UNIVERSITI PUTRA MALAYSIA

KINETIC STUDY OF AN AEROBIC DIGESTION WITH BIOMASS RETENTION B Y

ULTRAFILTRATION MEMBRANES

KHOR OOI HONG

FK 1997 10

KINETIC S TUDY OF ANAEROBIC DIGES TION WITH BIOMA S S RETENTION BY

UL TRAFIL TRA TION ME,MBR ANES

By

KHOR 001 HONG

Thesis Submitted in Partial Fulfilment of the Requirements for the D egree of Master of S cience

in the Faculty of Engineering, Universiti Putra Malaysia

April 1997

TO ALL "\II' TEACHERS

ACKNO'VLEDGEMENTS

First and foremost, I would like to thank my chairman, Dr.

Fakhru'l-Razi Ahmadun for constantly guiding and encouraging me

during the many crisis in this study. Also, I wish to thank members of

my supervisory committee, En. Ahmad Jusoh and En. !>.1egat Johari

!>.1egat Mohd. Noor, for their time and energy spent in making this a

better work.

My appreciations to En. Ismail Ghani of the Env. Eng. Lab for his

assistance and technical support throughout this study. I am grateful to

the staff of the Engineering Faculty , who have helped me a lot in one

way or another, and to the staff of GSa for their cheerfulness and

professionalism in handling their work.

Special thanks to Boon Yan, Abdullah and Aloysius for their kind

assistance . To my lab mates , I hope they don't break the last beaker.

Last but not least , I like to thank Mom, Dad, Ping, and Thai for

their love, understanding and steadfast support. May all be ings b e well

and happy. May they realise the wisdom and happiness that they seek.

111

TABLE O F CONTENTS Page

ACKNOWLEDGE!\1ENT S . . . . . . . . ...................... ................ ...... III LIS T OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vu LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . ix LIS T OF PLATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi LIS T OF ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xu AB S TRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . XUl AB S TRAK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv

CHAPTER

I

II

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

LITERA TURE REVIEW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

!\,1icrobiology of Anaerobic Digestion . . . . . . . . . . . . . . . . . . . . . .

Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Acidogenesis ..... . . . . . . . . . . . ... . . . . . . . . . . . . . ... . . . . . . . . . . .

Acetogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

M ethanogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Process Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conventional Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 5 6 6 7 9 1 2 1 4

Anaerobic Contact Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 5 Anaerobic Filter Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6

Kinetics and Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 7 Microbial Growth Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 8 Microbial Growth Rate and S ubstrate Utilisation Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 9 Effect of Substrate Concentration on Microbial Growth Rate . . . .. . . . . . . . . . . . . ... . . . . . . . . . . . . 20 Effect of Substrate Concentration on S ub strate Utilisation Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Steady S tates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Substrate Concentration and Solids Retention Time Minimum Substrate Concentration !\1icroorganism Concentration in Continuous Culture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

IV

23 23

24 2 5

2 6

[II

IV

Digestion Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Retention Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

MIxing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Membrane Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Pressure Driven Membrane Processes . . . . . . . . . . . . .

Concentration Polarisation and Fouling . . . . . . . . . .

Cleaning and 11embrane Lifetimes . . . . . . . . . . . . . . . . .

Membrane Anaerobic System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Treatment Technology and Cost for P011E . . . . . . . . . . . .

MATERIALS AND METHODS Experimental Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Crossflow Ultrafiltration Membrane (CUF) Unit Anaerobic Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Centrifugal Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

r..1embrane Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Feed Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .

Laboratory Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Feed, Reactor C ontent and P ermeate . . . . . . . . . . . . . Frequency of Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gas Measurement . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .

Start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Steady S tates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

First Attempt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Second Attempt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Problems Encountered During Steady States . . . . . . . . . . . .

Presence of S olids in Reactor . . . . . . . . . . . . . . . . . . . . . . .

RESULTS AND DISCUS SION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overall Performance of MAS Microbial Performanc e and Kinetics . . . . . . . . . . . . . . . . . . . . . .

Effect of Loading Rates and Retention Times on Utilisation Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Microbial Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Kinetic 110delling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Comparison with Existing Kinetic Constants . . . .

