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APPLICATION OF THE AIR BACKFLUSHING TECHNIQUE IN AMEMBRANE BIOREACTOR FOR SEPTIC WASTEWATER

TREATMENT

by

Sudtida Pliankarom

A thesis submitted in partial fulfillment of the requirements for degree ofMaster of Engineering.

Examination committee Dr. C. Visvanathan (Chairman)Mrs. Samorn MuttamaraDr. Byung-Soo Yang

Nationality ThaiPrevious Degree B.Sc. (Environmental Health Science)

Mahidol UniversityBangkok, Thailand

Scholarship Donor Government of Netherlands

Asian Institute of TechnologySchool of Environment, Resources and Development

Bangkok, ThailandAugust, 1996


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Acknowledgements

The author wishes to expressed her profound gratitude, great appreciation andindeptness to her advisor Dr. C. Visvanathan for his valuable guidance, encouragement,

support and sharing the knowledge throughout the research. Special thanks are extended toMrs. Samorn Muttamara (Co-chairman) and Dr. Byung-Soo Yang for their valuablesuggestions and guidance given for this study as members of the examination committee.

The author also thankful to the staff and the friend of the EnvironmentalEngineering Program for their prompt assistance and cooperation during this study.

A very grateful acknowledgment is extended to the Government of Netherlands forproviding the author with a scholarship to study the master program at AIT. The authorwishes to express her sincere gratitude to Prof. R. Ben Aim of Department of ChemicalEngineering, University de Technologic de Compiegne, France and Mr. Robert T. Wale ofMemtec Ltd., Australia for providing membrane and giving valuable technical information.

Finally the author express her profound gratitude to her parents for their strongencouragement and inspiration given to her.


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Abstract

In this study the possibility of application of air backflushing technique throughhollow fiber microfiltration was investigated. The process employed direct solid-liquid

separation by immersed two membrane modules with pore size of 0.2 m directly in theactivated sludge aeration tank of 80 L volume. This study was conducted with highconcentration of activated sludge and divided into short-term and long-term experiments.

In short term experimental runs, the optimum air backflushing and filtration cyclewas investigated. 15 minutes filtration and 15 minutes air backflushing provided the bestresult in term of flux improvement and stability. Due to the membrane module stabilitylimitation, the applied compressed air pressure of 1 bar was not sufficient to remove theclogging completely. However this cyclic operation provided higher flux stability comparedto operation without air diffusion.

In long-term experiments, the initial sludge concentration was 13,000 mg/L. Three

different hydraulic retention times (HRT) of 26, 18 and 10.5 hour which corresponds to thepermeate flux of 3.08, 4.44 and 7.62 L/m2. h were investigated. Here, it was noted that thefiltration pressure related to the MLSS concentration. Whereas the stable operation could beobtained at 26 and 18 hours. All experimental runs provided more than 90% removal ofCOD, BOD and TKN with final MLSS of 40,000 mg/L in the reactor. Although theoperation with daily sludge draining (1.6 L/d), the MLVSS/MLSS values seem slightlydecreased. However, such conditions could not effect significantly to the processperformance in term of physical, chemical, biological and bacteriological qualities ofmembrane bioreactor effluent.


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List of Abbreviations

AS - Activated SludgeASP - Activated Sludge Process

BOD - Biochemical Oxygen DemandCOD - Chemical Oxygen DemandCST - Capillary suction timeDO - Dissolved OxygenEA - Extended aerationED - ElectrodialysisPHF - Polyethylene hollow fiberEff - EffluentF-BOD - Filtered biochemical oxygen demandF-COD - Filtered chemical oxygen demandF / M - Food / Microorganism ratioHRT - Hydraulic retention time

Inf - InfluentJ - Permeate fluxk - Maximum rate of substrate utilization per unit mass of microorganismkd - Endogenous decay coefficient

KLa - Overall gas transfer coefficientKs - Half- velocity constant (saturation constant)MBR - Membrane bioreactorMF - MicrofiltrationMLSS - Mixed liquor suspended solidsMLVSS - Mixed liquor volatile suspended solidsNF - NanofiltrationNH3-N - Ammonia nitrogenNO2-N - Nitrite nitrogenNO3-N - Nitrate nitrogenNTU - Naphelometric turbidity unitRd - Membrane resistance due to the deposition of solidsRg - Rate of bacterial growthRg - Net rate of bacterial growthRm - Apparent membrane resistanceRmo - Initial membrane resistanceRm1 - Membrane resistance after first cleaningRm2 - Membrane resistance after second cleaningRO - Reverse osmosis

Rsu - Substrate utilization rateSRT - Solids retention timeSS - Suspended solids

TKN - Total kjedahl nitrogenT-N - Total nitrogenTP - Total phosphateTS - Total solids


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List of Abbreviations

TVS - Total volatile solidsUF - UltrafiltrationVSS - Volatile suspended solids

Y - Sludge growth coefficient - Dynamic viscosity

m - Maximum specific growth rate15:15 - 15 minutes of filtration then 15 minutes of air diffusion15:15* - 15 minutes of filtration then stop without sending air for 15 minutes


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Table of Contents

Chapter Title Page

Title Page iAcknowledgements iiAbstract iiiTable of Contents ivList of Tables viList of Figures viiAbbreviations ix

1. Introduction 1

2. Literature Review2.1 Fundamentals of Activated Sludge 32.2 Fundamentals of Microfiltration 32.3 Biological Nitrification 42.4 Denitrification 52.5 Phosphorrus Removal from Wastewater 52.6 Microfiltration Membrane in Domestic

Wastewater Treatment 72.7 Application of Membrane Bioreactors in Domestic

Wastewater Treatment 72.8 Clogging Mechanisms in Microfiltration 102.9 DecloggingTechniques 112.10 Fundamentals of Gas transfer 122.11 Applications of Gas Diffusion through Membrane 14

3. Experimental Set Up3.1 Measurement of Gas Transfer Efficiency of Aeration Units 173.2 Measurment of Initial Membrane Resistance 193.3 Short Term Experiments 213.4 Long Term Experiments 243.5 Analytical Methods 25

4. Results and Discussion

4.1 Gas Transfer Efficiency of Aeration Units and InitialMembrane Resistance 26

4.2 Short Term Experiments 304.3 Long Term Experiments 38


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Table of Contents

Chapter Title Page

5. Conclusion and Recommendations5.1 Conclusions 645.2 Recomendations for Future Work 65

References 66

Appendix A 68Gas Transfer Efficiency and Membrane ResistanceAppendix B 76Detail Results of Short Term ExperimentsAppendix C 87

Detail Results of Long Term ExperimentsAppendix D 98Detail Results of Typical Coefficients of Activated Sludge Processfor Domestic Wastewater TreatmentAppendix E 110Screening and Toxicity Methodololy for Measurement of Biomass ActivityAppendix F 112Detail Results of Nutrient RemovalAppendix G 116Detail Results of Activated Sludge CharacteristicsAppendix H 121Detail Results of Membrane Cleaning Efficiency


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List of Tables

Table Title Page

3.1 Characteristics of Raw Septic wastewater 213.2 Parameters Analyzed ( long tem experiments) 254. 1 Comparison of Performance of Different Suction Pressure

with 15:15* Operation Mode and with Air Diffusion 304.2 Comparison of Performance of Different Suction Pressure

with 15:15 Operation Mode and 1 bar Compressed Air 314.3 Comparison Performance of Different Mode of Operation

with Suction Pressure at 7 kPa and 1 bar Compressed Air 354.4 Comparison Performance of Different Compressed Air Diffusion

with Suction Pressure at 7 kPa and 15:15 Operation Mode 364.5 Kinetics of Acclimatized Sludge Growth 384.6 Transmembrane Pressure of each Experimental Runs 424.7 Average Value of Biological Solids Concentration of

each Experimental Runs 454.8a Nitrogen Mass Balance at Steady State 494.8b Calculated Data for Nitrogen Mass Balance 504.9a Phosphate Mass Balance at Steady State 534.9b Calculated Data for Phosphate Mass Balance 5 34.9c Characteristic of total Phosphate in Sludge from Bioreactor 5 44.10 Characteristics of Sludge from Conventional Activated Sludge

and Membrane Bioreactor Process 554.11.1.1 Microbiological Quality of Permeate and Effluent from

Conventional Activated Sludge4.12

Dynamics of Activated Sludge at Steady State for eachExperimental Runs 58


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L ist of Figures

Figure Title Page

2.1 Dead-End Filtration and Crossflow Filtration 42.2 Schematic of Pilot - Scale Membrane Bioreactor 92.3 Module Backflushing with Gas 112.4 Effect of Gas Backflushing during Wine Filtration 122.5 Schematics of Two-Film Theory of Gas Transfer 143.1 Membrane Module Used for All Experiments 183.2 Experimental Set Up for Gas Transfer Efficiency 183.3a Schematic of Membrane Bioreactor Set Up 233.3b Actual Membrane Bioreactor Set Up 234.1 Variation of Oxygen Concentration with Time when Using

Ambient Air (stone diffusers) 274.2 Variation of Oxygen Concentration with Time when Using

Pure Oxygen (stone diffusers) 274.3 Plot of Cs - Ctwith Time when Using Ambient Air

(stone diffusers) 284.4 Plot of Cs - Ctwith Time when Using Pure Oxygen

(stone diffusers) 284.5 Comparison of KLa (20)

o C at Different Flow Rates forAmbient Air and Pure Oxygen (stone diffusers) 29

4.6 KLa (20)o C at Different Pressure (membrane diffusers) 29

4.7 Relationship between Permeate Flux of Clean Water andTransmembrane Pressure (membrane module 1) 32

4.8 Relationship between Permeate Flux of Clean Water andTransmembrane Pressure (membrane module 2) 32

4.9 Variation of Average Permeate Flux with Time at EachHour for Different Transmembrane Pressure with 15:15*Operation Mode (* =without sending air) 33

