Pyrolysis of sewage sludge and use of pyrolysis coke

Pyrolysis of sewage sludge and use of pyrolysis coke

Journal of Analytical and Applied Pyrolysis, 28 ( 1994) 137- 155 Elsevier Science Publishers 137 B.V., Amsterdam Pyrolysis of sewage sludge and us...

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Journal of Analytical and Applied Pyrolysis, 28 ( 1994) 137- 155 Elsevier

Science Publishers

137

B.V., Amsterdam

Pyrolysis of sewage sludge and use of pyrolysis coke M.P. Rumphorst

* and H.D. Ringel

KFA Research Center, Institute of Chemical Technology, Jiilich (Germany) (Received

October

2, 1992; accepted

May 17, 1993)

ABSTRACT The properties of pyrolysis coke from sewage sludge and its possible applications are discussed. Pyrolysis of sewage sludge was carried out using laboratory-scale equipment at temperatures between 500 and 900°C. The conditions of sludge feed were varied. Treatment of sewage sludge by pyrolysis decreased the sewage sludge to half its original mass. The adsorption properties of the pyrolysis coke were investigated and its possible application in gas purification and biological purification of sewage are considered. If not used in other areas, the resulting coke was found to have useful disposal properties. Coke; pyrolysis;

sewage sludge.

INTRODUCTION

The treatment and disposal of waste material is becoming a problem of increasing urgency in industrialized societies. The situation in Germany is characterized by: (i) increasing amounts of waste; (ii) growing knowledge and increasing concern about harmful substance contained in waste material and those produced in the so-called thermal treatment of wastes; (iii) critical discussion about environmental evaluation for use or disposal of residual substances form thermal waste treatment plants; (iv) declining public acceptance of waste disposal plants. In this connection, sewage sludge takes on a special role since its origin would suggest that it could be used in agriculture. However, this method of utilization is made difficult, because sewage sludge also has a quality very welcome in waste water purification of concentrating and incorporating diffusely spread chemicals which cannot be degraded in the present process [ 11. With an annual 15 million t of chemicals used in the former Federal Republic of Germany without any regard to the environmental effects [2] this

* Corresponding Germany.

author.

Present

address:

0165-2370/94/$07.00 0 1994 - Elsevier SSDZ 0165-2370(93)00757-E

Johann-Jacob-Classen-Str.

Science Publishers

10, D-47906 Kempen,

B.V. All rights reserved

138

M.P. Rumphorst

and H.D. RingelI J. Anal. Appl. Pyrolysis

28 (1994) 137-155

inevitably leads to a pollution load with undesirable inorganic and organic substances. Thus, sewage sludge, the “symbol of recycling and fertility, becomes a waste material with a high pollutant content” [ 11. Therefore, as long as the content of noxious substances cannot be significantly reduced, the methods of so-called thermal treatment remain the only way to treat and then dispose of most types of sewage sludge. These are capable of destroying pollutants by high temperature or a reducing atmosphere. Various studies have been conducted in the field of sewage sludge pyrolysis. The aim has always been to consider pyrolysis not only as a waste treatment method but also as a means of obtaining a valuable product. In most studies in Germany, special attention has been given to the liquid and gaseous products of pyrolysis, whereas pyrolysis coke was considered to be a residual material to be disposed of. The purpose of this work is to concentrate on pyrolysis coke and its properties and possible applications. SEWAGE

SLUDGE

The quantity and composition of the basic organic starting material is of decisive significance for any pyrolysis. In considering the various sludges arising in a sewage plant, excess sludge from final clarification was chosen as the basic material for pyrolysis. The sludge was obtained from a medium-sized municipal sewage plant. It was mechanically dehydrated. In order to avoid an undesirable loss of the pollutant content by drying, the sludge was air-dried to constant weight over a period of about 40 days. Additional drying of this sludge at 105°C showed a further weight loss of 8.66 wt.%. The sludge was then roughly crushed and homogenized in a rolling machine. Samples were then ground for analysis. Table 1 shows the composition of the sludge as a mean of five samples. It is apparent that the organic substance, expressed by the-loss at red heat (i.e. 550”(Z), is essentially composed of an organic carbon fraction, as well as a containing sulphur, hydrogen, nitrogen and organic oxygen. The organic carbon was determined as the carbon fraction of the sample not soluble in hydrochloric acid. Water-soluble carbon components such as oxalates and surfactants were not detected by the acid-free washing, so that the total organic carbon content is probably somewhat greater. Table 2 shows the contamination of the sludge with inorganic pollutants expressed by the seven heavy metals according to Sewage Sludge Ordinance. By way of comparison, the limits of the revised Sewage Sludge Ordinance are also given. Owing to the ready volatility of mercury, the content of this element was determined separately by direct sampling from the wet sludge. All values, except those for cadmium, are clearly below the limits. The cadmium content increases due to the degradation of organic substance

