Kangas - Ecological Engineering - Principles and Practice - Chapter 2 doc

25 2 Treatment Wetlands INTRODUCTION The use of wetlands for treating wastewater is probably the best example of eco- logical engineering because the mix of ecology and engineering is nearly even. The idea is to use an ecosystem type (wetlands) to address a specific human need that ordinarily requires a great deal of engineering (wastewater treatment). This appli- cation of ecological engineering emerged in the early 1970s from a number of experimental trials and is today a growing industry based on a tremendous amount of experience as reflected by a large published literature. Although there is, of course, still much to be learned, the use of wetlands for wastewater treatment is no longer a novel, experimental idea, but rather an accepted technology that is beginning to mature and to diffuse throughout the U.S. and elsewhere. The focus of the chapter is on treatment of domestic sewage with wetlands, which was the first application of the technology, but many other kinds of wastewaters (urban stormwater runoff, agricultural and industrial pollution, and acid mine drainage) are now treated with wetlands. Domestic sewage probably is the least toxic wastewater produced by humans and, in hindsight, it was logical that ecologists would choose it as the first type of wastewater to test for treatment with wetlands. The dominant parameters of sewage that require treatment are total suspended solids (TSS), organic materials measured by biological oxygen demand (BOD), nutrients (primarily nitrogen and phosphorus), and pathogenic microbes (primarily viruses and fecal coliform bacteria). In a sense wetlands are preadapted to treat these parameters in a wastewater flow because they normally receive runoff waters from surrounding terrestrial systems in natural land- scapes. Wetlands are sometimes said to act as a “sponge” in absorbing and slowly releasing water flow and as a “filter” in removing materials from water flow; these qualities preadapt them for use in wastewater treatment. STRATEGY OF THE CHAPTER A principal purpose of this chapter is to review the history of the treatment wetland technology. This effort will search for the kinds of thinking that went on during the development of the technology and, thus, it will provide perspective on the nature of ecological engineering. This is important since ecological engineering is a new field with a unique approach that combines ecology and engineering. Hopefully, a careful examination of the history of this example will reveal aspects of the whole field. The chapter will not attempt to describe the state-of-the-art in wetland waste- water treatment, especially since this has been done so well by Kadlec and Knight (1996) and others. Rather, the emphasis will be on the early studies. Examination of these studies, which were conducted in the 1970s and which are the “ancestors” 26 Ecological Engineering: Principles and Practice of the present technology, should yield insight into the thought processes of ecolog- ical engineering. A summary of the old field of sanitary engineering from which conventional sewage treatment technologies have evolved is described first. This is followed by a discussion of the history of use of wetlands for sewage treatment, including the proposal of hypotheses about where the original ideas came from and who had them. It is suggested that ecologists played the critical role in the development of treatment wetland technology and that engineering followed the ecology. The conceptual basis of treatment wetlands is covered and the role of biodiversity is discussed with emphasis on several important taxa. A comparison is made of mathematical equations used to describe analogous decay processes in ecology and sanitary engineering, which indicates similarities between the fields. Finally, two variations of treatment ecosystems are examined in detail to demonstrate the design process: Walter Adey’s algal turf scrubbers and John Todd’s living machines. SANITARY ENGINEERING Modern conventional methods of treating domestic sewage use a sequence of sub- systems 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 even- tually 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 FIGURE 2.1 View of a septic tank and leaching bed. (From Clapham, W. B., Jr. 1981. Human Ecosystems. MacMillan, New York. With permission.) Sewer from house Septic tank Outlet sewer Perforated sewer Gravel leaching beds Treatment Wetlands 27 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 19th 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 FIGURE 2.2 Processes that take place in a conventional wastewater treatment plant. (Adapted from Lessard, P. and M. B. Beck. 1991. Environ. Sci. Technol. 