Estimation of S teady S tate Microorganism Concentration in Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Minimum S olids Retention T ime and Substrate Utilisation Efficiency

v

Page 27 27 2 8 29 30 3 1 3 2 3 4 3 5 36 40

42 42 43 -1-4 44 -1- 5 4 5 -1-7 -1-7 -1-7 48 5 1 5 2 5 2 5 3 5 4 5 5

5 9 5 9 6 1

62 66 68 73

77

8 2

Page Biogas C omposition and Production . . . . . . . . . . . . . . . . . . . . . . . 86

v

Effect of Cleaning and Biomass C oncentration on Permeate Flux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Comparison of Performance of M embrane Biological Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Comparison of Performance of POME-fed Anaerobic Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CONCLUSIONS Recommendation

REFERENCES

APPENDICE S A

B - 1 B - 2

Experimental R a w Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Reactor Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Example of Simulation Results . . . . . . . . . . . . . . . . . . . . .

88

93

9 5

9 7 1 00

1 02

1 08 III 1 1 2

BIOGR APHIC AL S KETCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 3

vi

Table

1

2

3

4

5

LIST O F TABLES

Geographic al Distribution of Full- and Pilot-scale Biogas Plants in the European Community and in Switzerland According to the Type of Waste Treated

Some Typical Operating Parameters of the Contact and Anaerobic Filter . . . . . : . . . . . . . . . .. . . . . . . . . . . . . . . . . . .

Kinetic 1\10dels Used in Anaerobic Treatment

1\1athematical Expressions of Specific S ubstrate Utilisation Rates for S everal Known Kinetic Models

Biomass Reactor Classification

6 Pressure-Driven 1\1embrane Processes and

Page

3

1 7

22

23

28

Membrane Characterictics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

7

8

9

1 0

Apparent Dimensions of Various P articles . . . . . . . . . . . . . . . . .

Application of Membrane Biological Reactors in Treating Several Type of Wastewater . . . . . . . . . . . . . . . . . . . .

Cost for Three 1\10st Common Treatment System for P 01\JE in J'vlalaysia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

S chedule and lvlethod of Ivfembrane Cleaning

1 1 Laboratory Analysis on Feed, Permeate and

3 3

3 8

4 1

4 5

Reactor Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... .. . 47

1 2 Designated Organic Loading Rates (OLR) and "lastage at Various Steady S tates . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3

1 3 Summary of Problems and Rectification . . . . . . . . . . . . . . . . . . . . 5 4

1 4 S ummary of Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Vll

Table

1 5 Results of the Application of Three Known

Page

Substrate Utilisation r-.10de1s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

16 Representative Values of Kinetic Constants for Anaerobic Digestion at 3 5 °C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

1 7 Summary of Kinetic Constants for \' arious Substrates Utilised in 1\1esophilic Anaerobic Treatment Processes as Compared to MAS Kinetic Constants for this Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 5

1 8 Kinetic Constants for S everal Mesophilic Anaerobic System Treating POr-.1E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

1 9 Characteristics of Three Known Models in Predicting SR T nun' Substrate Utilisation Efficiency, E and Effluent Substrate Concentration, S . . . . . . . . . . . . . . . . . . . . 8 3

20

2 1

2 2

23

Comparison of o'&CH4 in Biogas Produced from Several Anaerobic Digesters Treating POME

Comparison of Performance of S everal Membrane Biological Rea ctors . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Analytical Data for r-.lAS during S S l - S S6

Gas Composition and Production during S S l - S S6

Permeate Fio\vrate and Flux Data during S S 1 - S S6

V1l1

8 7

94

1 0 8

1 09

1 1 0

LIST OF FIGURES

Figure Page

1 Methanogenic Reactions 1 0

2 The Main �ficrobial Reactions in Anaerobic Digesters 1 1

3 S chematic Diagram of Three Main Types of Anaerobic Treatment Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3

4 Schematic Diagram of Three Existing V ariations of Membrane Bioreactor System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 9