4.10 Comparison of Permeate Flux with Time for 40 kPaSuction Pressure between Operation with and withoutAir Diffusion 33

4.11 Comparison of Flux with Different Suction Pressure 344.12 Variation of Permeate Flux with Time for Long - Run

Observation when using Transmembrane Pressure of 7 kPa 344.13 Comparison of Permeate Flux with Time between 10:10 and

15:15 Operation Modes for 7 kPa Suction pressure and1 bar Compressed Air 37

4.14 Comparison of Permeate Flux with Time between 20:20, 25:25and 30:30 Operation Modes for 7 kPa Suction pressure and1 bar Compressed Air 36


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L ist of Figures

Figure Title Page

4.15 Variation of MLVSS and Effluent Filtered-COD with Timeafter Sludge Acclimatization 38

4.16a Variation of Transmembrane Pressure (suction pressure) withTime for Different Experimental Runs 41

4.16 b Variation of Permeate Flux with Time for DifferentExperimental Runs 41

4.17 Variation of Effluent Turbidity and DO with Time for DifferentExperimental Runs 44

4.18 Variation of COD Concentration with Time for DifferentExperimentalRuns 44

4.19 a Variation of Biological Solids Concentration with Time forDifferent Experimental Runs 46

4.19 b Accumulation of Inert Materials with Time for DifferentExperimental Runs 46

4.20 Variation of F/M-ratio and BOD Concentration with Time forDifferent Experimental Runs 47

4.21 Visual Characteristic of Raw Influent, Sludge in Bioreactorand Effluent 47

4.22 Variation of TKN Concentration with Time for DifferentExperimental Runs 51

4.23 Variation of NO3-N Concentration with Time forDifferent Experimental Runs 51

4.24 Viation of Total Phosphate concentration with Time forDifferent Experimental Runs 54

4.25a Biological Flocs with Safanin-O at steady state of RUN 1,200x 59

4.25b Biological Flocs with Safanin-O at steady state of RUN 2,200x 59

4.25c Biological Flocs with Crystal Violet at steady state of RUN 3,200x 59

4.26a Relationship between Flux of Clean Water and TransmembranePressure (membrane module 1) 63

4.26b Relationship between Flux of Clean Water and TransmembranePressure (membrane module 2) 63


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Chapter 1

Introduction

1.1 General

The most common and classical wastewater treatment process which has beenused to treat domestic wastewater is the activated sludge process (ASP). In thissystem the organic and inorganic matters present in the suspended solid, colloidal andsoluble forms can be removed up to 95%.

However, there are some limitations in classical ASP when a high quality ofeffluent is required. In such situations, large secondary sedimentation tank is requiredto provide sufficient retention time. Moreover, there are various factors that must beconcerned to reach good settling characteristic. Therefore, various types ofcombination between ASP and membrane unit have been studied and adapted toovercome these problems and to obtain good effluent quality.

Membrane separation technology in water and wastewater treatment can becategorized into four classes according to the membrane, namely, reverseosmosis(RO), ultrafiltration (UF), microfiltration (MF) and electrodialysis (ED). UFand MF techniques are useful in removing macromolecule, colloids and suspendedsolids.

Membrane separation technology has been introduced for solid/liquidseparation in biological treatment system. The advantages of employing membraneseparation are minimum sludge wastage by maintaining low F/M ratio, reducing plantsize by maintaining higher biomass concentration in the reactor, and solid freeeffluent could be obtained. Complete retention condition could be maintained by

operation without sludge wastage since the solid/liquid separation could be doneregardless of sludge settleability.

For domestic wastewater treatment, combined activated sludge/membranefiltration can provide a high degree of treatment in terms of organic oxidation andnitrogen removal.

However, the power/energy consumption that has been reported is muchhigher than the value for conventional ASP. (Yamamoto,1989) As Yamamoto (1989)indicated, the process is not cost effective. The main reason for the high cost is due tothe recirculation pump, which connects the main reactor with a membrane unit andmaintain high crossflow velocity on the membrane surface to keep the flux

undeclined. The solution for this has been investigated by direct membrane separationusing hollow fiber in an activated sludge aeration tank which still give a stableoperation and good quality of effluent.

Considering the process performance, direct membrane separation in ASPwith continuous suction operation caused severe clogging of the membrane modulewhenever transmembrane pressure is increased. Using the intermittent suction


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operation enabled a stable flux to be maintained for suitable, particularconditions.(Yamamoto et al., 1989)

Cyclic operation with air diffusion has been investigated by Chiemchaisri(1990). Air backflushing technique was applied to achieved the recovery of permeate

flux and net cumulative volume. However, increasing the pressure applied for airbackflushing to achieve complete membrane cleaning may damage the membrane.

1.2 Objectives of the study

1. To investigate the possibility of using 0.2 m-membrane pore size for effluentfiltration and air diffusion purposes in alternative cycle.2. To compare the effect of produced gas bubbles by using ambient air and pureoxygen by consideration of gas transfer coefficient.3. To study the effect of operation cycle ( effluent filtration and air diffusion ) inmembrane bioreactor to prolong the operational life of membrane bioreactor.

4. To find out the optimum operating condition for 0.2 m-pore size of membrane.5. To study the treatment efficiency and operational stability of the membranebioreactor.

1.3 Scope of the study

1. This study was carried out in laboratory-scale.

2. Polyethylene hollow fiber (PHF) membranes of 0.2 m pore size were used in thisstudy.3. Actual septic wastewater collected from septic tank of public apartment in the areaof Pathumthani province was used as feed substrate.4. COD, MLSS, MLVSS, Turbidity, DO, pH, Temperature, TKN, NO3

--N, NO2--N

and total phosphate were monitored regularly to observe the reactor temporalperformance. In addition, the permeate flux and transmembrane pressure were alsomonitored to assess the reactor performance.


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Chapter 2

Literature Review

2.1 Fundamentals of Activated Sludge Process

The activated sludge process make use of the suspended biomass to stabilize,biochemically, organic waste in wastewater with the presence of oxygen. The aerobiccondition is achieved by the use of diffused or mechanical aeration, which also serves tomaintain the mixture called mixed liquor in completely mixed regime. After a specificperiod of time, the conversion of organic wastes to the more stabilized substances take placeand provide the desired quality of water.

Extended aeration is similar to the conventional activated sludge process with theexception of the operation in endogenous respiration of growth curve. The processoperations prefer the low organic loading, long aeration time and low F/M ratios. Due to thestated operations, the sludge problems can be overcome in view of small amount of wastesludge produced which need to be carried and good sludge characteristic for dewatering

unit.

2.2 Fundamentals of Microfiltration

Microfiltration membranes are applied for separation of particles within the range of

0.02-10m. High pressure driven force allows the passage of water through the membraneat the feed side and the tangential liquid flow promote the membrane cleaning at theinverse direction. Microfiltration membrane process is widely used in water and wastewatertreatment, which the present pollutants contain diverse particle sizes of colloids andsuspended solids.

Due to larger membrane pore size, higher flux is obtained in microfiltration systemcompare to the RO and UF. However, often MF membrane process faces possible internaland external pore clogging due to colloidal fraction, which lead to significant fluxreduction.

This problem can be overcome by selecting appropriate membrane pore size and byusing appropriate pretreatment techniques.

The operational mode of microfiltration can be classified into two types asshown in Figure 2.1.

1. Dead-end or conventional filtration (Vigneswaran et al., 1991)2. Crossflow filtration


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Figure 2.1 Dead- End Filtration and Crossflow Filtration

(Vigneswaran et al., 1991)

In conventional filtration the flow direction perpendicular to the filter medium,while the crossflow filtration has tangential flow to the membrane, the feed is along themembrane surface and the permeate is perpendicular to the feed. Thus this system is knownas cross-flow filtration.

2.3 Biological Nitrification

Nitrification is the conversion of ammonia nitrogen (NH4+-N) and some organic

nitrogen form to nitrate nitrogen (NO3--N) with nitrite (NO2

--N) formation as anintermediate and is performed by either heterotrophic bacteria or autotrophic bacteria.However, the major nitrifying bacteria are the autotrophic species, Nitrosomonas andnitrobacter which are common in soil and aquatic ecosystems. They derive energy for

growth from the oxidation of inorganic nitrogen compounds instead of oxidation of organicmatter.

The stoichiometric reaction of nitrification and assimilation become

55 NH4++5 CO2 +76 O2 ------ C5H7O2N +54 NO2

- +109 H++52 H2O

(2.1)

400 NO2- +195 O2 +5 CO2 +NH4

+ +2H2O------- C5 H7 O2 N +400 NO3- +H+

(2.2)

It is seen that approximately 3.22 mg O2 will be required for each mg of NH4+-N

oxidized to NO2--N, and 1.11 mg O2 will be need for each mg of NO2

--N oxidized to ofNO3

--N for a total of 4.33 mg O2 mg of NH4+-N oxidized all the way to NO3

--N.It is generally accepted that the specific growth rate ofNitrobacter is higher than the

growth rate ofNitrosomonas and hence there is no accumulation of nitrite in the process


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and the growth rate ofNitrosomonaswill control the overall reaction. (Medcalf & eddy,1991)

2.4 Denitrification

Denitrification is a biochemical reaction which involves the reduction of nitrate or

nitrite, present in water, to gaseous nitrogen compounds such as nitrogen gas, nitrous andnitric oxides and is carried out by facultative heterotrophic bacteria under anoxicconditions. There are also certain autotrophic bacteria that denitrify using an inorganicenergy source.

The principal genera arePseudomonas, Micrococcus, Achromabacter andBacillus,which were reported as abundant in sewage. Denitrification offers a mechanism of not onlyremoving nitrogen in a non-polluting form, but also oxidizing organic matters in theprocess. Thus the oxygen which has been supplied in nitrification can, in principle, beeffectively recovered and reused in denitrification. Nitrate readily replaces oxygen aselectron acceptor because the pathway for the transfer of electrons from the organicsubstrate to the final electron acceptor is similar, but the presence of dissolved oxygen acts

as a strong inhibitor on denitrification as it prevents the formation of the enzyme necessaryfor the final electron transfer to nitrate.

There are four conditions that are necessary for denitrification :(Medcalf & eddy, 1991)

1. Presence of nitrate

2. Absence of dissolved oxygen

3. Bacterial mass that can accept nitrate and oxygen as electron acceptor

4. Presence of a suitable electron donor ( energy source )

2.5 Phosphorus Removal from Wastewater

2.5.1 Chemical Phosphorus Removal

Chemical P removal can be achieved by addition of cation, which will causeprecipitation of phosphorus holding wastewater. Lime is less used now because it produceslarge quantity of sludge and alkaline effluent. Alum has also been used, however its activepH condition is slightly acidic. The optimum pH for activated sludge is neutral range so pHadjustment is then required prior to precipitate by alum. Metal precipitant, FeCl3 orFe(OH)3, or coagulant was then selected on the performance of phosphorus removal inwastewater treatment. Jar test have to be conducted in order to determine which chemical

and at what dosing level give optimum results. The stoichiometric of ferric salt was shownin the following equations.

FeCl3 Fe+3 +3 Cl- (2.4)


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Fe+3 +PO4-3 FePO4 (2.5)

Phosphorus can be removed by chemical dosing at:

1. Primary settling tank2. Activated-sludge aeration tank

3. Prior to secondary sedimentation tank

4. After secondary sedimentation tank

Coagulation upstream from primary settling tank results substantially reduction inorganic loads on secondary treatment unit because considerable proportion of solids andcolloids together with some soluble material were removed. However, greater chemicalusage is required and primary sludge production is increased significantly. It has been foundthat organically-bound phosphorus is not easily precipitated and therefore complete

phosphorus removal might not be achieved.

Chemical addition as a tertiary treatment is probably a valuable alternatives whenhigh quality of effluent is required to achieved reliable total Phosphorus standards below0.5 mg-P/L. However, addition of solids polishing facilities are then required in order toreduce solids loss to the effluent. Dosing directly into an aeration tank or prior to asecondary sedimentation tank is likely to be preferred in situations as: (Cooper et at., 1994)

1. It need less chemical

2. Organically-bound phosphorus is oxidized and precipitated in the aeration tank

3. Less excess sludge is produced

2.5.2 Biological Phosphorus Removal

The basic mechanisms is to create the alternative conditions of anaerobic andaerobic or oxic stages. Under the anaerobic conditions the growth of particular strains ofbacteria such asAcinetobacter is selected. Energy uptake under these condition is gained byhydrolysis of polyphosphates stored in the cells. The hydrolyzed polyphosphates are thenreleased out from the cell into the liquid as Orthophosphates. During the aerobic stage thesoluble phosphorus is taken up and stored as polyphosphate in order to produce energy fortheir cells.

The unit processes of biological phosphorus removal can be applied in differentways. Some of the processes include the anaerobic stage within the existing activatedsludge process. This is called water-line phosphorus removal process.

Other processes, particularly for PhoStrip, create an anaerobic stage outside theexisting activated sludge plant where some part of recycled activated sludge is stripped ofits phosphorus and then returned to the aerobic activated sludge plant to take up more


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phosphorus. This is called the sludge-line phosphorus removal process. (Cooper et at.,1994)

2.6 Microfiltration Membrane in Domestic Wastewater Treatment

Many researchers have studied the application of membrane technology in domestic

wastewater treatment. In conventional treatment, membranes can be inserted at threelocations; namely: (Vigneswaran, 1991)

1. After the primary sedimentation

2. In the activated sludge tank

3. After the secondary sedimentation, in the tertiary treatment with or without

pretreatment.

Combination of membrane together with the activated sludge process is used forseparating of liquid from solids. This process performance provide attractive results such as:

1. SS are totally eliminated through membrane separation

2. Settleability of the sludge has no effect on the quality of the treated water

3. Adequate sludge retention time (SRT) which allow the proliferation of low

growth rate microorganisms such as nitrifying bacteria

4. Maintain of high concentrations which over all activities level can be raised due

to high concentration of the dispersed microorganism maintaining in the bioreactor as long

as possible, high concentrations create a favorable environment for endogenous thereby

ensuring high treatment efficiency

5. This method can produce bacteria and virus free treated water.

Because of all solids are retained in the bioreactor and long SRT, dissolved organicsubstances with low molecular weights can be taken up, broken down and gasified bymicroorganisms or converted to polymers as bacterial cell. Thereby, improving the qualityof treated water could be achieved. The retained polymeric substances can be biodegraded,then provide less accumulation of substances within the treatment process. (Yamamoto,1994)

2.7 Application of Membrane Bioreactors in Domestic Wastewater Treatment

Talat (1988) investigated hollow fiber microfiltration for solid-liquid separationfrom the aeration tank of an activated sludge process. The variation of 3 parameters of pore

size (0.1, 0.2 and 0.45m), MLSS in the reactor (5000, 10,000 and 20,000 mg/L) and


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suction pressure (1.36, 2.72, and 7.5 m head of water) were conducted during a short termexperiment in order to find out the suitable mode of operation for long term experiment.

The short term result shown that at 10:10 intermittent operation provided the best conditionfor the stable flux.In long term experiments, membrane modules were regulated at constant flux of 1.5, 2.5and 3.5 L/m2.h and the corresponding increase in suction pressure was recorded.

Volumetric organic loading of 3 kg COD/m3.d shown critical condition toward theseparation process. However, loading of 2 kg COD/m3.d appeared to provide most suitablecondition since the COD removal efficiency was upto 95-97%.

Nitrification and denitrification was achieved 100% and 30-40% respectively. Undersimilar operating conditions, the removal efficiency were independent of the membrane

pore size. The 0.45 m membranes which operated at lower suction pressure than the 0.1

m membrane under similar operating conditions can provide the highest flux (3.5 L/m2.hor 0.084 m3/m2.d) and similar in clogging characteristic to others. Low value of Y, kd andF/M ratio showed very small sludge production. The 100% removal of fecal coliform can be

achieved by using 0.1 and 0.45m membrane filters.

Yamamoto et al.(1989) investigated direct membrane separation by using 0.1mpore size hollow fibers which was immersed in the aeration tank, regardless of usingsecondary sedimentation tank for solid/liquid separation. The treated water was filtered bysuction with various operation modes. Continuous suction exhibited dramatic flux decreaseas well as high MLSS together with high pressure difference. Intermittent suction at lowpressure (13 kPa) provided good result in order to prevent the unrecoverable clogging andto prolong the operating time without cleaning. COD removal can be achieved higher than95% while the nitrogen removal can be reached up to 60% by investigating the intermittentaeration mode.

Chiemchaisri (1990) investigated an activated sludge using 0.1m hollow fibermembrane modules for solid liquid separation. This study was conducted to treat lowstrength wastewater from AIT domestic wastewater. Comparison of the membranebioreactor under different operating conditions, such as non-aerated and aerated, withdifferent initial hydraulic retention time (HRT) of 1, 3 and 6 h which providedcorresponding permeate flux of 4.17, 1.38 and 0.7 L/m2.h was studied. The process wasoperated at 10:10 intermittent time. From the experiment, it can be seen that the non-aeratedbioreactor has an advantage over the aerated condition at an initial HRT of 3 and 6 h, sincelower energy consumption was required while giving similar effluent quality and processstability. However, at lower HRT of 1 hour (or higher permeate flux, 4.17 L/m2.h) aerationis required in order to prevent membrane clogging.

This highest flux of 4.17 L/m2.h seem to be a critical value since creating severe clogging

condition.

At lower flux, no clogging was observed under non-aerated and aerated conditions.The quality of permeate in term of COD was independent of the low volumetric organicloading at the range of 0.2-2 kg COD/m3.d. Because of the long solid retention time (SRT),the process was stable and steady, COD removal efficiency was similar in everyexperimental conditions.


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The performance of 0.03 m pore size with 9 m2 surface area of hollow fibermembrane was also investigated in pilot-scale unit. Two hollow fiber membranes modulewas immersed in an aeration tank which feed with diurnally AIT domestic wastewater. Thesuction pump was used at 10:10 minute intermittent operation to extract the permeatethrough the membrane as shown in Figure 2.2. For jet aeration, the effect of jet aerationperiod (1/2 and 1 h) and jet aeration pattern 15 minutes for two times a day and 30 minutes

for once a day was investigated. The jet aeration flow rate used was 20 L/min.

In the bioreactor consist of 2 zone: aerobic and anaerobic on top and at bottom ofthe reactor respectively. consequently, low MLSS in aerobic zone of which it could reducethe clogging problem of membrane.

Figure 2.2 Schematic of Pilot-Scale Membrane Bioreactor (Chiemchaisri, 1990)

The mean hydraulic retention time (HRT) was determined after the permeate fluxreached steady state. At the flux 4.17 L/m2.h has reached, average HRT of 1 day wasobtained under diurnal varied loading. Diurnal variation in loading play a minor role in thenitrification process since more than 80% nitrification can be observed throughout theexperiment.

The MLSS in the bioreactor was affected by the aeration flow rate and optimum air

flow rate in this experiment was taken as 7.5 L./min. which provided sufficient oxygen forthe microorganisms and maintain low MLSS in the aerobic zone.

Maythanukhraw (1995) applied 0.1 m hollow fiber membrane directly in thereactor for solid-liquid separation to treat domestic wastewater from AIT campus, Bangkok.For short term experiments, the effect of transmembrane pressures, intermittent mode ofoperation and duration of air diffusion were investigated to find out the optimum conditionswhich corresponding to high and constant flux obtained. Variation of transmembranepressures were studied with values of 13.3, 21.3, 32.0 and 41.0 kPa. It was found that using13 kPa transmembrane pressure was a limiting pressure for all experiments. The different of


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operation modes were studied by varied the duration of effluent filtration and air diffusion:5:5, 10:10, 15:15, 30:30, 60:60 and 15:15* (15* =15 minute without sending air).

The results shown that at the operational mode of 15:15 provided the best results.Although cyclic operation with air diffusion could not completely remove the clogging, airbackflush technique in this mode of experiment could improve the flux by up to 371 %compared to the continuous operation. At 15:15 operation mode, further experimental runwas continued to find out the optimum air diffusion duration on that mode. By varied thevalues of 15:5, 15:10 and 15:15, the best performance can be observed under 15:10 mode.Considering both recovery of permeate flux and net cumulative flux. To study the processperformance of long term experiment under the optimum conditions obtained from shortterm experiment , the effects of HRT at 12, 6 and 3 hr. attributed to variation of volumetricorganic loading. At 3 hr.- HRT provided fluctuation of volumetric organic loading in rangeof 1.9-2.1 kg.COD/m3-d.

Transmembrane pressure was increased according to the cake formation on themembrane. So, after the long period of experimental runs the steep increasing intransmembrane pressure can be observed even operated under air backflushing. The

periodically chemical membrane cleaning was needed in order to recover the permeate flux.The permeate flux contains good quality in term of the very low SS.

Since the infinite SRT was operated, more than 90% of COD with effluentconcentration below 20 mg/L was achieved in all runs. The TKN removal was more than 90%, and total phosphate removal around 50 % in all experimental runs. The MLVSS/MLSSin the bioreactor was in the order of 20-30%. Inorganic mass balance calculation indicated asteady accumulation within the reactor. The lower fraction of active microorganisms in thebioreactor did not show any significant effects on the process efficiency.

Nevertheless, it is anticipated that in longer run it might affect the process, thus it isadvisable to have periodic sludge draining.

To use the membrane as an air diffusers, the compressed air pressure should be highenough to produce steady stream of micro-air bubbles according to the bubble pointconcept. One way to overcome this problem was to use relatively large pore sizemembranes. The membrane cleaning process which was adopted in this study was found tobe adequate to remove mainly the external membrane resistance. It is necessary to havechemical cleaning procedure for complete elimination of internal and external resistances,which mainly caused by the macromolecular adsorption. Longer air diffusion will improvethe recovery of permeate flux. However, by considering both the recovery of permeate fluxand net cumulative flux 15:10 operational mode gave better results than the 15:15.

2.8 Clogging Mechanisms in Microfiltration

The following four different types of clogging mechanism are observed in afiltration system:

1. Complete blocking occurs when particles plug the capillaries;


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2. Cake filtration involves the formation of a porous layer on the membrane surface

which poses an additional resistance;

3. Standard blocking occurs when solids adhere to the walls of the capillaries which

reducing their internal diameter. Standard blocking was observed to be the most common in

the absence of cake filtration.4. The forth mechanisms was called intermediate blocking because of the rate of

blocking falls between cake filtration and standard blocking.

2.9 Declogging Techniques

In microfiltration system, particle deposition and internal clogging cause majoroperational problems in membrane filtration. These depositions cause permeate fluxreduction in addition to decrease membrane life span. There are two simple ideas ofdeclogging technique which are described as followed :

1. prevent particles reaching the membrane surface or

2. flush the deposited out

Backflushing technique is used in order to achieved higher membrane processefficiency. The rate of permeate flux would be increased by backflushing. A high pressureof air is applied from the permeate side in order to removed the deposits out of themembrane surface. For gas backflushing, gas is brought to pressure in the lumens frompermeate side and then explodes through the membrane wall whereby the boundarylayer is released and can easily be transported away as shown in Figure 2.3. This

results in a very efficient cleaning of the membrane. The effects of backflushing with gas inpermeate flux is shown in Figure 2.4


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Figure 2.3 Module Backflushing with Gas ( Peters & Pederson, 1990)

Figure 2.4 Effect of Gas Backflushing during Wine Filtration

(Vigneswaran et al., 1991)

2.10 Fundamentals of Gas Transfer

Gas transfer is defined as the process by which gas is transferred from gas phase toliquid phase. Oxygen transfer in the biological treatment of wastewater is the most commonapplication in field of wastewater treatment. Due to the low solubility of oxygen, normalsurface air-water interfaces can not provide sufficient oxygen. To satisfy therequirement of aerobic waste treatment, aeration devices are used to create additional gas-liquid interface.

The rate of molecular diffusion of a dissolved gas in the liquid depends on thecharacteristics of the gas and the liquid, temperature, concentration gradient and the cross-sectional area across which diffusion occurs. Equation (2.5) is used to explain Figure 2.5.

rm=Kg.A (Cs-C) (2.5)

where rm = rate of mass transfer ( mg/s.)

Kg = coefficient of diffusion for gas (L/m2.s.)

A = area through which gas is diffusing (m2)

Cs = saturation concentration of gas in solution (mg/L)


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C = concentration of gas in solution. (mg/L)

Under the conditions of mass transfer encountered in the field

rm = V.dC/dt (2.6)

So, Equation (2.6) can be written as :

rc =dC/dt =Kg(A/V) (Cs-C) (2.7)

where Kg(A/V) =KLa (2.8)

Therefore Equation (2.7) can be written as :

rc=dC/dt = KLa (Cs-C) (2.9)

where rc =change in concentration, mg/L.s

KLa =overall mass-transfer coefficient, s-1

Cs =saturation concentration of gas in solution, mg/L

C =concentration of gas in solution, mg/L

V = volume of gas (L)

Equation (2.9) could be modified to the practical form which is presented as follow :

Log (Cs-CL) = Log (Cs-C0) - (KLa/2.3)* t (2.10)

where,

C0= initial concentration of gas in liquid phase

CL = concentration of gas in liquid phase

For a given volume of water being aerated, oxygen-transfer can be evaluated on thebasis of the quantity of oxygen transferred per unit of air, which is introduced to the waterwith equivalent conditions. (temperature and chemical composition of the water, depth atwhich the air is introduced, etc.)


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To measure oxygen transfer in clean water, the accepted testing procedure involves

the removal of oxygen (DO) from a known volume of water by addition of sodium sulfitefollowed by reoxygenation near the saturation level belonging to the water temperature.

The DO of the water volume is monitored during the reaeration period by measuring DOconcentration at several different points. The data obtained were then analyzed to estimatethe apparent volumetric mass-transfer coefficient, K

La. These estimates of various point of

DO are adjusted to standard conditions.

Figure 2.5 Schematic of Two-Film Theory of Gas Transfer ( Metcalf & eddy, 1991)

2.11 Applications of Gas Diffusion through Membrane

Semmems et al. (1991) developed a bubbleless hollow-fiber membrane aerator andtested for oxygenation of water . The aerator houses a bundle of sealed, hollow, gaspermeable fibers that are filled with pure oxygen under pressure. By considering Equation(2.9) for mass transfer; raising the oxygen transfer efficiency could be accomplished byincreasing KLa and Cs values. Eventhough, practically the external surface area ofmembrane is fixed. Therefore large KLa can be achieved with the action of elongatedstationary bubbles of oxygen.

Very high value of Cs can also achieved if specially coated membrane are used, ofwhich it can be operated at pressures up to 60 psi with pure oxygen. At temperature of 10o

C and a flow rate of 75 gpm. through the pipe, dissolved oxygen of water is ranged from 0-

23.1 mg/L

Pierre et al. (1988) investigated bench-scale experiments using both dense polymermembrane and porous membrane as an aeration units. Bubble-free aeration usingmembranes has potential applications for wastewater treatment when conventional bubbleaeration gives unsatisfactory results, such as toxic volatile organic compound stripping orfoam production. In order to find out the design parameters of membrane aerators, bothtypes of membrane were immersed directly in well-mixed biological reactor.


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Three systems which were studied for these experiments based on different settingsof design parameters such as:

1. Specific oxygenation capacity

2. Mass transfer characteristics of the membranes

3. Type of gas and

4. Operating gas pressure

Three operation modes consist of A, B and C were conducted in order to define theessential design parameters for membrane aerator. Cases A and B used 5 and 9 bar. Case Cwas evaluated by using pressurized industrial oxygen.

The mass transfer analysis indicated that the high oxygen flux were based on surfacearea of the membrane. The optimal value of 38.3 m2-membrane per m3-aeration basin wassuggested to meet the aeration requirements of conventional bioreactor (100 g. O2 / m

3.h)while the high- rate bioreactor required 213 m2/ m3. The fraction of oxygen transferred is adesign choice. The value of 0.8 was much higher than that possible with conventionaldiffused or mechanical aeration. As the fraction of oxygen transferred is increased,however, the average driving force decreases.

Limitations of membrane aerator were due to the high capital cost according to themembranes. Furthermore, the membrane themselves seem to represent as an additionalresistance to oxygen transfer, which could be translated into high energy cost. However, itis suggested that applications should be developed with industrial oxygen in order to reducethe requirement of membranes.

