PHAs can be produced via two microbial systems: pure cultures or mixed microbial cultures (MMCs). Pure cultures involve specific bacterial strains such as Cupriavidus necator, Bacillus subtilis, or Pseudomonas putida, and require sterile conditions, controlled nutrients, and often pretreated wastewater to ensure process stability. While they offer high product yield and consistency, operational costs remain high. MMCs, on the other hand, rely on diverse, naturally occurring microbes capable of functioning under non-sterile conditions, making them more practical and economical for large-scale applications. Their ability to thrive in complex substrates makes MMCs particularly suitable for wastewater systems, where maintaining sterility is not feasible.
Due to their adaptability to fluctuating and impure feedstocks, MMCs are especially effective in biological treatment systems. They are commonly enriched through feast-famine regimes, which select for microbes capable of storing carbon as intracellular PHA during periods of substrate abundance. These microbes then metabolize stored reserves during starvation, improving overall PHA accumulation. Volatile fatty acids (VFAs), derived from anaerobic digestion of organic waste, serve as excellent carbon sources. Integrating this system into existing BTPs enables continuous PHA synthesis with minimal infrastructure investment.
Industries generating organic-rich wastewater—such as agro-industrial processing, food manufacturing, sludge treatment, biogas production, and the paper and pulp sector—produce effluents rich in sugars, fatty acids, and organic acids. These compounds serve as ideal substrates for microbial PHA biosynthesis. Utilizing existing biological treatment infrastructure in these industries not only reduces raw material and disposal costs but also mitigates greenhouse gas emissions and fossil plastic dependency. This broader applicability makes BTP-based PHA production a scalable and circular solution across multiple sectors.
The integration of PHA production into BTPs offers several environmental and economic benefits. It supports waste valorization by converting organic pollutants into biodegradable plastics, which can reduce treatment costs and create new revenue streams. Additionally, PHA production from waste emits significantly less greenhouse gas compared to fossil-based plastic production, contributing to Sustainable Development Goals (SDGs) such as climate action, responsible production, and industry innovation. The valorization of waste into PHA also strengthens the foundation of a circular bioeconomy, where biological waste loops are closed through value-added products.
Looking ahead, BTP-integrated PHA production presents a viable opportunity for industries to produce cost-effective bioplastics from waste. By combining MMC-driven synthesis, circular economy practices, and advanced bioprocessing tools, this model supports both environmental stewardship and economic resilience. Target applications include packaging, agriculture, and consumer goods, where low-cost, biodegradable materials can replace conventional plastics without compromising performance. With continued interdisciplinary collaboration and policy support, the biological treatment plant fermenter model could play a transformative role in reshaping the future of sustainable plastics.
A food processing industry sector is the initial industry sector targeted in the study. Initial PHA production involved wastes from fruit processing and the developments had been to have the dairy industry streams providing the bulk waste streams. To assess this potential, lignocellulosic waste from commonly processed fruits, papaya (Carica papaya), mango (Mangifera indica), watermelon (Citrullus lanatus), and pineapple (Ananas comosus) was evaluated as carbon sources for bacterial PHA synthesis. The bacterial strain Bacillus subtilis was selected for its known capacity to accumulate PHAs under nutrient-limited conditions. The fermentation was conducted over a 72-hour period under ambient temperature conditions using a modified LB agar medium. Preliminary screening with Sudan Black B staining confirmed intracellular PHA accumulation. Further validation through Fourier Transform Infrared Spectroscopy (FTIR) detected characteristic functional group peaks corresponding to PHA structures.
Among the tested substrates, papaya waste yielded the highest PHA production, indicating its potential as a cost-effective and locally available resource for biopolymer synthesis. This outcome underscores the feasibility of converting fruit processing waste into valuable materials while reducing waste management burdens on agro-industrial operations. Although the yield varied across different fruit types, all tested substrates supported measurable levels of PHA accumulation, reinforcing the applicability of diverse organic waste streams.
The fruit wastes will augment the OLR (organic loading rate) moving forward in this Process Fermenter – Biological WWTP model.