Charles Mpho Makgatha - SaniUP...To my family and friends back home; thanks for the many messages of...

EVALUATING A SUPERSATURATED AERATION SYSTEM FOR TREATMENT OF HIGHLY CONCENTRATED BLACKWATER IN

EMERGENCY SETTINGS

Charles Mpho Makgatha MSc Thesis MWI SE 2014-11

April 2014

EVALUATING A SUPERSATURATED AERATION SYSTEM

FOR TREATMENT OF HIGHLY CONCENTRATED

BLACKWATER IN EMERGENCY SETTINGS

Master of Science Thesis

by

Charles Mpho Makgatha

Supervisors Prof Damir Brdjanovic, PhD, MSc (UNESCO-IHE)

Mentors H.A. Garcia Hernandez, PhD, MSc (UNESCO-IHE)

C.M. Hooijmans, PhD, MSc (UNESCO-IHE)

P. Mawioo, MSc (UNESCO-IHE)

Examination committee

Prof D. Brdjanovic, PhD, MSc (UNESCO-IHE)

H.A. Garcia Hernandez, PhD, MSc (UNESCO-IHE)

C.M. Hooijmans, PhD, MSc (UNESCO-IHE)

M.A. Calzada Garzón, MSc

This research is done for the partial fulfilment of requirements for the Master of Science degree at the

UNESCO-IHE Institute for Water Education, Delft, the Netherlands

Delft

April 2014

©2014by Charles Mpho Makgatha. All rights reserved. No part of this publication or the information

contained herein may be reproduced, stored in a retrieval system, or transmitted in any form or by any

means, electronic, mechanical, by photocopying, recording or otherwise, without the prior permission of

the author. Although the author and UNESCO-IHE Institute for Water Education have made every effort to

ensure that the information in this thesis was correct at press time, the author and UNESCO-IHE do not

assume and hereby disclaim any liability to any party for any loss, damage, or disruption caused by errors

or omissions, whether such errors or omissions result from negligence, accident, or any other cause.

Cover photos source:

BlueInGreen SDOX® (http://blueingreen.com/sdox/ accessed 23 October 2013) and Syrian emergency

camp (http://www.foreignpolicy.com/articles/2014/01/07/the_power_of_mediation_syrian_refugee_crisis

accessed 02 April 2014).

http://blueingreen.com/sdox/http://www.foreignpolicy.com/articles/2014/01/07/the_power_of_mediation_syrian_refugee_crisis

vii

Abstract

Outbreak of disasters such as floods, earthquakes, tsunamis, and civil unrest can result in the destruction of

infrastructure and displacement of people to emergency camps. Water and sanitation are critical

determinants for survival of the affected individuals during emergencies. Often, the affected people suffer

from illness and even death; both strongly related to sanitation and water supplies. This makes wastewater

management one of the key areas that need attention during emergencies. The requirement to have proper

wastewater treatment cannot be over emphasised, more especially during emergencies where the volume of

faecal sludge and black water accumulates quickly. The membrane bioreactor (MBR) technology has

potential for application in the treatment of highly concentrated black water in emergencies.

MBRs are preferred due to the ability to operate at high mixed liquor suspended solids (MLSS); however

the oxygen transfer efficiency (OTE) of conventional fine bubble aerators significantly decreases in MLSS

concentration above 16 g/L. The Supersaturated Dissolved Oxygen (SDOX) delivery unit which uses

pressurised system to achieve higher OTEs is a potential replacement to the fine bubble aerators in MBR

systems. A recent study determined that the SDOX delivery unit can successfully dissolve oxygen in MLSS

of up to 34.2 g/L. The activated sludge used in the said study was in endogenous respiration. This study

evaluated the performance of the SDOX using activated sludge in exogenous respiration. The main aims of

this study were to determine optimal operational conditions of the SDOX unit, to test whether oxygen and

COD utilisation rates of biomass will increase with the increase in MLSS concentration under SDOX

aeration and also compare the SDOX unit with diffuse aerators. Additionally, the impact of the SDOX

operating pressure on biomass was assessed.

The optimisation of the SDOX unit involved determination of influent air pressure and process water flow

rates that resulted in minimal variation of water level inside the SDOX during operation and thus ensure a

constant supply of oxygen supersaturated water. Optimisation of the unit was achieved by trial and error.

Activated sludge samples were collected from the local treatment works, and aerated overnight to

endogenous respiration phase using diffuse aerators. The endogenous sludge samples were later aerated

with the SDOX unit during the experiment, synthetic blackwater was added into the reactor, oxygen and

COD utilisation rates were measured. The experiments were performed at sludge concentrations 4, 7, and

13 g/L. The experiment was then repeated at 4 g/L using diffuse aerators and performance between SDOX

unit and diffuse aerators was compared. The impact of the high operating pressure of the SDOX aeration

unit on biomass was assessed exposing activated sludge sample (7 g/L) to pressure in the unit for a

cumulative period of about 14 hours, repetitively dosing of synthetic blackwater, measuring oxygen and

COD utilisation rates.

The SDOX unit showed stable operation at influent air pressure of 58 – 65 psi, with the influent process

water flow rate ranging from 1585 mL/min to 1750 mL/min. It was also found that the oxygen utilisation

rate increased linearly with the increase in sludge concentration for sludge concentrations of up to 13 g/L

while aerated with the SDOX unit. The COD utilisation rate significantly increased with the increasing

sludge concentration. The SDOX unit exhibited a superior performance (based on oxygen and COD

utilisation rates) over ordinary diffuse aerator. The high pressure environment of the SDOX unit

temporarily affected the biomass; biomass undergo a ‘shock effect’ resulting in lower and irregular

microbial activities (based on oxygen and COD utilisation) during the initial period of operation. However,

the biomass later becomes acclimatised to the high pressure environment at which the unit operates.

Another observation was that biomass can withstand the high pressure (75 psi) in the SDOX unit for

cumulative duration of up to 14 hours. This undoubtedly illustrates the prospects of the SDOX technology

as the future aeration device for MBR application. Further studies to determine the performance of the

SDOX unit at MLSS concentration >13g/L are recommended.

Keywords: Supersaturated dissolved oxygen (SDOX) delivery unit, Membrane aeration, Emergency

sanitation, Blackwater treatment.

viii

Acknowledgements

I would like to extend my sincere word of gratitude to the Bill & Melinda Gates Foundation,

Netherlands University Foundation for International Cooperation (NUFFIC) and Tshwane

University of Technology for granting me the opportunity to pursue my MSc studies.

Special thanks to Prof Brdjanovic and all the lecturers under the Environmental Engineering

& Water Technology department for unselfishly sharing your valuable expertise.

To my mentors; Dr. Hector Garcia Hernandez, Dr. Tineke Hooijmans and Peter Mawioo; I

could not have asked for more. Your support and guidance throughout my research work was

beyond measure.

Ferdi, Berend, Peter and Lyzette your commitment and assistance in the laboratory has not

gone unnoticed; much appreciated.

The one and half year duration of study would not have been as easy without the warm

friendship from my fellow colleagues in the Sanitary Engineering specialisation, the

Tanzanian community in Delft and friends; Dennis, Kyomukama, Sebastiaan and Jana.

Thanks for the memorable time we shared.

Much appreciation to my fellow South African sisters with whom I have undertaken this

study journey; Fezeka and Happiness, it’s been great to have you guys.

To my family and friends back home; thanks for the many messages of encouragement and

the support throughout. To my mother; thanks for the love and prayers.

Special thanks to you Edith; your love, patience and support are immeasurable. Tshegofatso

and Tebogo; this work is dedicated to you.

