHA/g VSS)-[33]
20Wastewater from chocolate factoryPlasticicumulansacidivoransUpflow sludge reactor (60 L), anaerobic tank (1500 L), enrichment reactor (200 L), FBR (200 L), centrifuge0.76 gPHA/gVSS-[45]
21Paper industry wastewaterPlasticicumulansacidivoransUpflow sludge reactor (60 L), anaerobic tank (1500 L), enrichment reactor (170 L), FBR (200 L), centrifuge0.70–0.80 g PHA/g VSS42 days[2]
22Municipal solid waste-Fermentation CSTR (200 L), filter bag (5.0 μm porosity), SBR (140 L), PHA synthesis reactor55% -[46]
23Municipal solid wasteMMCFermentation CSTR (200 L), SBR-1 (100 L), SBR-2 (50 L), accumulation reactor49% (g PHA/g VSS)-[28]
24Municipal solid waste-Fermentation reactor (380 L), coaxial centrifuge (5.0 μm porosity, ultrafiltration membrane (0.2 μm porosity), CSTR (230 L), selection reactor (100 L), FBR (80–120 L)7.6% -[47]
25Activated sludge-SBR-1 (2500 L), nitrification SBR-2 (2500 L), precipitation reactor (150 L), buffer tank (2500 L)6.9–9.2% (gPHA/gTSS)439 days[48]
26Municipal solid wasteP. acidivoransSettling tank (1 m3), buffer vessel (1500 L), enrichment reactor (180 L), SBR, accumulation reactor (180 L), centrifuge77 ± 18% PHA757 days[49]
27Food waste-Fermentation reactor (380 L), coaxial centrifuge (5.0 μm porosity, ultrafiltration membrane (0.2 μm porosity), CSTR (230 L), selection reactor (100 L), FBR (80–120 L)7.6% -[47]
28Pulp–paper industry wastewater-2 bioreactors(15 L)39.6% dry sludge -[50]
29Oil mill wastewaterMMCSBR (16 L), FBR (2 L), solvent extraction74 ± 8%350 days[51]
30Activated sludge-Primary settler, activated sludge unit, secondary settler, thickening units, sludge digester, heat exchangers, buffer tanks, enrichment SBR, accumulation SBR, centrifuge--[52]
31Olive oil mill wastewaterMMC and Pseudomonas sp.Feed Tank (50 L), acidification reactor (20 L), fermentation SBR (50 L), aerobic reactor25%-[53]
32Wine grape wastePseudomonasputidaFermentation bioreactor (300 L), marine propeller tank, filter, autoclave, orbital shaker, batch centrifuge, extraction reactor 41%-[54]
33Food waste-Anaerobic bioreactor (34 L), enrichment reactor, PHA production reactor, SBR reactor23.7%-[55]
Table 3. A list of PHAs produced by different strains from different waste streams.
Table 3. A list of PHAs produced by different strains from different waste streams.
C-SourceN-SourceBacterial StrainPHAFermentation ModeOperating ConditionsPHA Yieldπ (g L−1 h−1)Ref.
CMNH4Cl, (NH4)2SO4 NH4NO3, ureaAlcaligenes sp. NCIM5085PHBBatchTemp. = 30 °C; pH = 6.54; agitation speed = 3.13 Hz; incubation time = 48 h8.8 ± 0.4 g L−10.19[17]
CM and WS(NH4)2SO4A. latus DSM1123PHBFed batch Temp. = 30 °C; pH = 7; agitation speed = 200–600 rpm; incubation time = 72 h; C/N ratio = 4–200 16.9 g L−1 L with 60-Brix syrup @ 500 rpm and 200 C/N0.234[73]
CM (NH4)2SO4A. eutrophusPHBBatchTemp. = 30 ± 1 °C; pH = 7; agitation speed = 250 ± 10 rpm; incubation time = 84 h0.78 g L−10.013 ± 0.022[15]
CMNH4ClA. eutrophus H16 and 5119, B. subtilis: R. eutrophus 5119PHBVBatchTemp. = 30 °C; pH = 7; agitation speed = 160 rpm; incubation time = 72 h2.30 g L−1-[16]
CM NH4Cl, CSL, (NH4)2SO4 NH4NO3
(NH4)3PO4		B. megateriumPHBBatchTemp. = 30 °C; pH = 7; agitation speed = 130 rpm; incubation time = 48 h46.2% per mg CDM-[74]
CM bacillus sp. Strain COl1/A6-Shaken flaskTemp. = 30 °C; agitation speed = 170 rpm; incubation time = 48 h54% CDW-[75]
CMureaB. megaterium BA-019PHBBatch and fed batchTemp. = 30 °C; pH = 7: agitation speed = 200 rpm; incubation time = 36 h; C/N = 10–10042%DCW @25 C/N1.27[22]
CM(NH4)3PO4B. cereus SPVPHBShaken flask and fed batchTemp. = 30 °C; pH = 6.8: agitation speed = 200 rpm; incubation time = 60 h6.63 g L−1-[76]
SCL monosodium glutamateP. fluorescens A2a5PHBbatchTemp. = 25 °C; pH = 7; incubation time = 144 h22 g L−10.23[24]
