Surface activity of heat–alkaline treated excess sludge

Biosurfactants are produced by fermenting pure strains but not treating excess sludge. For reducing the excess sludge, heat and alkaline treatment was reported as a feasible process. To develop a biosurfactant from treated sludge, the surface activity of heat–alkaline treated sludge was investigated in this study. Sludge, Escherichia coli, and extracted E. coli lipids were treated under pH 7, pH 11 and pH 13 conditions that were then combined with or without heat treatment. When the sludge was treated at pH 13 and then heat-treated for 18 h, the surface tension of the supernatant of the treated sludge was reduced by 26.6% from 72.5 mN/m to 53.0 mN/m. As a whole, the alkaline treatment more effectively reduced surface tension than heat treatment because the surface tension of the supernatant was lower than 55.4 mN/m after the sludge was treated at pH 13. N-hexadecane droplets were emulsified by the supernatant of treated sludge, and then collected using a designed sampling device. The droplet size distribution and cumulative frequency curve of n-hexadecane droplets showed that over half of the droplets were smaller than 80 μm after they were emulsified by supernatant from alkaline-treated sludge at pH 13 treatment condition no matter with or without heat treatment. These findings indicate that heat–alkaline treated sludge has both surface activity and emulsifying capability.

• We obtained surface activity from sludge treated by heat and alkaline. • Alkaline treatment contributed more to the surface activity. • The specific surface area of n-hexadecane droplets were increased 2.1 folds after emulsified by the treated sludge.