Effects of pH on the surface characteristics and molecular structure of extracellular polymeric substances from activated sludge

To understand the mechanisms of extracellular polymeric substances (EPS) influencing sludge characteristics, change in constituents of extractable EPS from municipal wastewater and beverage wastewater sludge at various pH was investigated. Fourier transform infrared (FTIR) spectrophotometry was used to analyze the effect of pH on the functional groups in EPS. Surface charge was measured by colloid titration. Finally, surface charge measurements and molecular structure analysis were evidenced by Scanning Electron Microscopy (SEM) observation of the sludge samples before and after EPS extraction at different pH. The results show that at strong acid condition (pH 3), the extractable EPS are lower than those at neutral condition by about 50% including 30% decreasing in polysaccharides and 65%-70% in protein. At strong alkali condition (pH 11), the extractable EPS are higher than those at neutral condition by 20%-30% including about 15% increasing in polysaccharides and 20%-50% in protein. Hydroxyl changes both at alkali and acid condition. Carboxyl acid, polysaccharides, phenolic groups, arid amino groups in protein disappear at acid condition (pH 3). Negative surface charge of EPS from both of the sludges increases with pH. Observation of sludge by SEM indicates that compared with alkali condition, acid condition is more effective in making the bacterial cell broken. The results suggest that pH changes the constituents, concentration and functional groups in EPS, consequently the surface charge of EPS. As a result, sludge characteristics are changed.