M.P. Rumphorst and H.D. Ringer 1 J. Anal. Appl. Pyrolysis 28 (1994) 137-155

139

TABLE 1 Major constituents

of sewage sludge a

Constituent

Wt.% DM

Ash Loss at red heat

42.6 57.5

Organic carbon (only one value) Total carbon Sulphur Hydrogen Nitrogen Organic oxygen

12.9 28.1 0.6 4.7 3.5 27.5

Total C+S+H+N+O Total organic C + S + H + N + 0

64.4 49.2

“Calorific value of sludge H,, = 10778 kJ/kg.

TABLE 2 Heavy metals in sewage sludge (mg/kg DM) Metal

This work

Limit pursuit of waste sewage sludge ordinance.

Pb Cd Cr cu Ni Hg Zn

138 * 4.92 12 f 0.52 29 _+3.42 185 f 3.77 77 f 11.75 1.9 f 0.08 1187 + 24.45

900 10/s = 900 800 200 8 2500/2000 a

a Applied to light soils and soils with a pH value of 5-6.

during the usual digestion process at the sewage works. The contamination of the sludge with polychlorinated biphenyls (PCBs) in Table 3 is given in mg/kg dry matter (DM). These are means from four samples. Only a cumulative parameter was specified in the Sewage Sludge Ordinance for these pollutants, the AOX (adsorbable organic halides) and for the PCBs the value of 0.2 mg/kg DM for each single congeneric, listed in the numbering system used by Ballschmiter [3]. No limit was defined for polyaromatic hydrocarbons (PAHs) but significant. amounts of these organic substances were also found. Up to 40% of the limit for PCBs has already been reached in the undigested sludge.

140

M.P. Rumphorst and H.D. RingelI J. Anal. Appl. Pyrolysis 28 (1994) 137-155

to outlet

rising tube heating sludgedosing

conoensate

mass spectra meter El--

3

Ar inertization

Fig. 1. Pyrolysis

equipment

process

LABORATORY

PYROLYSIS

flow chart.

EQUIPMENT

Figure 1 shows the flow chart of the laboratory pyrolysis equipment. It essentially consisted of the pyrolysis reactor, and gas condensation equipment with aerosol precipitation and gas analysis. All measured data were recorded by a data acquisition program which simultaneously carried out the gas-dependent correction of the gas quantity measurement. At the same time gas samples were taken for control purposes. Figure 2 shows the pyrolysis reactor. An indirectly heated shaft reactor of 10 cm diameter was selected so that during degasification the pyrolysis coke would undergo minimal movement. The furnace temperature was similarly controlled by the data acquisition program as a function of the temperature in the reactor. For purposes of inertization and in order to ensure a minimum flow through the gas analysis instruments, argon as a carrier gas was fed in through a filter plate in the bottom of the reactor. For the first series of tests the sewage sludge was weighed into the cold reactor before starting the experiment. In order to achieve the greatest possible similarity to conditions in a commerical pyrolysis plant it was also possible to feed sludge externally into the hot reactor. It was therefore possible to investigate the influence of the way in which sludge was fed into the reactor on the coke properties. The rising tube was heated to 400°C in order to prevent the condensate from flowing back into the reactor.

h4.P. Rumphorst and H.D. Ringeli J. Anal. Appl. Pyrolysis 28 (1994) 137-15.5

141

.rising tube

Fig. 2. Pyrolysis

reactor.

