Surfactant-like Properties of Alkaline Extracts from Wastewater Biosolids

In order to assess the potential for utilizing wastewater biosolids as a source of useful substances, the surface activity of materials extracted from wastewater biosolids (activated sludge) by simple incubation with sodium hydroxide solutions at room temperature was assessed. The surface activity, measured by surface and interfacial tension methods, of the extracts was shown to be dependent on the extraction pH and the concentration of the organic matter solubilized in the alkaline solution. Increasing the extraction pH increased the surface activity of the extract (lower surface tensions), which is linked to the presence of more hydrophobic species in the extract. After adjusting the pH to more acidic values (e.g., pH = 4), the extracts retained their surface activity. The apparent CMC (critical micelle concentration) of pH 12.6 extracts was approximately 1,000 mg/L (based on total organic carbon or TOC), and the surface tension after CMC approximately 35 mN/m. While the CMC of the extract is significantly higher, when compared to a conventional surfactant, sodium dodecyl benzene sulfonate (SDBS, CMC ~ 25 mg/L), its surface tension at CMC was comparable. Above its CMC, the pH 12.6 extract had similar interfacial tensions than SDBS against toluene, heptane and hexadecane. Furthermore, the extract and SDBS had similar detergency performance for the removal of hexadecane from cotton. Skin corrosivity tests of the neutralized extracts show that they have comparable toxicity to conventional anionic surfactants such as sodium dodecyl sulfate. The potential use of these extracts in commercial products is discussed.