Municipal sewage water contains considerable amounts of organic carbon which can be useful carbon-based materials like cellulose or lipids or converted into fatty acids. Polyhydroxyalkanoates (PHA) are produced in special processes, but neither sewage nor sewage sludge contains this substance in larger amounts without enrichment. Utilizing these valuable materials could reduce the use of natural resources and subsequent carbon dioxide (CO2) emissions and hence, realize a circular economy. In the WOW project, a techno-economic assessment on the PHA production plant from primary sludge as feedstock was performed which shows that a positive business case is possible.
The value chain consists of several steps including acidogenic fermentation, biomass selection/enrichment, PHA accumulation, PHA separation, drying, and PHA extraction. The recovered raw PHA material is mixed with other materials and used in an injection moulding process to produce end products. To be able to have an economically feasible PHA production, a decentralized strategy is required due to limited sludge treatment plant (STP) capacities. In this strategy, the selection and production of PHA-rich biomass are done at several STPs and transported to a centralized PHA extraction facility.
The virgin PHA minimum selling price (MSP) estimated is €3.54/kg PHA which is in parity with the current market prices and that reported in the literature. Several technical and economic parameters affect the overall performance of the plant. The most influential parameter is the PHA-rich biomass produced in the accumulation reactor. This value directly represents the solid content in the reactor. By optimizing the accumulation process to have 3 wt.% of solid content, the PHA could be produced at €1.5/kg, which is significantly cheaper than the market price. Another influential parameter is the PHA yield, which is an intrinsic process parameter. Though the reference value obtained from the pilot plant was around 35 wt.%, it was reported that the PHA yield can be altered by manipulating the carbon to nitrogen ratio, using inhibitors and novel fermentation strategies. If the PHA yield is increased to 70 wt.%, the reduction in the PHA production price could be up to 57%. Increase in the plant scale and a reduction in the capital investment required for the plant, the PHA production cost could be reduced to some extent.
When it comes to the end product, the raw PHA is required to be mixed with other polymers. In the WOW project, the blend consisting of 30 wt.% of PHA is tested with promising results. The blend ratio and the raw material price are the two main factors that affect the MSP of the end product. These two factors depend on the desired quality and the intended application of the end product. A cheaper raw material might not provide the best quality and vice versa. Therefore, an optimized value of blend ratio and the raw material price ensures the best quality product at a low cost. The performance of this value chain compared to other PHA production plants is promising and results in a positive business case under the assumptions made. It would be interesting to investigate further since it has the potential for reducing the MSPs by optimizing plant operation and efficient energy consumption.
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Figure: Bioplastic (PHA) production from sewage: