RIASSUNTO
Abstract
This paper introduces the subject of constructed (or treatment) wetlands. Itidentifies the many wetland applications that are currently in use for thetreatment and re-use of water. Treatment wetlands are compared with traditionalprimary, secondary and tertiary wastewater treatments. Constructed wetlandtypes are discussed along with estimated costs and design features. A nine-stepprocess to put a treatment wetland on-line is presented. This paper shares bestpractices for the design, construction and operation of treatment wetlands.
Introduction
A wetland revolution is underway. Not long ago, wetlands were drainedbecause of bad air (malaria). Today, wetlands are recognized for theirbiodiversity of flora and fauna, use as a nursery (for birds, shrimps, crabs,fishes, insects, and plants), and ability to treat wastewater. Scientists arejust beginning to unlock some of the secrets of the wetlands. We are nowbuilding and operating constructed wetlands for a variety of purposes:
* municipal, industrial and refinery wastewater,
* ground water treatment,
* storm water and non-point source wastewater treatment,
* surge capacity to prevent flooding,
* wildlife habitat restoration and mitigation banking,
* water re-use (irrigation or industrial),
* produced water treatment,
* agriculture (rice farming and feedlot runoff),
* aquaculture (shrimp and fish farming), and
* acid mine drainage treatment.
Why Wetlands
The impetus for selecting constructed wetlands (as the wastewater treatmenttechnology of choice in the petroleum and other industries) is not because itis politically correct, but rather, because it is economically correct. Whenland is available, a constructed wetland is generally the least expensivetreatment option to treat refinery wastewater, oilfield produced water, stormwater or sewerage. Mother Nature has evolved a system of plants, microbes, andsoils, which are capable of doing a remarkable job of treating water. Wetlandsare the world's kidneys. They were cleaning up storm water runoff long beforethe advent of any of today's environmental regulatory agencies. Wetlands candegrade organic compounds, chelate metals, filter solids, transformbiorefractory compounds, sequester a host of materials, and prevent flooding.Over the last century, we as engineers have developed a collection of physical,chemical, and biological wastewater treatment technologies (Table 1) that cando almost as much as a wetland can.
Wetland Types
There are a number of classification schemes used by biologists for naturalwetlands. These are based on the degree of salinity, plant community, and typeof adjacent body of water (river, lake, stream, ocean, etc.). Fortunately, forengineers the classification scheme for constructed wetlands is greatlysimplified (Table 2). The subsurface flow system (SFS) wetland is a rock/reedfilter with water flowing below the surface of the rocks. The free-watersurface (FWS) wetland is the classical shallow-water duck marsh with waterflowing through the plants. Hybrid system wetlands use a combination of SFS andFWS wetlands. One type of silviculture wetlands uses tree roots to clean upground water. Another type is a variation of a FWS wetland but with relativelyshort hydroperiods to avoid drowning the trees. All of the aforementionedwetland types may be set up to treat wastewater over a wide range of salinities(fresh water to seawater). By selecting the appropriate plant, soil, andmicrobial association, it is possible to treat most of the inorganic andorganic compounds that are found in wastewater.