ENVIRONMENTAL GYPSUM LIME SOFTENING MEMBRANES MINING WATER NANOFILTRATION REVERSE OSMOSIS SELENIUM SULFATE ULTRAFILTRATION WASTEWATER

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Abstract

The U.S. Environmental Protection Agency (EPA) lists 1,322 sites on the Final National Priorities List of hazardous Superfund sites in the United States. Of these, active and inactive mining lands have contributed to contamination of natural waters with selenium, sulfate, molybdenum, arsenic, and other constituents. In this work, low-pressure microfiltration (MF)/ultrafiltration (UF) and high-pressure nanofiltration (NF)/reverse osmosis (RO) membrane filtration technologies are applied in remediation efforts of contaminated waters associated with mining operations. This article offers an overview of membrane application in these unique process schemes, as well as data and practical considerations from pilot- and full-scale case studies for selenium and sulfate reduction and waste handling.

Selenium is a byproduct of coal, gold, phosphate, copper, and other mining operations. Selenium-related contamination is represented at an estimated 15% of active Superfund sites, with an additional 2,000+ unlisted abandoned mining lands with added hazardous waste potential. In surface waters, selenium contamination is of concern with reported cases of abnormalities in aquatic populations as a result of elevated concentrations. With consideration of selenium speciation, RO technology was applied in concert with biological metals treatment to achieve compliance with the EPA regulatory limits.

Sulfate (SO₄²-) is a divalent anion that is readily rejected by NF technology. However, in mining processes, elevated levels of additional heavy metals, including manganese, molybdenum, and arsenic can further complicate treatment schemes. Additionally, appropriate handling of the NF concentrate waste stream requires consideration. As a case study, extensive bench- and pilot-scale work was performed to evaluate multiple process scenarios for reduction of heavy metals and sulfate. These process scenarios consist of coprecipitation using calcium aluminate compared with enhanced chemical precipitation, UF, and NF. Results demonstrated successful use of membrane filtration to reduce overall sulfate concentrations by approximately 77% to exceed regulatory standards, leading to ultimate selection of this technology for full-scale application. Additional benefits of the membrane filtration process were determined, including ease of materials handling and process automation. 

In addition to a case study highlighting a specific project, process considerations are discussed, including pretreatment chemistry and contaminant speciation, as well as use of polymeric flocculant aids in membrane applications. Equipment design concepts are also explored, such as the use of an open-platform membrane filtration system in an industrial context and considerations for reducing operating costs and energy consumption.

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