Lastly and most importantly; I am grateful to the Almighty for the gift of life and wisdom to

produce this work.

ix

Table of Contents

Abstract vii

Acknowledgements viii

List of Figures xii

List of Tables xv

Abbreviations xvi

1. INTRODUCTION 1 1.1. Background of area of study 1 1.3. SDOX-MBR System 3 1.4. Problem Statement 4 1.5. General Objective 5 1.6. Specific Objectives 5

2. LITERATURE REVIEW 6 2.1. Emergency Sanitation 6 2.2. Characteristics of Blackwater 8 2.3. Oxygen Transfer Phenomenon 9

2.3.1. Oxygen transfer mechanism 9 2.3.2. Factors affecting oxygen transfer 11 2.3.2.1 Wastewater Characteristics 11 2.3.2.2 Mixing and aeration devices 11 2.3.2.3 Temperature 11 2.3.2.4 Mixed Liquor Suspended Solids and Sludge Retention Time 12

2.4. Aerobic Utilisation of COD 14 2.4.1. Organic substrate and aerobic bio-processes 14 2.4.2. COD utilisation kinetics 17

2.5. Oxygen Utilisation Rate 18 2.5.1. Measurement of the oxygen utilisation rate 18 2.5.2. Types of oxygen utilisation rates 20

2.6. Supersaturated Dissolved Oxygen (SDOX) Delivery System 21 2.6.1. SDOX® Operating Principle 22 2.6.2. Performance of SDOX unit for MBRs Application 23 2.6.3. Advantages of the SDOX Delivery Unit 23

x

3. RESEARCH METHODOLOGY 24 3.1. Experimental design 24 3.2. Optimisation of the SDOX unit, and determination of aeration capacity of the SDOX

using different spray nozzles 26 3.2.1. Materials 26 3.2.2. Experimental Set-Up 26 3.2.3. Optimisation of the Modified SDOX Experimental Set-Up 29

3.3. Dissolved oxygen delivery test using the SDOX unit 31 3.4. Determination of the oxygen and COD utilisation rates in sludge samples using diffuse

aerators to create aerobic conditions 34 3.4.1. Materials 34 3.4.2. Sample collection, Preservation and Preparation 34 3.4.3. Synthetic blackwater/external COD 35 3.4.4. Experimental procedure 37

3.5. Determination of the oxygen and COD utilisation rates in MLSS samples using the SDOX unit to create aerobic conditions 38 3.5.1. Materials 38 3.5.2. Experimental procedure 38

3.6. Impact of the of the SDOX unit on biomass to uptake COD and oxygen 40 3.6.1. Experimental methodology A 40 3.6.2. Experimental methodology B 41 3.6.3. Analytical methods 43

4. RESULTS AND DISCUSSION 46 4.1. Optimisation of the modified SDOX unit, and determination of aeration capacity of the

unit using different spray nozzles 46 4.1.1. Optimisation of the modified SDOX unit 46 4.1.2. Summary of optimisation results 51 4.1.3. Determination of aeration capacity of the SDOX unit using different spray

nozzles 52 4.1.4. Recommended optimum conditions 56

4.2. Determination of the oxygen and COD utilisation rates in sludge using diffuse aerator to create aerobic conditions 57 4.2.1. Sludge concentration 4.07 g/L 57 4.2.2. Sludge concentration 3.89 g/L 61 4.2.3. Discussion 64

4.3. Determination of the oxygen and COD utilisation rates in sludge using the SDOX unit to create aerobic conditions 66 4.3.1. Sludge concentration 3.71 g/L 66 4.3.2. Sludge concentration 3.88 g/L 70 4.3.3. Sludge concentration 7.07 g/L 74 4.3.4. Sludge concentration 13.6 g/L 77 4.3.5. Discussion 80 4.3.6. Experimental challenges encountered and solutions thereto 83

4.4. Comparison of the oxygen and COD utilisation rates between diffuse aerator and SDOX unit aeration devices 84

4.5. Impact of the of the SDOX unit on biomass to uptake COD and oxygen 87 4.5.1. Sludge concentration 3.88 g/L –Pre-exposed to SDOX unit and aerated with

diffuse aerator 87 4.5.2. Sludge concentration 7.07 g/L 90 4.5.3. Discussions 94

4.6. The SDOX-MBR system as a treatment technology during emergencies 95

xi

5. CONCLUSIONS AND RECOMMENDATIONS 96 5.1. Conclusions 96 5.2. Recommendations 97

6. References 98

xii

List of Figures

Figure 1: Schematic layout of the novel SDOX-MBR system ........................................................................ 3 Figure 2: Overview of potential treatment options for faecal sludge .............................................................. 7 Figure 3: Schematic representation of the two film theory of gas transfer ..................................................... 9 Figure 4: The relationship between alpha factor and MLSS concentration .................................................. 12 Figure 5: Dependency of Alpha factor on MLVSS ....................................................................................... 13 Figure 6: Biological pathway of substrate (COD) and oxygen in an aerobic culture .................................... 14 Figure 7: Transformation reactions of organic and inorganic wastewater constituents from particulate

and soluble forms in the solid and liquid phases to the solids phase as sludge, and gas and

liquid phase escaping to the atmosphere and with the effluent, respectively. ........................ 15 Figure 8: Relationship between substrate utilisation and microbial growth .................................................. 16 Figure 9: Typical experimental set-up for OUR determination .................................................................... 18 Figure 10: Illustration of a response curve for OUR measurements ............................................................. 19 Figure 11: Typical SOUR in response to decrease in different biodegradable COD fractions within the

reactor ..................................................................................................................................... 20 Figure 12: The laboratory scale SDOX unit .................................................................................................. 21 Figure 13: SDOX® Pressurised chamber...................................................................................................... 22 Figure 14: Dissolved oxygen delivered by the SDOX unit at different MLSS concentrations ..................... 23 Figure 15: Design of study ............................................................................................................................ 25 Figure 16: Standard SDOX Experimental Set-Up ......................................................................................... 27 Figure 17: Modified SDOX Experimental Set-Up ........................................................................................ 27 Figure 18: Instruments used; a. Peristaltic pump; b. Influent air pressure gauge; c. SDOX PLC during

operation; d. Bio-Controller. .................................................................................................. 28 Figure 19: General Approach: Optimisation of the modified SDOX experimental set-up ........................... 29 Figure 20: Illustration of the fluctuations in the SDOX operating pressure and volume during operation;

a. Increase in pressure and volume when electronic drain valve is closed, b. Decrease in

pressure and volume when electronic drain valve is opened. ................................................. 30 Figure 21: Process water delivery mode into the SDOX unit: a. No spray nozzle; b. Flat spray nozzle; c.

Helicoidal spray nozzle. ......................................................................................................... 31 Figure 22: Sampling and sample preparation: a. Sample collection at the WWTW; b. Solid residuals on

an 800 microns metal sieve; c. Membrane sheets after filtering the activated sludge

sample. .................................................................................................................................... 35 Figure 23: Experimental approach - Aeration by diffuse aerators ................................................................ 37 Figure 24: Experimental set-up using diffuse aerator for creating aerobic conditions .................................. 37 Figure 25: Experimental approach - Aeration by SDOX unit ....................................................................... 39 Figure 26: Experimental set-up- Aeration with SDOX unit .......................................................................... 39 Figure 27: Experimental approach - Pre-pressurised sample aerated with diffuse aerators .......................... 40 Figure 28: Experimental approach – Prolonged aeration with the SDOX unit (from top row, left to

right) ....................................................................................................................................... 42 Figure 29: The biological oxygen meter ....................................................................................................... 44 Figure 30: Active data transfer for DO measurement ................................................................................... 44 Figure 31: Operation of SDOX unit at influent air pressure of 58 psi .......................................................... 47 Figure 32: Operation of SDOX unit at influent air pressure of 65 psi .......................................................... 47 Figure 33: Operation of SDOX unit at influent air pressure of 65 psi .......................................................... 48 Figure 34: Operation of SDOX unit at influent air pressure of 65 psi .......................................................... 48 Figure 35: Operation of SDOX unit at influent air pressure of 87 psi .......................................................... 49 Figure 36: Operation of SDOX unit at influent air pressure of 58 psi .......................................................... 50 Figure 37: Operation of SDOX unit at influent air pressure of 65 psi .......................................................... 50 Figure 38: Operation of SDOX unit at influent air pressure of 101 psi ........................................................ 51

