EFFICIENCY OF AERATION SYSTEM IN
WASTEWATER TREATMENT PLANTS.
SRI RUTHIRA KUMAR
UNIVERSITY TEKNOLOGI MALAYSIA
ii
DEDICATION
I would like to dedicate this project report to my parents
(V. Amirthalingam and S.Sarojini Thevi), wife ( Lily) and
my beloved daughter (Sanjana Sri) for their constant love
and encouragement
iii
APPRECIATION
The author would like to extend his most sincere appreciation and gratitude to
Associate Professor Dr. Fadil Othman for his guidance and encouragement
throughout the course.
Special gratitude also goes to Indah Water Konsortium Sdn. Bhd for without its
financial sponsorship and the releasing of its professional staffs as lecturers, my
colleagues and my-self would not have completed this post-graduate course in
Wastewater Engineering.
Last but not least, I would like to record my most sincere gratitude to my colleague
Miss Monica who had taken a lot of her own time to type and proof read this project
report for me.
iv
ABSTRACT
The most important factor in selecting aeration equipment for a specific
application is the oxygen transfer rate. Other factors that are equally important are
reliability, serviceability, capital cost, system appurtenances and cost of operation and
maintenance. Although there are many systems designed to aerate and mix the waste
water, they vary in their effectiveness in providing uniform oxygen dispersion. It is
the intention of the study to evaluate the performance of different types of aeration
devices based on dissolved oxygen (DO) readings and costing. To achieve this
objective, experimental work were carried out on five different aeration devices
namely brush aerator, tornado, surface aerator, aspirator and diffusers on five different
sewerage treatment plant with average PE of 2000. Through the experiment it was
found that aspirator was able to achieve 1 to 2 mg/l dissolved level while meeting the
regulators requirement on biochemical oxygen demand level. In the terms of
electricity, aspirator needed the lowest consumption compared to the other type of
device system. A detailed study on costing was done for the last 6 months in term of
operating and maintenance on the aeration device and was found that aspirator was
the cheapest to maintain compared to the others. While meeting the biochemical
oxygen demand standard as require by the regulators, this outcome of the study would
be a crucial factor when selecting a suitable aeration device in sewerage industry in
future.
v
ABSTRAK
Kadar resapan oksigen merupakan faktor yang paling penting semasa
pemilihan peralatan
untuk pengudaraan Faktor-faktor lain termasuk realiabiliti,
servisabiliti, kos pembelian, kos peralatan sampingan serta kos operasi dan
penyelenggaran. Walaupun terdapat berbagai-bagai sistem direkabentuk untuk
mengudarakan serta mengadunkan kumbahan, ia berbeza dari segi kecekapan dalam
menghasilkan oksigen yang setara. Tujuan projek ini adalah untuk menilai kecekapan
pelbagai jenis peralatan pengudaraan melalui bacaan
oksigen terlarut serta
perbelanjaan penyelenggaraan dan operasi setiap loji kumbahan. Untuk mencapai
objektif ini satu kajian telah dijalankan terhadap lima jenis peralatan aeration yang
berbeza iaitu “brush aerator”, “tornado”, “surface aerator”, “aspirator” serta
diffusers yang terdapat pada lima loji kumbahan tersebut yang mempunyai penduduk
setara sekitar 2500. Melalui kajian ini, telah terbukti bahawa aspirator mampu
mencapai oksigen terlarut “dissolved oksigen” sebanyak antara 1 mg/L hingga 2
mg/L. Aspirator juga menggunakan kadar elektrik yang rendah berbanding dengan
peralatan pengudaraan yang lain. Satu kajian perbelanjaan terperinci telah dijalankan
selama enam
bulan untuk penyelenggaran peralatan pengudaraan dan terbukti
bahawa aspirator merupakan peralatan yang paling murah untuk diselenggarakan.
Keputusan dari kajian ini merupakan faktor terpenting dalam pemilihan peralatan
pengudara “aeration” yang sesuai dalam industri pembetungan pada masa hadapan.
vi
TABLE OF CONTENTS
CHAPTER
TITLE
DECLARATION
DEDICATION
APPRECIATION
ABSTRACT
ABSTRAK
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
LIST OF APPENDICES
PAGE
i
ii
iii
iv
v
vi
vii
viii
ix
CHAPTER 1 - INTRODUCTION
1.1.
1.2.
1.3.
1.4.
1.5.
1.6.
1.7.
1.8.
1.9.
MALAYSIAN SEWERAGE SYSTEM
IMPORTANCE OF A SEWERAGE SYSTEM
SEWERAGE SYSTEM
UNCONNECTED SEWERAGE SYSTEMS
(TRADITIONAL TOILET)
CONNECTED SEWERAGE SYSTEMS
MECHANICAL PLANTS
MONITORING EFFLUENT
OBJECTIVE
SCOPE OF STUDY
1
1
2
4
4
4
4
6
6
CHAPTER II - LITERATURE REVIEW
2.1.
2.2.
2.3.
2.4.
INTRODUCTION
OXYGEN TRANSFER
FACTORS AFFECTING OXYGEN REQUIREMENTS
AERATION
2.4.1.
DIFFUSED-AIR AERATION
DIFFUSES
POROUS DIFFUSES
NON-POROUS DIFFUSES
OTHER AIR-DIFFUSION
DIFFUSER PERFORMANCE
BLOWERS
AIR PIPING
2.4.2. MECHANICAL AERATORS
AERATOR PERFORMANCE
MECHANICAL AERATION
2.5. OXYGEN DIPERSION EFFICIENCY AND MIXING
7
7
9
10
13
13
15
16
17
17
20
22
22
23
23
24
CHAPTER III - METHODOLOGY
3.1. INTRODUCTION
3.2. MEASURING AERATION DISSOLVED OXYGEN
3.2.1
WINKLER TITRATION METHOD
3.2.2
MEMBRANE ELECTRODE METHOD
3.3. AERATION BASIN DISSOLVED OXYGEN
PROFILES
26
27
28
CHAPTER IV - RESULTS & DISCUSSION
4.1. DISSOLVED OXYGEN
4.2. COSTING
OPERATIONAL AND MAINTENANCE COSTING /
CAPITAL COST
4.3. ELECTRICITY CONSUMTPTION
CHAPTER V -
29
34
36
37
CONCLUSION AND SUGGESTION
5.1. CONCLUSION
5.2. SUGGESTION
40
41
REFERENCES
42
APPENDICES
43
vii
LIST OF TABLES
TABLES
TITLES
PAGES
1.1.
EFFLUENT STANDARD BY ENVIRONMENT
QUALITY ACT (1974)
5
2.1.
EFFICIENCY OF VARIOUS AERATION SYSTEM
IN KWH/KG
11
2.2.
DESCRIPTION OF COMMONLY USED DEVICES
FOR WASTEWATER AERATION
12
2.3.
DESCRIPTION OF COMMONLY USED AIR
DIFFUSION DEVICES
14
3.1.
TYPE OF AERATION
26
4.1.
DISSOLVED OXYGEN READINGS
AT TAMAN GERMUDA
31
4.2.
DISSOLVED OXYGEN READINGS AT
TAMAN PAKATAN JAYA
31
4.3.
DISSOLVED OXYGEN READINGS AT
TAMAN ANDA
31
4.4.
DISSOLVED OXYGEN READINGS AT
TAMAN DESA KEBUDAYAAN
32
4.5.
DISSOLVED OXYGEN READINGS AT
TAMAN MEDAN PENGKALAN IMPIAN
32
4.6.
CHART & SUMMARY OF AVERAGE
DISSOLVED OXYGEN FOR DIFFERENT PLANTS
33
4.7.
CAPITAL COST OF DIFFERENT TYPE OF AERATION
DEVICES
35
4.8.