Tariq & Semmems (1992) studied the mass transfer in a various pore diameter of

hollow fiber membrane aerator. Individually-sealed hollow fibers were filled with oxygenand immersed in a flowing stream of water. Three experiments have been checked bymeasuring the pressure drop, gas flow velocity and gas composition along the length of thefibers. Pressure drop was measured according to the difference between inlet and outlet.

The pressure drop due to friction across the 362 cm length was negligible. By calculating,the optimum operating pressure was below 1-3 psi. Gas flow velocity inside the fiberdepend on the mass transfer coefficient. The gas flow velocity was ranged from 0.008cm/sec to 0.03 cm/sec.

The decrease of oxygen partial pressure inside the fiber was observed along thefiber length. Better oxygen transfer could be achieved by pumping oxygen continuouslythrough the hollow fibers in addition to removing accumulated nitrogen due to the back

diffusion from outside to inside the fibers. Feeding of pure oxygen encourages nitrogenenter the fiber. Since partial pressure of nitrogen within the fiber is less than external partialpressure of approximately 0.79 atm, back diffusion along the fiber occurred.

However, the nitrogen back diffusion rate dose not increase significantly whenoperated with high water flow velocity outside the fiber and high oxygen feed flow rateinside the fiber in order to decrease the nitrogen concentration gradient and increase oxygenconcentration gradient between outside and inside the fiber respectively.


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Chapter 3

Experimental Set Up

The experiments that were carried out in this study can be classified into three partsas follows:

1. Gas transfer efficiency of aeration units and measurement of initial membraneresistance

2. Short term experiments3. Long term experiments

3.1 Measurement of Gas Transfer Efficiency of Aeration units

3.1.1 Materials Used

- Ordinary ceramic porous diffusers

- Micro filtration Membrane

For this study, a microporous hollow fiber membrane was used. It is produced from

high density polyethylene with the pore size of 0.2 m. Each membrane module wasassembled in plastic air tight cap to ensure the capability of operation under vacuumpressure as shown in Figure 3.1.

- Clean water

Tap water was prefiltered by using cartridge ultra-filter to produce clean water forthis test.

- Chemicals :

i) Sodium sulfite ( Na2SO3) : 60 mg/Lii) Cobaltous chloride ( CoCl2) : 0.2 mg/L

- Dissolved oxygen probe

One probe was used for measuring dissolved oxygen concentration. The samplingpoints were at the head, middle and at the end of reactor. The position of each point was atthe middle of water depth.

- Turbine mixer

One turbine mixer was used for promoting turbulence and homogenous mixture.Speed with value of 50 rpm. was used for this test.

3.1.2 Experimental Set up


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Figure 3.2 presents the experimental set-up. The porous diffusers and membrane are

immersed separately in acrylic-rectangular reactors which held a working volume of 80 L.The reactor was filled with clean water, and the increase in oxygen concentration wasmeasured by dissolved oxygen probe. Turbine mixer was used to promote turbulence.

Figure 3.1 Membrane Module Used for All Experiments

Figure 3.2 Experimental Set up for Gas Transfer Efficiency

3.1.3 Process Operation


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The effect of pressure was studied in batch operation in order to define theappropriate KLa. To compare the transfer efficiency of two aeration devices, the membranemodules and ordinary air diffusers, were operated as shown in Figure 3.1. The testingprocedure began with the removal of oxygen from water by addition of 60 mg/L sodiumsulfite (Na2SO3) with cobalt (CoCl2) as a catalyst. (Pierre, 1989)

The increase in oxygen concentration was measured during aeration under specifiedpressure, and the overall transfer coefficient was calculated from Equation. (2.10)

log (CS-CL) = log (CS - C0) - (KLa /2.3)* t (2.10)

when,

C0 = initial concentration of gas in liquid phase

CL = concentration of gas in liquid phase

The gas, which was sent to the testing unit, come from purified ambient air and pureoxygen. Dissolved oxygen was measured by using dissolved oxygen probe. Three samplingpoints are at the middle of water depth along the tank; head , middle and end. DO and watertemperature were recorded every three minute intervals until the dissolved oxygen reacheda constant level. The pressure which were applied in this study were 0.2, 0.4, 0.6, 0.8 and1.0 bar. The maximum KLa were then indicated the appropriate operating pressure for airdiffusion which was used for short term and long term experiments.

The effect of gas flow velocity which was sent to aeration devices was studied. Byvarying the velocity of 9, 14 and 18 L/min., the calculated maximum KLa then indicated theoptimum velocity which was used for short term and long term experiments.

3.2 Measurement of Initial Membrane Resistance

3.2.1 Material Used

- Membrane

Two microporous hollow fiber membranes produced from high density polyethylene

with the pore size of 0.2m was used in this study. Each membrane module is assembled in

plastic air tight cap to ensure the capability of operation under vacuum pressure.

3.2.2 Experimental Set up


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The measurement of initial membrane resistance was conducted by immersingmembranes in the rectangular reactor. Clean water was fed to the reactor. The speedcontrolled roller pump was used for extracting the permeate. The transmembrane pressurewas measured by mercury-filled manometer and the filtered water was recorded andreturned to the reactor to keep the volume constant during the experiment.


The relationship between the flux and transmembrane pressure is given in thefollowing equation :

J =P / Rm (3.1)

where,

J = flux (L/m2-h.)

= transmembrane pressure (kN./m2)

= viscosity (kN..s /m2)

Rm = apparent membrane resistance

= Rmo + Rd

where,

Rmo = initial membrane resistance

Rd = membrane resistance due to the deposition of solids

The modified equation to find the initial membrane resistance, when clean water is used, is:

P =.Rmo.J +Po (3.2)

Po = initial transmembrane pressure required to over come the air blocking

effect.

So, by modifying the transmembrane pressure and measuring the permeate flux, thevalue of Rmo could be determined.3.3 Short Term Experiments



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- Membranes

Microporous hollow fiber membranes produced from high density polyethylene with

the pore size of 0.2m were used in this study. The properties of the membrane was similarto that used in measurement of membrane resistance. Each membrane module is assembled

in plastic air tight cap to ensure the capability of operation under vacuum pressureapplication.

- Feed substrate

Glucose solution and tap water were used for concentrating or diluting thewastewater respectively to the desired COD concentration.

- Biomass culture

The seed microorganisms which was used in these experiments was obtained from

the acclimatization of biomass which initially exist in septic wastewater.

3.3.2 Experimental Set-up

- Acclimatization unit

Acclimatization procedure of these biomass to raw septic wastewater was properlyconducted by daily fill and draw operation before being placed in the reactor. Glucosesolution was fed in order to concentrate biomass mixture in the acclimatization unit. Byconsidering the initial characteristics of raw influent septic wastewater which presents in

Table 3.1, glucose solution was prepared and fed daily in proportion to maintain CODconcentration around 5,000 mg/L. Stock solution of 98.2 g-glucose/L is equivalent to 100 g-

COD/L.

Table 3.1 Characteristics of raw septic wastewater

Parameters Values Units

1. Settled solid2. Suspended solids3. COD4. Filtered COD5. BOD6. Filtered BOD

7. Total nitrogen8. Total phosphate9. pH

7002,200-3,5003,500-5,500

300-400600-1200158-560

240-35030-388.88

mL/Lmg/Lmg/Lmg/Lmg/Lmg/L

mg/Lmg/L

- Membrane bioreactor

The schematic diagram of the experimental set up was similar as presented inFigure 3.2. Membranes were immersed directly in the reactor of activated sludge system.


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Speed controlled roller pump was used for extracting the permeate while the gas was sentthrough the membrane module for declogging purpose. This operation was carried outalternatively. The intermittent extraction and air injection was then controlled byintermittent controller and solenoid valves. The transmembrane pressure was measured bymercury manometer. The filtered water was then returned to the reactor to keep the volumeand concentration constant during the runs.

Compressed air which was sent through membrane had to pass air filter to removeoil vapor and was maintained at 1 bar. Compressed air was also sent to the stone diffuserslocated at the bottom of the reactor. The air flow rate was controlled at 14 L/min. by airflow meter while the permeate flux was then recorded. The main purpose of aeration wereto :

1. Allow homogenous mixture in the reactor

2. Provide additional dissolved oxygen for biomass

3. Remove or shear off solids deposited on the membrane surfaces


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Figure 3.3a Schematic of Membrane Bioreactor Set Up

Figure 3.3b Actual Membrane Bioreactor Set Up

3.3 Process Operation

The effect of suction pressure was studied with 7, 23, 32 and 40 kPa. The operatingconditions for these test were then fixed at 15:15* and 15:15 operation mode. ( 15:15* =15


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minutes of filtration then 15* minutes without effluent filtration or sending any air diffusionwhile 15:15 =15 minutes of filtration then 15 minutes of air diffusion)

The effect of operation modes with air diffusion and effluent filtration were studied.The suction pressure of 7 kPa and 1 bar compressed air were kept for all the operationalmodes. All runs were conducted as batch experiments for a duration of five hours. Thefollowing five different modes were studied:

(1) 30:30 (2) 25:25 (3) 20:20 (4) 15:15 (5) 10:10

( 30:30 : indicates 30 minutes of filtration then 30 minutes of air diffusion)

The effect of compressed air for air diffusion was also studied. The operatingconditions for this test were kept at 15:15 operation mode with 7 kPa suction pressure. Fivedifferent runs were studied with 0.3, 0.5, 0.7, 1.0 and 2.0 bar

3.4 Long Term Experiments:


- Membranes

Two microporous hollow fiber membranes were used. The properties of membranewere similar to that used in short term experiments.

- Feed substrate

Septic wastewater collected by Department of Public Welfare of the LocalAuthority of Pathumthani Province was used as substrate for long term experiments.

- Biomass culture

The seed microorganisms which was used in these experiments was obtained fromthe acclimatization of biomass which originally exist in septic wastewater which obtainedfrom short trm experiments.