xiii

Figure 39: Reactor TSS and VSS .................................................................................................................. 57 Figure 40: Reactor pH and DO...................................................................................................................... 58 Figure 41: OUR curves as a function of DO concentration .......................................................................... 58 Figure 42: OUR obtained from the BOM cell ............................................................................................... 59 Figure 43: SOUR obtained from the BOM cell............................................................................................. 59 Figure 44: Decrease in soluble COD concentration in the reactor ................................................................ 60 Figure 45: Suspended solids over time .......................................................................................................... 61 Figure 46: Reactor pH and DO...................................................................................................................... 61 Figure 47: OUR curves ................................................................................................................................. 62 Figure 48: OUR over time ............................................................................................................................. 62 Figure 49: SOUR over time .......................................................................................................................... 63 Figure 50: Decrease in COD over time ......................................................................................................... 63 Figure 51: Suspended solids in the reactor .................................................................................................... 66 Figure 52: Reactor pH and DO...................................................................................................................... 67 Figure 53: OUR curves ................................................................................................................................. 67 Figure 54: OUR over time ............................................................................................................................. 68 Figure 55: SOUR over time .......................................................................................................................... 68 Figure 56: COD utilisation over time ............................................................................................................ 69 Figure 57: Suspended solids in the reactor .................................................................................................... 70 Figure 58: Reactor pH and DO...................................................................................................................... 71 Figure 59: OUR curves ................................................................................................................................. 71 Figure 60: OUR over time ............................................................................................................................. 72 Figure 61: SOUR over time .......................................................................................................................... 72 Figure 62: COD utilisation over time ............................................................................................................ 73 Figure 63: Suspended solids in the reactor .................................................................................................... 74 Figure 64: Reactor pH and DO...................................................................................................................... 75 Figure 65: OUR over time ............................................................................................................................. 75 Figure 66: SOUR over time .......................................................................................................................... 76 Figure 67: COD utilisation over time ............................................................................................................ 76 Figure 68: Suspended solids in the reactor .................................................................................................... 77 Figure 69: Reactor pH and DO...................................................................................................................... 78 Figure 70: OUR over time ............................................................................................................................. 78 Figure 71: SOUR over time .......................................................................................................................... 79 Figure 72: COD utilisation over time ............................................................................................................ 79 Figure 73: OUR curves at different sludge concentrations ........................................................................... 81 Figure 74: SOUR curves at different sludge concentrations ......................................................................... 81 Figure 75: Relationship between sludge concentration and SOUR............................................................... 82 Figure 76: Relationship between change in sludge concentration and CODr ............................................... 82 Figure 77: Comparison of average reactor TSS with the use of different aeration devices .......................... 84 Figure 78: Comparison of average reactor VSS with the use of different aeration devices .......................... 85 Figure 79: Comparison of OUR under diffuse aeration and SDOX aeration ................................................ 85 Figure 81: Comparison of average TSS under diffuse aeration for sludge pre-exposed to the SDOX unit

and non-exposed sludge ......................................................................................................... 87 Figure 82: Comparison of average VSS under diffuse aeration for sludge pre-exposed to the SDOX unit

and non-exposed sludge ......................................................................................................... 88 Figure 83: Comparison of maximum OUR and SOUR under diffuse aeration for sludge pre-exposed to

the SDOX unit and non-exposed sludge ................................................................................. 89 Figure 84: Comparison of COD utilisation rates under diffuse aeration for sludge pre-exposed to the

SDOX unit and non-exposed sludge ...................................................................................... 89 Figure 85: Suspended solids in the reactor for the 14 hour duration of experiment (Top row:

Experimental cycle 1 –and 2, Bottom row: Experimental cycle 3 and 4) .............................. 90 Figure 86: OUR curves over the 14 hours experimental period .................................................................... 91

xiv

Figure 87: OUR over time ............................................................................................................................. 91 Figure 88: SOUR over time .......................................................................................................................... 92 Figure 89: COD utilisation after repeated slug doses of soluble COD.......................................................... 92

xv

List of Tables

Table 1: Characteristics of blackwater ............................................................................................................ 8 Table 2: Typical kinetic coefficients for CAS systems ................................................................................. 17 Table 3: Operational conditions of the SDOX unit for the oxygen delivery test .......................................... 31 Table 4: Synthetic blackwater recipe ............................................................................................................ 36 Table 5: Experimental conditions using diffuse aerators for aeration ........................................................... 37 Table 6: Experimental conditions using SDOX unit ..................................................................................... 39 Table 7: Operational condition using diffuse aerators to aerate pre-pressurised sludge ............................... 40 Table 8: Analytical techniques and methods ................................................................................................. 43 Table 9: Dissolved oxygen delivered by SDOX unit without a spray nozzle ............................................... 52 Table 10: Relative oxygen saturation concentration of the SDOX unit operated without a nozzle .............. 52 Table 11: Dissolved oxygen delivered by SDOX unit with a flat spray nozzle ............................................ 53 Table 12: Relative oxygen saturation concentration of the SDOX unit operated with a flat spray nozzle ... 54 Table 13: Dissolved oxygen delivered by SDOX unit with a helicoidal spray nozzle .................................. 55 Table 14: Relative oxygen saturation concentration of the SDOX unit operated with a helicoidal spray

nozzle ...................................................................................................................................... 55 Table 15: Experimental conditions for the diffuse aerated sludge ................................................................ 57 Table 16: Endogenous and exogenous respirometry values: 4.07 g/L sludge ............................................... 60 Table 17: Endogenous and exogenous respirometry values for the 3.89 g/L sludge ................................... 63 Table 18: Experimental conditions for the 3.71 g/L sludge .......................................................................... 66 Table 19: Endogenous and exogenous values for the 3.71 g/L sludge .......................................................... 69 Table 20: Experimental conditions for the 3.88 g/L sludge .......................................................................... 70 Table 21: Endogenous and exogenous values for the 3.88 g/L sludge .......................................................... 73 Table 22: Experimental conditions for the 7.07 g/L sludge .......................................................................... 74 Table 23: Endogenous and exogenous respiration values for the 7.07 g/L sludge ........................................ 76 Table 24: Experimental conditions for the 13.6 g/L SDOX aerated sample ................................................. 77 Table 25: Endogenous and exogenous respiration values for the 13.6 g/L sludge ........................................ 79 Table 26: Challenges during experiments and remedial actions ................................................................... 83 Table 27: Average and total SOUR using different aeration devices at approximately 4 g/L MLSS ........... 86 Table 28: COD utilisation rates using different aeration devices at approximately 4 g/L MLSS ................. 86 Table 29: Experimental conditions for the 7.07 g/L sludge .......................................................................... 90 Table 30: Endogenous and exogenous values after repeated doses of soluble COD: 7.07 g/L sludge ......... 93 Table 31: Average OUR for different experimental cycles at 7.07 sludge concentration ............................. 94

xvi

Abbreviations

ASCE American Society of Civil Engineers

ATP Adenosine Triphosphate

bCOD Biodegradable Chemical Oxygen Demand

BOM Biological Oxygen Meter

bsCOD Biodegradable Soluble Chemical Oxygen Demand

BNR Biological Nutrient Removal

CAS Conventional Activated Sludge

COD Chemical Oxygen Demand

CODr COD utilisation rate

DNA Deoxyribonucleic Acid

F/M Food to microorganisms ratio

HRT Hydraulic Retention Time

KHP Potassium Hydrogen Phthalate

MBRs Membrane Bioreactors

MLSS Mixed Liquor Suspended Solids

MLVSS Mixed Liquor Volatile Suspended Solids

OTE Oxygen Transfer Efficiency

OTR Oxygen Transfer Rate

OUR Oxygen Utilisation Rate

PLC Programmable Logic Controller

psi Pounds per square inch

PVC Polyvinyl Chloride

RAS Return Activated Sludge

sCOD Soluble Chemical Oxygen Demand

SDOX Supersaturated Dissolved Oxygen

SDOX® Supersaturated Dissolved Oxygen Registered Trademark

SDOX-MBR Supersaturated Dissolved Oxygen Membrane Bioreactor

SRT Sludge Retention Time

TSS Total Suspended Solids

VSS Volatile Suspended Solids

xvii

List of Symbols

Xt initial biomass concentration, mg VSS/L

Xo Final biomass concentration( at time, to), mg VSS/L

t is reaction time, h

µ specific microbial growth rate

rsu rate of substrate concentration change due to utilisation, g/m3.d

k maximum specific substrate utilisation rate, g COD/g VSS·d

S growth limiting substrate concentration in solution, g/m3

K half velocity constant, g/m3

ro oxygen utilisation rate, g O2/m3·d

rg rate of biomass growth, g VSS/m3·d

1.42 COD of microbial cell tissue, g bsCOD/ g VSS

A total gas-liquid interfacial area.

ΔC average driving force (concentration gradient) between the bulk of the liquid.

kLa absorption coefficient, (h-1

) .

Β ratio of oxygen saturation concentration between the waste/process water and clean/tap

water.

ϴ Temperature correction factor, 1.024 (for ASCE standard)

α ratio of oxygen mass transfer rate between the waste/process water and clean/tap water.

T Temperature.

L Litre

g gram

mg milligram

s second

EVALUATING A SUPERSATURATED AERATION SYSTEM FOR TREATMENT OF HIGHLY CONCENTRATED BLACKWATER IN EMERGENCY SETTINGS 1

1.1. Background of area of study

Outbreak of disasters such as floods, tsunamis, tropical storms, hurricanes, fires and civil unrest can result

in destruction of infrastructure, livelihood and displacement of people to emergency camps. This have been

witnessed in the recent disasters that occurred; the Haiti earthquake in 2010, the Japan earthquake in 2011,

the floods that occurred in Thailand in 2011 and the recent November 2013 typhoon in the Philippines.

Water and sanitation are critical determinants for survival of the affected individuals in the initial stages of

an emergency. In the majority of cases affected individuals suffer from illness and even death; both

strongly related to sanitation and water supplies (The Sphere Project, 2011). In the case of Haiti, cholera

outbreak associated with poor sanitation resulted in the death of approximately 7000 people (Frerichs., et al

2012) whereas diarrhoea was reported to be amongst the five main causes of morbidity in the Philippines

(WHO n.d.).

