Atmospheric Emissions of Elements on Particles from the Parkway Sewage-Sludge Incinerator Robert R. Greenberg,?William H. Zoller, and Glen E. Gordon* Department of Chemistry, University of Maryland, College Park, Maryland 20742
Concentrations of about 30 elements were measured in dry sludge, fly ash, and suspended particulate matter from the stack of a fluidized-bed sewage-sludge incinerator equipped with a Venturi scrubber, a secondary scrubber, a gas cooler, and a demister. Although there is some enrichment of volatile elements on the suspended particles with respect to the sludge, the particulate emissions of elements observed in this study are quite small compared with other sources, e.g., the following: Pb, 2 glday; Cd, 0.17 glday; Se 0.14 glday; and Sb, 0.028 glday from a sludge reactor burning -600 kg of sludge per hour. Based on previous work the most significant of the toxic elements measured from this source is Hg vapor at a rate of -50 glday.
Introduction Sludge is produced by sewage treatment plants a t a rate of -90 g of dry sludge daily per person in the United States ( I ) . In numerous large urban centers the problem of disposal has become critical as land burial sites have been closed. If this sludge is thoroughly digested in wastewater treatment plants, its mass can be reduced by one half ( I ) . Although sewage sludge contains enough nutrients to be used as low-grade fertilizer, the concentrations of toxic metals such as P b and Cd are often so great as to raise serious questions about the use of this material on food crops, or in other ways in which the toxic metals may be released to the environment ( I ) . Thus, many communities have built or are planning construction of sewage-sludge incinerators that will burn the sludge for two basic purposes: volume reduction and solids sterilization (2). In view of the high concentrations of toxic metals such as P b and Cd in sewage sludge ( I - 3 ) , there has been concern that large amounts of these elements may be released to the atmosphere by incineration. We have, therefore, investigated the particulate emissions from a fluidized-bed sewage-sludge incinerator at the Parkway Wastewater Treatment Plant in Laurel, MD. Plant Description The Parkway incinerator consists of two identical fluidized-bed reactor systems each capable of incinerating from 540 to 610 kg of dry sludge per hour. Either primary or activated sludge can be incinerated. Activated sludge comes from the aeration basins in which aerobic bacterial activity has partially digested the sludge and removed nutrients from the liquid phase. Sewage sludge containing 5% solids passes from + Present address: Center for Analytical Chemistry, National Bureau of Standards, Washington, D.C. 20234. 64
Environmental Science & Technology
the thickener tank into the disintegrator, where the sludge is mixed with a polyelectrolyte to aid flocculation. The sludge is then centrifuged and the liquid portion returned to the sewage treatment plant's intake to be reprocessed with incoming raw sewage. As shown in Figure 1,the solid portion is pumped into the top of one of the fluidized-bed reactors, which are maintained at a minimum temperature of 730 "C to assure complete combustion of the sludge. The ash, which is less dense than the flint sand which composes the bed of the reactor, is swept out of the top of the reactor with the incoming air which enters at the bottom of the reactor. Before sludge can be incinerated, the reactor bed is heated to 760 "C by burning No. 2 fuel oil, which is cut off when the temperature is achieved and the sludge flow is initiated. After the combustion gases and suspended particles leave the reactor, they enter a Venturi scrubber with a pressure drop of 50 cm of water, where most of the particles are removed. The gases then pass through a secondary scrubber, located at the bottom of the gas cooler. The combined efficiency of the particle-control device is greater than 99%. The plant-process water is used by the gas cooler which cools the gases to a temperature of -32 "C. Excess water is removed by a demister before the gases and remaining suspended particles exit through one of the two stacks. Each stack has a diameter of 1 m and extends to a length of 4.6 m above the roof of the incinerator building and 16.5 m above ground level ( 4 ) .