25:30–39.) Influent Primary Treatment Sludge Digestion Secondary Treatment Suspended Growth Processes (Activated Sludge) Attached-Growth Processes (e.g., Trickling Filters or Rotating Biological Contactors) Sludge Disposal Effluent Storm Retention Thickening 28 Ecological Engineering: Principles and Practice 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 engi- neered 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 suc- cession. 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), Treatment Wetlands 29 The term “filter” is a misnomer, because the removal of organic material is not accom- plished with a filtering or straining operation. Removal is the result of an adsorption process which occurs at the surfaces of biological slimes covering the filter media. Subsequent to their absorption, the organics are utilized by the slimes for growth and energy. The gravel or other materials provide a surface for microbes that consume the organic material in sewage. The bed of gravel also provides an open structure that allows a FIGURE 2.3 The longitudinal succession of various ecological parameters caused by the discharge of sewage into a river. A and B: physical and chemical changes; C: changes in microorganisms; D: changes in larger animals. (From Hynes, H. B. N. 1960. The Biology of Polluted Waters. Liverpool University Press, Liverpool, U.K. With permission.) Outfall A Salt B.O.D. Suspended Solids NH 4 NO 3 PO 4 B C D Distance Downstream Oxygen Algae Protozoa Cladophora Asellus Bacteria Sewage Fungus Tubificidae Chironomus Clean Water Fauna 30 Ecological Engineering: Principles and Practice free circulation of air for the aerobic metabolism of microbes, which is more efficient than anaerobic metabolism. A relatively high diversity of organisms colonizes the tank because it is open to the air. Insects, especially filter flies (Pschodidae), are important as grazers on the “biological slimes” (Sarai, 1975; Usinger and Kellen, 1955). For optimal aerobic metabolism the film of microbial growth should not exceed 2 or 3 mm, and the invertebrate animals in the trickling filter help to maintain this thickness through their feeding. The overall diversity of trickling filters is depicted with traditional alternative views of ecological energy flow in Figure 2.6 and Figure 2.7. The food web (Figure 2.6) describes the network of direct, trophic (i.e., feeding) interactions within the ecosystem. Both the topology of the food web networks (Cohen, 1978; Cohen et al., 1990; Pimm, 1982) and the flows within the networks (Higashi and Burns, 1991; Wulff et al., 1989) are important subjects in ecological theory. The trophic pyramid (Figure 2.7) describes the pattern of amounts of biomass or energy storage at different aggregated levels (i.e., trophic levels) within the ecosystem. Methods for aggregation of components, such as with trophic levels, are necessary in ecology in order to simplify the complexity of ecosystems. For example, a trophic level consists of all of the organisms in an ecosystem that feed at the same level of energy transformation (i.e., primary producers, herbivores, FIGURE 2.4 Several views of the Streeter–Phelps model of biodegradation of sewage in a river ecosystem. (From Odum, H. T. 1983. Systems Ecology: An Introduction. John Wiley & Sons, New York. With permission.) Septic B Quantity Sun Time or Distance Downstream Dissolved Oxygen, % Sat. 100 80 60 40 20 0 0 5 10 Light Waste Load Heavy Waste Load Extremely Heavy Waste Load Time of River Flow, Days K 2 K 1 O 2 O 2 B =-K 1 B X =K 2 (A - X) - K 1 B D =K 1 B - K 2 D (e -K 1 t - e -K 2 t ) + D a e -K 2 t K 1 A O 2 in Air O 2 in Air X O 2 B BOD Waste Waste Consumers Water tr K Organic Matter Cons. R P Deficit: D = A - X D = K 1 B K 2 - K 1 Treatment Wetlands 31 primary carnivores, etc.). Magnitudes are shown visually on the trophic pyramid by the relative sizes of the different levels. A pyramid shape results because of the progressive energy loss at each level due to the second law of thermodynamics. Energy flow is an important topic in ecology though the concept of “flow” is an abstraction of the complex process that actually takes place. Colinvaux (1993) labels the abstraction of the complex process that actually takes place. Colinvaux (1993) labels the concept as a hydraulic analogy in reference to the simpler dynamics of water movements implied by the term, flow. McCullough (1979) articulated the abstraction more fully as follows, The problem concerns energy flux through the system; because it is unidirectional, and perhaps because of a poor choice of terminology, an erroneous impression has devel- oped. Ecologists speak so glibly about energy flow that it is necessary to emphasize that energy does not “flow” in natural ecosystems. It is located, captured or cropped, masticated, and digested by organisms at the expense of considerable performance of work. Far from flowing, it is moved forcibly (and sometimes even screamingly) from one trophic level to the next. Studies of energy flow, while imperfect in method, provide empirical measure- ments of ecological systems for making synthetic comparisons and for quantifying magnitudes of contributions of component parts to the whole ecosystem. FIGURE 2.5 View of a typical trickling filter system. The distributor arms, a, are supported by diagronal rods, b, which are fastened to the vertical column. c. This column rotates on the base, d, that is connected to the inflow pipe. e. The sewage flows through the distributor arms and from there to the trickling filter by means of a series of flat spray nozzles, f, from which the liquid is discharged in thin sheets. The nozzles are staggered on adjacent distributor arms in order for the sprays to cover overlapping areas as the mechanism rotates. The bottom of the filter is underdrained by means of special blocks or half-tiles, g, which are laid on the concrete floor, h. (From Hardenbergh, W. A. 1942. Sewerage and Sewage Treatment (2nd ed.). International Textbook Co., Scranton, PA.) a a b b c d f f e h h g g 32 Ecological Engineering: Principles and Practice The trickling filter is a fascinating ecosystem because of its ecological complex- ity and its well-known engineering details. Interestingly, Mitsch (1990), in a passing reference, suggested that some of the new constructed treatment wetlands have many characteristics of “horizontal trickling filters.” Perhaps a detailed study of the old FIGURE 2.6 Food web diagram of a trickling filter ecosystem. (From Cooke, W.G. 1959. Ecology. 40:273–291. With permission.) FIGURE 2.7 Trophic pyramid diagram of a trickling filter ecosystem. (From Hawkes, H.A. 1963. The Ecology of Waste Water Treatment. Macmillan, New York. With permission.) ESSENTIAL COMPONENTS NONESSENTIAL COMPONENTS SUBSTRATE PRODUCERS CONSUMERS TERMINAL PARASITES HERBIVORES CARNIVORES SCAVENGERS SAPROBES WORMS SNAILS FLYING INSECTS PRIMARY DECOMPOSERS SECONDARY DECOMPOSERS TRANSFORMERS GREEN PLANTS SEWAGE LIGHT MINERALIZED NUTRIENTS EFFLUENT DETRITUS NONLIVING COMPONENTS DETRITUS LIVING COMPONENTS Synthesis Death and Waste Product By-products of Respiration Insects and Worms Holozoic Protozoa Heterotrophic Bacteria and Fungi Saprobic Protozoa Humus Sludge Auto- trophic Bacteria Effluent Influent Dead Organic Solids Soluble Organic Waste Degraded Organic Matter Mineral Salts Flies Rotifera and Nematoda Treatment Wetlands 33 trickling filter literature will provide useful design information for future work on treatment wetlands. Other treatment systems have evolved that have more direct similarity to wet- lands (Dinges, 1982). Oxidation or waste stabilization lagoons are simply shallow pools in which sewage is broken down with long retention times (Gloyna et al., 1976; Mandt