SUMMARY OF OPERATIONAL AND MAINTENANCE
COSTING FROM NOV’06-APRIL’06
36
4.9.
SUMMARY OF ELECTRICITY CONSUMPTION
FROM NOV’06-APRIL’06
38
viii
LIST OF FIGURES
FIGURES
TITLES
PAGES
1.1.
WHAT ENTERS THE SEWERAGE SYSTEM
FROM HOUSEHOLDS
3
1.2.
MECHANICAL PLANTS
4
2.1.
PICTURES OF BLOWERS
19
4.1.
GRAPH DISSOLVED OXYGEN AT EACH TAMAN
33
4.2.
GRAPH CAPITAL COST OF AERATION DEVICE
35
GRAPH SUMMARY OF OPERATIONAL
AND MAINTENANCE COSTING
36
4.4
GRAPH SUMMARY OF ELECTRICITY
CONSUMPTION
38
4.5
RESULTS OF MIXING CAPABILITIES AND
FLOW PATTERN
39
ix
LIST OF APPENDICES
APPENDICES
TITLES
A.
SUMMARY OF SAMPLING RESULTS
FOR SEWERAGE PLANTS
43
PHOTOGRAPH OF VARIOUS AERATION
DEVICE
44
B.
PAGES
1
CHAPTER 1
INTRODUCTION
1.1
MALAYSIAN SEWERAGE SYSTEM
An effective sewerage system ensures sewage being treated and disposed in a
safe manner. Sewage includes human waste, urine and wastewater from kitchens,
bathrooms and laundries. Sewerage systems are designed to collect, transfer, treat and
dispose of human waste and wastewater. The system serves government, domestic,
commercial and industrial properties in economical and environmentally responsible
manner.
In some countries the sewerage systems are designed to treat commercial and
industrial sewage, toxic waste and manufacturing waste. However Malaysia’s
sewerage system treats only human waste and household wastewater. Industrial and
trade waste is treated separately by on site industrial waste treatment plant. None of
the industrial waste or trade effluent is allowed to be discharged into existing
sewerage system.
1.2
IMPORTANCE OF A SEWERAGE SYSTEM
In certain places in Malaysia, where there are no sewerage systems, sewage
ends up in waterways. This is usually due to toilets discharging straight into the
waterways or sewer pipes discharging into the sea. Irrespective of the manner, in
which the sewage ends up in our waterways, it can have detrimental effects on public
health and the environment. Untreated human waste may carry infectious pathogenic
organisms into our rivers. Such polluted rivers cause the spread of diseases like
cholera, typhoid and hepatitis A. polluted rivers will contaminate sea life, particularly
fish, cockles and prawns. People who eat this contaminated seafood can become
seriously ill. Incidents of waterborne diseases such as these are not uncommon in
Malaysia and have often been traced to sewage contaminated waters.
2
Other than the tremendous public health risk that untreated sewage poses, it
also pollutes our environment. This is because sewage is able to consume oxygen
normally found dissolved in river water, for example, will mean that eventually the
river will lack sufficient oxygen to allow aquatic life and plants to survive.
As a result of this, there will be drop in supply of seafood and aquatic plants.
Aquatic plants produce oxygen, which keep the river alive. This vicious cycle will
eventually result in the river being “dead”. A dead river emits an unpleasant odor, is
unsightly, poses a health risk and does not support any plant or animal life.
Sadly enough, today 72% of the rivers in Malaysia are polluted and 65% of all
pollution loads has been identified as raw sewage. A step towards keeping our rivers
clean is to treat all the sewage that is generated by the community.
1.3
SEWERAGE SYSTEM
A modern and efficient sewerage system is vital of a developing nation such
as ours if we are to successfully move towards Vision 2020. A reliable system will not
only ensure that our increasing population is kept away from unnecessary health risks
but also that our water resources are preserved for future generations.
Sewage comprises of various pollutants that enter the sewerage system from
domestic, commercial and industrial premises. It is more than just what goes down a
toilet as it also includes wastewater from kitchen, bathrooms and laundries.
Many of our activities at home generate pollutants that find their way into the
sewerage system. Unless treated at a sewage treatment plant, raw sewage and
pollutants can end up in our drains, rivers and coastal water. It risks public health,
contaminating water resources and polluting the environment.
In Malaysia, sewerage systems range from simple toilets providing little or no
treatment to sewage to modern sewage treatment plants that employ mechanical
means to treat large volumes of sewage to acceptable environmental standards.
3
WHAT ENTERS THE SEWERAGE SYSTEM FROM HOUSEHOLDS
TOILET
BATHROOM
Faeces, toilet paper,
Urine,
Sanitary goods,
medicine,
Bacteria + viruses
Disposable nappies,
Toys
Shower/ bath water
Soap
Hair
Nail Clippings
Toothpaste tubes
Toothbrushes
Blood
SEWERAGE LINE
KITCHEN
LAUNDRY
Sink Water
Leftover food
Fat + grease
Cutlery + Glass
Tea leaves
Coffee Grinds
Clothes Washing
Detergents
Lint
Household
Cleaning bleach
Figure : 1.1: -
Source : Indah Water Konsortium 1998
There are various sewerage systems that produce effluent of different
standards. There are simple toilets, where sewage undergoes no treatment causing it to
be highly damaging to our environment, to the more modern mechanical plants that
are able to produce Standard “A” effluent. Sewerage systems can be categorized into
two board categories that are unconnected sewerage systems and connected sewerage
system.
4
1.4.
UNCONNECTED SEWERAGE SYSTEMS (TRADITIONAL TOILETS)
Simple toilets come under this category. These toilets were very popular
before the modern day toilets came into the scene. Depending on its make, there are
two types of traditional toilets. Firstly, toilets that do not treat the sewerage and
secondly, toilets that partially treat the sewage.
1.5.
CONNECTED SEWERAGE SYSTEMS
In connected sewerage systems, sewage outlets from a number of premises are
connected to a sewage treatment plant via a network of underground sewer pipes.
Modern and efficient sewage treatment plants are the best way to treat sewage.
Connected sewerage system generally comprise of a network of underground sewer
pipes, pump stations, sewage treatment plants and sludge treatment facilities.
Connected sewerage system generally operates by gravity so sewage treatment plants
should be located at the outlet of drainage catchments to capture all the sewage from
the catchments without the need for costly pumping.
1.6.
MECHANICAL PLANTS
In Malaysia, 11% of treatment plants are made up of various types of
mechanical plants. These plants run on mechanical equipment that accelerates the
breakdown of sewage. In the long term it is hoped that Malaysia’s sewerage system
will be made more efficient by standardising the types of plants used.
The diagram below shows an extended aeration plant where air is bubbled
through sewage to accelerate the breakdown of sewage by bacteria.
Figure : 1.2.: - Source : Indah Water Konsortium 1998
5
1.7.
MONITORING EFFLUENT
Various pollutants in sewage are analyzed in order to understand how sewage
should be treated and to examine the effect of treated sewage (effluent) on the
environment.
Effluent from all sewage treatment plants is sampled at regular intervals and
analyzed in modern laboratories to ensure it complys the required standards. These
tests are carried out as part of a monitoring programmed to ensure that Indah Water
meets its operational license conditions and that its’ treatment processes are operating
efficiently. This provides for a cleaner and safer environment that improves the living
conditions of Malaysia.
The two most important parameters measured are biological oxygen demand
(BOD) and suspended solids (SS). BOD is a measurement of the amount of oxygen
sewage will consume over a given time. High BOD means that sewage will rapidly
consume all the oxygen naturally dissolved in streams, rivers and lakes killing all
aquatic life and turn the water septic and smelly.
SS is a measurement of the
undissolved material in sewage. High SS leads to sludge deposit in the waterways
causing significant environmental degradation.
Table 1.1.: – Effluent Standard by Environmental Quality Act (1974)
Biochemical
Suspended
Oxygen
Solids (SS)
Demand (BOD)
Oil &
COD
Grease
STANDARD A
20 mg/L
50 mg/L
0 mg/L
50 mg/L
STANDARD B
50 mg/L
100 mg/L
10 mg/L
100 mg/L
If the effluent is discharged upstream of a water supply intake point, it should
meet Standard A. For effluent that is discharged downstream, it should meet Standard
B. These standards are set by the Environmental Quality Act (1974).
6
1.8.
OBJECTIVE
The objective of this study is to determine the most effective aeration device
in wastewater industry by monitoring their oxygen transfer and costing which
includes operational and maintenance cost, the capital cost and electricity
consumption of each and every aeration device.
1.9.
SCOPE OF STUDY
The study consists of a thorough experimental work at five sewerage treatment
plants using five sewerage treatment plants with five different aeration devices. The
five sewerage treatment plants were observed based on their oxygen transfer level. It
was carried out by using dissolved oxygen meter on a daily basis for one week.
During the experiment, five sewerage treatment plants sampling were carried out in
order to monitor the BOD level. These would enable to verify the most efficient
aeration device while meeting the Standard as required by the regulators.
There was also a study on the energy saving of different type of aeration
device. It was carried out by monitoring the electricity consumption of aeration
device for a period of six months. Capital cost and the operation and maintenance cost
were also taken into consideration as factors before deciding the most effective
aeration device.
7
CHAPTER II
LITERATURE REVIEW
2.1.
INTRODUCTION
In the treatment of wastewater, an aeration system is ineffective in providing a
completed and uniform transfer of oxygen without the capability to disperse oxygen
throughout the entire process/basin. Although many systems are designed to aerate
wastewater, they vary in their effectiveness in providing uniform oxygen dispersion.
In this literature review, the importance of oxygen transfer and types of
aeration system that normally being practiced in Malaysian sewerage system were
discussed.
2.2.
OXYGEN TRANSFER
Oxygen is supplied to the mixed liquor in an aeration tank by dispersing air
bubbles through sub-merged diffusers or by entraining air into the liquid by
mechanical means. Air diffusers are porous plates, tubes or nozzles attached either to
air piping on the bottom of the tank or to pipe headers that can be lifted out of the
tank. Centrifugal blowers provide compressed air to the diffusers. Coarse-bubble
devices are orifices or nozzles designed so that the discharged air is broken up into
bubbles and dispersed in the surrounding liquid. Fine-bubble devices are porous
materials that release air as fine bubble. Although each kind of diffuser has individual
features, coarse-bubble nozzles are noted for maintenance-free operation, but finebubble diffusers have been the advantage of higher oxygen-transfer efficiency.
Mechanical aerators are horizontal paddle, vertical turbine, and vertical-turbine draft
tube. A horizontal rotor rotates partially submerged in an aeration channel. A vertical
turbine may be surface unit or completely submerged with compressed air supplied
under the rotating blade. For deep mixing, a vertical turbine may be in a draft tube so
that liquid from the bottom of the tank is drawn up through the tube and discharged at
the surface.
8
Oxygen transfer is a two-phase process. First, gaseous oxygen is dissolved in
the wastewater by diffused or mechanical aeration. Then the dissolved oxygen is
taken up by the microorganisms in metabolism of the waste organic matter. If the rate
of oxygen utilization exceeds the rate of dissolution, the dissolved oxygen in the
mixed liquor is depleted. The oxygen transfer from the air bubbles into solution.
The K-factor depends on wastewater characteristics and more important on the
physical features of knowledge of the basic concepts of oxygen transfer and uptake, is
helpful in understanding operational problems generally associated with aeration
processes. An oxygen deficiency in an aerating basin is possible if the rate of
biological utilization exceeds the capabilities of the equipment. For example, organic
overloading of an extended aeration system that is equipped with coarse-bubble
diffusers set a shallow depth can result in a dissolved oxygen level of less than 0.5
mg/l even though the tank contents are being vigorously mixed by air bubbles
emitting form diffusers. Perhaps a situation that occurs more frequently in practice is
uneconomical operation from over aeration, producing a dissolved oxygen level
greater than is necessary in the mixed liquor. Since biological activity is just as great
at low levels and the transfer rate from air to dissolved oxygen increases with
decreasing concentration, it is logical to operate a system as close to critical minimum
dissolved oxygen as possible. Operation of the air compressors at reduced capacity, or
even turning off one blower on weekends may be feasible to conserve electrical
energy with no adverse effects to the biological process. Automatic control of blowers
using dissolved oxygen probes has increased with improvements in control systems
and the reliability of dissolved oxygen meters. (Metcalf and Eddy. 1991)
9
2.2.
FACTORS AFFECTING OXYGEN REQUIREMENTS
There are many factors that affect oxygen requirements of the bacteria in the
aeration basin. But in this chapter, the discussion or explanation is only on two
factors. The first factor is the direct relationship between the influent BOD
concentration and the aeration basin dissolved oxygen. As the concentration of BOD
entering the aeration basin rises, the amount of oxygen required to maintain DO levels
rises also. If you don’t respond to increase influent BOD levels by increasing aeration
rates, the level of dissolved oxygen in the aeration basin will drop.
Some operators mistakenly assume that if dissolved oxygen drops, a toxic
material has entered the aeration basin and killed or inhibited the bacteria. Actually,
the opposite happens. Healthy bacteria are the agents that use the oxygen in the mixed
liquor. If you kill or slow them down, the aeration basin dissolved oxygen will
increase (Tim Hobson. 1992)
There is another relationship concerning dissolved oxygen and the amount of
bacteria in the aeration basin that you should be aware of. The amount of aeration
required to maintain a given level of dissolved oxygen is directly proportional to the
amount of bacteria you have in the aeration basin. As the concentration of bacteria in
the basin goes up, aeration rates must be increased to maintain a given level of
dissolved oxygen.
If you are having trouble-maintaining dissolved oxygen and your solids level
in the aeration basin is high, you can increase dissolved oxygen by washing more
sludge to bring solids levels down. (Tim Hobson. 1992).
10
2.4.
AERATION
Aeration is the process by which the area of contact between water and air is
increased, either by natural or mechanical means, resulting in air being suspended in
air. Aeration is the most important operation in the treatment process, to provide
oxygenation and mixing. The aeration facilities are designed to meet the calculated
oxygen demand of the process while maintaining in the aeration tank minimum
Dissolved oxygen of about 1-2 mg/l which is necessary for proper development of
biological sludge.
In addition to supplying dissolved oxygen, the aeration devices have also to
provide adequate mixing ad agitation so that the mixed liquor suspended solids do not
settle down. This way aeration increases the contact opportunity between the floc and
sewerage.
To summarize aeration serves the following three functions:
(i)
Oxygenation of the mixed liquor
(ii)
Flocculation of the colloids in sewage influent and
(iii) Suspension of activated sludge floc.
Following are the three methods, which are usually employed for the purpose of
aeration in activated sludge process:
(i)
Diffused air aeration
(ii)
Mechanical aeration
(iii) Combined diffused air and mechanical aeration
One of the pollutants of water is organic matters. The reason why organic
matters are considered water pollutants is that microorganism feed on them and in the
process used up the dissolved oxygen needed for aquatic life. If the organic matters
are in sufficient quantity, this can lead to nearly all the dissolved oxygen being used
up, aquatic life killed, and to anaerobic conditions in which anaerobic microorganism
produces hydrogen sulfite and other odorous constituents are produced.
