What is the "ideal" sewage treatment system? It would probably produce high quality effluent (discharge) at modest cost, be aesthetically pleasing, and would not create secondary environmental impacts. Yet the reality is that the vast majority of wastewater treatment systems developed over the past twenty-five years discharge highly toxic effluents, are ugly and/or have a strong odor (or both), and create secondary problems in air quality or sludge disposal. These plants may also consume large quantities of energy in processing, sludge drying, incineration, and other activities. It is becoming increasingly clear that "bigger" isn't necessarily "better" in terms of wastewater treatment efficiency or environmental impact.
A conventional sewage treatment plant typically discharges effluents containing relatively low amounts of biochemical oxygen demand (BOD) and suspended solids (SS) - often around 30 mg/L of each. But for every 10,000 people, a secondary plant discharges with the treated wastewater over 100 kg/day of microbial dry solids, an equal amount of oxygen-demanding substances, and enough nutrients to provide fertilizer to grow crops on over 300 ha. Where chlorination is practiced, a substantial amount of toxic chlorinated organic compounds are also discharged. In addition, two thirds of the dry matter received by the plant is only converted in form and discharged as sludge, on average about 12 cubic meters of wet residue for every 10,000 people. Because intensive aeration is practiced at most of these facilities, volatile organics - including some known carcinogens - are stripped during the treatment process and discharged into the local environment. In many cases, significant quantities (kg/day) of these materials are discharged into the atmosphere. And these conventional technologies aren't cheap: they typically cost each individual served about 1% of his or her annual income.
Some engineers and scientists are beginning to challenge the notion that sewage treatment facilities must be large, costly, and unsightly. They believe that "natural" technologies, like constructed wetlands, could offer better treatment at lower costs, with better aesthetics and less environmental impact, than traditional methods. Some authors have even used the term "sewage treatment park" instead of "sewage treatment plant."
These natural, sometimes called "alternative", wastewater treatment technologies are attracting considerable attention across Canada and elsewhere in the world. A variety of proprietary systems are now available on the market, and many municipalities, industries, and farms have begun testing constructed wetlands as a treatment alternative or an adjunct to conventional treatment. Constructed wetlands are now in use in the treatment of acid mine drainage, barn wastes, feedlot runoff in agricultural operations, urban stormwater runoff, various industrial wastewater treatment applications, and in the secondary and tertiary treatment of municipal sewage. Private applications of this technology are also on the rise, replacing traditional septic systems, especially in areas where those conventional approaches are not feasible or where dense development limits their treatment efficiency. A number of "community" systems have been developed for groups of ten to 200 homes, and some authors believe that this community approach to sewage treatment is likely to be an important trend in the future, reducing dependence on large, costly, centralized sewage treatment facilities.
There are various kinds of constructed wetlands. Some are housed in a greenhouse enclosure, where they can provide aesthetic and indoor air quality benefits as well as effluent treatment. Examples of these systems are the "solar aquatics" installations at the Ontario Science Centre, the head office of The Body Shop in Toronto, Ontario, and facilities at Bear River and Beaverbank Villa, Nova Scotia. (The term "solar aquatics" was coined by John Todd, an innovator in this field and the designer of the first installation of this type.) Some of these facilities have generated such public interest that they have become tourist attractions in their own right.
Other wetlands are modeled on natural systems and are located outdoors, without protection from the elements even in the coldest times of winter. Among these, are the Niagara-on-the-Lake experimental facility ("SWAMP"), which provides advanced treatment for the municipal sewage effluents in that town, and the Listowel Marsh, in west-central Ontario, which was the site of a long-term study of constructed wetland performance through the 1980s. These systems are usually planted with a variety of species (although single species "reed beds" are also sometimes used), with certain species, such as cattails, used in the initial stages and a mixture of other water- tolerant species used to "polish" the effluent in its final stages. Sometimes, as has been the case with the SWAMP project, additions to the wetland sediments (for instance, crushed recycled iron) have been used to improve the removal of phosphorus or other target constituents.
In general, constructed wetlands have been proven to perform well and often inexpensively relative to conventional treatment systems. Depending on the system and its configuration, problems may arise with insect or mammal pests - invasive muskrats may, for instance, chew off most of the emergent cattail growth and reduce treatment efficiency in a cattail marsh. In some solar aquatics systems, insect infestations have caused certain stages of the process to fail temporarily. But most systems have been designed with enough retention time that effluents can be held until an adequate discharge quality is achieved, so temporary upsets do not create serious problems with the process.
Constructed wetlands are very popular with the public, but regulatory agencies have been slower to support them, in part because of concerns that their performance may be less effective or less reliable than tried-and-true technologies. The public, and many engineers and scientists, would respond by saying that conventional technologies are inadequate in other ways, and there is a clear need for "alternative" technologies for wastewater treatment.
One of the most important advantages of constructed wetlands is their aesthetic appeal. Some researchers believe that the sewage treatment plant of the future could actually do double duty as a wastewater treatment facility and a recreational facility. Certainly, constructed wetlands, whether greenhouse-enclosed or out-of-doors, are an attractive alternative to the monolithic structures of most modern-day sewage treatment plants.
