The application of PHREEQCi, a geochemical computer program, to aid in the management of a wastewater treatment wetland
Date
1999
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Texas A&M University
Abstract
In the past decade, constructed wetlands have become popular for treating coal-generated acid mine drainage and leachate from coal-ash disposal areas. The goal of the wetland manager is to design a system in which the pH is neutralized, toxic metals are removed, and wetland discharge meets or exceeds discharge standards for water quality. This is typically accomplished by using a combination of wetlands, ponds, and limestone drains. The treatment capability of a constructed wetland is based on relationships among dissolved oxygen (DO), pH, and metal speciation. The aim of this research was to determine if PHREEQCi, a geochemical computer program, could be used in wetland management and design. The wetland site chosen for this study was at a Texas Municipal Power Agency (TMPA) plant located in Grimes County, Texas and was created to treat leachate from a solid waste disposal area where coal ash and SO₂ scrubber sludge was deposited. The leachate contains significant concentrations of sulfate, chloride, total dissolved solids (TDS), arsenic, and selenium. Using PHREEQCi, geochemical speciation models were created to study the interrelationships between critical chemical components at the TMPA site in order to establish an optimum set of conditions to improve treatment capability and to avoid wetland failure. The results of the geochemical speciation modeling indicated a challenging situation for a wetland manager because different species precipitate under contrasting environments. In order to apply the geochemical speciation results to the design of the TMPA site, two conditions must be recognized. First, metal removal is best accomplished by generating alkaline and oxidative conditions to promote metal-oxide precipitation. Second, sulfate can be controlled under reducing environments where it is converted to sulfide and metal sulfides precipitate. Chlorides are very soluble and no viable conclusions as to the most appropriate removal method could be postulated. TDS has an ambiguous composition and could not be modeled using PHREEQCi.
Description
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Includes bibliographical references (leaves 71-73).
Issued also on microfiche from Lange Micrographics.
Includes bibliographical references (leaves 71-73).
Issued also on microfiche from Lange Micrographics.
Keywords
geology., Major geology.