EXTRACTION OF METALS AND PHOSPHORUS FROM
INCINERATED MUNICIPAL SEWAGE SLUDGE
Robert C. Gabler, Jr., Research Chemist
David L. Neylan, Chemist
College Park Metallurgy Research Center
Bureau of Mines
U.S. Department of the Interior
College Park, Maryland 20740
INTRODUCTION
In the U.S. approximately 9.3 million dry tons [ 1 ] of municipal sewage sludge is
generated annually, of which 35% is incinerated.   As an extension of the Bureau of Mines
program to convert waste materials from a disposal problem into a secondary resource,
studies were undertaken to determine if the metals and phosphorus can be removed prior
to disposal of the ash.   By this approach, the ash is converted into an environmentally
acceptable material while valuable resources are conserved.   Although this secondary
source of metals and phosphorus will probably not be economically competitive with
conventional ores, part of the extraction-recovery cost could be borne as a disposal
expense.   This resource recovery study, being conducted at the Bureau of Mines College
Park Metallurgy Research Center, has two objectives—detailed characterization of incinerated sludge ashes from representative U.S. cities, and evaluation of low-cost extraction
processes.
Various investigators have shown an increasing interest in both the quantity of metals
present in sludge ash and the most efficient means of extracting these metals and phosphorus.   Furr and co-workers [2] and Salotto, Grossman and Farrell [3] determined the
metals in sludge samples from many municipal wastewater treatment plants.   The disposition of the metals, phosphorus, pesticides and polychlorinated biphenyls during the
incineration of sludges was studied by Farrell and Salotto [4];   Other researchers [5-9]
investigated the extraction of metals and phosphorus from both sludges and sludge ash.
Diosady [6] was of the opinion that "only phosphate recovery seems economically
warranted."   However, most of the previous investigators are in agreement that further
investigations into extraction processes are necessary because environmental considerations
may ultimately require the removal of toxic metals from the sludge ash prior to disposal.
In addition, the sludge ash from one of the U.S. cities investigated in this study contained
concentrations of precious metals that may justify removal and recovery.
Previous characterization studies by the authors [10] on incinerated sludge ash are
summarized in Table I.   Samples were obtained from 14 incinerators serving 11 U.S.
localities that range from cities with heavy industry such as Detroit, Michigan, Cleveland,
Ohio, and St. Louis, Missouri, to cities with small specialized industries and large residential areas such as San Mateo and Palo Alto, California.   The analytical data on these samples showed significant variations between cities in the major constituents, possible reflecting the various reagents-iron and aluminum slats, and lime-used to remove phosphorus
from the sewage stream.   As expected, there were also large variations in trace and minor
elements between cities.
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