Literature Cited (1) Hartung, R., Klingler, G . W., &hiron. sei. Technol., 4,407-10 (1970). ( 2 ) Seba, D. B., Corcoran, E. F., Pestic. Monit. J., 3, 190-3 (1969). (3) Walker, J. D., Colwell, R. R., A .P.P ~ Microbiol., . 27. 285-7 (1974). (4) Sayler, G. S., Colwell, R. R., Enuiron. Sci. Technol., 10, 1142 (1976). (.i) Walker, J. D., Colwell, R. R., Microb. Ecol., 1,63295 (1974).
(6) Walker, J. D., Austin, H. F., Colwell, R. R., J . Gen. A p p l . Microb i d , 21, 27-39 (1975). (7) Walker, J. D., Colwell, R. R., A p p l . Microbiol., 31, 198-207 (1976). Receiued for recieu: September 2,1975. Accepted M a y 12, 1976. Work supported by Contract N00014-75-C-0340 between t h e Office of Nacal Research and t h e Uniuersity o f Maryland and National Science Foundation Grant No. B M S 72-02227-AO2.
Combined Process of Pyrolysis and Combustion for Sludge Disposal Nobuo Takeda" and Masakatsu Hiraoka Department of Sanitary Engineering, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto, Japan 606
A combined process of pyrolysis and combustion of sewage sludge was examined by use of a double hearth incinerator. The effectiveness of the introduction of a pyrolysis stage to incineration was investigated and compared with the usual sludge incineration. The behavior of heavy metals and other toxic or unburnt gases was analyzed in connection with the pyrolysis temperature, showing the effectiveness of this process for the prevention of air pollution. Pyrolysis at low temperature minimized the vaporization of heavy metals contained in the sludge without sacrificing the reduction of bulk density. The analyses of heavy metals caught in the particulates and scrubbed water suggested that much of the vaporized metals escape as fumes. An adequate temperature control may suppress nitrogen oxide emission. In Japan, incineration is the prevalent method for sewage sludge disposal. As sludge production increases with the spread of sewer systems, new incineration plants are constantly being constructed. Indeed, the incineration method is excellent in the respect that perishable organic sludge can be converted into stable inerts, reduced in volume, and disposed under good sanitary conditions. But it is inevitably such a high temperature operation that the vaporization of some heavy metals and/or the production of toxic gases, e.g., sulfur oxides and nitrogen oxides, can cause serious air pollution problems. Moreover, the highly excess air necessary for incineration results in so much volume of flue gas that gas cleaning equipment becomes prohibitively expensive. A continuous sequence of drying, decomposition of organics, flame and char combustion is carried out simultaneously in a sludge incinerator. The lower limit of the combustion temperature is governed by the condition that no odorous matter is permitted to remain in the flue gas, and the higher limit that the residual ash does not melt. Hence, the combustion temperature is usually maintained between 700 and 1000 "C. T o prevent the heavy metals from vaporizing into the air, it is preferable to burn the sludge a t a temperature as low as possible. Also, a homogeneous gas-phase combustion is better than a heterogeneous gas-solid phase condition for reducing the flue gas volume. Thus, the authors planned a "two-stage combustion" in which the organics of the sludge were decomposed into gases in the first stage, and the destructed gases were burnt through a gas-phase reaction in the secondary stage. Some experimental results are reported in this paper, demonstrating the effectiveness of this process. Experimental
A double hearth incinerator, MHI-2SE (4 m$ X 5 m), with a secondary combustion furnace was used. T o maintain an