The high content of aerosols in the pyrolysis gas became apparent in preliminary experiments. In order to separate these aerosols and to close the mass balance more effectively, glass filter plates (DO, Dl, D2, D3) were built into the gas condensation equipment. PYROLYSIS

AND

PYROLYSIS

RESULTS

The entire experimental programme involved the variation of the following experimental parameters: (i) pyrolysis temperature, 500-900°C; (ii) type of sludge feed - into the cold reactor or into the hot reactor;

M.P. Rumphorst

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and H.D. RingelI J. Anal. Appl. Pyrolysis 28 (1994) 137-155

(iii) mixing the entire condensate with the sewage sludge before pyrolysis; (iv) pyrolysis of only mechanically dehydrated sludge. In some experiments the entire condensate from a pyrolysis was previously mixed with the appropriate quantity of sewage sludge for the next experiment, with the aim of both disposing of the condensate and also improving the coke quality. However, only the experiments with the addition of dried sludge into the hot reactor will be discussed here, although a comparison will occasionally be made with the addition of sludge into a cold reactor [4]. Between 100 and 200 g of dried sludge was fed into the cold reactor before the run, or during the first phase of the run, after heating up and inertization of the whole apparatus in order to remove oxygen. Pyrolysis was carried out in each case until degasification had been completed. This was determinated by the CO content in the pyrolysis gas. In the case of feeding sludge into the hot reactor, the time of material dosing was clearly indicated by slight fluctuations in reactor temperature and the subsequent degasification time. The pyrolysis gas consisted of approximately 20 vol.% hydrogen, 15 vol.% carbon monoxide, 7 vol.% CO, and 7 vol.% methane. The composition is made up to 100% by the cleansing gas and undetermined gas constituents. Coke was removed after cooling the furnace and reactor. The mass balance for sludge feed into the hot reactor is shown in Fig. 3 as a function of pyrolysis temperature. Depending on the pyrolysis temperature, the coke fraction is in the range 40-52 wt.%, whereas the total

100 ?g-

go-

<

80-

$

70-

u .>

60

lGasl

50 -

1Condensate] 40 30 20 10 o-

600

700

Pyrolysis

Fig. 3. Product

yields from

800

temperature

sludge

900

[“Cl

feed in the hot reactor

as a function

of temperature.

M.P. Rumphorst

and H.D. Ringer/J.

Anal. Appl. Pyrolysis

28 (1994) 137-155

143

60

Sludge

Sludge 40

-

30

1 500

Fig. 4. Comparison

feed

into

hot

feed

into

cold

reactor

reactor

I

I

I

I

600

700

800

900

Pyrolysis

temperature

of coke yields for different

temperatures

[“Cl

and methods

of sludge feeding.

condensate was lo-28 wt.% and the yield of pyrolysis gas was 9-40 wt.%. The total mass balance was about 90%. Apart from small condensate losses, this is probably largely due to inaccuracies in the gas analysis, especially since the gas constituents containing nitrogen and sulphur were not measured. The loss is thus probably mainly attributable to the gas quantity. Figure 4 shows a comparison of coke yields as a function of the method of sludge feed. Feeding sludge into the hot reactor apparently leads to a greater degradation of the organic substance, which is expressed in lower coke yields. The results of the other process engineering parameters will not be discussed here in detail. The studies on the possibilities of coke application were carried out with the coke resulting from the feed of dried sludge into the hot reactor. PROPERTIES

OF PYROLYSIS

COKE

The essential mechanical properties of pyrolysis coke for the addition of sludge into the hot reactor and subsequent brief crushing are the following. The particle size was less than 3 mm. The bulk density was in the region of 600 kg/m3 and the solid density about 2350 kg/m3. Major constituents of coke The major constituents of coke are ash and carbon. In Fig. 5 the carbon content of the coke is shown as a function of the pyrolysis temperature for

M.P. Rumphorst and H.D. RingelI J. Anal. Appl. Pyrolysis 28 (1994) 137- 155

144

TABLE 3 PCBs in sewage sludge (mg/kg DM) a Name according to Ballschmiter [3]

PCB content

No. No. No. No. No. No.

0.01 0.02 0.08 0.06 0.06 0.03

28 52 101 153 138 180

Total of six Total x 5

0.26 1.30

a Limit of determination: 0.01 mg/kg DM for each congeneric. sewage sludge ordinance: 0.2 mg/kg DM for each congeneric.

C--O 10

2: Sludge

feed

into

hot

Limit pursuant

reactor

r

i

I

/

I

500

600

700

800

9oc

Pyrolysis

to waste

temperature

[“Cl

Fig. 5. Carbon content of pyrolysis coke as a function of pyrolysis temperature.

sludge feed into a cold and hot reactor. It is apparent that the lower coke yields (Fig. 4) when sludge is fed into the hot reactor correspond directly to the lower carbon content of the coke (Fig. 5). The ash content was determined to be between 74 and 90 wt.% for the cold reactor and 75 and 95 wt.% for sludge fed into the hot reactor. The total of ash content, carbon, organic oxygen and hydrogen was always slightly above 100 wt.% (approximately 108 wt.%).

