Effect of cold plasma on breaking activated sludge and the output dominance of protein

Upon contacting with water, cold plasma should produce numerous ozone molecules and free electrons at room temperature. In this study, a cold plasma generator was used to break the walls of residual activated sludge obtained from domestic sewage. The impact was mainly influenced by the ozone generated. With 800 W power, sludge wastewater pH of 12.0, and under continuous treatment for 10 h, the system's reduction efficiency for the dry sludge was ≈90%. Furthermore, the organic matter content (especially protein) of the upper layer of the sludge solution increased a lot after the sludge digestion. This observation proved the reduction of sludge from both sides. Moreover, when the cold plasma technique was compared with thermal acid hydrolysis, thermal alkali hydrolysis, and ultrasonication for extracting protein from activated sludge, cold plasma wall-breaking sludge exhibited the highest efficiency, reaching 38.2% under ambient temperature. After the analysis, the toxic metal content in the extracted protein was near zero, which is a level other protein extraction methods via sludge breaking have not achieved to date, we attribute this efficiency to free electrons the cold plasma produce. These species promote the transformation of metal ions into atomic metals, thereby facilitating their removal.