oxygen-free atmosphere, the double hearth incinerator was preliminarily improved to prevent any air leakage. Sludge was supplied to the top of the incinerator and thermally decomposed through the double hearth incinerator; the pyrolysis gases were burnt in the secondary combustion furnace. For the pyrolysis experiment, the air supply to the preheater burner was minimized to prevent the advancement of the combustion reaction in the double hearth incinerator. After burn out in the secondary combustion furnace, the flue gas was mixed with cooling air prior to conduction to the stack. Part of the flue gas was delivered to a venturi-type scrubber to examine particulate collection efficiency. The entire experimental equipment is shown in Figure 1. The temperature of the lower stage of the double hearth incinerator was varied from 450 "C (Run I), 500 "C (Run 2), 600 "C (Run 3) to 800 "C (Run 4), while the secondary combustion furnace was kept between 700 and 820 "C. Run 4 was carried out as a control experiment under usual incinerating conditions, namely, sufficient air was supplied to burn out the sludge within the double hearth incinerator. The sludge consisted of a filtrated cake of mixed primary and surplus activated sludge from a municipal sewage treatment plant. The properties of this sludge were typical of an activated sewage sludge. A certain amount of heavy metal chlorides (nitrate for lead) was added to the sludge to point up the behavior of the heavy metals more clearly. Since the moisture content of the received sludge cake was too high for pyrolysis at a low temperature in this incinerator, it was initially dried to about 40% moisture content. This, however, was not done for Run 4. The sludge feeding rate was 100 kg-wet sludge/h for Runs 1-3 and 240 kg-wet sludgeh for Run 4; this corresponded to about 60 kg-dry sludge/h. Flue gas, 1000 nm3/h, was delivered to the scrubber at 200 "C and was scrubbed with 7-15 1. of water/min. The scrubber
F i g u r e 1. Schematic diagram of experimental equipment 1: Sludge feeder, 2: upper stage, 3: lower stage, 4: ash container, 5: preheater. 6: secondary combustion furnace, 7: fuel oil tank, 8: venturi scrubber, 9: water tank, 10: mist separator, 11: stack, FI: flow indicator, PI: pressure indicator, TR: temperature recorder, S: solid sampling point, G: gas sampling point, L: liquid sampling point
Volume
IO, N u m b e r 12, November 1976
1147
was 1900 mm high. The internal diameter of the venturi was 80 mm, whereas the linear velocity of the flue gas was about 100 m/s. The scrubber mist droplets were eliminated by impingement with buffles of the mist separator and recycled to the water tank. The following analyses and measurements were made so that the behavior of some toxic metals a t the time of pyrolysis and/or combustion of sewage sludge could be examined. Besides physical and calorific properties, cyanide and heavy metals were analyzed in the feed sludge and in the residual ash. Some toxic or unburnt gaseous components, particulate mass concentrations, and heavy metals in the particulates were measured as well as the more usual carbon dioxide and oxygen concentrations. pH, SS (suspended solids), COD, BOD, and heavy metal concentrations were measured for the scrubbed water. Some other items, e.g., temperature and gas flow rate, were continuously recorded for the control of the operation. Figure 1 shows the individual sampling points.
Results and Discussion Table I shows the complete analyses of the feed sludge, and Table I1 those of the residual ash. On Runs 1-3 the bulk density reduced to 68-74% of that of feed sludge. On Run 4 it reduced to 47%. The density of the residual ash was almost independent of the lower stage temperature between 450 and 800 "C, being approx. 50% of that of the received sludge. The ignition loss and the residual carbon and sulfur showed a tendency to decrease with temperature rise, reflecting that a high temperature is necessary for the decomposition of large molecular organics in a double hearth incinerator. The decrease of the ignition loss with temperature rise may be attributable to the vaporization of inorganic components as well as the decomposition of organics. The concentration of methane, ethane, propane, ethylene, and propylene a t the inlet of the secondary combustion furnace, with the exception of methane a t 500 "C, uniformly decreased with temperature rise as shown in Figure 2. The curve lines in Figures 2 and 3 were merely drawn in by eye. In the case of combustion a t 800 "C, no hydrocarbons except ethylene were found. The total amount of hydrocarbons as calculated from multiplication of the concentrations by the flue gas flow rates had the same temperature dependence as that of the concentrations themselves. Some previous results obtained by the authors ( 1 ) have shown that an increase in pyrolysis temperature results in a uniform increase in the production of such hydrocarbons as methane, ethane, and ethylene. The results of this paper are inconsistent with the previous results. This may be due to the fact that it was very difficult to keep the atmosphere perfectly devoid of oxygen in the incinerator, resulting in some of the hydrocarbons produced being burnt out a t the inlet of the secondary combustion furnace. Acetylene or any hydrocarbons above C4 were not found. Therefore, all hydrocarbons a t the inlet were burnt out with the sufficient oxygen in the secondary combustion furnace, thus not being detected a t the outlet. The sulfur oxide concentration a t the inlet of the secondary