Experimental Section Five in-stack, suspended-particle samples were collected at the Parkway incinerator during three different shakedown periods of sludge incineration which were designed to be representative of normal operating conditions. A standard EPA Method 5 stack sampling train was used; however, Nuclepore polycarbonate filters (90 mm) were substituted for the glass-fiber filters because of their lower blank values for the elements of interest. Because of the problems with wall losses using this type of sampling train, the glass liner of the probe was washed with acid, and the solution analyzed for trace metals. The results of these analyses showed that wall losses were not important for the samples discussed here. Sludge samples were collected from the feed line to the incinerator during two of these sampling periods, and fly ash was taken from the ash classifier. The sludge had been homogenized before introduction into the incinerator. Most of the elements were measured by instrumental neutron activation analysis (INAA) using the National Bureau of Standards reactor and following procedures previously described ( 5 , 6). P b and Cd were measured by atomic absorption spectrometry (AAS) using acid dissolution of the samples in a Teflon Paar bomb. In order to test and improve 0013-936X/81/0915-0064$01,00/0
@ 1981 American Chemical Society
f Stack
(and scrubber make-up
water)
Gas Cooler
I
+ Venturi
I
*rutbar
Secondary Scrubbu
~
wet
Cyclones
and Fluidizing Air
I
cScrubber Return
I
Sump Figure 1.
Schematic diagram of the Parkway sewage-sludge incinerator ( 4 ) .
our analytical procedures, our laboratory occasionally analyzes new NBS Standard Reference Materials (SRMs) for as many elements as possible and publishes the results (7,8).The accuracy of the techniques used on the incinerator samples was checked by analyzing NBS Fly Ash (SRM 1633), which yielded results (6) in good agreement with literature values (7).
Results and Discussion Elemental concentrations observed in the input sewage sludge are listed in Table I along with similar data from the Blue Plains plant in Washington, D.C., and an average of sludges from 16 US.cities for comparison ( 3 ) .These studies of sludges have shown high concentrations of many heavy metals such as Pb, Cd, Au, Ag, Cu, Zn, Se, and Sb in sludge relative to soils, which may in part originate from industrial inputs to the sewer system ( I , 3 ) . Since the inputs to the Parkway plant are almost entirely from residences, offices,
and small businesses, we thought the sludge might exhibit lower concentrations of some elements. Concentrations of some elements at Parkway are comparable to the average of sludges from 16 cities or a factor of 2 or 3 lower. The major exceptions to this are Na, C1, Cr, Cd, In, and Eu, which are much lower at the Parkway plant, and I and Mn, which are much higher at Parkway. The amounts of Na and C1 retained by the sludge depend on the amounts solubilized in prior treatment steps and may be influenced by salting of highways if storm-sewer water is mixed with waste water. Cr, Cd, In and Eu may be introduced in many cities by industrial operations, e.g., electroplating in the cases of Cr and Cd. The big difference for Eu is surprising, as the other rare earths do not exhibit such large differences. Perhaps it is related to industrial uses of Eu as a phosphor in color television tubes (9).We have no explanation for the two elements I and Mn, which have unusually high concentrations at Parkway. As Washington, D.C., borders on the area served by the
Table 1. Elemental Composition of bry Sludge from Parkway, Washington, D.C., and Other Cities concn, pglg (unless % Indicated) Washington av of 16 cities
element
Parkway a
Na
740 f 40 0.51 f 0.15 0.25 f 0.08( % ) 4.6 f 0.4( % ) 290 f 70 710 f 50 900 f 90 50 f 30 40 f 20 2.9 f 0.5( % ) 1.9 f 0.4 2100 f 100 25 f 3 d 130 f 10 1300 f 130d 4.1 f 0.9( % ) 17f5 500 f 100
cs Mg
Ca Sr Ba CI Br I AI
sc Ti
v Cr Mn Fe
co
cu a
1100 2.9
0.55( % ) 2.4( % ) 890 3400 27 1 .o 5.1 ( % ) 5.7 4300 90 1260 320 4.5 (Yo) 15 460
4400 1.1 0.60 ( % ) 3.6 ( % ) 200 620 3800 46 7.8 1.8( % ) 2.5 2300 40 1440 190 1.5( % ) 9.6 1350
concn, pg/g element
Parkway a
Washington
1480 6.6 1.9
Cd
960 f 90 8.5f 0.8d 1.9 f 0.2 29 f 5 12f 1
In