Constructed wetlands offer the potential to create habitat for bird and mammal populations, and thus to improve the ecological integrity of a wastewater treatment site. In an urban area, wildlife habitat can be scarce in any case, so the creation of additional habitat is often highly valued. When this creation occurs in conjunction with a socially- valuable function like sewage treatment, the facility may be particularly welcomed by local residents.
Evidence from a number of experimental sites suggests that the maintenance and labor costs associated with constructed wetlands are significantly lower than those for conventional treatment facilities designed to treat the same volume of wastewater. In other words, the costs of treating the sewage flows for 10,000 people are potentially lower, or much lower, with a constructed wetland system than they would be for a conventional treatment system, while effluent quality from those systems is as good or better than that from conventional systems. It should be noted, however, that treatment costs vary widely depending on the nature of the raw sewage stream (for instance, whether it contains a high proportion of inorganic and trace industrial pollutants from industrial discharges), on climate, and on other local factors.
Phosphorus removal is notoriously poor in many constructed wetland systems. The reasons for this are not entirely clear, but may be related to reduced oxygenation of the root zone in slow-moving waters. Although constructed wetlands may be able to remove BOD, suspended solids, and nitrogen compounds with reasonable effectiveness, they may not perform well in terms of phosphorus removal. Where phosphorus enrichment is a concern (for instance, because of the potential for eutrophication), constructed wetlands may cause problems rather than solve them.
A conventional wastewater treatment system is well protected from the elements and from disease and predators, but a constructed wetland is vulnerable to all three. Although even a conventional plant experiences occasional process upset, for instance from unexpectedly high loads of certain pollutants in the raw sewage, generally speaking, a conventional plant is well buffered from its environment. By definition, a constructed wetland is part of its environment, and is subject to the same forces. In cold climates, as would be the case in most of Canada, climatic variations in particular may affect the performance of a "natural" treatment system. In the spring and during summer thunderstorms, excessive rainfall can flood a wetland, decreasing its treatment effectiveness. Insect and other predators can reduce the populations of key wetland species, thus impairing the treatment function. Or simple upsets of temperature, pH or other factors can affect the health and removal efficiency of wetland plants. These factors create a potential for variability in effluent quality that is unacceptable in a public utility or a private treatment facility subject to legally-binding standards for discharge.
Although a conventional sewage treatment plant can have a life expectancy of many decades, if it is adequately maintained and upgraded, the life expectancy of a constructed wetland system is likely to be much shorter. For one thing, wetlands tend to accumulate suspended solids, so they will fill up with time, gradually reducing their volume and thus their treatment capacity - and effectiveness. Because of the relative newness of alternative technologies, long-term data for wetland performance is not yet available. Estimates suggest that wetlands may be limited to 15 to 20 years of life, compared to the 25 to 50 year life span possible with conventional facilities.
As wetlands accumulate sediments, they also accumulate the many pollutants that have a natural affinity for solids. These pollutants include some forms of phosphorus, many heavy metals, and some trace organic pollutants. When the wetland is "retired" at the end of its useful life, it is not an inert or innocuous component of the environment but rather a hazardous waste disposal site. Concentrations of hazardous materials in wetland sediments may be too high to permit the use of the site for other purposes, such as recreation. The treatment or removal of those contaminated sediments may therefore add significantly to the costs of building and operating a constructed wetland.
Regardless of how it is processed, treated effluent must comply with applicable discharge standards. In most of Canada, provincial standards would apply, either specific standards for a given industry or discharge point, or general prohibitions against pollution. The Canadian Federal Fisheries Act contains prohibitions on discharges that adversely affect the quality of fish habitat, and the Canadian Environmental Protection Act contains general prohibitions on environmental degradation. In most provinces, approval must be obtained for any effluent discharge before a new treatment works is allowed to begin operation. (This, in fact, has been the point at which some constructed wetlands have been refused permission to operate.) Application procedures and expectations are generally geared to more conventional technologies, so it may be difficult to persuade regulators that an innovative technology will be as good as or better than conventional treatment approaches - but the new technology will not be allowed to operate until such official permission is given. Local standards may apply for the discharge of stormwater treated in a constructed wetland, or for industrial or other effluents treated in this way and destined for discharge to municipal sewer systems. Municipal zoning restrictions may limit where new constructed wetland facilities may be built.
Page 12 discusses the general problem of habitat loss and alteration, and the associated problem of conserving biodiversity. Habitat creation is one of the benefits of constructed wetlands in urban systems. Page 272 discusses the general problem of water pollution in the context of the water cycle. Page 296 describes the process of eutrophication and the role of nutrient oversupply from sewage treatment, stormwater runoff, and agriculture. This discussion is continued on pages 300-306. Pages 306-308 discuss the problem of wetland loss and illustrate the relative productivity of wetland systems compared to grasslands and forest ecosystems. Pages 322-336 discuss the problems of sewage pollution and conventional approaches to sewage treatment. Pages 336-340 describe some alternative treatment systems, including constructed wetlands. Pages 340-342 describe some of the obstacles to using "alternative" treatment approaches.
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