The Recovery of Biomass or Sludge from Wastewater Treatment Plants (WWTPs) is currently of great interest, given the large amounts of this waste generated every year and the high costs associated with its disposal. In the current European system, around 309 million tons of sludge are produced annually in the EU and Turkey, with disposal costs reaching up to €400 per ton—representing as much as 50% of the operation and maintenance costs of a wastewater treatment plant.
What makes this topic particularly interesting is that these abundant and seemingly useless residues can in fact be exploited as a source of raw materials and energy. Such wastes, including conventional sewage sludge, microalgae biomass, and PPB (purple phototrophic bacteria) biomass, are composed of potentially valuable compounds such as non-toxic organics, nutrients, polymers, sugars, bioactive compounds, oils and fats, proteins, among others. Currently, their valorization is mainly based on energy production (e.g., methane) and use as fertilizers in agriculture. However, in many cases, this biomass, rich in high-value compounds that could be recovered, is ultimately discarded in landfills.
Within the framework of the PROPHACTION project, funded by the Spanish Ministry of Science and Innovation (PID2020-113544RB-I00), the main objective is the valorization of these wastewater-derived residues through the recovery of proteins and the production of polyhydroxyalkanoates (PHAs). This approach enables the development of a system aligned with the principles of the Circular Economy, aiming to give wastewater treatment residues a second life, conserve natural resources, and reduce the environmental footprint of the industry.
Why proteins? Proteins account for around 50% of the dry matter in sludge and microalgae biomass generated in wastewater treatment. The recovered protein fractions have promising applications as agricultural fertilizers, functional polypeptides in industry, or amino acids for animal feed. In addition, the growing demand for amino acids and peptides in the market has significantly increased interest in protein recovery.
Why PHA production? Polyhydroxyalkanoates are naturally produced polyesters synthesized by a wide variety of bacteria as storage compounds. They can be extracted from biomass derived from wastewater treatment plants and used to produce bioplastics, a biodegradable alternative with lower environmental impact. Despite the advantages of these polymers, their production costs are still not competitive. However, these costs could be reduced by developing processes based on alternative carbon sources, such as the hydrolysates obtained in earlier stages of the project.
In this context, PROPHACTION researchers are investigating how to separate proteins and produce PHAs from wastewater treatment residues using more sustainable and cost-effective techniques than those currently in use. The diagram provides a brief description of the methods and technologies under development to give these wastes—currently discarded as refuse and accumulated in the environment—a second life.