CM and CO P. PutidaPHAShaken flaskTemp. = 20–55 °C; pH = 3–9: agitation speed = 200 rpm; incubation time = 78 h35.63% CDW @ 37 °C and 7pH [77]
CMureaP. aeruginosaPHBBatchTemp. = 37 °C; pH = 7.0 ± 0.5: agitation speed = 150 rpm; incubation time = 72 h5.60 g L−10.12[78]
CM P. mendocinaP(3HO))BatchTemp. = 30 °C; pH = 6.8: agitation speed = 200 rpm; inoculum concentration = 10 %vv−1; incubation time = 48 h43.2% CDW [79]
PKO(NH4)2SO4Engineering Aeromonas caviae (PhaCAc)14.9 mol %P(3HB-co-3HHx)BatchTemp. = 30 °C; pH = 7; incubation time = 72 h14.1 ± 0.4 g L−1 [80]
WFO, WAF, IWAFUrea, NH4ClA. eutrophus H16 and recombinant strain of A. eutrophusPHB, P(3HB-co-3HHx)Batch and fed batchTemp. = 30 °C; pH = 6.8; agitation speed = 300–1200 rpm; incubation time = 72 h82% DCW PHB, 72% DCW P(3HB-co-3HHx)0.4[81]
OO-A. caviaeP(3HB-co-3HHx)BatchTemp. = 30 °C; pH = 7; incubation time = 24 h--[82]
CO, PKO, CPO, PO SBO, CO and POO -Recombinant strain of A. eutrophusP(3HB-co-70% 3HHx)BatchTemp. = 30 °C; agitation speed = 200 rpm; incubation time = 48 h1.3 g L−1-[83]
WPO(NH4)2SO4Pseudomonas Sp.Gl01mcl-PHAFed batchTemp. = 30 °C; pH = 7; agitation speed = 220 rpm; incubation time = 48 h1.6 g L−10.0907[25]
Tallow-based biodiesel-P. citronellolismcl-PHABatchTemp. = 30 °C; pH = 7; incubation time = 72 h26% DCW0.036[84]
WRO-Pseudomonas Sp.Gl01mcl-PHAFed batchTemp. = 30 °C; pH = 7; incubation time = 48 h2.0 g L−10.0374[85]
WFO P. chlororaphis PA23mcl-PHA (3HO,3HD,3HDD)BatchTemp. = 30 °C; pH = 7; incubation time = 48 h; shaking32.5% CDW [86]
CG, GBL and PA(NH4)2SO4A. eutrophus DSM 545P(3HB-4HB), P(3HB-4HB-3HV)Fed batch 36.9% DCW0.25[87]
CG(NH4)2SO4, yeast extractC. necator
DSM 545		PHBFed batch 70% CDW0.05[88]
CG(NH4)2SO4C. necator DSM 545PHB, PHBVShaken flask 5.260.12–0.15[89]
Sources, glucose, CGNH4ClB. cereus and B. thuringiensisPHAsShaken flask 2725 mgL−1 [90]
CG recombinant E. coilP(4HB)Fed batch 15 g L−10.207[91]
Biodiesel waste with glycerol P. oleovorans NRRL B-14682PHBShaken flask 13–27% DCW [92]
SIWW B. subtilis NG05PHBbatch 0.5 g L−1 [93]
Dairy wastewater effluents C necator DSM 13513PHBBatch and fed batch 1.34% [62]
Bioindustrial WW P. aeruginosa
B. subtilis		P (3HBco-3HV) Shaken flask 45%DCW [94]
Olive mill wastewater Pseudomonas sp., MMCPHBVBatch and 3-stage fermentation 64%DCW [95]
Molasses spent wash activated sludgePHB Shaken flask 31% DCW0.022[96]
PKO, palm kernel oil; POO, palm olein oil; CPO, crude palm oil; PO, palm olein; SBO, soybean oil; CO, corn oil; CO, coconut oil; OO, olive oil; SIWW, sugar industry wastewater; SCL, sugarcane liquor; WS, waste stream; π, productivity.
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Gautam, S.; Gautam, A.; Pawaday, J.; Kanzariya, R.K.; Yao, Z. Current Status and Challenges in the Commercial Production of Polyhydroxyalkanoate-Based Bioplastic: A Review. Processes 2024, 12, 1720. https://doi.org/10.3390/pr12081720

AMA Style

Gautam S, Gautam A, Pawaday J, Kanzariya RK, Yao Z. Current Status and Challenges in the Commercial Production of Polyhydroxyalkanoate-Based Bioplastic: A Review. Processes. 2024; 12(8):1720. https://doi.org/10.3390/pr12081720

Chicago/Turabian Style

Gautam, Shina, Alok Gautam, Juily Pawaday, Rekha Karshanbhai Kanzariya, and Zhitong Yao. 2024. "Current Status and Challenges in the Commercial Production of Polyhydroxyalkanoate-Based Bioplastic: A Review" Processes 12, no. 8: 1720. https://doi.org/10.3390/pr12081720

APA Style

Gautam, S., Gautam, A., Pawaday, J., Kanzariya, R. K., & Yao, Z. (2024). Current Status and Challenges in the Commercial Production of Polyhydroxyalkanoate-Based Bioplastic: A Review. Processes, 12(8), 1720. https://doi.org/10.3390/pr12081720

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