3.4.2 Experimental Set-up

This experimental set-up was similar to short term experiment. A working volumeof 80 L is maintained by using floating valve. Since the mixed level in the reactor droppeddue to the extraction of the permeate, then the same amount of substrate was then supplied

automatically from the influent storage tank to the reactor by using floating valve.

Roller pump was also used for extracting the permeate through one of the membranemodules conducted at the optimum operational frequency, identified in the earlier shortterm experiments. This operation was carried out alternatively which was controlled byintermittent controller and control valve. (Maythanukhraw, 1995)



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The effect of HRT was studied in this experiment. The initial HRT of membranebioreactor process was varied at 7, 12 and 26 hours by controlling the effluent permeatefluxes. The change in transmembrane pressure was monitored everyday to monitor theclogging of the membrane during the long term operations. The effect of sludge drainingwas also studied with SRT equal to 50 days. This could be achieved by draining 1.6 liters ofmixed liquor out of the bioreactor everyday.

3.5 Analytical Methods

The analysis during these experiments were carried out following the procedurestated in APHA, AWWA, JPWCF, 1991 except for NO3

--N and NO2- determination. Nitrate

and nitrite analysis were analyzed by sodium salicylate and napthylamine method,respectively. Total phosphate in sludge was determined by digestion or oxidation technique,which convert all forms of phosphate such as orthophosphate and condensed phosphate,both soluble and insoluble, and organic and inorganic species to the reactiveorthophosphate. Then reactive orthophosphate was measured during colorimetricdetermination steps. This step involves the formation of intermediates (molybdophosphoricacid), which is reduced to the intensely colored complex substances.

Table 3.2 Parameters Analyzed( Long Term Experiments)

Parameters Frequency Sampling pointsCODpHDO

TurbidityTemperatureMLSS & MLVSS

TKNNO2

--NNO3--N

Total- Phosphatestandard plate count

Total coliformFecal coliformSludge characteristics :

TS, TVS, CST, specificresistance etc.

everydayeverydayeverydayeverydayeverydayeverydayonce in two daysonce in two daysonce in two daysonce in two daysonce in each runonce in each runonce in each runonce in each run

Inf / EffInf / Reactor / EffReactorEffReactorInf / EffInf / EffInf / Reactor / EffInf / EffInf / EffEffEffEffReactor

Remarks : Inf =Influent ; Eff =Effluent


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26

Chapter 4

Results and Discussion

4.1 Gas Transfer Efficiency of Aeration Units and Initial Membrane Resistance

4.1.1 Gas Transfer Efficiency of Aeration Units

Before conducting the experimental runs, the measurement of gas transfer efficiency ofstone diffusers and membranes were determined. Their oxygen concentration versus time wasrecorded and gas transfer coefficient (KLa) values were then calculated corresponding to theEquation (2.10). Figure 4.1 and 4.2 present the relationship between oxygen concentration andtime of ambient air and pure oxygen at different flow rate. (Results are summarized in Table A-1 and A-3 in Appendix A)

It was found that the dissolved oxygen in water was increased with time until it reachedthe saturation level, which is related to the water temperature. For this experiment, thesaturation level of oxygen concentration when using ambient air is 7.423 mg/L (30.9 o C) and37.36-37.81 mg/L (Metcalf & Eddy, 1991) for pure oxygen. Both using ambient air and pureoxygen, the higher diffusion rate of air to water could be observed with the higher flow rates.

Figure 4.3 and 4.4 show the relation of Cs-Ct with time and based on this data the gastransfer coefficient, KLa, was calculated. Detail experimental results and calculations arepresented in Table A-2-1 to A-2-3 for ambient air and A-4-1 to A-4-3 for pure oxygen ofAppendix A. These results present higher slope with higher flow rate of gas which was sentthrough diffusers. It can imply that higher diffusion rate of gas phase to liquid phase wasobtained.

Figure 4.5 presents the comparison of gas transfer coefficient at 20 oC between ambientair and pure oxygen for stone diffusers. From these results, it can be deduced that the gastransfer coefficient of ambient air, from gas phase to liquid phase increases with higher air flowrates.

Figure 4.6 presents gas transfer coefficient when using ambient air with membrane

diffusers. By comparison these experimental results in Table 4.1 and 4.2, the maximum gastransfer coefficient was 28.05 h-1 when using 1 bar of ambient air. It clearly demonstrated theeffectiveness of the membrane module as an air diffuser in comparison to the conventionalstone diffusers.


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27

4.1.2 Initial Membrane Resistance

The relationship between filtration flux and transmembrane pressure was investigatedwith microfiltered water. The initial membrane resistance of each membrane module was thendetermined, prior to using them in the actual experimental runs.

As shown in Figure 4.7.1 and Figure 4.7.2 ( Results are summarized in Table A-7-1

and A-7-2 in Appendix A), the permeate flux was directly proportional to the transmembranepressure. This corresponds to the relationship given in Equation (3.2).

It was found that the initial membrane resistance (Rmo) of membrane module 1 andmodule 2 were 8.01 x 1012 m-1 and 9.26 x 1012 m-1 at 24.7o C respectively. The flux obtainedfor membrane module 1 and 2 were 310 L/m2.h and 350 L/m2.h respectively.

Rm values are useful not only for modeling purposes, but also for evaluating theeffectiveness of the cleaning procedure and for designing long-term operation stability of themembrane which is discussed in the next section.

4.2 Short Term Experiments

4.2.1 Effects of T ransmembrane Pressure on Permeate Flux

This test was conducted without air backflushing operation. Two membrane moduleswere used with alternative operation of effluent filtration and then stopping the filtrationprocess. The intermittent mode of 15:15* was used for this run.

Figures 4.8 and 4.9 show the relationship between permeate flux and time at differenttransmembrane pressure without sending air through the membrane ( Table B-1 to B-4 inAppendix B). The flux seem to be constant during 5-hour operation. At any rate, the higher fluxcould be obtained when the transmembrane pressure was increased. The final flux of thisexperimental performance at different suction pressures is presented in Table 4.1.

Table 4.1 Comparison of Performance of Different Suction Pressure with 15:15*Operation Mode and without Air Diffusion

Suction Pressure

(kPa)

Total Average Flux 1

(L/m2.h)

Final Average Flux 2

(L/m2.h)

7

23

32

40

3.42

5.67

5.79

7.46

3.63

5.54

5.83

7.25

Note:

Comment:


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28

15* = 15 minutes without sending air1 = Average flux at 5th hour2 = Total average flux in 5th hour

Figure 4.10 ( Appendix B) shows the comparison of permeate flux between applying 15minutes of air diffusion and not applying air diffusion. The flux was almost constant in both

applications. Figure 4.11 and Table 4.2 show the comparison of flux at different suctionpressure between the application with and without air diffusion. From these results, higher fluxcould be obtained when increasing suction pressure.

Table 4.2 Comparison of Performance of Different Suction Pressure with

15:15 Operation Mode and 1 bar Compressed Air

Suction Pressure

(kPa)

Total Flux

(L/m2.h)

Final Flux

(L/m2.h)

Percentage

Improvement in Flux

with without Air

Diffusion3

7

23

32

40

3.08

5.63

6.13

7.29

3.17

5.50

6.04

7.46

no improvement

no improvement

4

3

Note: 3 = Percentage improvement based on final flux

4.2.2 Effects of Operation Time

By considering data from Table 4.1 and 4.2, the operation without sending air seem toprovide higher flux when compared to the operation with air diffusion. The effect of operationtime was then studied by extent the operation time from five hours to ninety hours. Thisexperimental performance was conducted in order to clearly demonstrated the stability ofpermeate flux between applying and not applying air through membranes. This experimentalresult is presented in Figure 4.12. By operating without air diffusion, permeate flux declinedapparently after ten hours of operation. In turn, operation with air diffusion provide constantpermeate flux beyond this critical period of time. Due to the fixed membrane module 2 whichcaused lower performance ability, different value of permeate flux between module 1 and 2was obtained and presented in Figure 4.12.

4.2.3 Effects of I ntermittent Mode of Operation


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29

During short term experiments, the effects of different operation modes on permeateflux were investigated. Two membrane modules were used in alternative function of airdiffusion and effluent filtration. In all these experimental runs, transmembrane pressure wasmaintained at the critical value of 7 kPa. The MLSS values in all cases were maintained in therange of 12,500-12,875 mg/L.

Figure 4.13 to 4.14 (Table B-5 to B-9 in Appendix B) show the performance of

different modes of operation. There were 12 and 10 percent of flux improvement in 10:10 and15:15 operation mode respectively. Results are summarized in Table 4.3. Based on this results,it can be concluded that flux was improved with high frequency of intermittent operation. Asthe study which was conducted by Maythanukhraw (1995), air diffusion improves permeateflux by:

(a) It removes the surface deposition or external deposition and prevents thecompaction of cake layer under filtration pressure.(b) It removes the particles which clogged the pore of membranes.

Among different modes of operational test, 10:10 ( 10 minutes for effluent filtration and10 minutes for air diffusion) operation mode presents the highest improvement in flux. In anycase, accumulation of permeate volume was less because effluent filtration time was wastedduring the lag time to have filtered water through the module, after each removal operation. For

longer frequency, 60:60, provided longer air diffusion time which will increase the cleaningability, however this will allow high solids compaction on membrane surface due to longereffluent filtration time. By considering all these factors, 15:15 operation was selected as theoptimum condition for long term experiments.