Proper sanitation is effective at reducing waterborne illnesses (Fewtrell et al. 2005) and if properly

managed it can significantly prevent of the spread of waterborne illnesses. As part of the sanitation chain,

wastewater treatment is one of the key areas that need attention during emergencies. The requirement to

have proper wastewater treatment cannot be over emphasised, more especially during emergencies where

the volume of faecal sludge and blackwater accumulates quickly. The membrane bioreactors (MBRs) have

potential for application in the treatment of highly concentrated black water in emergencies.

1.2. Rationale of the study

Due to industrialisation, population growth and more stringent environmental legislation, technological

developments in the wastewater treatment industry are continuing to advance. These advancements are

vital for responding to the increased need to protect and ensure the sustainability of the available water

resources for future use. Thus far, the evolution of wastewater treatment technologies have evolved

significantly (Tchobanoglous et al,. 2003). Amongst others, is the growing advancement witnessed in

MBRs. The MBRs have potential for application in the treatment of highly concentrated black water in

emergencies. Apart from a wider application in wastewater treatment, MBRs are preferred due to the ability

to operate at high mixed liquor suspended solids (MLSS), relative small footprint, reduced sludge

production and high effluent quality (Cote et al., 2004; Henze et al., 2008).

CHAPTER 1

INTRODUCTION

INTRODUCTION 2

Biological treatment of wastewater with MBRs differs from the conventional activated sludge (CAS)

system. In the use of MBRs, solids separation is achieved by membrane microfiltration other than gravity

induced settling as it happens in the CAS system. Depending on the membrane type, nominal membrane

pore sizes range between 0.04 and 0.4 µm (de Carolis and Adham., 2007). Particles greater than these pore

sizes in the bioreactor are retained by the membrane and thus a high quality effluent is produced. Unlike the

CAS, the application of the MBRs can to a great extent be limited by high capital and operational costs as

well as aeration requirements (Henze et al., 2008). Most of the operational costs in the MBRs are

associated with aeration. Both studies by Gander et al. (2000) and Germain et al. (2007), concluded that

aeration costs are high due to the need to scour (physically clean) immersed MBRs surfaces and provide

oxygen to the biomass for growth. Coarse and fine bubble aerators provide the aeration for immersed

MBRs, where the former is for scouring and the latter is for biomass growth.

MBRs can be operated at MLSS concentration of up to 15g/L using fine bubble aerators for aerobic growth

of biomass. The higher the MLSS concentration at which MBRs are operated the smaller the footprint.

However, the oxygen transfer efficiency (OTE) decreases as MLSS concentration increases (Ando,. 2013).

The OTE in standard condition is also expressed as alpha factor (α-factor), which is the ratio of dissolved oxygen concentration between process/wastewater and clean/tap water, and as such α-factor also decreases with increasing MLSS concentration.

The Supersaturated Dissolved Oxygen (SDOX) delivery unit is a potential replacement of the fine bubble

aeration devices. Unlike fine bubble aerators which injects air bubbles into the treated wastewater for

biomass growth, the SDOX delivery unit dissolves air into treated wastewater under pressurised conditions

then injects and evenly distributes a stream supersaturated with oxygen into the wastewater (BlueInGreen

n.d.). The use of the SDOX delivery unit has proven to overcome the limitations of α-factor at higher MLSS concentrations at which fine bubble aerators are ineffective. This unique ability of the SDOX unit

may have a positive impact on the reduction of MBRs aeration costs in addition to the smaller footprint.

This study was follow up of the work completed by Bilal, (2013). Part of his work was to evaluate the

oxygen transfer capabilities of the SDOX unit and effects of increase in activated sludge concentration on

the OTEs during endogenous respiration. One of the findings of the said study was that the SDOX unit can

successfully deliver dissolved oxygen into sludge concentrations of up to 34.2 g/L MLSS. The focus of this

study was therefore to assess microbial activity of an activated sludge suspension in exogenous respiration

whilst the SDOX delivery unit was used to supply oxygen.


1.3. SDOX-MBR System

The SDOX unit is an aeration device with a unique operating principle. The aeration process is bubble free.

The unit aerates the desired/treated wastewater stream under pressurised conditions using two principal

mechanisms. The treated wastewater is sprayed into a saturation chamber wherein; 1) a spray nozzle

distributes the wastewater into tiny droplets thereby instantly allowing oxygen diffusion into the water

phase, 2) the pressure within the saturation chamber (higher than atmospheric pressure) increases the

interaction of oxygen molecules with the water droplet, and therefore increasing the dissolved oxygen

levels in the water droplets. This result in a super saturation of dissolved oxygen in the wastewater stream

leaving the saturation chamber. This unique aeration mechanism of the SDOX technology makes it

compatible for MBRs, since MBRs operation at high MLSS is deterred by aeration when conventional

aerators are used for aeration.

The SDOX-MBR system is a developing concept to couple MBR and the SDOX delivery unit. The use of

the SDOX technology for provision of aeration requirements can potentially enhance MBRs performance at

high MLSS concentration by eliminating the OTE limitations and present a robust treatment technology.

The SDOX-MBR system has the following possible advantages; compactness, operation at high MLSS,

increased OTE at high MLSS concentration and aerobic COD removal at high MLSS without any aeration

limitations, amongst others. Whereas the SDOX-MBR system may present the advantages above, it should

be noted that the air requirements for membrane scouring will still be required. Figure 1below indicates the

schematic layout of the novel SDOX-MBR system

Figure 1: Schematic layout of the novel SDOX-MBR system

INTRODUCTION 4

1.4. Problem Statement

MBRs require oxygen supply for two principal operational aspects; aerobic growth of microorganisms and

scouring of membranes, in the case of immersed membranes. Fine bubble aerators are employed to supply

air for aerobic growth of microorganisms, whereas coarse bubble aerators provide air for scouring of

membranes. MBRs designed for typical domestic wastewater strength influents are normally designed for

operating at around 10 g/L MLSS. An increase in the influent wastewater strength (increase in COD load)

in an MBR operating at any MLSS concentration, will require an increase of the MBR volume by the same

magnitude in order to maintain the same operational conditions. This implies that an MBR fed with highly

concentrated wastewater (such as in emergency situations) operated at typical MLSS concentrations (of

around 10 g/L) will require a larger volume. The increase in MBR volume can be avoided by increasing the

MLSS concentration at which the MBR is operated. On the contrary, the higher the MLSS concentration at

which an MBR is operated the lower the α-factor (Judd., 2011). The design of MBR plants is generally limited to MLSS concentrations around 10 g/L where fine bubble aerators are effective. Thus, BlueInGreen

developed the SDOX delivery unit with the view of substituting fine bubble aerators as an aeration device

for MBR systems, particularly for operation at MLSS concentrations above 10 g/L. The use the SDOX unit

can allow for the possibility of operating an MBR at higher MLSS concentrations without limitation in

OTE, this will further reduce the MBR footprint.

In the study by Bilal (2013) it was determined that the SDOX delivery unit can successfully dissolve

oxygen into wastewater of MLSS concentrations of up to 34.2 g/L. The MLSS used in the experimental

work was in endogenous respiration. During endogenous respiration biomass die off and all oxygen utilised

is not associated with growth of biomass but rather cell maintenance (Gujer., 2001). No further studies have

been conducted to evaluate the performance of the SDOX delivery unit under normal MBR operational

conditions wherein oxygen utilisation accounts for both endogenous and exogenous respiration. Knowledge

of whether or not the respiration rate of biomass will increase with the increase in MLSS concentration is

essential for further assessment of the applicability of the SDOX-MBR concept. This study considered

SDOX-MBR system under exogenous respiration (presence of external bCOD source) whereupon COD

and oxygen utilisation rates at different MLSS concentrations were measured. The ability to operate MBRs

at higher MLSS concentrations with the corresponding increase in COD loads and oxygen utilisation rates

can be a breakthrough in the development of the SDOX-MBR system for the treatment of highly

concentrated blackwater, more especially that the system compactness would also make it easily

deployable and a cheaper option during emergencies.


1.5. General Objective

The principal objective of this study was to evaluate the potential advantages of the SDOX unit (under

exogenous respiration) in the design of SDOX-MBR system for treating highly concentrated wastewater.

The application of the SDOX-MBR system can be useful in emergency situations where treatment of large

volumes of blackwater is required.

