Robert H Kadlec | PubMed Health

PubMed2005

Phosphorus removal in emergent free surface wetlands.

Constructed and natural wetlands are capable of absorbing new phosphorus loadings, and, in appropriate circumstances, can provide a low-cost alternative to chemical and biological treatment. Phosphorus interacts strongly with wetland soils and biota, which provide both short-term and sustainable long-term storage of this nutrient. Soil sorption may provide initial removal, but this partly reversible storage eventually becomes saturated. Uptake by biota, including bacteria, algae, and duckweed, as well as macrophytes, forms an initial removal mechanism. Cycling through growth, death, and decomposition returns most of the biotic uptake, but an important residual contributes to long-term accretion in newly formed sediments and soils. Despite the apparent complexity of these several removal mechanisms, data analysis shows that relatively simple equations can describe the sustainable processes. Previous global first order removal rates characterize the sustainable removal, but do not incorporate any biotic features. This article reviews the relevant processes and summarizes quantitative data on wetland phosphorus removal.

Adsorption↗

PubMed2005

Nitrogen farming for pollution control.

The use of free water surface treatment wetlands for nitrate reduction has an extensive basis in data from dozens of operating systems. Marshes are effective for denitrification, with first order areal annual rate constants centered on thirty-four meters/year. Performance improves at higher water temperatures, with a modified Arrhenius temperature factor of 1.090. Performance also increases with increasing hydraulic efficiency, created by prevention of short-circuiting, and reflected in values of the tanks-in-series parameter N > 5. Higher efficiencies are associated with submergent and emergent soft tissue vegetation, and lower efficiencies with unvegetated open water and forested wetlands. Hydraulic loadings of 2-7 cm/day can produce 30% nitrate load reductions, over the temperature range 6-20 degrees C. Carbon availability limits denitrification at high nitrate loadings, however, wetlands produce carbon in sufficient quantities to support the loads anticipated in the upper midwest. The conversion of agricultural lands to treatment wetlands focused on nitrate reduction is termed nitrogen (N) farming. (D.H. Hey, Nitrogen farming: harvesting a different crop. Restoration Ecology, 2002, 10 (1), 1-11). A demonstration project is indicated to address local issues and scale-up considerations. Such a project would require thorough monitoring for the purpose of optimizing and refining design models. Significant ancillary benefits of ecological diversity and wildlife habitat are certain to accompany the project, but are of secondary importance until the water quality functions are demonstrated. Regulatory issues include permitting and wetland classification. Economic issues include proper pricing of services and methods of revenue generation. Resolution of these potential difficulties may require modification of existing policies and institutions.

Agriculture↗