| Summary: | Wastewater activated sludge is produced during the biological treatment of wastewater. After treating the sewage, the sludge is allowed to settle. Part of the settled material is returned to the treatment process as return activated sludge (RAS) and the excess is removed as waste activated sludge (WAS). The handling and disposal of the sludge are energy and capital-intensive treatments, with a significant environmental impact. This work studies the possibility to utilize RAS (an example of wastewater sludge) as a source of surface active agents. The results indicate that higly surface active materials can be extracted from RAS, and that the RAS extract has potential applications as a detergent and wood adhesive. The results also suggest that recovering a suite of products from RAS, a biological heterogenous source, can be technically feasible.
An effective alkaline treatment was developed (at pH>12) that can extract up to 75% of the sludge’s organic matter, a yield higher than previously reported. Increasing the extraction pH increased the extract surface activity, which is linked to increasing the amount of higher molecular weight molecules and the presence of phospholipids. Increasing the extraction pH beyond 11 was also related to extensive cell lysis, increasing significantly the amount of recovered material and the surface activity of the extract.
The alkaline extract has properties comparable to commercial detergents. Without further purification, the extract has a low surface tension (37 mN/m on average) and performs similarly to synthetic detergents. Further assessment of the RAS extract (insensitivity to pH, surface tension, interfacial tension) suggests that it may be suitable for commercial applications.
The RAS extract can also be formulated into wood adhesives using glutaraldehyde as a crosslinker. The extract fraction with 10-50 kDa constituents at pH 9 achieves high adhesive shear strengths (4.5 MPa on average, at 30% relative humidity and 25°C) with 40% of wood failure. The adhesive strength of RAS-based adhesives is strongly correlated to its protein content.
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