Table 4.3 Comparison Performance of different Modes of operation withSuction Pressure at 7 kPa and 1 bar Compressed Air

Mode of Operation Total Average Flux

(L/m2.h)

Final Average Flux

(L/m2.h)

Percentage

Improvement in Flux

compare with Table

4.1

10:10

15:15

20:20

25:25

30:30

4.13

3.83

3.38

3.25

2.83

4.08

4.00

3.54

3.50

2.96

12

10

no improvement

no improvement

no improvement

4.2.4 Effects of Compressed Air Pressure for Backflushing


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30

The effects of compressed pressure air on the permeate flux stability were also studied.As shown in Table 4.4 ( Table B-10 to B-14 Appendix B), the highest flux could be obtainedat the performance by backflushing with 1 bar compressed air. Due to the limitation ofmembrane stability, application with 2 bar compressed air was not completely operated.Although membrane module has repaired with quick-set epoxy resins, it did not functionproperly as its original status.

In any case, increasing compressed air which was sent through membrane cantheoretical improving the surface cleaning of membrane. For the long term experiments, 1 bar-compressed air was selected to achieve the membrane backflushing.

Table 4.4 Comparison Performance of Different Compressed Air Diffusion

with Suction Pressure at 7 kPa and 15:15 Operation Mode

Compressed Air

(bar)

Total Average Flux

(L/m2.h)

Final Average Flux

(L/m2.h)

Percentage

Improvement in Flux

Compare with the

Table 4.1

0.3

0.5

0.7

1

3.92

3.75

3.29

3.83

3.92

3.54

3.38

4.00

8

no improvement

no improvement

10


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31

4.3 Long Term Experiments

4.3.1 Characterization of Acclimatized Sludge

Characteristics of sludge were determined after the sludge was acclimatized at the startof the experimental run. For this test, MLSS and COD were recorded with time in order todetermined the biomass built up rates and COD reduction or substrate utilization rates. The

results were presented in Figure 4.15 and Table 4.5.

400

500

600

700

800

900

1000

1100

1200

0 10 20 30 60 120 180 1380 1860

Time (min.)

EffluentFiltered-C

O

0

1000

2000

3000

4000

5000

6000

MLVSS

(mg/L

F-COD(mg/L) MLSS(mg/L)

Figure 4.15 Variation of MLSS and Effluent Filtered-COD with Time


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32

Table 4.5 Kinetics of Acclimatized Sludge Growth

Parameters Bioreactor Conventional ASP Units

Rate of bacterial growths1 , rg

Substrate utilization rate2, rsu

Sludge growth coefficient3, Y

Maximum specific growth

rate4 , m

Maximum rate of substrate

utilization per unit mass of

microorganism5 , k

Half-velocityconstant,

substrate concentration at

one-half the maximum growth

rate6, Ks

Endogenous decay

coefficient7 , kd

0.11

0.022

5.34

79.2

14.3

8651.5

2.68

-

-

0.4 - 0.8

-

2 - 10

25 - 100

0.025 - 0.075

mg-VSS/ L.s

mg-BOD5/s

mg-VSS/ mg- BOD5

d -1

d -1

mg -BOD 5/L

d -1

1 = rate of bacterial growths, rg

= ( Xi -Xo)/ (Ti -To)

2 = substrate utilization rate, rsu

= (So- Si)/ (Ti -To)

3 = sludge growth coefficient, Y

= (Xi -Xo)/ (So -Si )

4 = maximum specific growth rate, m

= (1/Ti)

5 = maximum rate of substrate utilization per unit mass of microorganism, k

= m/Y

6 = half-velocity constant, substrate concentration at one-half the maximum growth

rate, Ks


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33

=> rsu = - [k. Xi. Si ]/ [Ks +Si ]

7 = endogenous decay coefficient, kd

=> rg = -Y. rsu - kd . X

rg = net rate of bacterial growth, mg/L.s

= (Xii -Xo)/ (Tii -To)

By comparing typical kinetic coefficients of activated sludge process for septicwastewater with domestic wastewater (Appendix D), it was found that the values for septicwastewater are higher than domestic wastewater. These results are reasonable due to septicwastewater contained large portion of stabilized organic substances. It implies thatconventional activated sludge process for treating septic wastewater requires longer solid

retention time (c ) to meet effluent standard. Based on these results, membrane bioreactorwhich incorporated with physical and biological processes seem to be more appropriate fortreating this wastewater.

4.3.2 Effect of Hydraulic Retention Time

In these experiments, three different experimental runs (RUN 1, RUN 2 and RUN 3)

were conducted by keeping HRT at 26, 18 and 10.5 hours respectively. Reducing HRT wascontrolled by increasing permeate flow rate. In this study, septic wastewater was fed to thereactor continuously. Due to increasing permeate flow, average values of volumetric organicloading were fluctuated with 3.38 kg-COD/m2.d for RUN 1, 6.73 kg-COD/m2.d for RUN 2 and12.2 kg-COD/m2.d for RUN 3.

The intermittent operational mode of 15:15 (15 minutes filtration and 15 minutes airdiffusion) was used in all runs. In order to maintain log phase of biomass growth, activatedsludge was wasted 1.6 L everyday ( SRT =50 days).

4.3.2.1 Transmembrane Pressure

Figure 4.16a (Table C-4 to C-6 in Appendix C) shows the variation of transmembrane

pressure with time of membrane module 1 and 2 for different experimental runs. It shows thattransmembrane pressures increased with increasing permeate flow (Figure 4.16b). Theseincreased flux caused the increase in solid deposition of membrane surfaces. By consideringtransmembrane pressures in each run, it shows that transmembrane pressure in RUN 1 andRUN 2 of both membrane modules were almost constant through out the experiments.However, the sharp increasing of transmembrane pressure in RUN 2 can be observed after 14days of this run. After RUN 2 both membranes were cleaned ( 2nd cleaning) by air diffusion inclean water to remove the cake layer on the membrane surfaces. Transmembrane pressures ofRUN 3 were constant at early stage of this run, however transmembrane pressure began toincrease dramatically after 7th day.

Table 4.6 presents average values of transmembrane pressure for each experimentalruns. Once the membrane get clogged, it increases the transmembrane pressure which, again in


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turn, promotes membrane clogging. At this point, the air diffusion at 1 bar pressure isinsufficient to remove most of the deposited solid material on the membrane surface. Thisresults an unrecoverable dead end operation. Therefore periodic membrane cleaning was thennecessary in order to maintain required hydraulic retention time (HRT).

Table 4.6 Transmembrane Pressure of each Experimental Runs

Experimental Runs Transmembrane Pressure (kPa)

RUN 1

RUN 2

RUN 3

Module 1

13.9

38.6

58.8

Module 2

20.1

43.9

64.1

By considering biological solids concentration as shown in Figure 4.19a (Table C-7 toC-9 in Appendix C), activated sludge concentration increased with increasing volumetricorganic loading. Effect of activated sludge attachment on membrane surfaces was increasedwhen MLSS in the bioreactor became concentrated for each experimental runs. This occurrencecaused transmembrane pressure to be increased. In RUN 1 and 2, transmembrane pressureseem almost constant. The attached sludge on external membrane surfaces was detached due tothe agitation in the bioreactor and membrane air diffusion. It allows sludge to move and shearoff the solidified sludge from membrane which assist the cleaning abilities by membrane airdiffusion.

The rapid increasing of transmembrane pressure at the end of RUN 2 and in RUN3were due to sludge concentration in bioreactor became highly concentrated which close to thelimit concentration of activated sludge mixture. The limit for membrane filtration of activatedsludge mixture is 30,000 - 40,000 mg/L which called as Threshold Limits of SludgeConcentration (Yamamoto, 1994). This caused less abilities of sludge movement and

promoted cake compaction on membrane surfaces, especially among hollow fibers.

By considering percent suspended solids to total solids which is summarized in Table4.10, it was found that there were more than 98% of suspended solids to total solids. It imply

that most of contained solids in bioreactor has particle size more than 0.45 m which is largerthan membrane pore size. In order to maintain the stability of transmembrane pressure andpermeate flux, application of periodic jet aeration of clean water in membrane module orincrease the air diffusion pressure may assist to detach external deposition of sludge.

Figure 4.16b shows the variation of permeate flux with time for each experimental runs.These results have the same pattern with transmembrane pressure. Operating at lower HRTcaused higher permeate flux. Permeate flux of RUN 1and 2 were almost constant throughexperimental runs. For RUN 3 which was operated within threshold concentration of sludge,


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permeate flux was highly fluctuated. This fluctuation was due to the attachment andcompaction of sludge and detachment of solidified sludge. In any case, constant permeate fluxcan be observed for 5 days at the early stage of this run after membranes were cleaned.

4.3.2.2 Turbidity

Figure 4.17 (Table C-1 to C-3 in Appendix C) shows the effluent turbidity and

dissolved oxygen concentration for all three experimental runs. Influent turbidity is very highwhich contained more than 1000. There was no significant different in the permeate turbidity.The average turbidity for RUN 1, 2 and 3 were 1.1, 0.4 and 0.3 NTU, respectively.

This low effluent turbidity corresponds to 0 mg/L of SS, thus meeting well below the

required Thailand effluent standard of 30 mg/L. When compared to the study with 0.1 m byMaythanukhraw (1995), effluent turbidity by using 0.2 provided lower values for allexperiments. The reasons for this result can be delineated based on the operation with highconcentration of activated sludge. This operation caused high cake layer formation on theexternal surface of membranes. Cake layer assist to absorb the macromolecules whichcontribute to yellow color in the effluent. However, high yellowish color was observed at theinitial stage of membrane bioreactor operation, but later it became relatively clear which ispresented in Figure 4.21.

4.3.2.3 COD Removal

Figure 4.18 (Table C-7 to C- 9 in Appendix C) shows the influent and effluent COD ofRUN 1, 2 and 3. The influent COD was maintained in the range of 4000 - 6000 mg/L. Theremoval efficiency in all runs was maximum from the beginning which was greater than 90%.