1.6. Specific Objectives

This study evaluated the performance of the laboratory scale SDOX delivery unit for aerobic treatment of

highly concentrated blackwater, and the specific objectives are outlined below:

1) To determine the optimum operational conditions of the modified SDOX-MBR system at which

the SDOX unit can supply constant oxygen supersaturated water flow, and assess the oxygen

delivery capacity of the unit in tap water with the use of different spray nozzles.

2) To compare the oxygen and COD utilisation rates when using the diffuse aerators and SDOX

delivery unit both operated at the same MLSS concentration.

3) Determine both the COD and oxygen utilisation rates (microbial activity) of different MLSS

concentrations aerated with the SDOX delivery unit.

4) To assess the potential impact of the SDOX unit on biomass based on oxygen and COD utilisation

rates.

LITERATURE REVIEW 6

The use of the SDOX delivery unit as an alternative aeration device for MBRs, particularly for treatment of

blackwater during emergency situations require the understanding of the nature of emergency situations,

composition of blackwater, biochemical processes involved in wastewater treatment as well as the

operating principle of the SDOX delivery unit. This section gives an overview of these aspects, and set

them as a basis of this study.

2.1. Emergency Sanitation

Outbreak of disasters such as floods, tsunamis, tropical storms, hurricanes, fires and civil unrest can result

in destruction of infrastructure, livelihood and displacement of people to emergency camps. This have been

witnessed in the recent disasters that occurred; the Haiti earthquake in 2010, the Japan earthquake in 2011,

the floods that occurred in Thailand in 2011 and the recent November 2013 typhoon in the Philippines.

Water and sanitation are critical determinants for survival of the affected individuals in the initial stages of

an emergency. In the majority of cases affected individuals suffer from illness and even death; both

strongly related to sanitation and water supplies (The Sphere Project, 2011). In the case of Haiti, cholera

outbreak associated with poor sanitation resulted in the death of approximately 7000 people (Frerichs., et al

2012) whereas diarrhoea was reported to be amongst the five main causes of morbidity in the Philippines

(WHO n.d.).

Proper sanitation is effective at reducing waterborne illnesses (Fewtrell et al. 2005) and if properly

managed it can significantly prevent of the spread of waterborne illnesses. As part of the sanitation chain,

wastewater treatment is one of the key areas that need attention during emergencies. The requirement to

have proper wastewater treatment cannot be over emphasised, more especially during emergencies where

the volume of faecal sludge and blackwater accumulates quickly.

CHAPTER 2

LITERATURE REVIEW


Various wastewater treatment options have been proposed for treatment of faecal sludge. The options

proposed by Ingallinella et al., (2002) are indicated in Figure 2 below.

Figure 2: Overview of potential treatment options for faecal sludge

[Source: (Ingallinella et al. 2002)]

As shown in Figure 2 above, the proposed options also cater for the highly concentrated liquid fraction

after solid-liquid seperation. The selection and successful application of the treatment option for the highly

concentrated liquid fraction depends, amongst others, on availability and status of existing water and

sanitation infrastructure, legislative framework of the affected area and availability of other resources in the

emergency area. The SDOX-MBR system finds a niche for application during emergencies since the

system has advantages that most of these options do not offer. The majority of the above options have

larger land requirement when compared to the SDOX-MBR system. Additionally, the MBR delivers high

effluent quality available for reuse and is easily deployable to disaster areas due to its compactness. The

SDOX-MBR system is a potential treatment technology suitable for treating highly concentrated

blackwater and other wastewater streams in the absence of and during emergencies.

The sections below deal with the characteristics of blackwater, oxygen transfer phenomenon, COD

conversion, oxygen utilisation and operation of the SDOX unit.

LITERATURE REVIEW 8

2.2. Characteristics of Blackwater

Blackwater accumulates quickly during emergencies due to the high population density in emergency

camps, limited sanitation facilities and use of small flush water volumes. Understanding the composition of

blackwater is essential for successful application of the SDOX-MBR system for blackwater treatment. This

sub-chapter characterises blackwater in terms of COD and nutrients composition. However, research work

in characterising blackwater is ongoing and this increased knowledge will continue to fill the information

gap for proper design and management of wastewater treatment systems (Palmquist & Hanaeus 2005),

including the SDOX-MBR system.

Blackwater refers to the untreated mixture of faeces, urine, toilet paper and flush water (Knerr., et al 2011),

and forms 30 - 40 % of domestic wastewater. Depending on the amount of water used for flushing,

blackwater generally have much higher organic load and the pathogens, and as such poses the biggest

health risk (Paulo., et al 2013), and this risk can be more severe in emergency camps where little water is

available for flushing resulting in a much more concentrated blackwater stream. Blackwater characteristics

can vary widely mostly due to water use patterns and diet based on geographic location. Table 1 below

indicates the biochemical and physical characteristics of blackwater as reported by different authors.

Table 1: Characteristics of blackwater

Parameter Unit References

(Knerr et al.

2011)a

(Luostarinen et

al., 2007)b

Kujawa-

Roeleveld et al

(2005)c

(Coquin

2005)d

pH pH Unit 9.0±0.1 n.d n.d 8.81±0.2

TSS mg/L 1697±395 n.d n.d 3180

EC mS/cm 2.3±0.2 n.d n.d n.d

VSS (% TS) 96.1±1.8 n.d n.d 80.5

COD mg/L 2887±793 210 - 740 9503±6460 2260

BOD5 mg/L 524±118 300 - 600 n.d 1037e

TKN mg/L 273±39 n.d 1025±130 n.d

NH4-N mg/L 202±32 n.d 708±101 n.d

Nitrates (NO3-N) mg/L 2.2±0.7 n.d n.d n.d

TP mg/L 34.4±6.0 6 - 23 114±63 42.7

nd- not determined. a

Based on a 9L flush water by 15 inhabitants in Kaiserslautern (Germany),b Based on blackwater from

conventional flushing toilet c Based on 1 L flush water (as adopted from (Knerr et al. 2011)),

d Based on

blackwater collected from 110 houses in Skokaberg (Gothenburg - Germany), e BOD as BOD7

From Table 1 above, the influence of the amount of flush water on COD concentration is noticeable, more

especially between the COD reported by Luostarinen et al., (2007) using conventional flushing toilet and

Kujawa-Roeleveld., et al (2005) using only 1 L water to flush. Considering these two studies, it is evident

that using lower the amounts of flush water result in highly concentrated blackwater stream. Similar

concentrated blackwater streams in emergencies are common since available water use per capita is

normally far lower than under normal circumstances. This further motivates the need for innovative

blackwater treatment technologies for such highly concentrated blackwater.


2.3. Oxygen Transfer Phenomenon

Oxygen transfer is principal in the operation of aerobic wastewater treatment systems such as the novel

SDOX-MBR, the transfer of oxygen through the gas-liquid-solid interfaces is vital for microbial cell

growth and maintenance (Garcia-Ochoa., et al 2010) . The section below describes the gas transfer

mechanism, and other factors related thereto such as suspended solids, temperature and mixing regime

during aeration.

2.3.1. Oxygen transfer mechanism

Whereas there are other theories to explain the transfer of oxygen from the gas phase into the liquid phase,

the two-film theory is widely accepted and used to describe oxygen transfer between these two phases

(Garcia-Ochoa & Gomez 2009; Tchobanoglous et al., 2003). The two-film theory assumed that there are

stationery films of the gas/oxygen and liquid on both sides of the interface, and that the concentration

gradient between the two films is a driving force for the diffusion of the oxygen molecules from the gas

phase, through the interface, to the liquid phase (Lewis & Whitman, 1924), and lastly from the liquid phase

into the solid phase. Figure 3 describes this phenomenon in a form of a sketch.

Figure 3: Schematic representation of the two film theory of gas transfer

[Source:(Garcia-Ochoa et al., 2010)]

LITERATURE REVIEW 10

Garcia-Ochoa et al., (2010) described the two-film theory as shown in Figure 3 above as follow;

i. Molecular transfer from the interior of the bubble to the gas-liquid phase;

ii. Movement across the gas-liquid interface;

iii. Diffusion through the relatively stagnant liquid film surrounding the bubble;

iv. Transfer through the bulk liquid;

v. Diffusion through the relatively stagnant liquid film surrounding the cells;

vi. Movement across the liquid-cell interface through the flock or solid particles to the individual

cells;

vii. Transport through the cell cytoplasm to the site where reaction takes place;

viii. Biochemical reaction involving oxygen consumption and the production of carbon dioxide and

other gases;

ix. Transfer of the gases in the reverse direction.

The oxygen mass transfer rate per unit of reactor volume, OTR, is obtained multiplying the overall flux by

the gas–liquid interfacial area per unit of liquid volume, A:

OTR Eq.1

Eq.2

In Eq.1, the area A is the total gas-liquid interfacial area and ΔC is an average driving force (concentration

gradient) between the bulk of the liquid and the interface. The liquid film coefficient is generally

considered to be a constant value independent of liquid surface agitation due to stirring and air rate. It is the

specific interfacial area (a) that changes due to the change in gas hold-up and the mean bubble size.