5 Experimental S et-up 43

6 J-tube Gas Analyser 49

7 Observed S and E vs S R T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

8 S UR vs OLR 64

9 BLR, S S UR vs OLR 6 5

1 0 1 'S R T vs [r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

1 1 Monod �fodel : 1 L' vs 1 / S 69

1 2 C ontois �fodel : 1 / U vs XIS 70

13 Chen and Hashimoto Model : 1/U vs SiS . . . . . . . . . . . . . . . . . . . . 7 1

1 4

1 5

1 6

1 7

Observed X vs Predicted X

Monod Model: SRT, S and E

Contois :Model : S RT, S and E

Chen and Hashimoto Model : SRT, S and E

IX

7 8

7 9

8 0

8 1

Figure

1 8

1 9

%CH4, Gas and 11ethane Yield vs OLR

%CH4, Gas and Methane Yield vs SRT

Page

8 4

8 5

20 Permeate Flux and 11LVS S over Cumulative Day . . . . . . . . . 8 9

2 1

2 2

Specification of Reactor

Contois 110del : 11 U YS XI S ( Include S S4)

x

1 1 1

1 1 2

LI ST OF PLATES

Plates

1 Feeder, Gas Collector and Anaerobic Reactor of

Page

l\,1embrane Anaerobic System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6

2 CUF Unit of l\1embrane Anaerobic System 5 7

3 Reactor's Dome with Gas Hose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 8

-l Blockage is Suspected B etween the Conical S ection and the T -junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 8

5 Unused PCI Membrane Showing Pores with Size Ranging from 0 . 5 .urn to 0 . 0 5.um . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

6 Surface of Unwashed l\1:embrane ( not soaked in NaOH and HCI) is Overgrown with Fibrous, Net-like Gro\vths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

7 Surface of Washed r-.1embrane ( soaked in NaOH for 3 0 minutes and Hel in 2-1- hours) Reveal Larger Pores. The Smaller Pores are Still l\Iostly Covered by Organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Xl

BLR

COD

C STR

CUF

CUI\IAR

FFB

HRT

I\L-\S

I\IF

I\ IWC O

OLR

POME

PVC

RO

SEM

SRT

S S

SSUR

SUR

T S S

UF

VFA

\IS S

LI ST OF ABBREVIATIONS

Biological Loading Rate

Chemical Oxygen Demand

Completely I\1ixed, S tirred Tank Reactor

Crossflow Ultrafiltration Membrane

Crossflow Ultrafiltration I\,lembrane Anaerobic Reactor

Fresh Fruit B unch

Hydraulic Retention Time

I\1embrane Anaerobic System

lvIicrof iltration

Molecular \Veight Cut-off

Organic Loading Rate

Palm Oil Mill Effluent

Polyvynilchloride

Reverse Osmosis

S canning Electron Microscope

Solids Retention Time

Steady State

Specific Substrate Utilisation Rate

S ubstrate Utilisation Rate

Total S uspended S olids

Ultrafiltration

Volatile Fatty Acid

Volatile Suspended Solids

xu

Abs tract of thesis sub mitte d to the S enate of Universiti Putra Malaysia in partial fulfilment of the requirements for the degree of Master of S cience.

KINETIC STUDY OF ANAEROBIC DIGE STION WITH BIOMASS RETENTION BY ULTRFILTRATION MEMBRANES

By

KHOR 001 HONG

MARCH 1 997

Chairman : Dr. Fakhru'l-Razi Ahmadun

Faculty : Engineering

In this study, a 50 litre laboratory-scaled membrane anaerobic system

(1\1AS ) combining ultrafiltration (UF) membrane with anaerobic reactor

was used to treat palm oil mill effluent (POME) at ambient temperature .

S ix steady states were attained as part of a kinetic study. The results of

steady state 4 ( S S 4) was adversely affected by a long shutdown due to

pump leakage . The results of the five remaining steady states were

successfully fitted, above 96%, by Monod, Contois, and Chen and

Hashimoto models . Contois 1\t Iodel appeared to be the best at 99. 7�iO.