11
The purpose of aeration is to improve their physical and chemical
characteristics, to remove or reduction of objectionable taste and odor and to
precipitate inorganic contaminants such as iron and manganese. In wastewater
treatment, the purpose of aeration is to ensure continued aerobic conditions for the
microorganism to degrade the organic matters. The efficiency of aeration systems can
be measured in different ways. Different aeration systems have different efficiency.
(Metcalf and Eddy. 2004)
The exact efficiency of an aeration system is veries, depending on
circumstances under which it is measured such as liquid depth, density of diffuser,
energy level in the tank, etc.. The following Table 2.1 is a table of the efficiency of
various aeration systems adapted to give values in kilowatt-hour per kilogram of
oxygen. It is adapted from the table given by Environmental Dynamics.
Table 2.1.: - Efficiency of various aeration system in kWH/kg
Aeration system
kWh/kg
Mechanical aeration systems
Brush aerators surface aeration
Slow speed surface
High speed splash surface aeration
Induced surface aeration
0.47-0.66
0.47-0.55
0.51-0.66
1.10-1.64
Combination systems
Submerged turbine
Jets (pumps with compressors)
0.66-1.10
0.47-0.82
Diffused Aeration, Coarse Bubble
Static tubes
Wide band grid
Misc. coarse bubble
0.47-0.82
0.47-0.66
0.47-0.82
Diffused Aeration, Fine Pore
Ceramic disc or ceramic dome grid
Flexible membrane disc
Advanced technology membrane
0.23-0.33
0.23-0.41
0.14
12
Table 2.2.: - Description of commonly used devices for wastewater aeration
Classification
Submerged:
Fine-bubble
(fine-pore)
system
Description
Use or application
Bubbles generated with ceramic, plastic, or All types of activated-sludge
flexible membranes (domes, tubes, disks, processes
plates, or panel configuration)
Coarse bubble Bubbles generated with orifices, injectors All types of activated-sludge
and nozzles, or shear plates
processes, channel and grit
(nonporous)
chamber aeration and aerobic
system
digestion
Sparger turbine
Low-speed turbine and compressed-air All types of activated-sludge
injection
processes and aerobic digestion
Static tube mixer Short tubes with internal baffles designed Aerated lagoons and activatedto retain air injected at bottom of tube in sludge processes
contact with liquid
Jet
Surface:
Low-speed
turbine aerator
Compressed air injected into mixed liquid All types of activated-sludge
as it pumped under pressure through jet processes, equalization tank
mixing and aeration, and deep
device
tank aeration
Large-diameter turbine used to expose Conventional activated-sludge
processes, aeration lagoons, and
liquid droplets to the atmosphere
aerobic digestion
High-speed
floating aerator
Small-diameter propeller used to expose Aerated lagoons and aerobic
liquid droplets to the atmosphere
digestion
Aspirating
Inclined propeller assembly
Aerated lagoons
ditch,
channel
Rotor-brush or Blades or disks mounted on a horizontal Oxidation
central shaft are rotated through the liquid. aeration and aerated lagoons
rotating-disk
Oxygen is induced into the liquid by the
assembly
splashing action of the rotor and by
exposure of liquid droplets to the
atmosphere
13
2.4.1. DIFFUSED-AIR AERATION
The two basic methods of aerating wastewater are (1) injection of air or pure
oxygen into the wastewater with submerged diffusers or other aeration devices or (2)
to agitation of the wastewater mechanically so as to promote solution of air from the
atmosphere. A diffused-air system consists of diffuses that are submerged in the
wastewater, header pipes, air mains and the blowers and appurtenances through which
the air passes. The following discussion covers the selection of diffusers, the designs
of blowers and air piping.
Diffuses
In the past, the various diffusion devices have been classified as either fine
bubble or coarse bubble, with the connotation that fine bubbles were more efficient in
transferring oxygen. The definition of terms and the demarcation between fine and
coarse bubbles, however, have not been clear, but they continue to be used. The
current preference is to categorize the diffused aeration systems by the physical
characteristics of the equipment.
14
Three categories are defined: (1) porous or fine-pore diffusers, (2) nonporous
diffusers, and (3) other diffusion devices such as jet aerators, aspirating aerators and
U-tube aerators. The various types of diffused-air devices are described in Table 2.3
Table 2.3.: - Description of commonly used air diffusion devices
Type of diffuser or
device
Transfer efficiency
Description
Porous
Disk
High
Rigid ceramic disks mounted on airdistribution pipes near the tank floor.
Dome
High
Dome-shaped ceramic diffusers mounted
on air-distribution pipes near the tank
floor.
Membrane
High
Flexible porous membrane supported on
disk mounted on air-distribution grid
Panel
Very High
Rectangular panel with a flexible plastic
perforated membrane
Orifice
Low
Devices usually constructed of molded
plastic and mounted in air-distribution
pipes.
Slotted tube
Low
Stainless-steel
tubing
containing
perforations and slots to provide a wide
band of diffused air
Static Tube
Low
Stationary vertical tube mounted on basin
bottom and functions like an air-lift
pump
NonPorous
Fixed orifice
15
Porous Diffuses
Porous diffuses are made in many shapes, the most common being domes,
disks and membrane. Tubes are also used. Plates were once the most popular but are
costly to install and difficult to maintain. Porous domes disks and membrane has
largely supplanted plates in new installations. Domes, disks, or tube diffuses are
mounted on or screwed into air manifold, which may run the length of the tank close
to the bottom and along one or two sides, or short manifold headers may be mounted
on movable drop pipes on one side of the tank. Dome and disk diffusers may also be
installed in a grid pattern on the bottom of the aeration tank to provide uniform
aeration throughout the tank.
Numerous materials have been used in the manufacture of porous diffusers.
These materials generally fall into the categories of rigid ceramic and plastic materials
and flexible plastic, rubber or cloth sheaths. The ceramic materials consist of rounded
passageways through which compressed air flows. As the air emerges from the
surface pores, pore size, surface tension and air flow rate interact to produce the
bubble size. Porous plastic materials are newer developments. Similar to the ceramic
materials, the plastics contain a number of interconnecting channels or pores through
which the compressed air can pass. Thin, flexible sheaths made from soft plastic or
synthetic rubber have also been developed and adapted to disks and tubes.
Air passages are created by punching minute holes in the sheath material.
When the air is turned on, the sheath expands and each slot acts as variable aperture
opening; the higher the air flow rate, the greater the opening.
Rectangular panels that use a flexible polyurethane sheet are also used in
activated-sludge aeration. The panels are constructed with a stainless steel frame and
are placed on or close to the bottom of the tank and anchored.
16
Advantages cited for aeration panels are:
(1) Ultra-fine bubbles are produced that significantly improve oxygen transfer and
system energy efficiency
(2) Large areas of the tank floor can be covered, which facilities mixing and oxygen
transfer, and
(3) Foul ants can be dislodged by “bumping”, i.e. increasing the airflow to flex the
membrane.
Disadvantages cited for aeration panels are:
(1) The panel is a proprietary design and thus lacks competitive bidding,
(2) The membrane has a higher head loss, which may affect blower performance in
retrofit applications, and
(3) Increased blower air filtration is required to prevent internal fouling.
With all porous diffusers, it is essential that the air supplied be clean and free
of dust particles that might clog the diffusers. Air filters, often consisting of viscous
impingement and dry-barrier type, are commonly used. Precoated bag filters and
electrostatic filters have also been used. The filters should be installed on the blower
inlet. (S.K. Gang, 2004)
Nonporous Diffuses
Several types of nonporous diffusers are available. Nonporous diffusers
produce larger bubbles than porous diffusers and consequently have lower aeration
efficiency; but the advantages of lower cost, less maintenance and the absence of
stringent air-purity requirements may offset the lower oxygen transfer efficiency and
energy cost. Typical system layouts for orifice diffusers closely parallel the layouts
for porous dome and disk diffusers; however single and dual roll spiral patterns using
narrow or wide-band diffuser placement are the most common. Application for orifice
and tube diffuser includes aerated grit chambers, channel aeration, flocculation basin
mixing, aerobic digestion and industrial waste treatment.