The effluent COD can be explained by the following equation.

S =Ks (1+kd. c) (4.1)

c (yk - kd) -1

S = concentration of waste surrounding the microorganisms (mg/L)

Y = growth yield coefficientc = mean cell residence time (d)Ks = waste concentration at which rate of waste utilization per unit weigh of

microorganism in one-half the maximum rate (mg/L)k = maximum rate of utilization per unit weight of microorganisms (d-1)kd = microorganism-decay coefficient (d

-1)

Although 98% of COD removal was achieved by bioreactor, however more than 30mg/L of COD can be observed in effluent for all experimental runs. The remaining COD ineffluent probably presented in form of color.


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4.3.2.4 BOD

Figure 4.20 shows the variation of F/M ratio and BOD concentration for differentexperimental runs. The average value of F/M ratio for RUN 1, 2 and 3 were 0.25, 0.36 and 0.43kg-COD/ kg-MLVSS.d. Effluent BOD5 concentration was measured weekly. During all theexperimental runs the effluent BOD was maintained below the expected standard of 20 mg/L.

This means that effluent BOD5 is independent from the variation of F/M ratio. As this study

was operated with long SRT, c =50 days, therefore adequate sludge retention time allowsdissolved organic substances with both high and low molecular weights can be taken up,broken down and gasified by microorganisms or converted into polymers as constituents ofbacterial cells, thereby raising the quality of treated effluent.

4.3.2.5 MLSS

Figure 4.19a shows variation of biological solids concentration at different HRT. Table4.7 shows the average value of biological solids concentration of each experimental runs.

Table 4.7 Average Value of Biological Solids Concentration of each Experimental Runs

Experimental Runs Average value of Biological Solids

RUN 1

RUN 2

RUN 3

MLSS (mg/L)

17,181

25,675

39,560

MLVSS (mg/L)

13,485

18,757

28,781

% MLVSS/ MLSS

79.1

73.3

72.7

By increasing volumetric loading, MLSS and MLVSS were also increased whilepercent of MLVSS/ MLSS were decreased. The general decreasing trend of MLVSS/ MLSS inthe bioreactor could be explained by the following inorganic mass balance.

Input Output

Bioreactor

Inorganic solid =(SS - VSS) mg/L Inorganic solid = 0 mg/L

Input of Inorganic =Accumulation of Inorganic


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Hence, the following Figure 4.19b (Table C-7 to C-9 in Appendix C) presents theaccumulation of inert material in the reactor. This figure clearly indicates the slightly increaseof inert materials within the reactor. Although sludge draining was conducted to maintain 50days residence time, accumulation of inorganic matter can be observed. However percent ofMLVSS/ MLSS was maintained above 70% , normal condition for healthy conventionalactivated sludge, for all runs ( Metcalf & Eddy, 1991). It should be suspected that further

continuation of experimental runs will probably lead to relatively low MLVSS/ MLSS ratio.Such situation might hinder the biomass growth and possible reduction of MLVSS andreduction of organic removal ability.

In order to avoid such phenomenon in long runs, it is advisable to investigate theoperation with shorter sludge retention time. The details of method to measure biomassactivities in bioreactor is presented Appendix E.

4.3.2.6 Nitrogen Removal

Figure 4.22 (Table C-10 to C-12 in Appendix C) shows the influent and effluent ofTKN for different experimental runs. It was found that TKN removal efficiency wasmaintained above 90% for all runs. High degree of nitrification was due to long sludge age

condition which promoted the enrichment of low growth rate bacteria such as nitrifyingbacteria. Membrane bioreactor also do not allow the washout of nitrifying bacteria. In order toexplain the removal of nitrogen inside bioreactor, mass balance of total nitrogen wasinvestigated as followed.

4.3.2.6.1 Mass Balance of Total Nitrogen

Three principal mechanisms responsible for the removal of nitrogen are assimilationnitrification, denitrification and sedimentation.

During nitrification process, ammonia nitrogen is oxidized to nitrite and nitrate bynitrifying bacteria under aerobic conditions. As discussed in 2.3 and 2.4 Chapter II, it was

found that more than 95% of influent TKN (total ammonia nitrogen) was converted to nitriteand nitrate nitrogen. In order to determine the presence of denitrification reaction, nitrogenmass balance was then calculated based on the following equations: (Thanasupsin, 1995)

TKNi +NO2i+NO3 i = TKNe+NO2 e+NO3 e +N2

+assimilation by bacterial cell (4.3)

N2(denitrified- N) = (TKNi - TKNe) +(NO2 i - NO2 e) + (NO3 i - NO3 e)

- (BODi - BODe) (4.4)


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where = conversion factor (ratio of nitrogen to BOD which is

required by microorganisms)

The term of(BODi - BODe ) represents as the removal of nitrogen by assimilation.

The value of was calculated based on BOD : N =100 : 5. So, value of 0.05 was applied inEquation (4.4). Mass balance of nitrogen at steady can be derive as presented in the following:

Table 4.8 a

Table 4.8a Measured Nitrogen Mass Balance at Steady State

Influent (mg/L) Effluent (mg/L)

Parameters HRT (hours) HRT (hours)

TKN

NO2

NO3

BOD

26

166.8

0.17

1.58

2400

18

352.8

0.27

0.063

2200

10.5

146.4

1.32

1.22

3000

26

0.28

0.2

164.2

1.5

18

0

0.53

337.24

12

10.5

0

0.45

30.4

8

Table 4.8b Calculated Data for Nitrogen Mass Balance

HRT TKNi -TKNe NO2i - NO2e NO3i - NO3e (BODi - BODe) N2

26

18

10.5

166.52

352.8

146.4

- 0.03

- 0.26

0.87

- 162.62

- 337.20

- 29.18

119.93

109.4

149.2

- 116.06

- 94.06

- 31.11

As shown in Table 4.8a, material balance of nitrogen shows that about 99% ofnitrification was obtained by maintaining dissolved oxygen concentration above 1.5 mg/L.


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(Chiemchaisri et al. 1992)Based on extended aeration of activated sludge process, high degreeof nitrification was due to long sludge age condition of the system which allowed nitrifyingbacteria to be grown up and membrane biorector which does not allow any loss of nitrifyingbacteria from the system.

Based on these results, denitrification process occurred. Because this operation containshighly concentrated activated-sludge, agitation inside the bioreactor might not be sufficient.

Therefore, there was possible tendency of sludge attachment ,accumulation and settlement indead-zone which can be observed at the bottom of bioreactor. Besides this occurrence, therewas the compaction of cake layer on membrane surface which cause the anoxic conditions andlimitation of substrates and nutrients at inner part of the attached sludge among hollow fibers.Chang and Moo Yang (1988) has been studied the oxygen penetration depth for animal cell

immobilized in hollow fibers. The oxygen penetration was about 500 - 1,000m (0.5-1.0 mm).As Chang and Moo Yang indicated, the shortage of oxygen concentration was probably due tolimitation of oxygen penetration from both sides of membrane surface. Hence, Nitrate wasreduced during denitrification steps. Gaseous products were stripped by air bubbles andreleased out from the system.

4.3.2.7 Phosphate Removal

Figure 4.24 ( Table C-10 to C-12 in Appendix C) shows the variation of influent andeffluent total phosphate for each run. The removal efficiency fluctuated with value of 28.3,48.6 and 97.1% for RUN 1, 2 and 3, respectively. In order to explain total phosphate removal,mass balance of total phosphate was investigated as followed.

4.3.2.7.1 Mass Balance of Total Phosphate

Total phosphate removal mechanisms can be categorized into two means. One is theassimilation by microorganisms for cell synthesis and the other is sedimentation of totalphosphate inside the reactor. (Thanasupsin, 1995)

TPi = TPe+ (BOD i - BODe) + (VSSi - VSSe) (4.5)

where = conversion factor (ratio of phosphorus to BOD which

microorganisms require for their growth)

= conversion factor (ratio of phosphorus to VSS)

i , e = influent and effluent, respectively

The term of (BOD i - BOD e) and (VSSi - VSS e) represent the removal ofphosphorus by assimilation and sedimentation, respectively. For this study, phosphorus was


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determined in term of total phosphate. Therefore, term of (VSSi - VSSe) which indicated theeffect of biomass sedimentation is neglected.

Mass balance of total phosphate at steady state which is used for this study become asfollowed:

TPi = TP e + (BOD i - BODe) (4.6)

The value of was put as 0.01 based on BOD: P =100: 1. Mass balance of totalphosphate at steady state can be derived as followed.

Table 4.9a Data of Total Phosphate Balance

HRT Influent

TP i

(mg/L)

BOD i

Effluent

TP e

(mg/L)

BOD e

26

18

10.5

36.5

38.5

82.9

2400

2200

3000

12.8

16.6

3.0

1.5

12

8

Table 4.9b Calculated Data for Total Phosphate Mass Balance

HRT TP i

(mg/L)

[1]

TPe

(mg/L)

[2]

(BOD i -

BODe)

[3]

[2] +[3] =

(mg/L)

[4]

(mg/L)

[5]=[1]-[4]

26

18

10.5

36.5

38.5

82.9

12.8

16.6

3.0

23.99

21.88

29.92

36.79

38.48

32.92

-0.29

0.02

49.98

In column [5], it is expected to be zero. Based on the above results of HRT 26 and 18hours, it can be described that phosphorus was removed by biomass assimilation. Only theactive soluble phosphorus, ortho-phosphate, was consumed by the activated sludge in thebioreactor for their growth. For HRT 10.5 hours, phosphate removal was not balance. It can beconcluded that there were some amounts of phosphorus which retained and then accumulatedin the bioreactor. Due to the operation with high concentration of activated sludge in the


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