M.P. Rumphorst

500

and H.D. Ringer 1 J. Anal. Appl. Pyrolysis

145

28 (1994) 137-155

I

I

I

600

700

800

I 900

Fig. 6. Organic oxygen, nitrogen, hydrogen and sulphur constituents

in pyrolysis coke.

The organic oxygen, nitrogen, sulphur and hydrogen in the coke are represented in Fig. 6 as a function of the pyrolysis temperature for sludge fed into the hot reactor. Taking into consideration coke yield and analysis of the sludge, it can be seen that even at a pyrolysis temperature of about 500°C approximately 70% of the nitrogen, almost 80% of the organic oxygen and almost 90% of the hydrogen has been volatilized. Sulphur is hardly degraded at all by pyrolysis. In contrast, it is concentrated from 0.6 wt.% in the sludge to 1.4 wt.% in the coke due to the decrease of the organic material with increasing pyrolysis temperature, which corresponds to a sulphur recovery rate of 83-94% in the coke. These constitutents do not show any significant dependence on the nature of sludge fed into the reactor.

M.P. Rumphorst

146

TABLE

and H.D. RingelI

J. Anal. Appl. Pyrolysis

28 (1994) 137-155

4

Inorganic pollutant balance for sludge feed into hot reactor: to pollutant content in sludge Na

K

Mg

Ca

Cr

Ni

Cu

Zn

112 104 109 115 100

82 116 132 124 101

95 111 111 108 102

100 101 101 101 94

67 59 58 58 67

86 65 64 46 54

93 92 88 87 86

deviation 10 14

1 10

7 11

6062 9 52

37

9

Temperature

Cd

pollutants

in coke (%) relative

Hg

Pb

Mn Fe

Co

P

14 93 78 91 102 93 95 101 97 114

0 0 0 0 0

28 74 100 97 101

79 85 91 90 98

96 99 94 95 99

103 94 102 86 101

101 102 102 102 97

120 15 14

0

3 11

7 9

4 9

11 12

6 8

(“C) 900 800 700 600 500 Standard Min. Max.

Inorganic constituents and pollutants Of the inorganic constituents and pollutants, the alkaline and alkaline earth elements, the seven heavy metals specified in the Sewage Sludge Ordinance and a few further elements were analysed. The pyrolysis balance for these elements is shown in Table 4. The recovery rates for the elements are shown relative to their content in the initial sewage sludge material. The mean values of the samples of the sewage sludge were used for the balance. Particular attention should be paid to the considerable fluctuations of the Cr and Ni values, apparently attributable to special components in the initial sewage sludge material. The alkaline and alkaline earth elements were recovered quantitatively in the coke, uninfluenced by pyrolysis. Even at a pyrolysis temperature of 5OO”C, mercury was no longer detectable in the coke; whereas Zn, Cd and Pb persisted almost completely in the coke at low pyrolysis temperatures and their content in the coke only decreased at pyrolysis temperatures of 700 to 800°C. Particularly striking is the clearly lower content of Zn and Pb at a pyrolysis temperature of 900°C. A slightly increasing recovery rate for Cu was actually determined with increasing temperature. Almost 100% of the iron, cobalt and phosphorus was found in the coke, whereas the manganese content decreased slightly with increasing temperature. Organic pollutants Table 5 shows a comparison of some organic pollutants in sewage sludge and in pyrolysis coke. The basis for this comparison is the means of random samples of the sewage sludge. It is clear that with respect to PCBs in the remaining coke, pyrolysis, particularly above tempertures of 700-8OO”C,

M.P. Rumphorst

and H.D. RingelI J. Anal. Appl. Pyrolysis

28 (1994) 137-155

147

TABLE 5 PCBs in sewage sludge and in pyrolysis coke a @g/kg DM) Name according to Ballschmiter [ 31

No. No. No. No. No. No.

