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 generation time) realm of biodiversity, making up about the lower quarter of the graph in Figure 2.15. Do other realms of biodiversity have roles to play in existing or possible treatment wetlands? The consensus from many engineers and treatment plant operators seems to be that these roles, to the extent that they even exist, are minor. Another perspective is that the use of biodiversity in treatment wetlands is in the early stage of development and broader roles may be self-organizing or may be designed in the future for more effective performance. For example, Cowan (1998) found more species of frogs and toads in a treatment wetland in central Maryland as compared with a nearby reference wetland. Is this high amphibian diversity playing a functional role in treatment wetland performance? Ecology as a science may be able to lead the design of biodiversity in treatment wetlands through ecological engineering. Several examples of important taxa are discussed below.


The term microbe includes a number of different types of organisms that occur at the microscopic range of scale. The ecology and physiology of microbes is much different from macroscopic organisms, because of their small size and because their surface-to-volume ratios are so much larger (Allen, 1977). Thus, the methods of study for microbes are almost completely different from methods used for larger organisms. These qualities separate microbial ecologists from other ecologists and, to some extent, limit interaction between the two groups. The ecology of microbes in general is introduced by Margulis et al. (1986), Allsopp et al. (1993), and Hawk-sworth (1996), while references focusing on bacteria are given by Fenchel and Blackburn (1979), Pedros-Alio and Guerrero (1994), and Boon (2000). Microbes perform the main biological work of waste treatment in their metabolism. This is especially true for carbon and nitrogen, though less applicable for phosphorus. Organic materials, such as BOD, are consumed through aerobic or anaerobic respiration reactions, and nitrogenous compounds are ultimately converted to nitrogen gas through nitrification and denitrification reactions. Thus, microbes may be thought of as the principal functional forms of biodiversity in treatment wetlands. The basic theory in wastewater treatment engineering considers the dynamics of pollutants, such as BOD, and microbial communities within bioreactors (Figure 2.16), and this approach is used as a starting point for understanding the behavior of treatment wetlands.

Microbes can be either attached to surfaces or suspended in the wastewater. Attached microbes form biofilms (Characklis and Marshall, 1990; Flemming, 1993; Lappin-Scott and Costerton, 1995). These are the "slimes" mentioned earlier (Ben-Ari, 1999). Suspended microbes are important where artificial turbulence is applied as in fluidized beds and activated sludge units.

In natural ecosystems microbes are usually found attached to particles of detritus. Two historic views of the relationship are shown in Figure 2.17. In practice, it is difficult or impossible to separate the living microbial organisms from the nonliving detritus particles, and they are often treated as a complex in ecological field work. From the perspective of detritivores who consume detritus, Cummins (1974) suggested that the complex is like a peanut butter cracker. In this anthropocentric metaphor, the microbes are the nutritious peanut butter because of their low carbon to nitrogen ratio, while the detritus particle is the nutritionally poor cracker because of its high carbon to nitrogen ratio (see the composting section in Chapter 6 for more discussion of the carbon to nitrogen ratio). Thus, a detritivore obtains more nutrition from the microbe than from the detritus particle itself, but both must be ingested because they form a unit. The detritus complex is an important part of most ecosystems. It is associated with soils and sediments but it can be suspended, as in oceanic systems where it is termed marine snow (Silver et al., 1978). General reviews of the ecology of detritus are given by Melchiorri-Santolini and Hopton (1972),

Biological Reactor

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