37    MEETING STRINGENT METALS REMOVAL
REQUIREMENTS WITH IRON
ADSORPTION/COPRECIPITATION
Mark A. Manzione, Engineer
Brown and Caldwell Consulting Engineers
Pleasant Hill, California 94523
Douglas T. Merrill, Engineer
Brown and Caldwell Consulting Engineers
Pleasant Hill, California 94523
Mary McLearn, Project Manager
Electric Power Research Institute
Palo Alto, California 94303
Winston Chow, Project Manager
Electric Power Research Institute
Palo Alto, California 94303
INTRODUCTION
The Water Quality Act of 1987 set the stage for establishing a far more stringent set of metals-
removal discharge requirements than industry and municipalities have previously faced. These
requirements cover elements the USEPA designates as priority pollutants such as arsenic, antimony,
selenium, beryllium, cadmium, chromium, copper, lead, nickel, silver, and zinc. (For convenience,
these elements are called "metals" here, even though not all exhibit metal-like behavior.) Some states
also restrict discharge of other metals, such as molybdenum and vanadium.
Many industrial and municipal facilities discharge wastewaters that contain these metals. The new
metals limits are beginning to come into effect now, and managers of these facilities are seeking cost-
effective ways to remove these metals. The iron adsorption/ coprecipitation process can help these
managers meet the new metals limits by removing both dissolved and suspended forms of these metals
simply and economically.
Treatment involves adding an iron salt such as ferric chloride to the water, unless the water contains
sufficient dissolved iron already. Iron oxyhydroxide precipitate then forms, as shown in Equation 1.
FeCl3 + 3H20 - Fe(OH)3(s)i + 3H+ + 3C1" (1)
The metals are trapped within (coprecipitated) and adsorbed onto the precipitate, which then settles
out, leaving a purified effluent.
Treatment equipment is the same as that used in conventional physical/chemical water treatment
systems, typically a reaction mix tank, flocculation chamber, and a clarifier. Chemical feed and sludge
handling equipment is also needed. This technology's main advantages are:
1. Simplicity. The operator merely adjusts pH and iron dose to remove the metals of choice
to the extent desired.
2. Selectivity. Generally, major components such as calcium, magnesium, and bicarbonate
do not compete with the metallic elements for adsorption sites.
3. Effectiveness. The technology can reduce metals concentrations to the parts per billion
levels regulatory agencies now require. These levels are far below those that can be
achieved by precipitation, the traditional method for metals removal.
4. Low cost. The ability to remove only the components needed, low chemical demands,
and elimination of the need for pretreatment or post-treatment make costs low relative
to costs of other metals removal processes.
5. Versatility. The process can be retrofitted into existing wastewater and water treatment
44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.