The stabilization of sewage sludge by pulverized fuel ash and related materials

Environment

Pergamon

International, Vol. 22, No. 6, pp. 705-710, 1996 Copyright 01996 Elsevier Science Ltd Printed in the USA. All rights reserved 0160-4120/96 $15.00+.00

PII SO160-4120(96)00061-X

THE STABILIZATION OF SEWAGE SLUDGE BY PULVERIZED FUEL ASH AND RELATED MATERIALS C.S. Poon Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Horn, Kowloon, Hong Kong

M. Boost Department of Health Sciences, The Hong Kong Polytechnic University, Hung Horn, Kowloon, Hong Kong

EI 9602-120 M (Received 26 February 1996; accepted 12 July 1996)

In Hong Kong, with the implementation of the Strategic Sewage Disposal Strategy (SSDS), there will be a substantial increase in the generation of sewage sludge. An alternative method using pulverized fuel ash (PFA) generated from a local coal tire power station and lime to stabilize the sludge for potential beneficial uses is studied. The effects of the stabilizing agents on the leaching of heavy metals and pasturing of coliform are studied. The results show that PFA together with a small percentage of lime addition has the potential to reduce heavy metal leaching and reduce the total cohform content of the stabilized sewage sludge. Copyright e1996~/~~1,ierscience L:d

INTRODUCTION

landfill shortage for the disposal of other types of waste. In view of the above, alternative methods must be developed to reduce the quantity of sewage sludge that requires disposal. Pulverized fuel ash (PFA) is a by-product derived from burning coal during electricity generation. The two power companies in Hong Kong are currently producing 3000 tonnes (3000 Mg) PFA/d. Although a small proportion of the material can be utilized by the construction industry for the production of blended cement/ concrete and used as an engineering till material, a large proportion of the generated quantity remains largely unused and is currently stored in large ash lagoons constructed out at sea. This storage (disposal) arrangement is considered unsatisfactory as it may cause serious marine pollution. Lime has traditionally been used to stabilize or pasteurize sewage sludge before the latter is applied to land (USEPA 1979). The stabilization and pasteur-

Sewage works in Hong Kong are currently producing 60 000 m3 of sludge (at 30% dry solid) per year. With the implementation of the Strategic Sewage Disposal Strategy (SSDS), more sludge will be produced when new sewage treatment works become operational in the coming years. It has been estimated that the quantity of sewage sludge generated in 1996 will be 1 200 000 m3. The disposal of this large quantity of sludge is a major problem for the government. Currently, there are only two available disposal routes for sludge in Hong Kong, namely, disposal at sea and landfill disposal. Changing legislation and increasing environmental pressure will lead to the phasing out of sea disposal of sludge in the near future. Disposing sewage sludge at landfill sites requires wet sludge of certain solid contents in view of filling and engineering considerations. This is usually achieved by dewatering and drying which require an extensive energy input. The disposal of large quantities of sludge at landfills can also expedite the problem of 705

C.S. Poon and M. Boost

706

Table 1. Typical constituents of PFA (Source: China Light & Power Co. Ltd.). Normal range

Constituents

SiO 2

38 -77

54%

Alumina

A’,&

14-46

29%

Iron oxide

Fe203

1.2 - 18

Silica

5%

Calcium oxide

CaO

~0.1 - 16

5.1%

Magnesium oxide

MgO

~0.1 - 2.6

1%

N%O & K,O

~0.1 - 2.5

1.3%

~0.1 - 0.8

0.6%

Alkalis Sulphuric anhydride

so3

CI

Chloride Loss on ignition

(LOI)

Table 2. Mix proportions. Proportion (g/kg) Sample reference

Typical value

Sludge

PFA

CaO

A

590

369

41

B

590

364

46

C

590

359

51

D

590

351

59

E

590

342

68

F

590

410

0

G

590

0

410

ization reactions are believed to take place through the increase of pH in sludge to over 12 upon the addition of lime. But by itself, lime cannot induce a pH over 12 as pebble or granular lime particles are poorly soluble in water. It has been demonstrated by Tabasaran (1979; 1980) and Thormann (198 1) that the use of quicklime (anhydrous lime) could overcome the above problem. They further demonstrated that the exothermic hydration reaction of anhydrous lime with water induced a temperature rise in sludge to about 70°C within 10 min of mixing. This increase in temperature helped to render microbial populations innocuous and cause a rise in solid contents of 30-40% within 30 min. Sludge treated in this way has been shown to have better character-


