Experimental and theoretical analyses on the impacts of ionic surfactants on sludge properties

A large amount of excess waste activated sludge is produced in municipal wastewater treatment plants and should be further disposed to avoid environmental pollution. Ionic surfactants are being widely used for sludge conditioning, but how and to what extent ionic surfactants change the sludge properties remain unclear. In this work, the impacts of two typical ionic surfactants on the flocculability and stability of sludge were investigated by using experimental and theoretical analyses. The treatment of anionic sodium dodecyl sulfate (SDS) resulted in more detachment of extracellular materials and fluorophores from sludge compared to the treatment of cationic trimethyl ammonium bromide (DTAB). Fourier transform infrared spectra analysis indicates the promoted release of proteins and polysaccharides induced by the surfactants. Deteriorated flocculability of sludge was observed for the SDS-treated sludge, while treatment of DTAB slightly affected the overall sludge flocculability. The sludge floc structure became less stable after treatments of both surfactants, as confirmed by the elevated dispersed mass concentration of small particles in shearing tests. The relationship between the content of extracellular polymeric substances, surfactant dose and sludge properties were evaluated and the roles of released extracellular polymeric substances and surfactant dose in affecting sludge flocculability were examined. Extended DLVO approach was adopted to explore the stability of sludge. By using such an integrated approach, the impacts of ionic surfactants on sludge flocculability and stability were revealed, which is useful in understanding the mechanisms of sludge conditioning by surfactant treatment.