17
In the static tube aerator air is introduced at the bottom of a circular tube that
can vary in height from 0.5 to 1.25 m (1.5 to 4.0 ft). Internally, the tubes are fitted
with alternately placed deflection plates to increase the contact of the air with the
wastewater. Mixing is accomplished because the tube aerator acts as an airlift pump.
Static tubes are normally installed in a grid-type floor coverage pattern.
Other Air-Diffusion
Devices Jet aeration combines liquid pumping with air diffusion. The pumping
system re-circulates liquid in the aeration basin, ejecting it with compressed air
through a nozzle assembly. This system is particularly suited for deep (>8 m) tanks.
Aspirating aeration consists of a motor-driven aspirator pump. The pump draws air in
through a hollow tube and injects it underwater where both high velocity and
propeller action creates turbulence and diffuses the air bubbles. The aspirating device
can be mounted on a fixed structure or on pontoons. U-tube aeration consists of a deep
shaft that is divided into two zones. Air is added to the influent wastewater in the
down comer under high pressure; the mixture travels to the bottom of the tube and
then backs to the surface.
The great depth to which the air-water mixture is subjected results in high
oxygen transfer efficiencies because the high pressure forces all the oxygen into
solution. U-tube aeration has particular application for high-strength wastes.
Diffuser Performance
The efficiency of oxygen transfer depends on many factors, including the type,
size, and shape of the diffuser; the air flow rate; the depth of submersion; tank
geometry including the header and diffuser location; and wastewater characteristics.
Aeration devices are conventionally evaluated in clean water and the results adjusted
to process operating conditions through widely used conversion factors. Typical clean
water transfer efficiencies and air flow rates for various diffused-air devices.
18
Typically, the standard oxygen transfer efficiency (SOTE) increases with
depth; the transfer efficiencies for the 4.5-m (15-ft) depth, the most common depth of
submergence. Data on the variation of SOTE with water depth for various diffuser
types can be found in WPCF (1988).
The variation of oxygen transfer efficiencies with the type of diffuser and
diffuser arrangements. Additional data on the effects of diffuser arrangement on
transfer efficiency are reported in V.S. EPA (1989).
Oxygen transfer efficiency (OTE) of porous diffusers may also decrease with
use due to internal clogging or exterior fouling. Internal clogging may be due to
impurities in the compressed air that have not been removed by the air filters.
External fouling may be due to the formation of biological slimes or inorganic
precipitants. The effect of fouling on OTE is described by the term F. The rate at
which F decreases with time is designated ƒF which is expressed as the decimal
fraction of OTE lost per unit time. The rate of foul will depend on the operating
conditions, changes in wastewater characteristics, and the time in service.
The fouling rates are important in determining the loss of OTE and the
expected frequency of diffuser cleaning. Fouling and the rate of fouling can be
estimated by (1) conducting full-scale OTE tests over a period of time, (2) monitoring
aeration system efficiency and (3) conducting OTE tests of fouled and new diffusers.
Factors commonly used to convert the oxygen transfer required for clean
water to wastewater are the alpha, beta, and theta factors. The alpha factor, the ratio
of the KLa of wastewater to the KLa of clean water, is especially important because
alpha factor varies with the physical features of the diffuser system, the geometry of
the reactor, and the characteristics of the wastewater. Wastewater-constituents may
affect porous diffuser oxygen transfer efficiencies to a greater extent than other
aeration devices, resulting in lower alpha factors. The presence of constituents such as
detergents, dissolved solids, and suspended solids can affect the bubble shape and size
and result in diminished oxygen transfer capability.
19
Values of alpha varying from 0.4 to 0.9 have been reported for fine-bubble
diffuser systems (Hwang and Stenstrom, 1985). Therefore, considerable care must be
exercised in the selection of the appropriate alpha factors.
Another measure of the performance of porous diffusers is the combination of
the alpha and fouling factors, designated by the term αF In a number of in-process
studies, the values of ∂F have ranged widely, from 0.11 to 0.79 with a mean of < 0.5,
and were significantly lower than anticipated (U.S. EPA, 1989).
The variability of αF was found to be site-specific, and demonstrated the need
for the designer to investigate and evaluate carefully the environmental factors that
may affect porous diffuser performance in selecting an appropriate α or αF factor.
Because the amount of air used per kilogram (pound) of BOD removed varies
greatly from one plant to another, and there is risk in comparing the air use at
different plants, not only because of the factors mentioned above but also because of
different loading rates, control criteria, and operating procedures. Extra-high air flow
rates applied along one side of a tank reduce the efficiency of oxygen transfer and
may even reduce the net oxygen transfer by increasing circulating velocities. The
result is a shorter residence time of air bubbles as well as larger bubbles with less
transfer surface. (Metcalf and Eddy. 2004)
Methods of cleaning porous diffusers may consist of refining of ceramic
plates, high-pressure water sprays, brushing, or chemical treatment with acid or
caustic baths. Additional details on cleaning methods may be found in U.S. EPA
(1989). .
Figure 2.1 - Picture of blowers
20
Blowers
There are three types of blowers commonly used for aeration: centrifugal,
rotary lobe positive displacement and inlet guide vane-variable diffuser. Centrifugal
blowers (see Figure 2.1) are almost universally used where the unit capacity is greater
than 425 m3/min (15,000 ft3/min) of free air. Rated discharge pressures range
normally from 48 to 62 kN/m2. Centrifugal blowers have operating
characteristics similar to a low-specific-speed centrifugal pump. The discharge
pressure rises from shutoff to a maximum at about 50 percent of capacity and
then drops off.
The operating point of the blower is determined, similar to a centrifugal
pump, by the intersection of the head-capacity curve and the system curve.
In wastewater-treatment plants, the blowers must supply a wide range of
airflows with a relatively narrow pressure range under varied environmental
conditions. A blower usually can only meet one particular set of operating conditions
efficiently. Because it is necessary to meet a wide range of airflows and pressures at a
wastewater treatment plant, provisions have to be included in the blower system
design to regulate or turn down the blowers.
Methods to achieve regulation or turndown are:
(1)
Flow blow off or bypassing,
(2)
Inlet throttling
(3)
Adjustable discharge diffuser
(4)
Variable speed driver, and
(5)
Parallel operation of multiple units. Inlet throttling and an adjustable discharge
diffuser are applicable only to centrifugal blowers; variable-speed drivers are
more commonly used on positive-displacement blowers. Flow blow off and
bypassing is also an effective method of controlling surging of a centrifugal
blower, a phenomenon that occurs when the blower operates alternately at zero
capacity and full capacity, resulting in vibration and overheating. Surging occurs
when the blower operates in a low volumetric range.
21
For higher discharge pressure applications [> 55 kN/m2] and for capacities
smaller than 425 m3/min (15,000 ft3/min) of free air per unit, rotary-lobe positive
displacement blowers are commonly used. The positive-displacement blower is a
machine of constant capacity with variable pressure. The units can throttle, but
capacity control can be obtained by the use of multiple units or a variable speed drive.
Rugged inlet and discharge silencers are essential.
A relatively new blower design, the inlet guide vane-variable diffuser that
developed in Europe, mitigates some of the problems and considerations associated
with standard centrifugal and positive-displacement aeration blowers. The design
based on a single-stage centrifugal operation that incorporates actuators to position
inlet guide vane and variable diffusers to vary blower flow rate and optimize
efficiency
The blowers are especially well suited to applications with medium to high
fluctuations in inlet temperature, discharge pressure, and flow rate. Blower capacities
range from to 1700 m3/min (3000 to 60,000 ft3/min) at pressures up to 170 kN/m2.