5%

istics, particularly with respect to its disposal, than mechanically dewatered sludge. Logan and Burnham (199 1) and Hampton (1991) used a slightly modified process by adding cement kiln dust to the quicklime/sludge mixture. The end product, after certain procedures of curing and windrowing, was shown to be able to be used as an agricultural product. Modification was also done by Westphal and Christensen (1983) who added lime slurry to ironconditioned sludge prior to vacuum filtration and added dry lime to dewatered conditioned sludge cake. This study aimed to investigate the effect of modification of the process by the use of a mixture of quicklime and PFA as the stabilizing and pasteurizing agent. MATERIALS AND METHOD Materials

The sewage sludge was obtained from the Tai PO sewage treatment work in which primary and activated sludge were digested anaerobically followed by a dewatering process by a belt filter. The dry solid content of the sludge was determined and was in the range from 12.9%-14.1% with an average of 13.7%. Digested sludge prior to dewatering with a dry solid content of 3% was also used in coliform determination. The PFA collected from a local power station was used. The chemical composition of the ash is influenced by coal properties and burning conditions. A typical chemical composition of PFA produced in Hong Kong is shown in Table 1. Anhydrous calcium oxide (quicklime) was

Stabilization of sewage sludge by pulverized fuel ash

707

Table 3. pH values of

mixes.

PH Week after mixing

Sample 0

Raw

Preparation

3

4

8

7

7

7

7

12.5

12.5

12.5

12.5

12.5

B

12.5

12.5

12.5

12.5

12.5

C

12.5

12.5

12.5

12.5

12.5

D

12.5

12.5

12.5

12.5

12.5

E

12.5

12.5

12.5

12.5

12.5

G

by dehydrating

2

A

F

prepared lime.

1

industrially

9.5 13

9.5 13

available slaked

of mixed samples

The mixed proportions of the samples prepared for the investigation is shown in Table 2. The samples were prepared by manually mixing the sewage sludge, PFA, and lime until a homogenous mass was obtained. The mixed samples were then placed in containers without lids for curing inside the laboratory. The temperature and humidity of the laboratory were controlled at 20°C and 60-70%, respectively. Test methods The dry solid contents of the mixed samples were determined according to the Standard Methods (APHA 1993). The pH values of the solid and semi-solid samples were determined by moistened pH papers scaled I- 14. The temperature profiles of the samples were recorded by a thermocouple and a data-logger. The determination of metals was carried out in accordance with the Standard Methods by atomic absorption spectroscopy. Solid samples were first digested by concentrated nitric acid using the microwave heating method. The United States Environmental Protection Agency’s (USEPA) Toxicity Characteristics Leaching Procedure (TCLP) was used to assess the leaching of heavy metals from the raw and stabilized sludge. The sludge samples were extracted with an amount of extraction fluid (buf-

9.5

9.5

9.5

12.5

12.5

12.5

fered acetic acids at pH 2.8) equal to 20 times the weight of the samples in a rotary extractor device for 18 h. The mixture after extraction was then filtered, the pH and the heavy metal contents of the filtrate measured. Multiple extraction, in which fresh portions of the extraction fluid was used to extract the leached samples, was carried out to assess the change of metal leaching upon subsequent extractions. A total of five extractions were carried out. Total and faecal coliform counts were performed in accordance with the Standard Methods’ (APHA 1993) most probable number method. A wider variety of concentration of PFA and CaO were used in these trials to investigate the effects on the bacterial count. The coliform count was determined 24 h and 1 week after treatment. The pH was determined at both sampling times as a pH of 11 .O or more, shown to be necessary to prevent bacterial regrowth (Courts and Shukrow 1994; Farrell et al. 1974; Pedersen 1983). RESULTS The initial solid content of the sludge was about 140 g/kg. When the sludge was mixed with the various proportions of PFA and lime, the solid contents of the products increased to over 500 g/kg. High temperature (about 95°C) could only be recorded initially for Sample G, but the temperature decreased gradually to the ambient value after 6-8 h. No significant temperature increase (maximum up to 35 “C) was recorded for other mixes.

