Use of Industrial Wastes as Media in Constructed Wetlands and Filter Beds—Prospects for Removal of Phosphate and Metals from Wastewater Streams

Abstract

Commonly used filter media such as sand and gravel have low capacities to adsorb phosphate and heavy metals so that constructed wetlands are often ineffective for their removal. Industrial wastes such as blast furnace slag, steel slag, and alum water treatment sludge, which have reactive Fe/Al hydrous oxide and CaCO₃ adsorption surfaces, have been used successfully as active filter materials for P removal in pilot scale and/or full-scale wetlands/filters. Their use and effectiveness can, however, be affected by conditions in the wetland. The most important factors include changes from reducing to oxidizing conditions and vice versa, changes in pH, the presence of growing macrophytes and accumulation of soluble and solid-state organic matter in the filter material. The extent to which these factors interact with, and affect, P and metal adsorption by active filter materials still requires quantification. Because P and metals are sequestered in the filter material, their possible remobilization under changed conditions (e.g. changes in pH and redox potential) is of environmental concern and also requires ongoing investigation.

KEY WORDS:

• adsorption
• heavy metals
• oxidation
• phosphate
• precipitation
• reduction
• wastewater treatment