Bioplastic PHA from wastewater
Production of bioplastics may be improved by using the nutrients and minerals in wastewater as a feedstock for microbial fermentation. Strategies like feast-famine culture, mixed-consortia culture, and integrated processes have been developed for PHA production from highly polluted wastewater with high organic loads. Various process parameters like organic loading rate, organic content (volatile fatty acids), dissolved oxygen, operating pH, and temperature also have critical roles in PHA accumulation in microbial biomass.
Research advances are also going on in downstream and recovery of PHA utilizing a combination of physical and chemical (halogenated solvents, surfactants, green solvents) methods. This review highlights recent developments in upcycling wastewater resources into PHA, encompassing various production strategies, downstream processing methodologies, and techno-economic analyses.
The microbial synthesis of PHA stands as a remarkable example of nature’s innovation, offering a sustainable avenue for producing biodegradable bioplastics. Microorganisms produce PHA as intracellular carbon reserves in response to nutrient imbalances, particularly an excess of carbon and limited nitrogen or phosphorus, storing them as granules.
The biosynthetic operon for PHA consists of a cluster of genes, including PHA synthase, β-ketothiolase, and NADPH-acetoacetyl-CoA reductase, organized in close proximity .Carbon sources are converted into acetyl-CoA, a precursor for PHA synthesis, through pathways such as glycolysis or beta-oxidation of fatty acids.
The synthesis of polyhydroxybutyrate (PHB) involves the condensation of two acetyl-CoA molecules to form acetoacetyl-CoA, catalyzed by the enzyme β-ketothiolase. Subsequently, acetoacetyl-CoA is reduced to (R)-3-hydroxybutyryl-CoA by (R)-specific acetoacetyl-CoA reductase.
Finally, (R)-3-hydroxybutyryl-CoA is polymerized into PHB by the enzyme PHA synthase. This pathway utilizes the organic components available in wastewater, such as oil, volatile fatty acids (VFAs), glucose, fructose, xylose, and amino acids, transforming them into PHA and intermediates.
Strategies for converting wastewater into PHA involve several key factors. These include selecting the right microbial strains, their metabolic abilities, and their ability to adapt to the wastewater environment, all of which affect the variability in PHA production. The design and setup of the bioreactor system also play a significant role in determining the efficiency of the wastewater treatment process.
Current research is focusing on improving bioreactor design, optimizing microbial communities, and discovering new microbial species to enhance the efficiency of converting wastewater to PHA. Integrating wastewater treatment with PHA production represents a sustainable strategy that aligns with circular economy concepts by turning waste into a valuable resource.
To achieve this, several optimization strategies are employed. These include the Feast-Famine process, batch and continuous-flow systems, the use of microbial consortia, co-feeding techniques, and single or multi-stage processes, which are detailed below.
Conclusion
Polyhydroxyalkanoates (PHA), a biodegradable polymer, presents a promising alternative to traditional plastics, offering potential solutions for reducing packaging waste. The wastewater generated by industrial, agri-horticultural, and municipal activities is rich in organic and inorganic compounds, including carbon, nitrogen, phosphorus, and minerals, serving as a natural resource for microorganisms to produce PHA.
The selection of microbes through methods like the feast and famine approach, along with the utilization of microbial consortia, enhances the efficiency of PHA production processes. However, challenges persist in the downstream processing of PHA, as existing extraction methods often yield lower quantities and purity, affecting the material’s natural properties.
Utilization of wastewater as feedstock is advantageous as it doesn’t require any pretreatment like lignocellulosic biomass and the process can be more economical. However, long-term sustainability and feasibility must be scrutinized to prevent the further evolution of new pollutants. Further, wastewater characterization and selection of efficient PHA producers with easy and eco-friendly PHA recovery methods are areas that need attention to make the process feasible at a large scale.
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