Digested sewage sludge: characterization of a residual and modeling for its disposal in the ocean off Southern California

Abstract

Sewage sludge is a concentration of the residues that our modern municipal and industrial society discharges to the sewerage system. The sludge solids are removed from the sewage flow by sedimentation and then partially stabilized by biological digestion. Typical digested sludges are 95 to 98 percent water, with the majority of their potential environmental contaminates such as trace metals associated with the two to five percent solids fraction. This particulate matter has usually been characterized only for such gross constituents as total solids, total trace metals, and pesticides. For this work, the sludge particle system was approached on a much more detailed level. There is strong evidence that the particles in digested sludge may cause serious deleterious effects when discharged near the productive surface waters of the ocean. Such effects include disruption of light penetration into the water column. This can drastically reduce photosynthesis, the primary productivity in the ocean. Sludge particles discharged to the ocean are heavier than the surrounding water and tend to settle. They may "blanket" the bottom, interfering with the normal life cycles of bottom-dwelling organisms. The digested sludge, only partially stabilized by treatment processes before discharge, may also drastically alter the chemistry of the sediments where it settles. The very fine material in the sludge does not settle easily and may be carried many kilometers by the prevailing ocean currents. The trace metals and other components incorporated into the particles are also then carried great distances. Since many marine organisms such as bivalves and zooplankton are filter feeders, the presence of sludge-particulate matter of the right size in the water column may lead to uptake of sludge contaminants in the food chain.

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