Natural wastewater treatment refers to a category of technologies that specifically and substantially utilize natural methods to reduce contaminants from wastewaters in large open fields without incurring the cost of energy-intensive mechanical equipment operations for major treatment responses. The natural wastewater treatment systems have enjoyed a revival, not only in developing countries, but also in the United States; the impetus for this renewed interest in natural systems for our residual problem of modernity is a combination of cost-consciousness and a new mindset of recycling and reuse of yesterday's rejects.

Large-scale land application of wastewater has been practiced for more than 150 years. Initially, municipal and industrial wastewaters were routinely discharged into rivers and lakes. As the population of major cities in Western Europe and America grew exponentially due to the urban migration from rural areas and natural growth of population with the industrial revolution moving full speed ahead, the practice of discharging wastewater into rivers or lakes became a public scourge and the source of epidemic outbreaks. In London during those precarious times, the famed river Thames was constantly filled with human feces and other unmentionables; the city was drenched with stinking air. The House of Parliament was said to smartly soak the drapes with chlorine of lime to ward off the overwhelming odor during parliament sessions. It wasn't until Sir Edwin Chadwick that the suffering of this indignity finally came to an end. Sir Chadwick advocated separation of sewers by practicing the principle of "the rain to the river and the sewer to the soil." This was believed to be the beginning of large-scale land applications of municipal and industrial wastewaters.

The benefits of this practice of sewer disposal were quickly realized with its fertilizing capability. By the turn of the twentieth century, almost all wastewater generated in Western European cities and the North American continent was applied to the land. Today, natural systems including land applications are still used for wastewater treatment and management in many parts of the nation, albeit with different regulations and ordinances enforced in many different jurisdictions.

The natural systems for wastewater treatment are different from the wastewater treatment technologies in the previous chapters in the sense that the natural components of the treatment systems accomplish the majority of the process objectives. This means that natural systems for waste-water management and treatment do not involve large-scale energy and materials input. On the contrary, physical and biological wastewater treatment processes are often complex operations requiring intensive energy input (for mechanical devices/equipment) and/or material input (e.g., floc-culants and oxidants), even though these processes also utilize the natural components of the processes (for example, gravity for sedimentation and screening and microorganisms for BOD and nutrient removal). The natural systems for wastewater treatment also provide silent, odor-free, and robust treatment processes; they do, however, require a larger swath of land than those of conventional and more energy-intensive treatment processes. Overall, the natural systems for wastewater treatment and management are categorized based on environs of the systems: aquatic, terrestrial, and wetland.

Constructed wetlands, aquacultural operations, and sand filters are generally the most successful methods of polishing the treated wastewater effluent from the stabilization lagoons. These systems have also been used with more traditional, engineered primary treatment technologies, such as Imhoff tanks, septic tanks, and primary clarifiers. Their main advantage is to provide additional treatment beyond secondary treatment where required.

In recent years, there has been a revival of the uses of the natural systems for agricultural wastewaters from intensive animal farming. North Carolina and other southern states of the United States of America have renewed interests in employing aquatic plants to treat animal wastewaters that contain large amounts of nitrogen compounds and phosphorus compounds; one of the plants used in the region is duckweed, which looks like "oversized" algae floating on a pond or river. Duckweed is one of the smallest flowering plants in the world and can be used as food for fishes, birds, and of course, ducks. The other application of the high-strength wastewater in growing biomass for value-added biobased materials or energy also attracts interests in the animal farming industries in the region.

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