Bioleaching of heavy metals from sludge

BIOLEACHING OF HEAVY METALS FROM SLUDGE

by

ASHISH PATHAK

Centre for Energy Studies

Submitted

In fulfillment of the requirements of the degree of Doctor of Philosophy

to the

Indian Institute of Technology Delhi

India

CERTIFICATE

This is to certify that the thesis entitled "Bioleaching of heavy metals from sludge" submitted by Mr. Ashish Pathak is worthy of consideration for the award of the degree of Doctor of Philosophy and is a record of the original bonafide research work carried out by him under our supervision and guidance. The results contained in this thesis have not been submitted in part or full to any other University or Institute for the award of any other degree.

Dr. M.G.Dastidar (Supervisor) Professor

Centre for Energy Studies

Indian Institute of Technology Delhi

Dr. T.R. Sreekrishnan (Co-Supervisor)

Professor

Department of Biochemical Engineering and Biotechnology

Indian Institute of Technology Delhi

ACKNOWLEDGEMENTS

I express my deepest sense of gratitude to my supervisor Prof. (Mrs.) M. G. Dastidar to offer me this great opportunity to conduct my Ph.D at prestigious Indian Institute of Technology Delhi.

Her expert guidance, untiring efforts and valuable suggestions were a great help throughout my research work. She had been a source of inspiration and without her cooperation this work would not have reached this stage.

I am equally grateful to my co-supervisor Prof. T. R. Sreekrishnan, Department of Biochemical Engineering and Biotechnology, for his valuable guidance, constant support and useful suggestions during my research work. I immensely appreciate his support and efforts for clarifying my thoughts throughout my research work.

Grateful thanks are due to Prof. S. C. Kaushik, the Head, CES, IIT Delhi for providing me the necessary facilities to carry out the research work.

Heartfelt thank to Dr. P. K. Roy Choudhury (Department of Biochemical Engineering and Biotechnology), Dr. K. Gadgil and Prof. R. P. Sharma (Centre for Energy Studies) for their valuable suggestions and help in conducting my research work.

Special thanks are given to my research colleagues Dr. (Mrs). Mousumi Sen, Mr. Kapil Kumar, Mr. G.H.V.Chary, Mrs. Reena Pundir, Ms. Alka Thapliyal, Mr. C.H. Biradar, Dr (Mrs), Chahya Singh and Ms. Shweta Puri for their kind help and cooperation throughout my research work.

There are no words that will suffice to thank my family members who always have been a moral and mental support, without which the work would not have been completed.

ABSTRACT

The present work is an attempt towards development of a suitable bioleaching process for removal of heavy metals from the sewage sludge using sulfur-oxidizing and iron-oxidizing microorganisms indigenous to the sludge. The bioleaching of heavy metals initially carried out in batch bioreactors using indigenous sulfur-oxidizing microorganisms under different experimental conditions indicated that 2 g/l is the optimum sulfur concentration for bringing down the pH sufficient for solubilization of metals. The batch studies also suggested that indigenous microorganisms were capable of growing within a wide range of initial pH (3-7) of the sludge leading to significant solubilization of metals. By using 2 g/l of sulfur, bioleaching of metals can be carried out from the sludges containing solids up to 20 g/l concentration and at temperature up to 50°C. Further, batch studies using indigenous iron-oxidizing microorganisms show that 10 g/l is the optimum concentration of ferrous sulfate to bring down the pH to the level needed for solubilization of metals. The iron-oxidizing microorganisms can also work within a wide range of initial pH and up to 20 g/l solids content of sludge. The batch experiments on speciation of metals in the raw sludge and in the bioleached sludge show that in the raw sludge (before bioleaching) Cu, Zn and Cr existed majorly in the oxidizable fraction. After bioleaching, a significant reduction in the above forms of metals was observed and most of the metals present in the bioleached sludge were in the stable fraction (residual fraction). The experiments conducted for recovery of metals from the leachate obtained after bioleaching using indigenous sulfur and iron-oxidizing bacteria suggest that almost complete recovery of all the metals (98- 100%) is possible by increasing the leachate pH up to 8. Studies conducted on simultaneous sewage sludge digestion and metal leaching (SSDML) under mesophilic condition indicated that

SSDML process was efficient in removal of 29% suspended solids (SS), 38% volatile suspended solids (VSS), 77% Cu, 85% Zn, 51% Ni and 57% Cr. Under thermophillic conditions, higher reduction in SS (56%) and VSS (58%) was observed. Based on the batch data obtained, further studies were conducted in continuous stirred tank reactor (CSTR) under mesophilic conditions at different hydraulic retention times (4-10 days). The results of the above study suggested that an HRT of 8 days is the optimum in which almost 58% Cu, 52% Ni, 72% Zn and 43% chromium were solubilized using sulfur-oxidizing microorganisms and about 51% Cu, 46% Ni, 62% Zn and 41% Cr were removed from the sludge using iron-oxidizing microorganisms. The results obtained by operating CSTR under thermophilic conditions using sulfur-oxidizing microorganisms suggest that an HRT of 8 days was found to be optimum where about 54%Cu, 49% Ni, 66% Zn and 44% Cr were solubilized. The solubilization efficiencies of metals achieved were comparable to what was achieved with CSTR operated under mesophilic conditions using indigenous sulfur-oxidizing microorganisms. An attempt was also made to operate the CSTR using immobilized sulfur. At an HRT of 10 days, about 57% Cu, 49% Ni, 68% Zn and 45% Cr were solubilized. The scanning electron microscopic images (SEM) also confirmed the attachment of Acidithiobacilli on the sulfur coated plates.