Turndown rates of up to 40 percent of maximum capacity are possible without
significant reduction in operating efficiency over the range of operation. Principal
disadvantages are high initial cost and a sophisticated computer control system to
ensure efficient operation.
The performance curve for a centrifugal blower is a plot of pressure versus
inlet volume and resembles the performance curve for a centrifugal pump. The
performance curve typically is a falling-head curve where the pressure decreases as
the inlet volume increases. Blowers are rated at standard air conditions, defined as a
temperature of 20°C (68°F), a pressure of 760 mm Hg, and a relative humidity of 36
pen Standard airs has a specific weight of 1.20 kg/m3. The air density aft the
performance of a centrifugal blower; any change in the inlet air temperature
barometric pressure will change the density of the compressed air.
22
The greater the density, the higher the pressure will rise. As a result, greater
power is needed for compression process. Blowers must be selected to have adequate
capacity for a hot summer day, and be provided with a driver with adequate power for
the cold set winter weather.
Air Piping
Air piping consists of mains, valves, meters, and other fittings that port
compressed air from the blowers to the air diffusers. Because the pressure [less than
70 kN/m2 J, lightweight piping can be used.
The piping should be sized so that losses in air headers and diffuser manifolds
small in comparison to the losses in the diffusers. Typically, if head losses in the air
piping between the last flow-split device and the farthest diffuser are less than 10
percent of the head loss across the diffusers, good air distribution through the aeration
basin can be maintained. Valves and control orifices are an important consideration in
design (WEF, 1998b).
2.4.2. Mechanical Aerators
Mechanical aerators are commonly divided into two groups based on major
design and operating features: aerators with vertical axis and aerators with horizontal
axis. Both groups are further subdivided into surface and submerged aerators. In
surface aerators, oxygen is entrained from the atmosphere; in submerged aerators,
oxygen is entrained from the atmosphere and, for some types, from air or pure oxygen
introduced in the tank bottom. In either case, the pumping or agitating action of the
aerators helps to keep the contents of the aeration tank or basin mixed. In the
following discussion, the various types of aerators will be described, along with
aerator performance and the energy requirement for mixing. (D.Lal. 2004)
23
Aerator Performance
Mechanical aerators are rated in terms of their oxygen transfer rate expressed
as kilograms of oxygen per kilowatt-hour (pounds of oxygen per horsepower-hour) at
standard conditions. Standard conditions exist when the temperature is 20°C, the
dissolved oxygen is 0.0 mg/L, and the test liquid is tap water. Testing and rating are
normally done under non-steady-state conditions using fresh water, deaerated with
sodium sulfite. Commercial-size surface aerators efficiency ranges from 1.20 to 2.4
kg 02kW·h.
Oxygen transfer data for various types of mechanical aerators. Design
engineer should accept efficiency claim for aerator performance only when they are
supported by actual test date for actual model and size of aerator under consideration.
Mechanical Aeration
Two major types of mechanical aeration equipment are commonly used for
post-aeration systems: low-speed surface aerators and submerged turbine aerators.
Low-speed surface aerators are preferred because they are usually the most
economical, except where high oxygen transfer rates are required. For high oxygen
transfer rates, submerged turbine units are preferred. Most installations consist of two
or more aerators in rectangular basins. Detention times for post-aeration using either
mechanical or diffused-air aeration is usually 10 to 20 min at peak flow rates.
24
Aeration rates in mechanically aerated tanks center controlled in several ways:
•
Changing aerator speed
•
Changing the amount of “bite” that the aerator makes in the mixed liquor. This
increases of decreases the amount of energy transferred to the water (and the
amount of aeration)
•
Changing the numbers of aerators in services
•
Changing the amount of time the aerators are “ON” in a given period. When using
this type of aeration control, be careful to keep “OFF” times under an hour in
duration. An “OFF” time of 10 – 15 minutes is even better because it is not
advisable for the bacteria to spend much time in an anaerobic environment and
unaerated mixed liquor organisms or it will start to active aneraobically.
2.5.
OXYGEN DISPERSION EFFICIENCY AND MIXING
In the treatment of wastewater, an aeration system is ineffective in providing a
complete and uniform transfer of oxygen throughout the entire basin. Although many
systems are designed to aerate wastewater, they vary in their effectiveness in
providing uniform oxygen dispersion.
Typical aeration systems such as diffused air, surface splashers, and rotor have
limited areas of influence, causing short-circuiting, dead zones, and only partial
aeration. Because the Triton aeration/mixer technology procedures a horizontal and
circular flow pattern, the equipment provides whole basin circulation.
Conventional splashing type system, pump water upward and throws it into the
air, creating a high aerosol environment. Overcoming gravity also consumes large
amounts of energy. The area of influence is confined. Short-circuiting and dead spots
may occur due to inadequate basin mixing. Sludge deposits typically accumulate at
corners and between units in the basin, creating and even greater oxygen demand.
25
Blower/diffuser systems introduce compressed air through diffusers into the
water from the bottom of the basin. More horsepower (higher energy consumption) is
required to overcome the water head resistance. The mixing pattern is a limited
vertical column as air rises from the diffuser heads to the water’s surface. Over time,
the system’s diffuser heads clog as solids and biofilm accumulated. This reduces
oxygen transfer efficiency.
Rotor system proper water into the air creating an aerosol environment that
releases offending odour into the air. These systems are expensive to operate both in
electrical consumption and maintenance. Rotors are inefficient in suspending solids
uniformly, having similar mixing constraints of splasher type aerator.
The aspirator a horizontal circular flow pattern is created and controlled for
maximum treatment efficiency. (Mark J. Hammer. 2004)
26
CHAPTER III
METHODOLOGY
3.1.
INTRODUCTION
The study was carried out by choosing five different sewerage treatment plants
(STP) with five different aeration devices. The STP and type of aeration specification
is stated in Table 3.1.
Table 3.1. – Type of Aeration
STP
Type
PE
Aeration Devices
HP
Taman Germuda
Oxidation Ditch
1465
Brush aerator
2 nos x 5.5kw
Taman Anda
Aerated Lagoon
2300
Tornado
2 nos x 5.5kw
Depa Kebudayaan
Aerated Lagoon
1420
Surface aerator
2 nos x 5.5kw
Taman PakatanJaya
Oxidation Ditch
2465
Aspirator
2 nos x 3.7kw
1050
Diffusers
2 nos x 5.5kw
Taman
Idaman
Pengkalan Extented Aeration
(Blowers)
All the above Sewerage Treatment Plants which are operating with different
aeration device operates 16 hours daily except Taman Pakatan Jaya operates only 8
hours were monitored by using membrane electrode dissolved oxygen meter on a
daily basis for a week to determine the device efficiency. Samplings were also done
on each Sewerage Treatment Plant while the above study was being carried out. The
samples were then sent to Taiping Indah Water Konsortium laboratory to determine
the effluent results mainly on BOD. Subsequently the dissolved oxygen readings were
compared with the sampling result to determine the most efficient aeration system.
27
3.2.
MEASURING AERATION DISSOLVED OXYGEN
There are two common methods for measuring aeration basin dissolved
oxygen, the Winkler Titration Method and the Membrane Electrode Method. As for
this study case I used the membrane electrode method at difficult Sewerage Treatment
Plants
3.2.1. WINKLER TITRATION METHOD
This method is commonly used when the Dissolved Oxygen meter cannot be
obtained. If you use this method, be sure that you use the modification specifically
designed for measurement of aeration basin of dissolved oxygen.