The microbial kinetic constants are Y = 0 . 8 3 gVS S/gCOD and b = 0 . 1 5

day·I. Minimum solids retention time, e cmin obtained from the three

simulation models range from 6-1 4 . 3 days . Maximum total gas yield was

measured at 0 . 6 2 1 litre/g COD at an organic loading rate ( OLR) of 5 . 0

kgCOD/m3/d. %CH4 composition decreases from 75 . 7% at OLR of 1 . 8

XUl

kgCOD/m3/d, to 62 . 3 % at OLR of 6 . 0 kgCOD/m3/d. The percentages of

COD removal were achieved between 99.0%-88.9% over a range of

mixed liquor suspended solids of 1 0, 033-22, 1 75 mg/I. The final

hydraulic and solids retention time, e and e have been reduced to 8.3 _ c

days and 1 2 . 5 days, respectively during S S6 . Under scanning electron

microscope ( S EM), the effective pores of the membrane was found to be

pores larger than 0 . 1 f-Lm. Layers of fibrous growth on the membrane

surface increase separation efficiency. 1\10re efficient and frequent

cleaning is required to inhibit membrane fouling and increas e p ermeate

flux. Overall, this study indicated that 1\1A S is capable of treating higher

OLR when e. is maintained above 20 days .

XIV

Abstrak tesis yang dikemukakan kepada S enat Universiti Putra Malaysia sebagai memenuhi sebahagian daripada syarat untuk Ijazah 1>.1a ster S ains .

KAJIAN KINETIK KE ATAS PENC ERNAAN ANAEROB I K D ENGAN P ENAHANAN BIOMAS OLEH MEMBRAN

ULTRATURASAN

Oleh

KHOR 001 HONG

MAC 1997

Pengerusi : Dr. Fakhru'l-Razi Ahmadun

Fakulti : Kejuruteraan

Dalam kajian Int, suatu sistem anaerobik membran (MA S )

berskala-makmal salZ 5 0 liter, yang menggabungkan membran

ultraturasan (UF) dengan reaktor anaerobik, telah digunakan untuk

merawat efluen kilang kelapa sawit (P01>.'fE) pada suhu persekitaran.

Enam tahap tetap (steady s tate) telah dicapai sebagai sebahagian

daripada kajian kinetik . Keputusan tahap tetap 4 ( S S 4 ) telah mengalami

gangguan, kesan daripada p emberhentian lama setelah pam mengalami

kebocoran. Keputusan tahap tetap yang selebihnya berjaya digunakan

untuk mendapat fit pada 96% ke atas, bagi model-model Monod,

Contois, s erta Chen dan Hashimoto. l\.1odel Contois didapati p aling baik

dengan 99.7%. Koefisien kinetik mikrobial yang didapati a dalah Y = 0 . 8 3

gVS S/gCOD dan b = 0.15 hari"l. Masa tahanan pepejal minimum, ecmin

yang didapati daripada simulasi ketiga-tiga model mempunyai julat an tara

xv

6- 1 4.3 hari . lumlah biogas maksimum ialah 0 . 6 2 1 liter/hari b agi kadar

beban organik ( OLR) 5 . 0 kgCOD/m3/hari . %CH4 berkurang daripada

7 5 . 7% pada OLR 1. 8 kgCOD/m3/hari, kepada 62 .3% p ada OLR 6 . 0

kgCOD/m3/hari. P eratusan penyingkiran COD telah dicapai di antara

99. 0%- 8 8 . 9% untuk pepej al terampai larutan campuran (ML S S ) antara

1 0, 033-22, 1 7 5 mg/I. Masa tahanan hidraulik dan p epejal, e and ec telah

dikurangkan kepada 8 . 3 hari dan 1 2 . 5 hari, masing-masing pada S S6 . Di

b awah mikroskop elektron scanning ( SEM), liang membran yang

berkesan adalah liang b es ar b erukuran lebih daripada 0 . 1 )..lm . Lapisan

tumbesaran berfiber di atas permukaan membran meningkatkan

kecekapan pemisahan. Pembersihan membran y ang lebih cekap and kerap

diperlukan untuk menghalang membran daripada tersumbat (fouling)

serta meningkatkan kadar alir permeate. S ecara keseluruhan, kajian ini

menunjukkan b ahawa MAS mampu untuk merawat OLR yang lebih

tinggi dengan pengekalan e c lebih daripada 20 hari.