Combustion furnace was 18 ppm a t the lower stage temperature of 450 "C, increasing with the lower stage temperature rise to range from 30 to 36 ppm a t 600 "C and from 36 to 228 ppm a t 800 "C. At the same time, the outlet concentration of the secondary combustion furnace ranged from 65 to 88 ppm a t 450 "C, from 36 to 48 ppm at 600 "C, and from 48 to 76 ppm was recorded a t 800 "C. The source of sulfur was the sludge, since the auxiliary fuel oil did not contain any sulfur. The amount of sulfur oxides produced per unit weight of dry sludge showed a uniform increase with the rise in temperature at the inlet of the secondary combustion furnace; on the contrary, it remained almost constant a t the outlet. The sulfur compounds dissociated from the sludge may have been oxidized in the secondary combustion furnace, since the amount of sulfur oxides is less a t the inlet than a t the outlet of the secondary combustion furnace. The higher the temperature a t the lower stage of the incinerator, the lesser the amount of difference between the sulfur oxide concentrations at the inlet and a t the outlet of the secondary combustion furnace. This may due to the fact that.the combustion reaction had partly proceeded in the incinerator as mentioned above for the hydrocarbons. The insignificant increase of sulfur oxides a t the outlet of the secondary combustion furnace with the temperature rise may be due to more dissociation of sulfur compounds a t higher pyrolysis temperatures. The conversion of sulfur into sulfur oxides, relative to that in the feed sludge, was about 40% in the range between 450 and 600 "C and from 40 to 65% a t 800 "C. In Runs 1-3 more hydrogen chloride was detected a t the inlet than a t the outlet. Some of the hydrogen chloride produced in the incinerator may react with the alkalis to produce alkali-chloride salts. The amount of hydrogen chloride increased in the secondary combustion furnace in Run 4 (800 "C). A detailed analysis of the production-decomposition of hydrogen chloride a t any temperature and atmospheric conditions has been left for future investigation. Hydrogen cyanide and ammonia increased a t the inlet of the secondary combustion furnace while the lower stage temperature rose between 450 and 600 "C. The temperature in the incinerator was controlled by the feed rate of the fuel oil. The oxygen concentrations a t the lower and upper stages of the incinerator were maintained almost constant in Runs 1-3. The decrease in the amounts of hydrogen cyanide and ammonia a t 800 "C (Run 4) may be due to an increase in the oxygen supply. This increase in hydrogen cyanide and ammonia with pyrolysis temperature rise has been confirmed in previous experiments by the authors. The concentrations of hydrogen cyanide and ammonia from sewage sludge in an inert atmosphere of helium uniformly increased with pyrolysis temperature. As expected, few nitrogen oxides were detected in this experiment. Figure 3 shows a sudden increase in the nitrogen oxides a t the inlet of the secondary combustion furnace between 600 and 800 "C. At the outlet they decreased between 450 and 600 "C, but increased between 600 and 800 "C. Nitrogen oxides may be produced in the secondary combustion stage when the pvrolvsis temperature is low (between 450 and 500 "C) or may
Table I . Analyses of Feed Sludge
Run
1 2,3 4
1148
Bulk Moisture density, content, kg/m3 wt %
700 760 1000
40.7 37.8 76.7
Dry basis heating value, kcallkg
Wt % on dry basis
mg/kg on dry basis
Ash
19-loss
C
H
S
Higher
Lower
CN
Cd
Pb
Mn
Cu
Zn
Cr
67.0 68.5
33.0 31.5
15.1 14.2
2.2 2.0
1.1 1.1
1410 1370
1290 1260
0.06 0.10
169 155 347
412 369 320
898 917 933
384 340 292
1554 1552 1762
53 59 50
...
...
Environmental Science & Technology
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...
...
...
...
. ..
