Experimental work was performed to delineate the system of digestedsludge particles and associated trace metals and also to measure theinteractions of sludge with seawater. Particle-size and particle numberdistributions were measured with a Coulter Counter. Numbercounts in excess of 1012 particles per liter were found in both the Cityof Los Angeles Hyperion mesophilic digested sludge and the Los AngelesCounty Sanitation Districts (LACSD) digested primary sludge. More than90 percent of the particles had diameters less than 10 microns.
Total and dissolved trace metals (Ag, Cd, Cr, Cu, Fe, Mn, Ni, Pb,and Zn) were measured in LACSD sludge. Manganese was the only metalwhose dissolved fraction exceeded one percent of the total metal.Sedimentation experiments for several dilutions of LACSD sludge in seawatershowed that the sedimentation velocities of the sludge particlesdecreased as the dilution factor increased. A tenfold increase indilution shifted the sedimentation velocity distribution by an orderof magnitude. Chromium, Cu, Fe, Ni, Pb, and Zn were also followedduring sedimentation. To a first approximation these metals behavedlike the particles.
Solids and selected trace metals (Cr, Cu, Fe, Ni, Pb, and Zn)were monitored in oxic mixtures of both Hyperion and LACSD sludges forperiods of 10 to 28 days. Less than 10 percent of the filterablesolids dissolved or were oxidized. Only Ni was mobilized away fromthe particles. The majority of the mobilization was complete in lessthan one day.
The experimental data of this work were combined with oceanographic,biological, and geochemical information to propose and model thedischarge of digested sludge to the San Pedro and Santa Monica Basins.A hydraulic computer simulation for a round buoyant jet in a densitystratified medium showed that discharges of sludge effluent mixtureat depths of 730 m would rise no more than 120 m. Initial jet mixingprovided dilution estimates of 450 to 2600. Sedimentation analysesindicated that the solids would reach the sediments within 10 km ofthe point discharge.
Mass balances on the oxidizable chemical constituents in sludgeindicated that the nearly anoxic waters of the basins would become whollyanoxic as a result of proposed discharges. From chemical-equilibriumcomputer modeling of the sludge digester and dilutions of sludge inanoxic seawater, it was predicted that the chemistry of all tracemetals except Cr and Mn will be controlled by the precipitation of metalsulfide solids. This metal speciation held for dilutions up to3000.
The net environmental impacts of this scheme should be salutary.The trace metals in the sludge should be immobilized in the anaerobicbottom sediments of the basins. Apparently no lifeforms higher thanbacteria are there to be disrupted. The proposed deep-water dischargeswould remove the need for potentially expensive and energy-intensiveland disposal alternatives and would end the discharge to the highlyproductive water near the ocean surface.
