PHYSICAL-PROPERTIES OF SEWAGE PARTICLES IN SEAWATER

Samples of primary-treated effluent were examined by laser particle sizer, photographic image-analysis and in settling/rinsing experiments in order to determine particle sizes and falling/rising speeds through seawater. Using a 1.7 m column and 24 h experiments, it was found that approximately 22% of the particles settle out of suspension, 3% rise to the surface and 75% remain in suspension. Between 80 and 100% of the falling particles reach the bottom in less than 6 h. A good correlation (r = 0.95) was found between the cumulative mass of particles at the base of a settling column and particle falling velocity. In contrast, a poor correlation (r = 0.57) was found between median particle size and falling velocity. The falling particle sizes did not follow a log-normal distribution as might be expected. This is probably explained by the fact that larger particles are composed of flocculated material and vary in density from the smaller particles. Particles with a diameter of < 80 mu m make up 90% of the mass of material that sinks. A fairly good correlation was found between percent cumulative mass and rising velocity (r = 0.74). However, the relationship between particle size and rising velocity is more complex. There are three main categories of rising particles: < 52 mu m; between 52 and 150 mu m; and those with an average size > 150 mu m. The smallest class of particles (< 52 mu m) are by far the most abundant, comprising on average 95% of the total number of rising particles. The small particles show a weak logarithmic relationship between size and rising velocity (r = 0.53). The mid-size particles (52-150 mu m) comprise approximately 4% of the total number of particles and are the most difficult to categorize as there appears to be no relationship between particle size and rising velocity. The large size particles, although the least abundant (similar to 1%), are the most predictable; there is a good correlation between particle size and rising velocity (r = 0.77). This is probably because the particles are fairly homogeneous and largely composed of oil and grease.