and Bell, 1982; Middlebrooks et al., 1982). This is a very effective technique that relies on biotic metabolism for wastewater treatment (Figure 2.8). Perhaps even closer to the wetland option is land treatment in which sewage is simply sprayed over soil in a grassland or forest (Sanks and Asano, 1976; Sopper and Kardos, 1973; Sopper and Kerr, 1979). In this system sewage is treated as it filters through the soil by physical, chemical, and biological processes. AN AUDACIOUS IDEA The use of wetlands for wastewater treatment was begun in the early 1970s. Whose idea was this? It is important to understand the origin of this application since it will reveal information on the nature of ecological engineering. One hypothesis is that the origin of treatment wetlands was a result of the technological progress of sanitary engineering systems (Figure 2.9). This is a reasonable hypothesis in that the pathways require no especially dramatic technical jumps and in each case ecosystems are used to consume the sewage. Of course, sewage was originally just released into streams as Streeter and Phelps had studied in the early 1900s. This is exactly the same approach taken with wetlands in the 1970s but with one treatment ecosystem (the river) being changed for another (the wetland). Although this hypoth- esis is reasonable, there is much more to the history. Rather than a gradual progression of technological steps, there was an explosion of ideas, all at about the same time, for combining wetlands and sewage for waste- FIGURE 2.8 Metabolic cycling that takes place in oxidation stabilization ponds during waste- water treatment. (Adapted from Oswald, W. J. 1963. Advances in Biological Waste Treatment. W. W. Eckenfelder, Jr. and J. McCabe (eds.). MacMillan, New York.) Waste Soluble Organics Sludge Aerobic Bacteria Anaerobic Bacteria Photosynthetic Bacteria Excess Bacteria S Algae Sunlight Algae CH 4 H 2 S CO 2 + NH 3 + PO 3- 4 SO 2- 4 34 Ecological Engineering: Principles and Practice water treatment (Figure 2.10). An examination of the literature shows that, starting in the early 1970s and extending through the decade, a large number of studies were conducted over a relatively short period of time to test wetlands as a system for FIGURE 2.9 Hypothetical pathways of technological evolution of the use of wetlands for wastewater treatment from sanitary engineering systems. FIGURE 2.10 The “big-bang” model of a technological explosion of early treatment wetland projects. Use of Wetlands for Wastewater Treatment Conventional Technology of Septic Tanks, Activated Sludge, Trickling Filters, etc. Land ApplicationOxidation Ponds Dumping Raw Sewage in rivers Georgia salt marsh (Haines 1979) Wisconsin constructed marsh (Fetter et al., 1976) North Carolina swamp (Brinson et al., 1984) Florida marsh (Dolan et al., 1981) Michigan peat wetland (Tilton and Kadlec 1979) South Carolina river swamp (Kitchens et al., 1975) H.T. Odum’s Morehead City mesocosms Water Hyacinth scientists K. Seidel’s wetlands Tinicum Marsh studies late 1960’s Canadian Ontario marsh (Murdoch and Capobianco 1979) Mississippi constructed marsh (Wolverton et al., 1976) Massachusetts salt marsh (Valiela et al., 1973) Minnesota constructed peat bed (Osborne 1975) Wisconsin marsh (Lee et al., 1975) Florida cypress domes (Odum et al., 1977a) Florida river swamp (Boyt et al., 1977) Canadian mesocosm (Lakshman 1979) New Jersey tidal marsh (Whigham and Simpson 1976) South Florida marsh (Steward and Ornes 1975) Wisconsin marsh (Spangler et al., 1976) Canadian marsh (Hartland−Rowe and Wright 1975) New York constructed marsh (Small 1975) Central [...]... WETLAND CONCEPT Basically, the same physical/chemical/biological processes are used to treat domestic sewage in both conventional wastewater treatment plants and treatment wetland systems The differences occur mainly in dimensions of space and time: wetlands 40 Ecological Engineering: Principles and Practice TABLE 2. 