CONTENTS

Page No.

CERTIFICATE--- --- ----i

ACKNOWLEDGEMENT---ii

ABSTRACT---iii

LIST OF FIGURES ---ix

LIST OF TABLES---xv

ABBREVIATIONS---xvi

Chapter 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 INTRODUCTION AND OBJECTIVES Generation of sewage sludge--- 1

Characteristics of sewage sludge--- 2

Disposal of sewage sludge--- 5

Effects of sludge disposal on environment--- 7

Fixation and speciation of heavy metals in sewage sludge--- 8

Physicochemical techniques for metal removal from sludge--- 12

Bioleaching--- 14

Need of the present work--- 14

Objectives ---

Chapter 2 LITERATURE REVIEW 2.1 2.2 2.2.1 2.2.2 2.2.3 Bioleaching of heavy metals from sewage sludge-- Microorganisms--- Mesophiles--- Thermophiles--- ---- 17

---- 18

---- 22

--- 23

---- 24

2.3 Mechanisms 24

2.4 Process parameters--- 25

2.4.1 pH and ORP--- 25

2.4.2 Nutrients and substrates--- 27

2.4.3 Gases--- 28

2.4.4 Total solids--- 28

2.4.5 Temperature--- 29

2.4.6 Organic matter--- 30

2.5 Processes--- 31

2.5.1 Bioleaching in batch mode--- 31

2.5.2 Bioleaching integrated with sludge digestion--- 34

2.5.3 Bioleaching in continuous mode--- 36

2.5.4 Recovery of metals and economy of the process--- 40

2.6 Analytical techniques--- 42

2.6.1 Atomic absorption spectrophotometer--- 42

2.6.2 Scanning electron microscopy--- --- 42

Chapter 3 MATERIALS AND METHODS 3 .1 Materials--- 45

3.1.1 Sewage sludge--- 45

3.1.2 Microorganisms--- 45

3.1.3 Energy source--- 45

3.1.4 Chemicals--- 46

3 .2 Methods--- 46

3.2.1 Physicochemical characterization of sludges--- 46

3.2.2 Enrichment of microorganisms--- 46

3.2.3 Culture for acclimatized microorganisms--- 47

Powdered sulfur--- 48 3.2.5.2 Iron-oxidizing microorganisms using ferrous

sulfate and ammonium ferrous sulfate--- 49 3.2.5.3 Speciation of metals--- 51 3.2.5.4 Simultaneous sludge digestion and metal

bioleaching using sulfur-oxidizing microbes--- 52

Recovery of metals--- 52 3.2.6 Bioleaching in continuous stirred tank reactor--- 52 3.2.6.1 Sulfur-oxidizing microorganisms using

powdered sulfur--- 52 3.2.6.2 Iron-oxidizing microorganisms using ferrous sulfate - 55 3.2.6.3 Sulfur-oxidizing microorganisms using

immobilized sulfur--- 55 3.2.6.4 Sulfur-oxidizing microorganisms using powdered

sulfur under thermophilic condition--- 56 3.3 Assay techniques--- 57 3.3.1 pH and total solids--- 57 3.3.2 Total kjeldahl nitrogen, total phosphorus and organic matter- 57 3.3.3 Heavy metals analysis--- 58 3.3.4 Scanning electron microscopy--- 58

Chapter 4 RESULTS AND DISCUSSION

4.1 Physicochemical characterization of sludges--- 59 4.2 Bioleaching in batch reactors--- 67 4.2.1 Sulfur-oxidizing microorganisms using powdered sulfur--- 67 4.2.2 Iron-oxidizing microorganism using ferrous sulfate and

ammonium ferrous sulfate--- 102

4.2.3 Speciation of heavy metals--- 124

4.2.4 Comparative study of batch bioleaching using elemental

4.2.5 Simultaneous sludge digestion and metal leaching (SSDML)

using sulfur-oxidizing microorganisms--- 135

4.2.6 Recovery of metals--- 144

4.3 Bioleaching in continuous stirred tank rector--- 145

4.3.1 Sulfur-oxidizing microorganisms using powdered sulfur--- 145

4.3.2 Iron-oxidizing microorganisms using ferrous sulfate--- 153

4.3.3 Sulfur-oxidizing microorganisms (15 g/1 sludge solids)--- 159

4.3.4 Sulfur-oxidizing microorganisms using immobilized sulfur 166 4.3.5 Sulfur-oxidizing microorganisms using powdered sulfur under thermophilic conditions--- 174

4.3.6 Comparison of bioleaching of heavy metals in different modes of operation--- 179

Chapter 5 SUMMARY AND CONCLUSIONS---182

REFERENCES

---

APPENDICES

--