28 52 101 153 138 180

Total Detection limit

Sewage sludge

10 17.5 75 62.5 62.5 32.5 260 10

Pyrolysis coke 700°C

800°C

900°C

2.87 5.23 18.77 11.67 13.97 3.63

N.d. N.d. 4.43 4.58 5.33 4.82

N.d. 1.56 4.5 2.89 2.29 0.53

55.14

19.16

11.77

0.4

0.4

0.4

a N.d., not detected (below detection limit).

can be regarded as a means of destruction for these substances. Their total content in the pyrolysis coke from 800°C is less than one-tenth of the sludge concentration and in the case of pyrolysis at 900°C even less than one-twentieth. The PAHs measured in the coke were all below the detection limit for those experiments with a pyrolysis temperature above 800°C. Elution behaviour First of all, the pH value of the coke is of significance in assessing the capability of heavy metals mobilizing in the pyrolysis coke matrix with respect to a possible landfill disposal or intern storage. Depending on the pyrolysis temperature, the pH is between 8.7 and 11.6. This value means that favourable elution behaviour by the coke is to be expected. The pyrolysis coke samples were examined according to the Swiss Eluate Test [5] to investigate the elutability of the various elements. This involved the coke being eluted twice for 24 h in distilled water through which a constant quantity of CO2 was passed. The distilled water through which CO2 was passed had a pH value of 4-4.5. After stirring in the coke, a pH value of 6-6.5 was reached after a few minutes and remained constant throughout the entire elution period. The elution rates of the elements mentioned above are listed in Table 6. The values give the percentage contents of the elements found in the eluate relative to their contents in coke. It can be seen that the alkaline and alkaline earth elements display clear elution rates of l-3% in the same way as Mn. The heavy metals are clearly less readily eluted. Of these Cr, Cu and Zn are the most poorly eluted with elution rates of less than 0.1%. Ni displays similar values of below 0.15%. In contrast, Pb and Cd are more readily mobilized with elution rates of up to 1%. Elutability

0.63 0.37 1.75 3.04 3.40

900 800 700 600 500

0.82 0.57 0.77 1.47 1.05

K

3.26 3.71 4.08 4.21 3.52

Mg

4.30 4.33 5.14 5.14 5.97

Ca

a N.d., not detected (below detection limit).

Na

Temperature (“C) 0.05 N.d. N.d. N.d. N.d.

Cr

0.15 0.03 0.09 N.d. N.d.

Ni

0.06 0.10 N.d. N.d. N.d.

Cu

0.06 0.01 0.01 0.03 0.03

Zn

0.81 1.57 0.51 N.d. N.d.

Cd

0 0 0 0 0

Hg

0.97 0.16 N.d. N.d. N.d.

Pb

2.43 0.72 0.83 1.05 0.80

Mn

1.67 2.32 1.02 0.06 0.00

Fe

Elution rates according to the Syiss Eluate Test for sludge feed into a hot reactor (% relative to content in coke) a

TABLE 6

1.05 N.d. N.d. N.d. N.d.

Co

0.01 0.02 0.01 0.01 0.09

P

0.97 0.67 0.56 1.49 5.13

S

E

M.P. Rumphorst

and H.D. Ringer 1 J. Anal. Appl. Pyrolysis

28 (1994) 137-155

149

TABLE 7 Eluate values of pyrolysis coke compared with German limits for landfill of urban refuse, class 3 (mg/l) a Pyrolysis coke

Limit for landfill class 3

8.7-11.3 < 0.06 < 0.05 < 0.02 < 0.05 GO.5 N.d. GO.07

5.5-12.0 <2 GO.5 < 10 < 10 < 10 GO.05
PH Pb Cd Cr total cu Ni Hg Zn

a N.d., not detected (below detection limit).

increases with rising pyrolysis temperature. Mercury was not found. Table 7 shows the assignment values of the eluate criteria for storage above ground of hazardous waste (special waste landfill) determined ‘according to the German Standard Method [6] in comparison with the author’s own measurements according to the Swiss Eluate Method. The elution values of coke reach a maximum of 25% of these limits with the element Ni. All other heavy metals are far below this limit.

h: ::

20 ,

*

C---O 0

1: Sludge feed into cold reactor 2: Sludge feed into hot reactor

,

I

I

I

I

500

600

700

800

900

Pyrolysis

temperature

[“Cl

Fig. 7. Specific surface of coke after BET as a function of pyrolysis temperature.

