Laboratory analysis of settling velocities of wastewater particles in seawater using holography

摘要

Ocean discharge of treated sewage and digested sludge has been a common practice for the disposal of municipal and industrial wastewaters for years. Since the particles in the discharge cause much of the adverse effect on the marine environment, the transport processes and the final destinations of particles and the associated pollutants have to be studied to evaluate the environmental impact and the feasibility of disposal processes. The settling velocity of particles and the possible coagulation inside the. discharge plume are among the most important factors that control the transport of particles. ududA holographic camera system was developed to study the settling characteristics of sewage and sludge particles in seawater after simulated plume mixing with possible coagulation. Particles were first mixed and diluted in a laboratory reactor, which was designed to simulate the mixing conditions inside a rising plume by varying the particle concentration and turbulent shear rate according to predetermined scenarios. Samples were then withdrawn from the reactor at different times for size and settling velocity measurements. Artificial seawater without suspended particles was used for dilution. ududAn in-line laser holographic technique was employed to measure the size distributions and the settling velocities of the particles. Doubly exposed holograms were used to record the images of particles for the fall velocity measurement. Images of individual particles were reconstructed and displayed on a video monitor. The images were then digitized by computer for calculating the equivalent diameter, the position of the centroid, the deviations along the principal axes, and the orientation of particles. A special analysis procedure was developed to eliminate sampling biases in the computation of cumulative frequency distributions. The principal advantages of this new technique over the conventional settling column (used in the early part of this research) are that: (1) the coagulation and settling processes can be uncoupled by use of extremely small concentrations (less than 2 mg/l) in the holographic sample cell, and (2) the individual particle sizes and shapes can be observed for correlation with measured fall velocities. ududFour sets of experiments were conducted with blended primary/secondary effluent from the County Sanitation Districts of Los Angeles County and the digested primary sludge from the County Sanitation Districts of Orange County (proposed deep ocean outfall) using different mixing processes. Experimental results show that the sludge and effluent particles have very similar settling characteristics, and that particle coagulation is small under the simulated plume mixing conditions used in these experiments. The median and 90-percentile fall velocities and the fractions of particles with fall velocities larger than 0.01 cm/sec of the digested primary sludge and the effluent are summarized in the following table. The experimental results from the conventional settling column are also included for comparison. In general, the holographic technique indicates slower settling velocities than all the previous investigations by other procedures.