Modern conventional methods of treating domestic sewage use a sequence of subsystems in which different treatment processes are employed. At the scale of the individual home, septic tanks with drain fields are used (Figure 2.1). This is a simple but remarkably effective system that is used widely (Kahn et al., 2000; Kaplan, 1991). Physical sedimentation occurs in the septic tank itself and the solid sludge must be removed periodically. Anaerobic metabolism by microbes occurs inside the tank, which initiates the breakdown of organic matter in the sewage. Liquids eventually flow out from the tank into a drain field of gravel and then into the surrounding soil where microbes continue to consume the organic matter and physical/chemical processes filter out pathogens and nutrients. The larger-scale sewage treatment plants (Figure 2.2) use similar processes for primary treatment (sedimentation of sludge) and secondary treatment (microbial breakdown of organic matter) in a more highly engineered manner. Processes can be aerobic or anaerobic depending on basic design features. Not shown in Figure 2.2 is a final treatment step, usually chlorination in most plants or use of an ultraviolet light filter, which eliminates pathogens. Note
also that nutrients are not removed and are usually discharged in the effluent unless some form of tertiary treatment is employed.
The technologies discussed above are used throughout the world to treat human sewage and are the products of a long history of sanitary engineering design. Sawyer (1944), in an interesting paper which represents one of the first uses of the term biological engineering, traces the origins of the conventional technologies back to 19 th century England and the industrial revolution, but the formal origin of the field of sanitary engineering seems to be the early 20th century United States. In his classic work on stream sanitation, Phelps (1944) places the origin at the research station of the U.S. Public Health Service, opened in 1913 in Cincinnati, Ohio. He calls this station an "exceptional example of the coordinated work of men trained in medicine, engineering, chemistry, bacteriology, and biology" which gives an indication of the interdisciplinary nature of this old field. The station was later named the Robert A. Taft Sanitary Engineering Center and it housed a number of important figures in the field.
Sanitary engineering developed the kinetic and hydraulic aspects of moving and treating sewage with characteristic engineering quantification. The field also involved a great deal of biology and even some ecology, which is particularly relevant in the context of the history of ecological engineering. Admittedly most of the biology has involved only microbes and, in particular, only bacteria (Cheremisinoff, 1994; Gaudy and Gaudy, 1966; Gray, 1989; James, 1964; Kountz and Nesbitt, 1958; Parker, 1962; la Riviere, 1977). Moreover, sanitary engineers seemed to have their own particular way of looking at biology as witnessed by their use of terms such as slimes (see Gray and Hunter, 1985; Reid and Assenzo, 1963). Even though this term is quite descriptive, a conventional biologist might think of it as too informal. Another example of their view of biology (see Finstein, 1972; Hickey, 1988 as examples) is the use of the name sewage fungus to describe not a fungus but a filamentous bacterium (Sphaerotilus) with a gelatinous sheath. Ecologists usually tend to be a bit more precise with biological taxonomy than this [though Hynes (1960) used the term sewage fungus in his seminal text on the biology of pollution]. These semantic issues are easily outweighed by the contributions of sanitary engineers to the biology and ecology of sewage treatment. It is significant that sanitary engineers were viewing sewage treatment much differently compared with conventional ecologists. To them sewage was an energy source and their challenge was to design an engineered ecosystem to consume it. This attitude is reflected in a humorous quote attributed to an "anonymous environmental engineer" that was used to introduce an engineering text (Pfafflin and Ziegler, 1979): "It may be sewage to you, but it is bread and butter to me." Meanwhile, more conventional ecologists wrote only on the negative effects of sewage on ecosystems as a form of pollution (Hynes, 1960; Warren and Doudoroff, 1971; Welch, 1980). Because of the negative perspective, this form of applied ecology was not a precursor to the treatment wetland technology.
One important example of classic sanitary engineering is the understanding of what happens when untreated sewage is discharged into a river. This was the state-of-the-art in treatment technology up to the 20th century throughout the world and it is still found in many lesser-developed countries. The problem was worked out by Streeter and Phelps (1925) and is the subject of Phelps' (1944) classic book. The river changes dramatically downstream from the sewage outfall with very predictable consequences in the temperate zone (Figure 2.3), in a pattern of longitudinal succession. Here succession takes the form of a pattern of species replacement in space along a gradient, rather than the usual case of species replacement in one location over time (see Sheldon, 1968 and Talling, 1958 for other examples of longitudinal succession). Streeter and Phelps developed a simple model that shows how the stream ecosystem treats the sewage (Figure 2.4). In the model, sewage waste creates BOD, which is broken down by microbial consumers. The action of the consumers draws down the dissolved oxygen in the river water resulting in the oxygen sag curve seen in both Figure 2.3 and Figure 2.4. Sewage is treated when BOD is completely consumed and when dissolved oxygen returns. This process has been referred to as natural purification or self-purification by a number of authors (McCoy, 1971; Velz, 1970; Wuhrmann, 1972). It is important because it conceptualizes how a natural ecosystem can be used to treat sewage wastewater and is a precursor to the use of wetland ecosystems for wastewater treatment.
Other early sanitary engineers contributed ecological perspectives to their field. A. F. Bartsch, who worked at the Taft Sanitary Engineering Center, wrote widely on ecology (Bartsch 1948, 1970; Bartsch and Allum, 1957). H. A. Hawkes was another author who contributed important early writings on ecology and sewage treatment (Hawkes, 1963, 1965). Many of the important early papers written by sanitary engineers were compiled by Keup et al. (1967), and Chase (1964) provides a brief review of the field.
Unlike most sanitary engineering systems, which focused solely on microbes, the trickling filter component of conventional sewage treatment plants has a high diversity of species and a complex food web. The trickling filter (Figure 2.5) is a large open tank filled with gravel or other materials over which sewage is sprayed. As noted by Rich (1963),
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