C.S. Poon and M. Boost

708

Table 4. Heavy metal contents of constitutents. Metal analysis Metal (concentration Sample

in mg/kg of drv solid)

Cd

Cr

Cu

Pb

Ni

Zn

3.0

74.0

879.0

114.0

72.0

1130.0

PFA

co.1

25.0

27.0

22.3

23.0

20.0

Lime

2.0

9.0

17.0

5.0

4.0

8.0

Raw sludge

Table 5. Results of TCLP tests. Cu(mg/ka;) +

Sample

pH

Zn(mg/L) *

Zn(mg/kg) +

Cu(mg/L) *

Raw sludge

3.5

3.74

519.4

0.15

20.8

A

9.5

1.16

24.3

0.46

9.6

B

10

0.85

32.1

0.66

24.9

C

11.5

0.54

15.9

1.35

39.8

D

11.5

0.48

16.4

1.29

44.0

E

11.5

0.29

10.6

2.09

76.7

F

8.5

3.19

83.8

0.72

18.9

G

12.5

0.07

1.9

1.14

30.7

* mg of metal per L of leachate. + mg of metal per kg of dry solid of mix. Table 6. Percentages of metal leached after multiple extraction. Total % of leaching after multiple extraction Zn

Cu

Raw sludge

94.2

20.5

C

24.2

10.7

Sample

The pH profile of the samples as monitored by moistened pH papers are shown in Table 3. All samples which contained lime and PFA have pH values >12. Sample F which contained PFA only had lower pH values. The total metal contents of the digested sewage sludge, PFA, and lime are given in Table 4. The raw sludge contained significant amounts of Zn and Cu. Both the PFA and lime used contained trace amounts of Zn and Cu.

The pH values of the leachate and the leachable heavy metal concentrations of the samples after TCLP are given in Table 5. Based on the measured metal concentrations, the leachable metal in terms of mg/Kg dry solid were calculated and are presented in Table 5. The percentages of metal leached from the raw sludge and Sample C after the multiple TCLP extraction test are given in Table 6. Nearly 95% of total Zn was extracted from the raw sludge while the stabilized sludge leached about 25% of its Zn content. For Cu, the respective values are 20% and 11%. The results of the total and faecal coliform determination are tabulated in Table 7. DISCUSSION

The addition of PFA and lime into the dewatered sludge cake (14% dry solid) immediately increased the solid contents. The increase in solid contents is not only the result of adding dry substances, but is also due to the

Stabilization of sewage sludge by pulverized fuel ash

709

Table 7. Total and faecal coliform enumeration.

Specimen

Digested

Sludge

WA

CaO

pH 24 h

1 LZek

MPN (coliforms) 24 h

MPN (E. coli) 24 h

MPN (coliform)

MPN (E. coli)

1 week

1 week

> 18 000

> 18 000

> 18 000

1

1000

0

0

9

8

2

590

102

308

13

13

3

590

154

257

13

13

4

590

200

200

13

13

5

590

257

154

13

12.5

80

80

6

590

300

102

13

12.5

60

80

7

590

360

50

13

12.5

120

150

8

590

410

0

9

8

> 18 000

5500

>I8000

> 18 000

Raw

9

100

0

0

8

7

> 18000

> 18000

>18000

> 18 000

(cake)

10

590

360

50

13

12

11

590

310

100

13

12

590

205

205

13

770

155

75

sludge

1300

2200

12.5

40

130

13

13

20

12

10.5

water sorption capacities of PFA and lime and the exothermic reaction between anhydrous calcium oxide and water which resulted in elevated temperatures to dry off some of the water. The increase in solid contents would greatly improve the handling characteristics of the sludge with respect to transport and land application. However, there is a drawback that the addition of foreign materials would also increase the final amount of the materials that need handling. Although the relationship between volume and weight increases has not been quantified, it has been observed that the corresponding increase in volume is only about a fifth of the increase in weight, mainly due to the high sorption capacities of PFA. The temperature logging showed that the mixing of quicklime with sewage sludge produced heated mass. A higher lime content resulted in a higher recorded temperature. This was due to the exothermic reaction of anhydrous lime with water. The increase in temperature could not be sustained for a long period even when pure lime was used as the additive. Mixing PFA with the sewage sludge alone did not produce the necessary exothermic reactions for temperature elevation. The pH value of sewage sludge is almost neutral and the recorded pH value was about 7. On the other hand,

