Table

Stages in the Evolution of the Treatment Wetland Technology

1970s "Optimism and Enthusiasm"

An explosion of ideas takes place; tests are performed in a variety of wetland types using different experimental strategies.

1980s "Caution and Skepticism"

Many of the original studies are discontinued; long-term treatment ability (especially for phosphorus removal) is questioned (see Richardson's many papers and Kadlec's "aging" concept); many review papers are written.

1990s "Maturation"

An almost exclusive emphasis emerges on the use of constructed wetlands rather than natural wetlands for wastewater treatment; Kadlec and Knight's book entitled Treatment Wetlands is published; management ideas evolve to address limitations brought up in the 1980s.

2000s "Commercialization"

The technology of treatment wetlands expands, especially in less developed countries throughout the world; constructed wetlands become a widely accepted alternative technology for certain scenarios of wastewater treatment.

need significantly more space and more time than conventional plants to provide treatment. The trade-off is economic with the wetlands option being cheaper in utilizing a higher ratio of natural vs. purchased inputs (Figure 2.12), at least conceptually.

A key factor in wastewater treatment is hydraulic residence time, as noted by Knight (1995):

The typical hydraulic residence time in a modern AWT (advanced wastewater treatment) plant is about 12 hr., and solids residence time might be only about 1-2 days. In a typical treatment wetland, the minimum hydraulic residence time is greater than 5 days and in some is over 100 days. Solids residence time is typically much longer as organic material slowly spirals through the system undergoing numerous transformations.

Knight's use of the verb spiral is significant in the above quote. Spiralling is a metaphor used to describe material processing in stream ecosystems that combines cycling and transport. In the classic sense, materials cycle through an ecosystem along transformation pathways between abiotic and biotic compartments (Pomeroy, 1974a). The study of these cycles is termed variously biogeochemistry (Schlesinger, 1997), mineral cycling (Deevey, 1970), or nutrient cycling (Bormann and Likens,

High Low

High Low

FIGURE 2.12 Locations of various wastewater treatment technologies along gradients of energy input. (From Knight, R. L. 1995. Maximum Power: The Ideas and Applications of H. T. Odum. C. A. S. Hall (ed.). University Press of Colorado, Niwot, CO. With permission.)

1967). In terms of abiotic compartments, some elements, such as carbon, nitrogen, and sulfur, have gaseous phases while others, such as phosphorus, potassium, and calcium, are primarily limited to soil and sediment phases. Most elements are taken up by plants for use in the organic matter production of photosynthesis and are released either from living tissue or after deposition as detritus (i.e., storage of nonliving organic matter) through respiration. Thus, each element has its own cycle through the ecosystem, though they are all coupled. Traditionally, cycling was essentially considered to occur at one point in space. This conception makes sense for an aggregated view of a forest or lake ecosystem where internal cycling quantitatively dominates amounts flowing in or out at any point. However, in stream and river ecosystems internal cycling is less important because of the constant movements due to water flow. Stream ecologists developed the spiraling concept (Figure 2.13) to account for both internal cycling and longitudinal transport of materials in a two- or three-dimensional sense as opposed to the one-dimensional sense of internal cycling as a point process (Elwood et al., 1983; Newbold 1992; Newbold et al., 1981, 1982). Wagener et al. (1998) have extended the spiraling concept to soils, and as indicated by Knight's quote, this may be the appropriate perspective for material processing in treatment wetlands. It is such complex system functioning that characterizes treatment of sewage in wetlands.

Sewage is discharged in a treatment wetland usually at a series of points (often along a perforated pipe) rather than at a 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.

FIGURE 2.13 The spiraling concept of material recycling in stream ecosystems. (From Newbold, J. D. 1992. The Rivers Handbook: Hydrological and Ecological Principles. Vol. 1. P. Calow and G. E. Petts (eds.). Blackwell Scientific, Oxford, UK. With permission.)

The efficiency of treatment wetlands is evaluated by input-output methods which quantify assimilatory capacity. A mass balance approach is most useful, which demonstrates percent removal of TSS, BOD, nutrients, and pathogens. Usually this is done by measuring water flow rates (for example, million gallons/day) and concentrations of sewage parameters (usually mg/l for TSS, BOD, and nutrients and numbers of individual organisms per unit volume for pathogens). When water flow rates are multiplied by concentrations, along with suitable conversion factors, the total mass of input can be compared with the total mass of output and uptake efficiencies calculated. If water flow rates cannot be quantified, comparisons between inputs and outputs can be made with concentration data alone, but this approach is not as complete as the full mass balance approach.

The dominant processes that remove the physical-chemical parameters of sewage in wetlands are shown in Figure 2.14 and highlighted in Table 2.3. Many kinds of transformations are involved in these treatment processes and much is known about their kinetics. In general, treatment efficiencies are variable but high enough for the technology to be considered competitive.

The treatment wetland technology works best in tropical or subtropical climates where biological processes are active throughout the annual cycle. An open question still exists about year-round use of treatment wetlands in colder climates where biological processes are reduced during the winter season, but some workers believe that the technology can be utilized in these regions (Lakshman, 1994; Werker et al., 2002). It also is most appropriate for rural areas where waste volumes to be treated

Discharge

FIGURE 2.14 Energy circuit diagram for the main processes in a treatment wetland.

Discharge

FIGURE 2.14 Energy circuit diagram for the main processes in a treatment wetland.

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