Laboratory analysis of settling velocities of wastewater particles in seawater using holography
Creators: name: Wang, Rueen-Fang Theresa
Publisher: name: California Institute of Technology
Year: 1988

description: 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. 

A 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 Samples were then withdrawn from the reactor at different times for size and settling velocity measurements.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. 

An 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. 

Four 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.
description: © 1988 Environmental Quality Laboratory. California Institute of Technology. 

First and foremost, I would like to thank my advisor, Professor Norman H. Brooks. This dissertation would never have come to be without his supporting and guiding hand. Also, I would like to thank Dr. Robert C. Y. Koh for his stimulating discussions and many constructive suggestions and criticisms. The following professors kindly serve on my examination committee: Allan J. Acosta, Richard C. Flagan, E. John List and James J. Morgan. I thank them for their time and help through my course of study. 

Appreciation also goes to Dr. Tim J. O'Hern who taught me the experimental techniques of holography and helped me started on the design of my own system. I would like to thank John Yee-Keung Ngai, my best friend at Caltech. His friendship and words of encouragement helped me through the most difficult time in my study. Technically, he introduced me to the fancy world of computation, discussed my problems, and made available to me his computer for my research work. I am grateful to my brothers, Yuan-Fang and Jih-Fang, who taught me the fundamentals of digital image processing and helped me build the interface and image analysis routines which greatly simplified the image processing task in this research. 

The assistance of Elton Daly, Joe Fontana, Rich Eastvedt and Hai Due Vu at the Keck Lab Shop is of particular importance in the construction and troubleshooting of the experimental apparatus. The continuing friendship and help of Joan Mathews, Luise Betterton, Rayma Harrison, Gunilla Hastrup, Elaine Granger, ans Sandy Brooks are greatly valued. I would like to thank Nancy Tomer, whose professional skill and agreeable personality make my thesis preparation much easier. I also thank personnel of the County Sanitation Districts of Orange and Los Angeles Counties for providing sludge and effluent samples. Special thanks go to Professor Nobuo Mimura for his interest in this work as well as his valuable discussions and suggestions. For all my friends and colleagues, especially Liyuan Liang and David Walker, I thank them for their support and understanding. 

I gratefully acknowledge the financial support provided by the United States National Oceanic and Atmospheric Administration (Grants no. NA80RAD0055 and NA81RACOO153), the County Sanitation Districts of Orange County, the County Sanitation Districts of Los Angeles County, the Andrew W. Mellon Foundation, the William and Flora Hewlett Foundation and EQL gifts. 

I cannot express in words my feeling for my family whose love and devotion never fail to encourage me and propel me through my study, and to whom, I dedicate this dissertation.
description: Submitted - EQLReport27.pdf