Description:

Metals precipitation from contaminated water involves the conversion of soluble heavy metal salts to insoluble salts that will precipitate. The precipitate can then be removed from the treated water by physical methods such as clarification (settling) and/or filtration. The process usually uses pH adjustment, addition of a chemical precipitant, and flocculation. Typically, metals precipitate from the solution as hydroxides, sulfides, or carbonates. The solubilities of the specific metal contaminants and the required cleanup standards will dictate the process used. In some cases, process design will allow for the generation of sludges that can be sent to recyclers for metal recovery.

Coagulants and Flocculation

In the precipitation process, chemical precipitants, coagulants, and flocculantation are used to increase particle size through aggregation. The precipitation process can generate very fine particles that are held in suspension by electrostatic surface charges. These charges cause clouds of counter-ions to form around the particles, giving rise to repulsive forces that prevent aggregation and reduce the effectiveness of subsequent solid-liquid separation processes. Therefore, chemical coagulants are often added to overcome the repulsive forces of the particles. The three main types of coagulants are inorganic electrolytes (such as alum, lime, ferric chloride, and ferrous sulfate), organic polymers, and synthetic polyelectrolytes with anionic or cationic functional groups. The addition of coagulants is followed by low-sheer mixing in a flocculator to promote contact between the particles, allowing particle growth through the sedimentation phenomenon called flocculant settling.

Flocculant settling refers to a rather dilute suspension of particles that coalesce, or flocculate, during the sedimentation operation. As coalescence or flocculation occurs, the particles increase in mass and settle at a faster rate. The amount of flocculation that occurs depends on the opportunity for contact, which varies with the overflow rate, the depth of the basin, the velocity gradients in the system, the concentration of particles, and the range of particles sizes. The effects of these variables can only be accomplished by sedimentation tests.

Synonyms:

Applicability:

Limitations:

• As with any pump and treat process, if the source of contamination is not removed (as in metals absorbed to soil), treatment of the ground water may be superfluous.
• The presence of multiple metal species may lead to removal difficulties as a result of amphoteric natures of different compounds (i.e., optimization on one metal species may prevent removal of another).
• As discharge standards become more stringent, further treatment may be required.
• Metal hydroxide sludges must pass TCLP prior to land disposal.
• Soluble hexavalent chrome requires extra treatment prior to coagulaation and flocculation.
• Reagent addition must be carefully controlled to preclude unacceptable concentrations in treatment effluent.
• Efficacy of the system relies on adequate solids separation techniques (e.g., clarification, flocculation, and/or filtration).
• Process may generate toxic sludge requiring proper disposal.
• Process can be costly, depending on reagents used, required system controls, and required operator involvement in system operation.
• Dissolved salts are added to the treated water as a result of pH adjustment.
• Polymer may need to be added to the water to achieve adequate settling of solids.
• Treated water will often require pH adjustment.
• Metals held in solution by complexing agents (e.g., cyanide or EDTA) are difficult to precipitate.

Data Needs:

Bench-scale treatability tests should be conducted to determine operating parameters and characteristics [i.e., reagent type and dosage, optimum pH, retention time, flow rate, temperature, mixing requirements, flocculent (polymer) selection, suspended solids, precipitate settling and filtration rates, and sludge volume and characteristics].

Performance Data:

Because of its success in meeting requirements for discharge of treated wastewater, metals precipitation is recognized as a proven process for use in remedial activities such as ground water treatment. Precipitation (combined with sedimentation, and/or flocculation and filtration) is becoming the most widely selected means for heavy metals removal from ground water in pump and treat operations.

Cost:

This cost estimate is does not include the costs for additional treatment steps which may be necessary when employing precipitation/coagulation/flocculation technology such as the sludge dewatering and disposal.  For budgetary purposes, sludge disposal may be estimated to increase operating costs by approximately $0.50 per 1,000 gallons of ground water treated.  The key cost driver information and cost analysis was developed using the 2006 version of the Remedial Action Cost Engineering and Requirements (RACER) software.

Key Cost Drivers 

·        No sensitivity analysis possible as only variable is influent flow rate.

Cost Analysis

The following table represents estimated costs (by common unit of measure) to apply precipitation/coagulation/flocculation technology at sites of varying size and complexity.   A more detailed cost estimate table which includes specific site characteristics and significant cost elements that contributed to the final costs can be viewed by clicking on the link below.

Detailed Cost Estimate

References:

Innovative Remediation Technologies:  Field Scale Demonstration Project in North America, 2nd Edition

Abstracts of Remediation Case Studies, Volume 4, June 2000, EPA 542-R-00-006

Guide to Documenting and Managing Cost and Performance Information for Remediation Projects - Revised Version, October, 1998, EPA 542-B-98-007

Battelle Memorial Institute, 1995. "ReOpt. V3.1", by Battelle Memorial Institute for DOE under Contract DE/AC06/76RLO 1830.

Bricka, R. Mark, 1988. "Investigation and Evaluation of the Performance of Solidified Cellulose and Starch Xanthate Heavy Metal Sludges", USACE-WES Technical Report EL-88-5.

EPA, 1980. Control and Treatment Technology for the Metal Finishing Industry: Sulfide Precipitation, EPA/625/8-80/003.

EPA, 1990. Innovative and Alternative Technology Assessment Manual, EPA, Office of Water Program Operations, EPA/430/9-78/009.

Federal Remediation Technologies Roundtable, 1998. Remediation Case Studies: Innovative Groundwater Treatment Technologies, EPA/542/R-98/015.

• Coagulation/Flocculation/Dissolved Air Flotation and Oleofiltration™ at the Coastal Systems Station, AOC 1, Panama City, Florida

NEESA, 1993. Precipitation of Metals from Ground Water. NEESA Document Number 20.2-051.6, Naval Energy and Environmental Support Activity, Port Hueneme, CA.

Tchobanoglous, G. and F.L. Burton, 1991. "Wastewater Engineering - Treatment, Disposal and Reuse," Third Edition. Metcalf & Eddy, Inc.

• EPA Demo Palangana Uranium Mine Site, TX
• Coakley Landfill New Hampshire
• Stringfellow Acid Pit Site CA
• Winthrop Landfill Winthrop, ME
• EPA Removal Action Crown Plating, MO
• Coastal Systems Station, AOC 1, Panama City, FL
• 

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A list of vendors offering En Situ Physical/Chemical Water Treatment is available from  EPA REACH IT which combines information from three established EPA databases, the Vendor Information System for Innovative Treatment Technologies (VISITT), the Vendor Field Analytical and Characterization Technologies System (Vendor FACTS), and the Innovative Treatment Technologies (ITT), to give users access to comprehensive information about treatment and characterization technologies and their applications.

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