5400

> 18 000

3500

the PFA used in this study contained sufficient alkaline materials to render the pH of its slurry to be about 12. But the alkaline contained in the PFA alone could not produce a highly alkaline environment for the stabilization of sewage sludge as shown by Sample F. When lime was added, all the mixes became highly alkaline, showing pH values of > 12 for a long period of time after mixing. The lowest lime dosage was in Sample A with about 4%. The sustained high pH values would show that the sludge/PFA/lime mixture would be able to comply with the USEPA’s Process that Significantly Reduce Pathogen (PSRP) criteria which specify that stabilization of sludge could be achieved by a pH of 12 for more than 72 h. The measured metal concentrations of sludge, PFA, and lime showed clearly that the major contribution of heavy metal in the system was the sludge. By adding PFA and lime, the heavy metal concentrations were diluted. The leaching test results showed that the stabilized sludge had a much lower leaching potential than the raw sludge. The ability of the stabilized sludge to withstand leaching is further demonstrated by the results of the multiple extraction test. Using the total and faecal coliform as indicators, the results showed clearly that the addition of PFA and lime

710

will significantly reduce the number of pathogenic bacteria in sewage sludge. According to the USEPA’s PSRP, the addition of PFA and lime can be regarded as a PSRP as there was a ‘2 log’ reduction of faecal coliform after stabilization for all combinations containing both PFA and lime at above 50 g/kg and 600 g/kg sludge. The pasteurization action is believed to be due to a combination of stresses, including high pH and accelerated drying. Due to the timing of the study, the effect of the stabilization agents on bacterial enteric pathogens, enteric viruses, and helminth ova have not been studied. Further research is being conducted to quantify this aspect. The results suggest that the use of PFA in conjunction with lime is sufficient to comply to the USEPA’s criteria for PSRP in addition to producing a final product with improved handling characteristics than that produced by lime stabilization alone. The process reduced leaching potential of the raw sludge. REFERENCES APHA (American Public Health Association). Standard methods for examination of water and wastewater. 17th edition. New York, NY: APHA; 1993. Courts, CA.; Shuckrow, A.S. Lime stabilised sludge: Its stability and effect on agricultural land. Nat. Environ. Res. Center U.S. EPA, Cincinnati, OH; 1974.

C.S. Poon and M. Boost

Farrell, J.B.; Smith, J.; Hathaway, S.; Dean, R. Lime stabilisation of primary sludges. J. Water Pollut. Control Fed. 46: 113-122; 1974. Hampton, N. The Simon N-viro sludge to soil sludge pasteurisation process. In: Proc. Polmet 1991, pollution in the metropolitan and rrban environment, Hong Kong. Vol. 2.; 1991: 259-274. Hong Kong: The Hong Kong Institution of Engineers. Logan, T.J.; Bumham, J. N-Viro soil: Advance alkaline sludge stabilisation. In: Proc. Polmet 1991, pollution in the metropolitan and urban environment, Hong Kong. Vol. 2.; 1991: 223-240. Hong Kong: The Hong Kong Institution of Engineers. Pedersen, D.C. Effectiveness of sludge treatment processes in reducing levels of bacteria, viruses and parasites. In: Wallis, P.M.; Lehmann, D.L., eds. Biological health risks of sludge disposal to land in cold climates. Calgary, Canada: University of Calgary Press; 1983: 9-3 1. Tabasran, 0. Stabilisation of raw and digested sludge by means of quicklime. Osterreichische Abwasser-Rundschau 24: 1 l-16; 1979. Tabasaran, 0. Stabilisation of primary and digested sludges with quicklime. Ingegneria Ambientale 9: 304-3 10; 1980. Thormann, A. Treatment of sewage sludge with quicklime-requirement and possibilities. Characterisation, treatment and use of sewage sludge. In: Proc. 2nd European symposium, Vienna 1980. CEC, Brussels; 1981: 140-148. USEPA (United States Environmental Protection Agency, Centre for Environmental Research Information). Process design manual, sludge treatment and disposal. EPA 625/l-79-01 1. Washington, D.C.: EPA; 1979. Westphal, P.A.; Christensen, G.L. Lime stabilisation: Effectiveness of two process modifications. J. Water Pollut. Control Fed. 55: 1381-1386; 1983.