In the standard Winkler procedure for measuring of dissolved oxygen, aerobic
bacteria in the mixed liquor sample continue to remove dissolved oxygen from the
sample even while you are preparing to run the test. This causes measured dissolved
oxygen levels to be lower than true levels. In the dissolved oxygen procedure
modified for measuring aeration basin dissolved oxygen, the technician adds a
solution containing copper sulfate and sulfuric acid to the sample container before
collecting the mixed liquor sample. As the sample is collected the copper sulfatesulfuric acid solution kills aerobic organisms and prevents them from using dissolved
oxygen during the test. The procedure for the Winkler method is readily available in
most texts and lab manuals for wastewater treatment plant operation.
3.2.2. MEMBRANE ELETRODE METHOD
This is other common way of determining aeration basin dissolved oxygen. I
used this method because of it reliability speed and also easier to handle. The
equipment consists of a meter, probe and cable and is available at any major scientific
equipment supplier.
After calibration, I measured the dissolved oxygen in the aeration basin on all
the five Sewerage Treatment Plants by directly placing the probe where I want a
dissolved oxygen reading. I attached the probe to a light-weight pole so it can be
positioned exactly where they want it. The probe is often attached to the pole upside
down (membrane end up).
28
This prevents air bubbles from getting trapped against the membrane and
causing high readings. After positioning the probe in the mixed liquor, switch the
meter to get a temperature reading. I recorded the temperature of the mixed liquor for
later reference. Now switch the meter and read the dissolved oxygen. It may take as
much as a minute for the meter to stabilize around a specific dissolved oxygen
reading.
3.3.
AERATION BASIN DISSOLVED OXYGEN PROFILES
Most aeration basins won’t have dissolved oxygen concentration that is the
same all through the tank. Since the dissolved oxygen reading is high and low, I
took few readings with a minimum of 0.5mg/l. The bacteria in the basin failed to
perform properly when the dissolved oxygen drops below that level for more than a
few minutes. Therefore, a reading at the location, of the lowest dissolved oxygen
concentration is the most important factor. To find this location, you need to have a
dissolved oxygen profile in the basin. To perform an aeration basin dissolved oxygen
profile, you will need a dissolved oxygen meter with a 4.5m cord on the probe.
Mark the probe cord in one meter intervals and attach to a long pole so I can position
it precisely where I want it.
Testing of Dissolved oxygen concentrations should be done at selected points
across the length and width of the tank. Measure the Dissolved oxygen at designated
depths (intervals of 2-3 meters) for each selected sampling point. Plot the cross
sectional measurements along the length of the tank that results from plotting the
Dissolved oxygen results. As you can see the Dissolved oxygen in the example tank,
varied a great deal from one location to another. The location for taking regular
Dissolved oxygen measurement, in this case, should be in the center, near the bottom
of the tank where the region of lowest Dissolved oxygen is located. Adjust aeration
rates to maintain dissolved oxygen of around 1.0 mg/L here and you can be
reasonable certain there is enough Dissolved oxygen in the rest of the aeration basin.
There was a comparison on costing in terms of energy saving of the aeration device
while maintaining the required Dissolved oxygen level. Wastewater treatment
facilities must meet strict effluent discharge permit standards to stay in compliance
with government regulations.
29
CHAPTER IV
RESULTS & DISCUSSION
4.1.
DISSOLVED OXYGEN
From the case study the below data were collected for further analysis in order to
achieve the objective of this study. The date collected for all five sewerage treatment
plants are attached as Table 4.1, Table 4.2, Table 4.3, Table 4.4 & Table 4.5.
T a b le 4 .1 . D IS S O L V E D O X Y G E N R E A D IN G S A T T M N G E R M U D A
L O C A T IO N
TM N GERM UDA
TYPE
S A M P L IN G D A T E
OD
8 .3 .0 7
D IS S O L V E D O X Y G E N IN M G /L A T G IV E N D IS T A N C E IN F R O N T O F
A E R A T IO N D E V IC E (B R U S H A E R A T O R )
DEPTH O F 3M
8 .3 0 A M
T IM E
LENG TH
1 .0 0 P M
4 .3 0 P M
0 .5 m
1 .5 m
0 .5 m
1 .5 m
0 .5 m
1 .5 m
0 6 -0 3 -0 7 D O R E A D IN G
4 .6
4 .2
4 .5
4 .2
5
4 .3
0 7 -0 3 -0 7 D O R E A D IN G
4 .9
4 .4
3 .9
3 .7
5 .3
4 .5
0 8 -0 3 -0 7 D O R E A D IN G
3 .8
3 .5
3 .7
3 .1
4 .2
3 .8
0 9 -0 3 -0 7 D O R E A D IN G
5 .2
4 .8
5 .7
5 .1
4 .9
4 .6
1 0 -0 3 -0 7 D O R E A D IN G
5
4 .7
4 .8
4 .3
5 .2
4 .7
T a b le 4 .2 . D IS S O L V E D O X Y G E N R E A D IN G S A T T M N P A K A T A N J A Y A
L O C A T IO N
TAM AN PAKATAN JAYA
TYPE
OD
S A M P L IN G D A T E
2 8 .2 .0 7
DEPTH O F 3M
T IM E
LENG TH
2 4 -0 2 -0 7 D O R E A D IN G
D IS S O L V E D O X Y G E N IN M G /L A T G IV E N D IS T A N C E IN F R O N T O F
A E R A T IO N D E V IC E (A S P IR A T O R )
8 .3 0 A M
1 .0 0 P M
4 .3 0 P M
0 .5 m
1 .9
1 .5 m
1 .7
0 .5 m
2 .1
2 5 -0 2 -0 7 D O R E A D IN G
2
1 .8
1 .9
2 6 -0 2 -0 7 D O R E A D IN G
5 .1
1 .9
2 .3 6
2 7 -0 2 -0 7 D O R E A D IN G
2 .1
2
2 .3
2 8 -0 2 -0 7 D O R E A D IN G
1 .8
1 .6
2
1 .5 m
1 .9
0 .5 m
2 .1
1 .5 m
2
1 .8
1 .8
1 .6
2 .1
2 .3
2
2 .1
2 .3
2 .1
1 .8
2 .1
2
T a b le 4 .3 . D IS S O L V E D O X Y G E N R E A D IN G S A T T M N A N D A
L O C A T IO N
TM N ANDA
TYPE
AL
S A M P L IN G D A T E
5 .3 .0 7
D IS S O L V E D O X Y G E N IN M G /L A T G IV E N D IS T A N C E IN F R O N T O F
A E R A T IO N D E V IC E (T O R N A D O )
DEPTH O F 3M
8 .3 0 A M
T IM E
LENG TH
1 .0 0 P M
4 .3 0 P M
0 .5 m
1 .5 m
0 .5 m
1 .5 m
0 .5 m
1 .5 m
0 3 -0 3 -0 7 D O R E A D IN G
5
4 .6
5 .3
5 .1
5 .3
5 .2
0 4 -0 3 -0 7 D O R E A D IN G
4 .8
4 .4
5
4 .8
4 .9
4 .7
