The waste treatment systems described in this book are specifically designed to utilize natural responses to the maximum possible degree when obtaining the intended waste treatment or management goal. In most cases, this approach will result in a system that costs less to build and operate and requires less energy than mechanical treatment alternatives.
All waste management processes depend on natural responses, such as gravity forces for sedimentation, or on natural components, such as biological organisms; however, in the typical case these natural components are supported by an often complex array of energy-intensive mechanical equipment. The term natural system as used in this text is intended to describe those processes that depend primarily on their natural components to achieve the intended purpose. A natural system might typically include pumps and piping for waste conveyance but would not depend on external energy sources exclusively to maintain the major treatment responses.
Serious interest in natural methods for waste treatment reemerged in the United States following passage of the Clean Water Act of 1972 (PL 92-500). The primary initial response was to assume that the "zero discharge" mandate of the law could be obtained via a combination of mechanical treatment units capable of advanced wastewater treatment (AWT). In theory, any specified level of water quality could be achieved via a combination of mechanical operations; however, the energy requirements and high costs of this approach soon became apparent, and a search for alternatives commenced.
Land application of wastewater was the first "natural" technology to be rediscovered. In the 19th century it was the only acceptable method for waste treatment, but it gradually slipped from use with the invention of modern devices. Studies and research quickly established that land treatment could realize all of the goals of PL 92-500 while at the same time obtaining significant benefit from the reuse of the nutrients, other minerals, and organic matter in the wastes. Land
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