150

M.P. Rumphorst

and H.D. RingelI J. Anal. Appl. Pyrolysis

28 (1994) 137-155

TABLE 8 Comparison

of adsorption

characteristics

BET (m*/g) Iodine value (mg/g)

Pyrolysis coke

Lignite coke dust

61-84 160-193

247-270 304

Adsorption characteristics

With respect to a possible utilization of coke in adsorption technology, apart from the pollutant contents some other adsorption characteristics of coke were also determined. In order to make a comparison with other adsorbents, in particular it seemed appropriate here to determine the specific surface area via BET and the iodine adsorption value. Figure 7 shows the curve of the specific surface area as a function of the pyrolysis temperature. The values are relative to the total coke and not, as occasionally mentioned in the literature, only to the carbon in the coke. For sewage feed into the hot reactor at between 700 and 800°C the values reach a maximum of approximately 70 m2/g, whereas in the case of sludge feed into the cold reactor increasing specific surface areas of up to 100 m2/g were measured with increasing pyrolysis temperature. The iodine values also dispaly a clearly different curve as a function of temperature. The maximum for both methods of sludge feed is in the region of 800°C and reaches values of up to 180 mg/g. Table 8 shows these values in comparison with lignite coke dust, an adsorbent produced industrially. APPLICATIONS

FOR PYROLYSIS

COKE

Gas adsorption experiments

In order to examine possible uses of pyrolysis coke, the adsorption of benzene and phenanthrene onto pyrolysis coke was studied in comparison with adsorption onto lignite coke. A sieved grain fraction (l-2 mm) of the pyrolysis coke was used for this purpose. The coke had been produced at 800°C by feeding sludge into the hot reactor. Adsorption was carried out at a temperature of 12O”C, unfavourable for this process, in order to create conditions similar to those in industrial equipment. 50.5 mg/m3 of benzene in air was passed through the adsorber bed with a velocity (measured in the empty column) of 9.83 cm/s. Figure 8 shows as an example the breakthrough curve for benzene adsorption. The first breakthrough was established for pyrolysis coke after about 0.5 h, whereas this was only the case for lignite coke after approximately 3 h. The break-

M.P. Rumphorst and H.D. RingelI J. Anal. Appl. Pyrolysis 28 (1994) 137-155

7 <

0.9

2

0.8

b n b $j

151

oading: 6.69mg/lOg

0.7 0.6

a f 0.5 ;r 0 0.4 2

0.3

c? _: Liz

0.2 0.1 0

I

I

0

2

k

k

b

lb

lk

lh

Time

[h]

Fig. 8. Breakthrough curve for benzene adsorption. c: actual concentration absorption. c,,: concentration of benzene after absorption.

of benzene after

through curve for pyrolysis coke displays a steep slope, which indicates a good utilization of the coke. Particular attention should be paid to the ratio of charges of coke to the specific surface area. For both adsorbents, the loading is directly related to their specific surfaces areas. The loads also behaved towards each other like the specific surface areas for phenanthrene adsorption. Experiments on coke utilization in sewage treatment plants a With the pyrolysis of sewage sludge, it is natural to consider the use of the resulting coke in the sewage plant. There are many references in the literature to an application of activated charcoal in various sectors of a sewage plant. In order to study the suitability of pyrolysis coke as an auxillary agent in the biologically activated stage, toxicity tests were first carried out in order to examine possible negative impacts of this coke on microorganisms. However, no evidence of this was found, enabling a further step to be taken. Two model sewage plants were operated in parallel on a laboratory scale in order to examine the influence of addition of pyrolysis coke on (i) bulking sludge formation, (ii) sludge volume index, (iii) sewage plant effluent values and (iv) dehydration behaviour. a Work carried out at the Institute for Biotechnology 3 of KFA.

M.P.

152

Start

of

coke

Rumphorst

and H.D. RingelI

J. Anal. Appl. Pyrolysis

28 (1994) 137-155

feed<

25 20

_

F 2

E

NO3

2

E !i = iYAi

1 I

0

10

2’0

30 Duration

of

40 experiment

S’O [d]

Fig. 9. Influence of coke feed on nitrification and phosphate

elimination

(d = days).

The two facilities were first operated without any addition of coke and their operating values measured. After about three weeks, 50 mg of pyrolysis coke per litre of waste water was added to one facility, and the same quantity of lignite coke dust was fed into the other. Figure 9 shows the influence of the addition of coke on nitrification and phosphate elimination. Ammonium nitrite was measured as the first nitrification product in the outlet of the model sewage works and nitrate as the second nitrification product. The nitrate data for the addition of lignite cokes are given in the top half of the Figure, and in the lower half the values for the addition of pyrolysis coke are given. An improvement of nitrification as a means of N-reduction was achieved in both cases. This means that not only was nitrite formed from ammonium nitrite, but also the addition of coke considerably improved further nitrification to nitrate. Thus the amount of

M.P.

