Settling speeds of sewage sludge in seawater

Abstract

w 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 ( N 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 pm, with at least 5% exceeding 250 pm. Median settling velocities of this fraction ranged from 6 X to 3 X 10-1 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 X lo4 to 3 X cm/s, with less than 10% of the particles settling more slowly than 10" cm/s. IntroductionThe ability to predict the fate of sewage sludge after ocean discharge requires knowledge of its physical properties, particularly the settling speeds of its constituent particles. Yet this measurement has not been satisfactorily made because of the difficulties of settling measurements in situ and because laboratory studies are beset with their own special set of problems. Specifically, adequate laboratory settling velocity measurements bearing on the fate of ocean discharges of sludge depend on (1) the creation of flocculated particles resembling those found in the ocean, a creation process that is dependent on poorly known ocean mixing conditions, and on (2) being able to prevent the change in the physical properties of the particles after the formation process and during the settling measurements. Few results of settling experiments for sludge particulates are not confounded by continued flocculation during the measurement period. The laboratory work described here primarily addresses the second aspect of this problem, though an attempt was made to create flocculated particles under low shear and during increasing dilution, conditions qualitatively resembling those found in the ocean during the floc formation period.The key to preventing flocculation during settling is to restrict the settling stage to low particulate concentration for which the frequency of particle-to-particle contact is low. Ozturgut and Lavelle (I) found that the settling of clays in seawater was not affected by flocculation when concentrations were 7 ppm or less. On the basis of that result, particulate concentrations near 10 mg/L (-5-10 ppm) were deemed suitable for conducting these measurements. This is also a desirable choice of concentrations because such concentration levels are near those expected in the ocean tens of minutes past the time of discharge. For example, concentrations of 10-40 mg/L were measured

Use of Trialkylamines as a Marker of Sewage Addition into the Marine Environment

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