Table
II. Analyses of Residual Ash
Bulk density,
Run$/
1 2 3 4
510 520 560 470
W t % on dry basis
Ash Ig-loss C
S
mg/kg on dry basis
Cd
87.0 13.0 5.8 1.7 184 87.8 12.2 4.0 1.6 139 93.9 6.1 1.8 1.4 159 98.1 1.9 0.9 1.3 131
Pb
Cu
Mn
Zn
Cr
399 1100 474 1943 59 296 1146 404 1973 68 267 974 425 1554 70 264 1056 399 1710 62
60
LZ
sn - w'
t340 ZLL
s>
5320 W
Yo, o w
Vu,
0
500 600 700 800 LOWER STAGE TEMPERATURE, C
Figure 2. Hydrocarbon concentrations at inlet of secondary furnace
S E C O N D AI ~ I~ FURNACE 0 INLET OUTLET
l0.5; O'
500 600 760 8h'O LOWER STAGE TEMPERATURE, C
Figure 3. NO, emission from sludge pyrolysis/combustion
become suppressed or decompose at higher temperatures (between 500 and 800 "C). Secondary combustion has no effect on the increase or decrease in the amount of nitrogen oxides around the temperature 550 "C. The production-decomposition rate of nitrogen oxides may be affected by many atmospheric conditions, e.g., the concentration of hydrogen cyanide and ammonia. In the course of an increase in hydrogen cyanide and ammonia with pyrolysis temperature rise, reactions between nitrogen oxides and these gaseous components may occur in the secondary combustion furnace so that the apparent concentration of nitrogen oxides is thought to have decreased. Kondo et al. (2) have pointed out that several heavy metals diffused into the air during incineration. For a quantitative treatment of the distribution of some components of sludge in the air and in ash during pyrolysis and/or combustion, the authors defined the term "residual
ratio" as the ratio of the amount remaining in the ash to that of the feed. Both residual ratios of ignition loss and carbon were about 30% at 450 "C, 30 and 22% a t 500 "C, and 14 and 9% a t 600 "C individually, and both were 3.5% a t 800 "C. The value of ignition loss of the ash was 13.0% a t 450 "C, 12.2%a t 500 "C, 6.1% a t 600 "C, and 1.9% at 800 "C. An abrupt decrease was observed with temperature rise. The decomposition of the organics may be thought to have been sufficiently accomplished a t 450 "C in view of the land filling requirement. In Japan the criterion on ignition loss is "under 15 percent for land filling". The distribution of heavy metals in the residual ash, in the scrubbed water, and in the particulates which passed through the scrubber is shown in Figure 4. Cadmium and lead were lost to a large extent in the pyrolysis and/or combustion process. On the other hand, for manganese, copper, zinc, and chromium, the residual ratios were high. Some of the heavy metals contained in wastewater are adsorbed by the activated sludge in sewage treatment plants. If they are given off into the air or into the effluent (e.g., in the scrubbed water) during incineration, the heavy metals will eventually be recycled in the environment. The residual ratio of all heavy metals decreased with a rise in the scrubbed water, and in the particulates increased in proportion to those vaporized; thus, there was also an increase in the unrecovered metals. Cadmium was observed to be readily vaporized at 450 "C, its residual ratio being 80%. The higher the lower stage temperature, the lower the residual ratio; in fact, it was only 25% at 800 "C. Much of the cadmium will be lost in the air as fumes, since it is not caught by the usual dust collectors. The residual ratio of lead was about 75% a t 450 "C, about 60% at 500 "C, and remained almost constant at 50%between 600 and 800 "C. The lead scrubbed was small, and that contained in the particulates was inclined to increase with a rise in the lower stage temperature; however, the maximum was only 10%of that in the feed material. About 35-40% of lead was unrecovered between 600 and 800 "C, reflecting the increase in that vaporized. Manganese did not vaporize a t all between 450 and 500 "C. The residual ratio was about 80% at 600 "C and about 75% at 800 "C; 80-85% to that in the feed material was recovered. Copper vaporized very little up to 600 "C; a t 800 "C the residual ratio was still about 85%. Zinc vaporized slightly between 450 and 500 "C; however, the residual ratio decreased to 70% a t 600 "C and to 60% a t 800 "C. Chromium showed a similar behavior to zinc. The effectiveness of water scrubbing as a means of dust collection is shown by the fact that in spite of the inlet high concentrations of particulates ranging from 0.127 to 0.227 mg/nm3, the outlet concentrations were kept to below 0.004 mg/nm3, with the overall efficiency being above 97.5%. The scrubbed water was recycled at a speed of 7-15 l./min as previously mentioned, and the water supply tank held 360 1. The hourly variations in the quality of the water in the tank were analyzed. As the time elapsed, the pH values decreased to pH 4 on Run 1 (lower stage temperature was 450 "C) and down to pH 3 on Run 4 (800 "C). This could be due to the absorption of some acidic gases such as sulfur oxides. SS values accumulated to 100-300 mg/l. on Runs 1-3 over 7 h. On Run 4 with such a low pH value, it is remarkable that the SS value was so low as 50 mg/l. The BOD values were less than 3 mg/l. or so, not showing any dependence on the lower stage temperature. The COD value ranged from 20 to 30 mg/l. on all runs. The heavy metal concentrations in the scrubbed water are shown in Figure 5 , indicating analyses a t three different times. One was of the feed sludge, and the other two were done during the course of the incineration runs. The blank parts of the histoVolume 10, Number 12, November 1976
1149
100 '
O
O
m
BO
80
6olj
€0
40
MANGANESE
500 603 703 803
i
loow
100
2100 I
almost constant. For lead and copper they were markedly high. The scrubbed water contained the heavy metals in relatively high concentrations; however, the total amount of metals was quite small compared with the total mass balance (Figure 4). The particulate collection efficiencies of the scrubber for the vaporized heavy metals were 0.7-2.7,0.6-1.6,O.g-2.3,3.9-4.7, and 1.845%for cadmium, lead, manganese, copper, and zinc, respectively. Thus, the heavy metals were scarcely caught in the scrubbed water. An additional table and eight figures have been deposited with the ACS Microfilm Depository Service.