Because ‘a’ cannot be easily measured in most practical applications, it is combined with kL to form the

absorption coefficient, kLa (h-1

) (Judd., 2002), as indicated in Eq.2 above.


2.3.2. Factors affecting oxygen transfer

There are numerous factors that influence oxygen transfer mechanism in biological wastewater treatment

processes. Wastewater characteristics, DO, temperature, type of aeration device, mixing regime, suspended

solids concentration and reactor geometry all affect oxygen transfer, including operational parameters such

as sludge retention time (SRT) and hydraulic retention time (HRT). The impacts of these factors on oxygen

transfer are well documented in literature and their brief discussion in this study would therefore be limited

to wastewater characteristics, mixing regime, temperature and mixed liquor suspended solids concentration.

2.3.2.1 Wastewater Characteristics The correction factor β is used to correct the oxygen transfer rate (OTR) for impacts due to constituents in the water such as salts, particulates and surface active substance. This factor can vary between 0.7 and 0.98,

and for wastewater treatment 0.95 is used.

Eq.3

In a review paper by Stenstrom & Gilbert (1981), it is reported that variance is OTR can largely be

attributed to the change in influent raw wastewater characteristics over time, and can fluctuate by up to

50% for various wastewaters.

2.3.2.2 Mixing and aeration devices Alpha factor (α) is used to correct OTR effects induced by mixing intensity, tank geometry and the type of

aeration devices used. Typical α value vary from 0.4 to 0.8 and 0.6 to 1.2 for diffused and mechanical

aerators, respectively and 0.4 to 1.1 for the SDOX unit. Alpha factor (α) is the ration between the mass

transfer coefficients of wastewater and clean water (kLa wastewater /KLa clean water), as expressed in Eq.4

below.

Eq.4

2.3.2.3 Temperature Effects of temperature on the mass transfer rate of oxygen in wastewater are corrected by use of the van't

Hoff-Arrhenius relationship in Eq.5;

Eq.5

Eq.5 above can be converted to process conditions by considering wastewater properties that impact on

OTR, as discussed above. For wastewater Eq.6 can be used to compute OTR as follows:

Eq.6


2.3.2.4 Mixed Liquor Suspended Solids and Sludge Retention Time Operation of an MBR system at high SRT results in increased MLSS concentrations within the reactor, and

lowers α-factor. Figure 4 below indicate the reduction of alpha (α) as a function of increasing MLSS concentrations reported by Krampe and Krauth., (2003), Germain., (2005) and Bilal., (2013). As compared

to others, the minimal effect of high MLSS concentration on OTR (expressed as Alpha factor) with the use

of the SDOX delivery unit is noticeable.

Figure 4: The relationship between alpha factor and MLSS concentration

[Source: (Bilal 2013)]

It is evident from the figure above that α-factor of 0.4 for the SDOX unit is far higher as compared to approximately 0.0 reported by other authors for MLSS concentration of approximately 35 g/L. This unique

property of the SDOX technology makes it favourable for application in MBR systems operating at high

MLSS concentrations.

According to Henkel et al., (2009) a large variation of α- factors at similar MLSS concentrations can be found, mainly due to sludge characteristics like respiration rate. The better method to correlate α- factor to suspended solids (at high SRT) is by the use of MLVSS instead of MLSS, as increase in MLVSS

concentrations have shown a clear correlation with α-factor irrespective of sludge characteristics or origin. Figure 5 below indicates the correlation of α-factor and MLVSS reported by different authors.


Figure 5: Dependency of Alpha factor on MLVSS

[Source: (Henkel et al. 2009)]

The relationship between MLVSS and α-factor can further be expressed by construction a mass balance of

oxygen over an aerated mixed bioreactor using the following expression:

Eq. 7

where dC/dt is the accumulation oxygen rate in the liquid phase, OTR stands for the oxygen transfer rate

from the gas to the liquid, and OUR is the oxygen utilisation rate by the microorganisms (MLVSS).


2.4. Aerobic Utilisation of COD

Under aerobic conditions and in the presence of biodegradable COD (bCOD) and other favourable

environmental conditions biomass utilise the bCOD for cell growth, mobility and cell maintenance (Gujer.,

2001). The section below briefly describes this phenomenon and much attention would be placed on

environmental conditions.

2.4.1. Organic substrate and aerobic bio-processes

In engineered aerobic biological treatment systems; an active mass and variety of microorganisms

particularly bacteria (fungi, protozoa, rotifers, etc) are retained within a bioreactor under favourable

environmental conditions, dissolved oxygen, nutrients and bCOD. The activated sludge model No. 1

(ASM1) and its subsequent models have been widely used to characterise the fractions of substrate

available for this biomass. COD within the bioreactor can be physically differentiated as soluble or

particulate, and biodegradable or non-biodegradable based on their ease to be rapidly utilised by

microorganisms (Wentzel., et al 1995; Gujer., 2001; Vanrolleghem et al., 2003; Henze et al., 2008). The

models recognises the affinity that biomass have to soluble bCOD over particulate bCOD substrates. In

suspended growth processes, like MBRs, the biomass is thoroughly mixed to enhance chances of physical

contact with the substrate and its homogenous distribution within the aerobic reactor. Upon contact the

substrate is consumed by the microorganisms for growth, mobility and cell maintenance (Lin 2007) . Figure

6 below indicates a typical pathway of biodegradable organic substrate from the liquid interface into the

microbial cell, and related biological processes.

Figure 6: Biological pathway of substrate (COD) and oxygen in an aerobic culture

[ Source: (Garcia-Ochoa et al., 2010)]


Organic fractions that are not consumed by the biomass follow different pathways from the biodegradable

organic substrates in an aerobic treatment system. Surplus non-biodegradable organics particles are rejected

from the treatment system as waste activated sludge, whereas the soluble non-biodegradable fraction is

carried through with the effluent. Figure 7 below summarises the pathways of organic substrates and

describes their related reactions.

Figure 7: Transformation reactions of organic and

inorganic wastewater constituents from particulate and soluble forms in the solid and liquid phases

to the solids phase as sludge, and gas and liquid phase escaping to the atmosphere and with the

effluent, respectively.

[Source:(Henze et al., 2008)]

Utilisation of organic substrate whether soluble or particulate under aerobic conditions as described above

and shown in Figure 6 and Figure 7 can be expressed mathematically as follow;

Eq.8

From Eq.8, biomass accumulates in a bioreactor due to consumption of organic substrate under aerobic

condition. The accumulation of biomass (new cell tissue) in within a bioreactor can be measured by volatile

suspended solids (VSS), protein content, ATP and DNA test. VSS is commonly used due to the ease of the

method and minimal time for analysis.


The relationship between organic substrate utilisation and biomass growth is further illustrated by Figure 8

below.

Figure 8: Relationship between substrate utilisation and microbial growth

[Source: (Henze et al. 2008)]

Biomass growth is characterised by four distinct phases as indicated in the graph above. During the

exponential growth phase biomass growth (new cells) reproduction rate changes quickly up to its maximum

possible, and can only be limited by the amount of bCOD available within the reactor (Judd., 2002).

In a batch reactor where there is no organic substrate limitation or depletion and no loss of cells due to

endogenous metabolism or death, growth of biomass will be exponential during the exponential growth

phase, and the specific growth rate (µ) can be calculated by using the Equation below.

Eq.9

The growth rate of microorganisms is at its highest when the bCOD is utilised at its maximum rate, and this

relationship can also be described by Eq.10 below.

Eq.10


2.4.2. COD utilisation kinetics

The rate of bCOD utilisation and increase in biomass within a reactor is governed by growth kinetics of

microorganisms. The rate of bCOD utilisation is first order with the limiting substrate until to a maximum

specific growth rate, after which microbial growth is unaffected by increase in substrate concentration, as

shown in Eq.11 below (Tchobanoglous et al., 2003).

Eq.11

From stochiometric relationship of the COD utilisation rate and microbial growth rate, the oxygen

utilisation rate can be determined with the expression in Eq.12 below.

Eq.12

The expressions used to illustrate substrate utilisation and growth of microorganisms (k, Ks, Y) in

Equations above are empirical, and can vary depending on wastewater characteristics, temperature and

microbial population. It is therefore essential to determine coefficient values under each operational

condition, including the SDOX-MBR system. Table 2 below presents the typical coefficients for CAS

systems.

