FY 1989

Settling speeds of sewage sludge in seawater

Lavelle, J.W., E. Ozturgut, E.T. Baker, D.A. Tennant, and S.L. Walker

Environ. Sci. Technol., 22(10), 1201–1207, doi: 10.1021/es00175a013 (1988)

Laboratory analyses of sludges from four treatment plants were performed to determine solid content, size distributions, and settling spectra of sludge particulates in seawater. The settling experiments were conducted after partitioning samples by sieving into coarse and fine particulate fractions. For the fine fraction, experiments proceeded after controlled mixing of samples with filtered seawater to particulate concentrations low enough ( 10 mg/L) that flocculation was limited during subsequent measurement periods. Results show that 14.6-47.3% of the sludge particulates by weight had diameters greater than 63 µm, with at least 5% exceeding 250 µm. Median settling velocities of this fraction ranged from 6 × 10 to 3 × 10 cm/s. The bulk of the particulates were smaller in diameter, and these flocculated in seawater. Resulting aggregates had median settling velocities by volume ranging from 7 × 10 to 3 × 10 cm/s, with less than 10% of the particles settling more slowly than 10 cm/s.