Rumphorst

and H.D. Ringer 1 J. Anal. Appl. Pyrolysis

Lignite

coke

28 (1994) 137-155

153

dust

15 -

Pyrolysis

Mean: 5.86 +/0 0

coke

1.6

I

I

I

I

I

I

10

20

30

40

50

60

Duration

of experiment

[d]

Fig. 10. Influence of coke feed on capillary suction time (d = days).

N03, measured in the effluent, is an indicator of the quality of nitrification. In a sewage plant NO3 is later oxidized to NZ. The same Figure shows the influence on biological phosphate elimination. This was also slightly improved for both types of coke. Finally, in times of increasing “thermal disposal” of sewage sludge, the question of the possibility of mechanical dehydration becomes increasingly important. As yet there is no standardized measuring procedure for determining this sludge property. The most promising approach is a measurement of the so-called capillary suction time (CST). This involves measuring the time taken for a sludge to release its water to a water-absorbing measuring paper; this time is related to the sludge dry matter. Figure 10 represents the influence of the addition of both types of coke on the CST value. It can be seen that both types of coke bring about a similar improvement in this dehydration characteristic.

154

CONCLUDING

h4.P. Rumphorst and H.D. RingelI J. Anal. Appl. Pyrolysis 28 (1994) 137-155

REMARKS

By way of summary, the following points have been established with regard to the properties of pyrolysis coke and the resulting possible applications. (1) The mass of the sewage sludge is decreased to about half by pyrolysis thus reducing capacity requirements at landfill sites. (2) The very alkaline pH value of the coke suggests that it might be used as a buffer and neutralizer in sewage sludge monowaste landfills, as recently demanded by the ATV (Abwassertechnische Vereinigung, i.e. German Waste Water Treatment Association). This solid will probably also have a favourable effect on the mechanical stability of such sites. (3) The very favourable elution behaviour of the coke similarly emphasizes its suitability for landfill disposal; furthermore, this property also minimizes the danger of a release of inorganic pollutants during further handling or interim storage of the coke. (4) It is important for disposal and also for interim storage or further utilization of the coke that most organic pollutants in the sewage sludge are decisively reduced by pyrolysis. (5) The adsorption capabilities of the coke would justify consideration of its limited application in gas purification, if it is assumed that: (i) short transport paths exist between coke production and application; (ii) the slight specific surface area is compensated by a larger or longer adsorber bed; (iii) after loading, the coke is no longer regenerated but rather incinerated. However, the mechanical strength of the coke would have to be further improved for such an application. (6) Particular attention should be paid to the use of pyrolysis coke in the biological purification stage of a sewage treatment plant, since in this case significance is apparently attached less to the adsorptive capabilities of the coke and more to the growth surfaces created for microorganisms which colonize coke grains readily and rapidly. The coke grains thus become the centre of sludge flakes which then become heavier, are able to precipitate more effectively during final clarification, and improve the dehydration behaviour of the sludge. According to the measured criteria, the effect of pyrolysis coke can be compared with commercially available lignite coke dust for this application. REFERENCES 1 W. Schenkel and R. Butzkamm-Erker, gungen und kiinftige Entsorgungswege,

Kkirschlammentsorgung, die neuen RahmenbedinKorresp. Abwasser, 37 (1990) 1037-1053.

M.P. Rumphorst and H.D. RingelI J. Anal. Appl. Pyrolysis 28 (1994) 137-155

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U. Lahl and B. Zeschmar-Lahl, Kllrschlammentsorgung-die Spielregeln iindern, Korresp. Abwasser, 37 ( 1990) 164- 174. K. Ballschmiter and M. Zell, Fresenius’ Z. Anal. Chem., 302 (1980) 20-31. M.P. Rumphorst, Ph.D. Thesis, University of Aachen, in preparation. Bundesamt fur Umwelt Wald und Landschaft, Schweiz, Technische Verordnung iiber Abf5lle (TVA), 1991. DIN 38414, Part 4, Deutsche Einheitsverfahren zur Wasser-, Abwasser- und Schlammuntersuchung Bestimmung der Eluierbarkeit mit Wasser (54), October 1984.