82    MANAGING SOLID RESIDUES FROM A TREATMENT
PROCESS FOR PRIORITY METALS
Douglas T. Merrill, Managing Engineer
Brown and Caldwell Consultants
Pleasant Hill, California 94523
Mary E. McLearn, Senior Project Manager
Electric Power Research Institute
Palo Alto, California 94303
Iron adsorption/coprecipitation is an economical process that, under the appropriate conditions,
can remove dissolved and particulate priority metals to the extremely low concentrations now being
required by regulatory agencies.'•2'3,4 Treatment involves adding an iron salt such as ferric chloride to
the water, unless the water contains sufficient dissolved iron already. An amorphous iron oxyhydrox-
ide precipitate then forms, as shown in Equation 1.
FeCl3 + 3H20 *-Fe(OH)3   \  +3H++3CL (1)
The metals are trapped within (coprecipitated) and adsorbed onto the precipitate, which then settles
out, leaving a purified effluent. Removals are pH-dependent, with anionic species, (for example,
arsenic and selenium) removed best at pH 6 or below and cationic species (for example, zinc, cadmium, and nickel) best removed at pH 7 and above.
The technology's metal-removal effectiveness is due in part to iron oxyhydroxide's large adsorptive
surface. The literature reports specific surface values of approximately 200 to 800 m2/gr.' However,
such large-surface solids are a challenge to concentrate, because they release water grudgingly. In our
studies, underflow solids concentrations from sedimentation tanks were 1 to 2% at best. We have
used gravity thickening to increase solids concentrations to 5 to 8%, but concentrations of 1 to 3%
were more typical. Solids concentrations for cakes produced by pressure filtration varied widely. We
have produced cakes with concentrations as high as 40% solids and as low as 6% solids.
This paper summarizes what we have learned about management of iron oxyhydroxide sludges
from 12 years of bench- and pilot-scale testing. The paper covers:
• Methods for predicting solids production
• Solids thickening techniques
• Solids dewatering and drying procedures
• Results of hazardous waste toxicity evaluations
• Effects of sludge leachate on clay landfill liners and the effects of the liners on leachate
composition
• Reuse of iron oxyhydroxide solids
ESTIMATING SOLIDS PRODUCTION
In this paper, we define solids production as the quantity of solids (free water excluded) generated
by waste treatment operations. Total solids production includes native wastewater solids, solids
generated by chemical precipitation, bound water* associated with hydroxide sludges, and sludge
conditioners. The solids produced are discharged in the waste sludge and in the process effluent. If
solids separators are performing adequately, solids production is approximately the weight of solids
(free water excluded) that the sludge management system must process and dispose of.
Equation 2 is a simplified formula for estimating approximate solids production:
Pp = [S+H(l+A)] [1+CJ (2)
45th Purdue Industrial Waste Conference Proceedings, © 1991 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
♦Bound water is defined here as water that is not released from the solids at the temperature of the standard moisture
analysis (105° C ) but is released upon heating at 550° C. Bound water is counted as a solid in the standard moisture
analysis.
705