Table 2: Typical kinetic coefficients for CAS systems

Valuea

Unit Range Typical

k g bsCOD/g.VSS.d 2 - 10 5

Ks mg/L BOD 25 - 100 60

mg bsCOD/L 10 - 60 40

Y mg VSS/ mg BOD 0.4 - 0.8 0.6

mg VSS/ mg bsCOD 0.3 - 0.6 0.4

kd g VSS/g VSS.d 0.06 - 0.15 0.10

aValue reported for 20°C

[Source: (Tchobanoglous et al., 2003)]


2.5. Oxygen Utilisation Rate

This Section compliments the previous sub-chapter, since aerobic COD removal is dependent on oxygen

transfer and its utilisation.

Oxygen utilisation rate (OUR) also referred to as respiration rate, is the measure of the amount of oxygen

utilised per unit volume and time for various oxidation reactions, predominantly organic matter removal in

an activated sludge plant. OUR is a simpler method to quantify microbial activity and substrate

characteristics in wastewaters (Rog., et al 1988b; Kristensen., et al 1992) and maximum aeration capacity

required for activated sludge processes (Huang., et al 1985; Rog., et al 1988a) . The OUR is the most

sensitive variable to validate activated sludge processes (Spanjers., 1993), as such the quantification of

OUR is of crucial importance for the modelling and design of wastewater treatment systems ( Sollfrank &

Gujer., 1990; Wentzel., et al 1995), including MBR based systems. The section below discusses the OUR

measurement and its theory.

2.5.1. Measurement of the oxygen utilisation rate

Sollfrank & Gujer., (1990), Kappeler & Gujer., (1992), Spanjers., (1993) and APHA., (2012) described

various methods to determine OUR, amongst others, is the closed batch respirometer. In the closed batch

respirometry a portion of the activated sludge under investigation is directed into an airtight completely

mixed cell, aerated, discontinue the aeration and recording of the change in DO over time occurs. The

frequency of recording DO measurement should not be more than one minute apart and at least six

measurements should be recorded, all depending of the speed with which the oxygen decreases over time.

The slope of the change in dissolved oxygen over time is OUR. Figure 9 below shows the typical

experimental set-up for OUR measurement.

Figure 9: Typical experimental set-up for OUR determination

[Source:(Kappeler & Gujer., 1992)]


As mentioned above, the OUR (mg O2/L.hr) is the slope of the oxygen consumption over time, and can be

calculated using the expression indicated below:

Eq. 13

The notation k in the equation above is the OUR, and CiO2 and CfO2 are the initial and final dissolved

oxygen concentrations, respectively, after time, t. A graphical response of the OUR measurement is a

typical linear curve with a negative slope, as illustrated in Figure 10 below.

Figure 10: Illustration of a response curve for OUR measurements

The OUR can further be expressed as specific oxygen utilisation rate (SOUR); oxygen utilised per gram of

VSS contained in the sample. The SOUR (mg O2/g VSS.hr) completely depend on the source and

characteristics of the wastewater, temperature and microbial population present. Since the microbial

activity depends on temperature, the SOUR measured at other temperatures (T) can be corrected to 20 °C

using the equations below.

Eq.14

Eq. 15

Where θ is the temperature correction factor; T< 20°C θ is 1.07 and1.05 when T> 20°C.

Dis

solv

ed

oxy

gen

, mg/

L

Time

CfO2 = CiO2 - OUR.t


2.5.2. Types of oxygen utilisation rates

Two types of cell respiration exist in biological wastewater treatment processes. These types of respiration

depend on the absence or presence of bCOD. Endogenous oxygen utilisation or respiration is defined as the

stage when microorganisms consume their own cells to obtain energy for cell maintenance (Rog et al.,

1988a; Tchobanoglous et al., 2003), or oxygen uptake in the absence of bCOD. Exogenous respiration

accounts for oxygen consumed during oxidation of bCOD. The sum of exogenous and endogenous oxygen

utilisation rates is known as the total or maximum oxygen utilisation rate. The ratio between the exogenous

and endogenous SOUR is known as a spiking factor (Exogenous SOUR/Endogenous SOUR), and can be

used to indicate the factor by which respiration increases when bCOD is introduced to the starved biomass

(Strotmann., et al 1999). Amongst others, the spiking factor depends on the ease with which the bCOD is

consumed once introduced to the system, and therefore differs.

As mentioned in paragraph 2.4.1 above, biomass has a particular order of preference for consumption of the

available biodegradable substrates/COD. Oxygen utilisation is a direct measure of the rate at which the

substrate is consumed, and as such the specific oxygen uptake profile of a mixed reactor with different

fractions of bCOD will reflect the order of substrate preference. Figure 11 below shows the SOUR in

response to the different bCOD fractions as a function of time.

Figure 11: Typical SOUR in response to decrease in different biodegradable COD fractions

within the reactor

[Modified from (Hagman & Jansen., 2007)]


2.6. Supersaturated Dissolved Oxygen (SDOX) Delivery

System

The SDOX® is designed to take a portion of water from the main treatment process, saturate it with oxygen

using the bubble-free technology, and re-inject the dissolved oxygen “supersaturated” stream back into the

main process such that it is effectively mixed and distributed. The SDOX® is able to cost-effectively

provide of aeration through oxygen dissolution and injection for wastewater treatment processes and other

applications such as ecological remediation, aquaculture and post aeration (BlueInGreen n.d.). Figure 12

shows the laboratory scale SDOX delivery unit.

Figure 12: The laboratory scale SDOX unit

The operating principle of the SDOX delivery unit is explained in the next Section. The explanation

focuses on the SDOX pressurised chamber shown in Figure 12 above.


2.6.1. SDOX® Operating Principle

The operation principle of the SDOX delivery unit is illustrated in Figure 13 below. Furthermore, each unit

operation is described in reference to the numbering indicated on the figure.

Figure 13: SDOX® Pressurised chamber

[Source:(BlueInGreen n.d. Accessed October 2013)]

1. Oxygen/air source

2. Pressure regulator within the oxygen/gas feed line- regulates the pressure within the headspace of

the SDOX unit indicated in 4 on the figure above. Possible pressure settings for the lab-scale unit

are 30 - 90 psi.

3. Feed line of the process/wastewater to be treated, flow regulated by a peristaltic high pressure

pump.

4. Pressurised headspace filled with air/oxygen (pressure may vary as indicated in 2. above); the

wastewater is sprayed through the oxygen headspace instantly getting supersaturated with

dissolved oxygen. There are two mechanisms responsible for the super saturation;

The nozzle sprays water in tiny droplets thereby immediately allowing the oxygen to be dissolved

in the water;

The pressure within the headspace (which is above the atmospheric pressure) increases the

interaction of oxygen molecules with the water droplet, and therefore increasing the dissolved

oxygen levels in the water droplets.

5. Operational liquid volume super saturated with oxygen, the levels of this volume differ with either

increase or decrease with operating pressure (and the electronic drain valve opening durations- in

the case of the modified SDOX set-up).

6. Bubble-free supersaturated stream leaves the SDOX delivery unit to the main treatment process.

This stream is added to the target process/wastewater that requires oxygenation. The

supersaturated water is quickly added and mixed with the process water faster than oxygen gas can

occur at normal atmospheric pressure. This is achieved by discharging this stream at the bottom of

the target process/wastewater.


2.6.2. Performance of SDOX unit for MBRs Application

The SDOX delivery unit has proven to be applicable for oxygenation of high MLSS concentration

wastewaters. In a latest study by Bilal., (2013), amongst others, aimed at evaluating the performance of the

SDOX delivery unit in aerating MLSS concentrations for MBR operational ranges 6 - 35 g/L, It was

determined that the SDOX delivery unit can successfully oxygenate >30 g/L MLSS. Figure 14 below

indicate the dissolved oxygen levels delivered by the unit operated at different MLSS concentrations

(Bilal., 2013). Most aeration devices are unable to reach the DO levels the SDOX technology has proved

possible in high sludge concentration.

Figure 14: Dissolved oxygen delivered by the SDOX unit at different MLSS concentrations

[Source: Bilal., (2003)]

2.6.3. Advantages of the SDOX Delivery Unit

The SDOX delivery unit has the following advantages over other aeration systems (BlueInGreen n.d.):

Bubble free technology, other than other aeration systems, no bubbles would leave the water

column.

Compact, easily deployable.

Operational control modes that allow the pump(s) to be turned down to save energy/costs.

Offers wide application (aeration of natural water resources and engineered systems).

