Page 170 - Wastewater Solids Incineration Systems

Emission Control and Monitoring 137

tower, impingement tray scrubber alone, and Venturi alone. Despite the divergence
in type of incinerator and APC equipment, relatively high metal control efficiencies
were recorded for all metals except mercury. U.S. EPA metal data are representative
of many of the older, existing incinerators equipped with only wet scrubber systems.
Recent APC systems which include mercury control are presented at the end of this
section under “Current APC Systems.”
The last column in Table 7.1 lists typical metal control efficiencies that an inciner-
ator manufacturer would provide as a performance guarantee for a new fluid bed
incinerator equipped with just a Venturi and impingement tray scrubber. The guar-
anteed metal control efficiencies are conservative and reflect the inherent risks
involved with providing emissions guarantees for a new facility.
For new facilities, state regulatory agencies often will require low metal emis-
sions as a result of air dispersion modeling and health risk assessments. Ensuring
low metal emissions is a two-step procedure for the incinerator design engineer.
First, representative maximum metals concentrations in the incinerator feed cake
must be determined for each metal. Second, guaranteed metal control efficiencies
must be selected for each metal. To account for the high variability of the feed cake
metal concentration, it is recommended that the last two to three years of metals
data be assessed and the maximum concentrations for each metal used as the design
basis. Metals data should be readily available from the WWTP, which are required
by Part 503 regulations (U.S. EPA, 1993), to obtain monthly composite solids sam-
ples and have them analyzed for metals. The typical metal control guarantees can be
used as a starting point for determining control efficiencies (see Table 7.1). If lower
emissions are required by the regulatory authority, slightly greater control efficien-
cies than those shown in Table 7.1 are possible for some metals. However, it is essen-
tial that agreement from the incinerator supplier be obtained before going to higher
control efficiencies.
If a higher metal control efficiency is required than can be obtained with a con-
ventional Venturi and impingement scrubber, additional particulate control would
have to be added to the system. The additional control might be obtained with a
higher-pressure drop Venturi scrubber or wet ESP. A dry collector such as a fabric
filter or dry ESP also could be considered. In this case, however, the flue gas handling
system would have to be significantly reconfigured so that the incinerator exhaust is
first cooled in a heat recovery boiler before the dry collector. A heat recovery boiler
adds significantly more cost and complexity to the incineration system, but this
option is becoming more justifiable and attractive as energy costs keep increasing.