80
Conclusions
I-60
z
1
20 COPPER
500 600
FHROMIUM~
5% 600 7 0 0 800 500 600 7co 803 LOWER STAGE TEMPERATURE, C
700 800
Figure 4. Distributions of heavy metals (neither zinc in particulates nor chromium in particulates and scrubbed water was measured)
0 450 500
100
600 80 v 45
200
300
15 00
I-
LI a800r1"'15
W
a
16.45
I U 13
1645
Oh,* 450 500 600 8001
_?0,00 4000
60,oO
8000
loo00 ZINC
6 40
"Two-stage combustion" investigated by the use of a double hearth incinerator showed some possibility in improving sludge incineration. Some important points are mentioned below. The bulk density of the feed sludge can be reduced by about 50% by pyrolysis at a temperature as low as 450 "C. The ignition loss value of the residual ash at 450 "C satisfies the Japanese criterion for land filling. Pyrolysis a t low temperature minimizes the vaporization of heavy metals in the air, whereas incineration a t high temperature may cause heavy metal air pollution problems, especially of cadmium and lead. Mercury was not investigated, but loss would be appreciable because of the low boiling point of this metal. Incineration a t 800 "C will lose about 80% of cadmium and about 50% of lead to the air. Only some of the vaporized metals will be caught in a dust collector, but much will remain unrecovered as fumes. Sludge pyrolysis produces some hydrocarbons, which may be utilized as a new energy source. In an incinerator these can be burnt out in a secondary combustion furnace to prevent any hydrocarbon pollution. The amount of sulfur oxides cannot be appreciably suppressed by low-temperature operation. The sulfur compounds dissociated from the sludge will be oxidized to sulfur oxides in an atmosphere of sufficient oxygen. The sulfur contained in the sludge was converted into sulfur oxides up to about 40% between 450 and 600 "C and 40-65% a t 800 "C. Hydrogen chloride produced by pyrolysis may react with alkalis. Nitrogen oxide emissions can be suppressed by controlling the product gases of pyrolysis; this can be balanced with the amount of hydrogen cyanide and ammonia in the pyrolysis gas. An optimal temperature of 600 "C minimized the nitrogen oxide emissions for the incinerator used. Water scrubbing was effective for dust collection. However, more care was taken on the design and operation of the scrubber as a countermeasure for the vaporized heavy metals. Literature Cited
'I"
a15 00
5
(1) Hiraoka, M., Takauchi, M., Takeda, N., J . J p n . Sewage W o r k s
CONCENTRATION, pGl L Figure 5. Heavy metal concentrations of scrubbed water
Assoc., (in Japanese), 10 (log), 30 (1972). (2) Kondo, J., Kunikane, S., Kurisaka, N., Sato, K., Takeda, T., Hane, A., Hamada, R., Maeda, M., Yamada, M., Kobayashi, H., Yamashita, S., W a t e r Res., 7,375 (1973).
grams indicate the dissolved concentrations, and the hatched parts, the concentrations of the insoluble heavy metals (insoluble according to a No. 5B filter paper). An accumulation of heavy metals in the scrubbed water can be seen as the elapsed time. The cadmium concentrations increased with a rise in the lower stage temperature, reflecting the increase in the amount of metals vaporized. The copper and manganese concentrations were almost constant, notwithstanding the variations in the lower stage temperature. Zinc, however, shows a reverse trend against the other metals. For the individual metals the ratios of unaissolved to total metal were
Received for review October I O , 1975. Accepted M a y 13,1976. Work supported by a grant f r o m t h e Ministry of Public Welfare.
1150
Environmental Science & Technology
Supplementary Material Available. One table and eight figures will appear follouing these pages in the microfilm edition of this volume of t h e journal. Photocopies of t h e supplementary material from this p a p e r only or microfiche (105 X 148 m m , 24X reduction, negatiues) containing all of t h e supplementary material for t h e papers i n this issue m a y be obtained from t h e Business Operations Office, Journals Department, American Chemical Society, 1155 16th S t . , N . W., Washington, D.C. 20036. R e m i t check or money order for $4.50 f o r photocopy or $2.50 for microfiche, referring to code number ES&T- 76-1147.