RESEARCH METHODOLOGY 24

3.1. Experimental design

This study was divided into four phases. The first phase of the study was to establish the optimum

operational conditions of the SDOX unit, i.e. determine the corresponding flow rates and influent air

pressures at which the SDOX unit will deliver a pseudo-stable supersaturated stream. Additionally,

determine the aeration capacity at the optimal conditions. The second part of the study was to use of diffuse

aerators for aeration and benchmark the COD and oxygen utilisation rates. The other phase, principal to

this study, was to determine if the oxygen and COD uptake rates of the biomass will increase linearly with

the increasing MLSS whilst aerated with the SDOX unit. Lastly, this study sought to assess the potential

impact of the SDOX unit on biomass, in particular; the biomass ability to uptake oxygen and COD. The

experimental design presented in Figure 15 below.

CHAPTER 3

RESEARCH METHODOLOGY


Optimize operation of SDOX unit.

Identify a spray nozzle with better aeration

capacity.

Conclusions and Recommendations

Data analysis

Determine the oxygen & COD utilisation rates in

approximately 4 g/L MLSS using diffuse aerators

to create aerobic conditions.

Determine the potential impact of the SDOX unit

on biomass to uptake oxygen COD utilisation rate.

Determine the oxygen & COD utilisation rates in

different MLSS concentrations using SDOX unit,

(and comparison with the diffuse aerators for the 4

g/L MLSS)

Figure 15: Design of study


3.2. Optimisation of the SDOX unit, and determination of aeration capacity of the SDOX using different spray nozzles

The aim of the first part of the research was to determine the optimum operational conditions of the

modified SDOX unit, as well to identify the spray nozzle with better aeration ability. The spray nozzle that

demonstrated better performance was used in the following phases of this study.

3.2.1. Materials The following materials were used during the experiments.

1. SDOX unit with accessories.

2. ADI 1030 Bio-Controller.

3. Electronic drain valve (model: MCDV-25-120 AVS).

4. Spray nozzles: flat and helicoidal nozzle.

5. Reactor, with mechanical mixer.

6. A calibrated Hach HQ30d DO meter.

7. Peristaltic high pressures pump (model: NEMA AX-IP66).

8. Stop watch.

9. Compressed air.

10. Sodium sulphite salt.

3.2.2. Experimental Set-Up

The standard laboratory scale SDOX experimental set-up as shown in Figure 16 below, presented some

operational challenges during the study by Bilal., (2013). During the sludge tests, the orifice on the delivery

side of the SDOX pressurised chamber was used to control the flow rate from the SDOX unit, and a sudden

drop of the level of the supersaturated liquid inside the SDOX pressurised chamber was observed. This

drop (and therefore increased flow from SDOX) was associated with the expansion of the orifice edges due

to friction between solid particles and the PVC orifice cap. A metal orifice cap was later used to avoid the

recurrence of this problem. Whereas the metal orifice cap averted the orifice expansion problem, in turn it

presented a clogging problem. The clogging of the metal orifice cap deterred delivery of a continuous

supersaturated dissolved oxygen flow into the reactor. The influent process water spray nozzle inside the

SDOX pressurised chamber was also removed as it was suspected that it also contributes to the clogging

problem on the delivery side of the SDOX unit.


Figure 16: Standard SDOX Experimental Set-Up

In order to prevent the orifice clogging and therefore ensure constant flow of oxygen supersaturated water

into the reactor, the standard SDOX experimental set-up was modified, see Figure 17 below. The orifice

was removed, and replaced with an electronic drain valve on the delivery side of the SDOX pressurised

chamber; the electronic drain valve received signals to close and open at set intervals from the ADI Bio-

Controller, thereby allowing a semi-continuous and uninterrupted flow from the SDOX unit. A by-pass of

the electronic drain valve was also provided to allow for the emptying of the SDOX unit via the manual

valve.

Figure 17: Modified SDOX Experimental Set-Up


A NEMA AX-IP66 peristaltic high pressure pump was used to pump process/tap water from the reactor

into the SDOX pressurised chamber, in parallel, the air flow into the chamber was released by opening the

influent air valve after the desired minimum liquid volume in the pressurised chamber was met. The air

pressure gauge was used to set the air pressure at chosen levels. Amongst other functions, the SDOX PLC

was used to monitor the operating pressure and supersaturated liquid volume in the SDOX unit. It should

be noted that the operating pressure is the pressure induced by both the air and inflowing process water into

the SDOX pressurised chamber, whereas the influent air pressure is the pressure of the influent air flow

before entering the SDOX unit.

As indicated in Figure 17 above, the SDOX pressurised chamber continually receives influent air and tap

water separately and the release of the supersaturated water into the reactor is controlled by an MCDV-25-

120 AVS electronic drain valve. The drain valve was plugged to an ADI 1030 Bio-Controller which was

used to open or close the drain valve for desired durations, this ADI Bio-Controller can vary the open or

closing durations ranging from 0 second - 9999 seconds. Figure 18 below shows some of the apparatus

used for the experimental set-up.

a.

b.

c.

d. Figure 18: Instruments used; a. Peristaltic pump; b. Influent air pressure gauge; c. SDOX PLC

during operation; d. Bio-Controller.


SDOX Optimization

Set the influent air pressure gauge at

desired pressure.

By trial and error:

Adjust the process water flow rate until a

pseudo-stable SDOX liquid volume is

maintained.

Repeat steps above for the next influent air

pressure.

Record the SDOX operating parameters

from PLC:

Upper & lower values for pressure and

volume.

Volume of air

headspace & water

level in SDOX unit is

pseudo-stable.

Yes

No

Select drain valve setting.

3.2.3. Optimisation of the Modified SDOX Experimental Set-Up

Given the modification of the SDOX experimental set-up as described above, it was essential to determine

the optimum operational conditions of the modified SDOX experimental set-up, in particular, to determine

optimal influent air pressure(s) and process water flow rates at which the SDOX unit will supply constant

oxygen supersaturated water into reactor, whilst SDOX unit operate at a pseudo-stable (not too much

variable, and stable over time) water level. The operational parameters considered during optimisation

were; influent air pressure, influent process water flow rate, operational pressure and supersaturated water

level within the SDOX pressurised chamber as well as the open-close intervals of the electronic drain

valve. Figure 19 below indicate the approach followed.

Figure 19: General Approach: Optimisation of the modified SDOX experimental set-up


Tap water was used for all trials performed during optimisation and the volume of the water in the reactor

was 10 L. The trials were carried out for influent air pressures between 50 psi and 100 psi and tap water

flow rates varied between 1400 mL/min - 2100 mL/min (SDOX unit HRT 1.35 – 0.9 minute). The

electronic drain valve open and close intervals were set at either of the two settings. In setting 1 the drain

valve opened and closed for 1 and 5 seconds, respectively and in setting 2, the drain valve opened for 2

seconds and closed for 10 seconds.

In order to achieve a pseudo-stable (not too much variable, and stable over time) supersaturated water level

in the SDOX pressurised chamber, by trial and error, the influent tap water flow rate was varied, whereas

the influent air pressure and electronic drain valve open-close intervals were fixed. Initially, the pressure

induced by the influent air on the supersaturated water inside the SDOX pressurised chamber led to almost

immediate emptying of the chamber at the start of each trial. It was later observed that filling the

pressurised chamber approximately 50 % of its capacity before the influent air was opened helped to retain

an almost constant supersaturated water level in the chamber for the duration of operation. As already

mentioned the electronic drain valve was also set to continually close and open for defined intervals during

each trial, the continual opening and closing of the electronic drain valve caused fluctuation within 10% of

average water level and operating pressure in the SDOX unit. In each occasion when the drain valve closed

both the water level and pressure in the SDOX chamber gradually increased and the opposite occurred

when the drain valve opened. The upper and lower levels to which both the supersaturated water level and

pressure increased and decreased were recorded every minute during each trial. The average of the upper

and lower values of the water level and pressure were used to plot the pattern of the average operating

liquid volume and pressure of the SDOX for the duration of each experimental trial. See Figure 20 below.

a. b.

Figure 20: Illustration of the fluctuations in the SDOX operating pressure and volume during

operation; a. Increase in pressure and volume when electronic drain valve is closed, b. Decrease in

pressure and volume when electronic drain valve is opened.

Pressure (psi) Volume (%) Pressure (psi) Volume (%)


3.3. Dissolved oxygen delivery test using the SDOX unit

It was also necessary to determine the oxygen delivery capabilities of the SDOX unit under different

optimum operational conditions of the modified SDOX experimental set-up. As such, a test to determine

the capacity of the SDOX unit to dissolve oxygen into tap water was performed. The aim of this test was to

determine the impact of the following parameters on the performance of the SDOX unit; operating pressure

and electronic drain valve settings, whilst operating the SDOX unit with and without a spray nozzle. In this

study, a helicoidal spray nozzle and flat nozzle were used for the tests. Figure 21 below shows the different

ways influent process water could be delivered into the SDOX unit; without spray nozzle and with the two

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