2 Stages in the Evolution of the Treatment Wetland Technology 1970s “Optimism and Enthusiasm”... wetlands Allen (1973) called these systems “sewage farming” and many examples exist (Allen and Carpenter, 1977; Costa-Pierce, 1998; 56 Ecological Engineering: Principles and Practice Treatment Capacity + + Nutrient Uptake + Denitrification − + Primary Production + Aeration + + Burrow Construction + − − Decomposition + Mound Construction Herbivory + + Muskrats FIGURE 2. 22 Causal diagram of direct and. .. of treatment wetlands as alternating sinks and sources for nutrients They are sinks during the growing season when uptake dominates the mass balance, and they are sources during the winter and early spring when decomposition and seasonal flushing dominate the mass balance Harvest can cause treatment wetlands to be primarily sinks, 48 Ecological Engineering: Principles and Practice O2 Source Nutrients... wetlands because they handle large flows with small area requirements 44 Ecological Engineering: Principles and Practice Log Generation Time (Days) 5 Sequoia Human Elephant 10 Years 4 1 Years 3 1 Day 1 Hour 1 0 −1 2 Whale Fox Kelp Frog Rat 2 1 Month Fir Bee House fly Daphnia Stentor Spirochaeta Paramecium Pseudomonas Tetrahymena Escherichia −5 −4 −3 2 −1 0 1 2 3 4 5 Log Length (cm) FIGURE 2. 15 A scale graph... constructed and partly because of the environmental impacts caused by the changes to natural saltmarshes In fact, some old mosquito control systems are currently being restored, which is another ecological engineering challenge (Axelson et al., 20 00) A related problem is the design of irrigation systems in regard to pest populations (Jobin and Ippen, 1964)  52 Ecological Engineering: Principles and Practice. .. Bastian (1993), Brown and Reed in a series of papers (Brown and Reed, 1994; Reed and Brown, 19 92; Reed, 1991), Cole (1998), Ewel (1997), and Tchobanolous (1991) BIODIVERSITY AND TREATMENT WETLANDS Most engineering- oriented discussions of treatment wetlands focus on microbiology, but other forms of biodiversity are, or can be designed to be, involved Microbes occupy the smallest and fastest (in terms... of constructed treatment wetlands differ in having either surface or subsurface water flows The state of the art is given in book-length surveys by Campbell and Ogden (1999), Kadlec and Knight (1996), Reed et al (1995), and Wolverton and Wolverton (20 01), and in a number of edited volumes (Etnier and Guterstam, 1991; Godfrey et al., 1985; Hammer, 1989; Moshiri, 1993; Reddy and Smith, 1987) Other useful... protozoans and bacteria which generate a classic oscillating pattern over time (see also Figure 4.5) The interaction between predators and prey is an important topic in ecological theory (Berryman, 19 92; Kerfoot and Sih, 1987), and knowledge of the subject will provide ecological engineers with an important design tool (see also the discussion of top-down control in Chapter 7) Because bacteria and other... single point, and it moves by gravity as a thin sheet-flow through the wetland This kind of flow, either at or below the surface, allows adequate contact with all ecosystem components involved in the treatment process Channel flows, with depths greater than about 30 cm, will not allow adequate treatment because they reduce residence time  42 Ecological Engineering: Principles and Practice FIGURE 2. 13 The spiraling... M W et al 19 72 Nutrients in Natural Waters H E Allen and J R Kramer (eds.) John Wiley & Sons, New York With permission.) Pomeroy (1980), Rich and Wetzel (1978), Schlesinger (1977), Sibert and Naiman (1980), and Vogt et al (1986) HIGHER PLANTS Higher plants, especially flowering plants, are an obvious feature of wetlands including treatment wetlands (Cronk and Fennessy, 20 01) Although wetlands can be . Sat. 100 80 60 40 20 0 0 5 10 Light Waste Load Heavy Waste Load Extremely Heavy Waste Load Time of River Flow, Days K 2 K 1 O 2 O 2 B =-K 1 B X =K 2 (A - X) - K 1 B D =K 1 B - K 2 D (e -K 1 t - e -K 2 t ). PA.) a a b b c d f f e h h g g 32 Ecological Engineering: Principles and Practice The trickling filter is a fascinating ecosystem because of its ecological complex- ity and its well-known engineering details NH 3 + PO 3- 4 SO 2- 4 34 Ecological Engineering: Principles and Practice water treatment (Figure 2. 10). An examination of the literature shows that, starting in the early 1970s and extending