XVI

CHAPTER I

INTRODUCTION

Anaerobic digestion IS a naturally occurnng microbiological

process in the environment; best observed in swamps, and deep reaches

of s ediments in water and soil . The confin ement and optimisation of the

naturally occurring anaerobic digestion process leads to the pioneering

use of anaerobic digestion in treating human excreta in septic tanks .

Since then, anaerobic digestion has moved into other areas of waste

reduction, such as agriculture, farming and industry.

The 19 70s energy cnSlS revealed another role of anaerobic

digestion - that of providing methane gas as an alternative fuel . The

crisis stimulated world-wide research and development in anaerobic

digestion. In highly industrialised and populated countries in Europe,

considerable research efforts were spent in this field. The European

Commission, for example, predicted that 1 0% of Europe's energy needs

could be met by renewable energy and thus, invested £ 1 00M in research

and development at one tim e ( Hobson, 1 993) . However, with the drop in

oil price, there remained no immediate economic reason for alternative

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energies . Furthermore, the energy contribution from digesters were

below expectation. The total value of the fraction of biogas that was

effective as an energy source was much less than the amount spent on

research and development programmes (Coombs, 1 990) .

From then on, the continuing research on anaerobic digestion was

fuelled by growing awareness of pollution control. An early 1 9 80s

survey of biogas plants in Europe ( Table 1 ) found that mos t of the plants

were used to treat agricultural waste. Apart from that, over 8 0% of the

plants were in fact full-scale operating plants . The widespread attraction

of anaerobic digestion technology may be attributed to its ability to treat

concentrated waste with lower energy requirement .

In Malaysia, the heightened conSCIOusness that waste treatment is

necessary to avoid environmental pollution, was reinforced in the recent

7th Malaysia Plan (RM7) . Anaerobic digestion will have a bigger role to

play in treating large volumes of high to medium-range conc entrated

wastes. This is especially so if the agricultural development, which

focuses on l arge-scale production of food and high-value produce, goes

according to the RM7.

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Table 1

Geographical Distribution of Full- and P ilot-S c ale Biogas Plants in the European C ommunity and in Switzerland According to the Type of \:Vaste Treated.

Country Type of Waste

Agricultural Energy Domestic Industrial Total

Crops residues

(landfills)

Full- + Pilot-scale Full- + Full- + Full- +

pilot -scale pilot-scale pilot-scale pilot-scale

Belgium 21 + 4 6 + 4 27 + 8

Denmark 22 + 1 3+3 25 + 4

FRG 75 10 12 97

France 02 + 12 2+ 3 10 + 5 74 +20

Greece 3 + I 1 4+ I

Ireland 2 + 3 1 + 3 3 + 6

Italy 58 + 5 1 1 11 + 2 70 + 8

Netherlands 21 + 1 3 + 8 22 + 1 46 +10

UK 12 + 9 7 + 2 3 + 2 22 + 13

Switzerland 102 + 6 102 + 6

Total 378 +42 1 23 + 13 69 + 20 470 +76

(Ferranti, 1 9 8 7 )

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Likewise , there is continuous expansion m other sectors such as

the local manufacturing and industrial sectors, and solid and hazardous

waste disposal. At the same time, the industrial s ector will find more

stringent standards and imposition of fees for treated waste discharges

with the implementation of the RM7. This will in turn create the demand

for more efficient and better waste treatment systems .

Therefore, there are plenty of reasons for commg up with more

innovative and improved waste treatment facilities . In the design of

anaerobic digestion alone, there are many such variations . Among them,

there is a Membrane Anaerobic System (MAS ) that combines membrane

technology with anaerobic digestion ( Tan, 1 99 5; Fakhru'l-Razi, 1 994) .

The membrane serves to retain the slow-gro\ving active biomass in the

digester while allowing the production of high quality effluent.

Objectives

Therefore, it is the purpose of this study on the treatment of palm

oil mill effluent ( POJ\1E) u sing Membrane Anaerobic System (MAS) to : -

(i) evaluate the overall microbial kinetics, and

(ii) evaluate the applicability of three known kinetic models .

C HAPTER II

LITERATURE REVIEW

Detailed knowledge of microbiology is not necessary m order to

run an anaerobic digester. However, general knowledge of the

microbiology of digestion is important. It is necessary to find out which

part of the interdependent complex processes are limiting and therefore

require control, and improvement in operation or digester design.

Therefore, the following sections hope to bring forth that useful and

vital background knowledge needed in this study.

�1icrobiology of Anaerobi c Digestion

The microbial ecology of a digester consists of anaerobic bacteria

that stabilise organic matter in the absence of free oxygen. Although

there is a gradation in oxygen tolerance , most of the diges ter's bacteria

are among the least tolerant of oxygen. Therefore , any exposure to air or

oxygen will kill or inhibit these obligate anaerobes .

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Classifying by their functions, anaerobic digestion of organic

matter to methane involves the interaction of s everal groups of bacteria.

Hydrolysis

The hydrolytic bacteria excrete extracellular enzymes to convert

complex particulate matter into soluble compounds. In the digestion of

particulate or polymeric waste, hydrolysis i s often found to be the

rate-limiting process (Archer and Kirsop, 1 990; S leat and Mah, 1 98 7 ) .

Acidogenesis

Archer and Kirsop ( 1 990) chose to classify the acidogens under

the s ame group as hydrolytic bacteria . However, the acidogenic group

was separately mentioned in another study by Haandel and Lettinga

( 1 994) . In B oone and Mah ( 1 98 7) , acidogens were also known as

fermentative bacteria, and were classified together with acetogens as

transitional bacteria .

The manner of classification by different researchers only seek to

emphasise the complex interspecies activitie s among the digester's

anaerobes . However, they all agreed that the hydrolytic products were

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taken up in the cells of these fermentative bacteria and further converted

to simpler organic compounds, such as volatile fatty acids , and gaseous

compounds, such as CO2, and H2•

Acetogenesis

B ased on the classification by Archer and Kirsop ( 1 990), the

acetogenic bacteria \vere divided into obligate proton-reducing (hydrogen

producing) species and non-obligate proton-reducing species . B oth the

obligate and non-obligate proton-reducing acetogens produce the

methanogenic substrates, acetate, H2 and CO2, from the intermediate

compounds . The important distinction between these two types of

bacteria is the ability of the non-obligate proton-reducing bacteria to

grow unhampered in an environment of high Hz concentration.

In an environment with high H2 concentration, non-obligate

proton-reducing bacteria produce 2 main reduced fermentative products,

i . e butyrate and propionate . When the H" level is low enough, the main

product is acetate. This is accompanied by the release of H2 (which acts

as proton-reducer) . Under this non-obligate proton-reducing group,

homoacetogens were identified to be c apable of producing acetate from

H2 and CO2 under certain conditions ( Archer and Kirshop, 1 990) .

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On the contrary, the obligate proton-reducing acetogens oxidise

reduced fermentative products , such as butyrate and propionate, to form

acetate and grow only by producing H2" Therefore, they can only survive

in an environment where the Hz-utilising bacteria co-exist (interspecies

Hz transfer), to keep the Hz concentration at a low _level .

According to B oone and Mah ( 1 9 8 7) , obligate proton-reducing

acetogens were only one of the three groups of bacteria classified under

transitional bacteria_ The other two were fermentative bacteria

(acidogens) and homoacetogens_

The obligate proton-reducing acetogen-mediated oxidising

reactions of propionate and butyrate. to acetate and H, or CO._ as

proposed III literatures are presented below. According to Haande1 and

Lettinga ( 1 994) , most of the reactions m sewage treatment follow the

general equation 2. If we consider butyric acid or propionic acid as

substrates, where the H:O ratio is l arger than 2 (y > 2z), this would hold

true. S imilarly, B oone and �fah ( 1 9 8 7 ) proposed the oxidation of

propionate (Eq. 3 ) and butyrate (Eq. 4) release H2 in the production of

acetate .