0 5 -0 3 -0 7 D O R E A D IN G
5 .4
5 .6
5 .1
4 .6
5 .3
5 .1
0 6 -0 3 -0 7 D O R E A D IN G
5 .3
5
4 .9
4 .4
5
4 .8
0 7 -0 3 -0 7 D O R E A D IN G
5
5 .1
4 .9
4 .6
4 .8
4 .6
30
Table 4.4. DISSOLVED OXYGEN READINGS AT TMN DESA KEBUDAYAAN
LOCATION
TMN DESA KEBUDAYAAN
TYPE
AL
SAMPLING DATE 21.3.07
DISSOLVED OXYGEN IN MG/L AT GIVEN DISTANCE IN FRONT OF
AERATION DEVICE (SURFACE AERATOR)
DEPTH OF 3M
8.30AM
TIME
LENGTH
1.00PM
4.30PM
0.5m
1.5m
0.5m
1.5m
0.5m
1.5m
19-03-07 DO READING
5.6
5.1
5.8
5
5.9
5.1
20-03-07 DO READING
5.8
5
6.1
5.5
5.8
5
21-03-07 DO READING
4.6
4
5.2
4.6
6
5.1
22-03-07 DO READING
5.2
4.6
5.6
4.9
6.1
5.2
23-03-07 DO READING
4.9
4.4
5.9
5.2
6
5.3
Table 4.5. DISSOLVED OXYGEN READINGS AT MEDAN PENGKALAN IMPIAN
LOCATION
MEDAN PENGKALAN IMPIAN
TYPE
EA
SAMPLING DATE
DEPTH OF 3M
TIME
LENGTH
DISSOLVED OXYGEN IN MG/L AT GIVEN DISTANCE IN FRONT OF
AERATION DEVICE (DIFFUSER WITH BLOWERS)
8.30AM
1.00PM
4.30PM
0.5m
1.5m
0.5m
1.5m
0.5m
1.5m
19-03-07 DO READING
4.8
5.1
6.2
5
5.6
5.1
20-03-07 DO READING
5.2
4.7
5.6
5.5
5.8
5
21-03-07 DO READING
4.6
6
5.2
4
6
5.1
22-03-07 DO READING
5.2
4.6
4
4.9
5.9
5.6
23-03-07 DO READING
4.9
4.4
5.9
5
6.1
5.3
31
TABLE 4.6: CHART & SUMMARY OF AVERAGE DISSOLVED OXYGEN FOR 5 DIFFERENT PLANTS
TMN GERMUDA
TMN PAKATAN
(BRUSH
JAYA (ASPIRATOR)
AERATOR)
TMN
AVE.DO (MG/L)
1.9
4.5
MEDAN
PENGKALAN
TMN ANDA
IMPIANA
(TORNADO)
(DIFFUSER
WITH BLOWER)
TMN DESA
KEBUDAYAAN
(SURFACE
AERATOR)
5.3
4.9
BCOD~25 MG/L (REFER TO SAMPLING DATA)
DISSOLVED OXYGEN (MG/L)
6
5
4
3
2
1
0
TMN PAKATAN
JAYA
(ASPIRATOR)
TMN GERMUDA
(BRUSH
AERATOR)
AVE.DO (MG/L)
TMN DESA
KEBUDAYAAN
(SURFACE
AERATOR)
TMN ANDA
(TORNADO)
TAMAN
FIGURE 4.1 : GRAPH DISSOLVED OXYGEN AT EACH TAMAN
MEDAN
PENGKALAN
IMPIANA
(DIFFUSER WITH
BLOWER)
5.3
32
Dissolved oxygen readings were collected for each and every STP on different
week so it was easier on the sampling purpose. This is because it takes 3 to 4 days for
the sampling results to be analysed. The sampling results from Indah Water
Konsortium Sdn. Bhd. (IWK) were sent to Taiping laboratory so that a comparison
can be done on each and every aeration device. In actual fact air was added to an
aeration basin to keep the wastewater and activated sludge mixed. In these way
bacteria was exposed to fresh food all the time.
The other important reason for adding air to the aeration basin was to provide
dissolved oxygen which is actually oxygen which has been dissolved in water. In real
fact, oxygen is not very soluble in water. At 20 degrees Celsius only 9.2 mg/l of
oxygen can be dissolved into water.
We need to add to the aeration basin to provide an environment in the aeration
basin that will encourage the growth of many bacteria. The Dissolved oxygen test
was very crucial because it measures the amount of oxygen available to the facultative
activated sludge organisms. In general aeration rates that provided dissolved oxygen
of between 1.0 and 2.0 mg/l were adequate for maintaining efficient, healthy activated
sludge organisms.
33
If the Dissolved oxygen drops below 1.0 and especially below 0.5 mg/l , the
treatment efficiency was begin to suffer because the activated sludge organisms was
starting to function anaerobically. On the other hand, if the dissolved oxygen was
maintained above 2.0 mg/l, it would be wasting power (that power providing
dissolved oxygen to the aeration basin) dissolved oxygen. Therefore, careful
measurement and control of dissolved oxygen in the aeration basin were necessary to
provide efficient treatment without wasted energy.
This was evident with the sampling result that was done on all type of aeration
devices that was discussed earlier. It was actually inter-related between sampling
results, dissolved oxygen and electricity consumption. From the result it was evident
that 2 numbers 3.7kw aspirator operating only 8 hours a day in Taman Pakatan Jaya
could achieve dissolved oxygen of 1.9 mg/l compared to the other, achieving higher
dissolved oxygen while operating on 16 hours daily. The brush aerator at Taman
Germuda was recorded at 4.5 mg/l dissolved oxygen. The aerator at Taman Desa
Kebudayaan was recorded at 5.3 mg/l dissolved oxygen while the tornado at Taman
Anda and the diffusers with blowers at Medan Pengkalan Impiana were recorded at
4.9 mg/l and 5.3 mg/l respectively.
34
4.3.
COSTING
Capital cost and comparison of each and every aeration system in RM is shown
in Table 4.7 and Figure 4.2. It may differ from time to time due to financial
constraints. Only 5 models are captured that mainly being operated in the waste water
industry:TABLE 4.7 : CAPITAL COST OF DIFFERENT TYPE OF AERATION DEVICE
Model (kw)
3.7kw
5.5kw
7.0kw
OBSELETE
OBSELETE
OBSELETE
Tornado c/w set
30,000
43,000
50,000
Aspirator c/w set
25,000
40,000
45,000
Surface Aerator c/w set
15,000
20,000
25,000
Diffuser (blowers cost)
10,000
15,000
20,000
Type
Brush Aerator
CAPITAL COST OF AERATION DEVICE
60,000
50,000
TOTAL
40,000
30,000
20,000
10,000
0
Tornado c/w set
Aspirator c/w set
Surface Aerator
c/w set
Diffuser (blowers
cost)
3.7kw
5.5kw
TYPE OF AERATION DEVICE
7.0kw
FIGURE 4.2 : GRAPH CAPITAL COST AERATION DEVICE
35
TABLE 4.8 : SUMMARY OF OPERATIONAL AND MAINTENANCE COSTING FROM NOV'06 - APRIL'07
MONTH NOV'06
DEC'06
JAN'07
FEB'07
MAC'07 APR'07
TOTAL (IN
RM)
TAMAN
TMN PAKATAN JAYA (ASPIRATOR)
594
TMN GERMUDA (BRUSH AERATOR)
1,220
6,880
TMN DESA KEBUDAYAAN (SURFACE
AERATOR)
1,260
1,500
TMN ANDA (TORNADO)
1,890
780
870
2,560
MEDAN PENGKALAN IMPIANA
(DIFFUSER WITH BLOWER)
515
714
3,043
8,580
3,485
18,945
650
200
573
3,983
1,800
600
3,270
300
5,730
SUMMARY OF OPERATIONAL & MAINTENANCE COSTING
20,500
18,500
COSTING (RM)
16,500
14,500
12,500
10,500
8,500
6,500
4,500
2,500
500
TMN PAKATAN
JAYA
(ASPIRATOR)
TMN GERMUDA
(BRUSH
AERATOR)
TMN DESA
KEBUDAYAAN
(SURFACE
AERATOR)
TOTAL (IN RM)
TMN ANDA
(TORNADO)
MEDAN
PENGKALAN
IMPIANA
(DIFFUSER W ITH
BLOW ER)
TAMAN
FIGURE 4.3 : GRAPH SUMMARY OF A OPERATIONAL & MAINTENANCE COSTING
Long term expenditure that a wastewater treatment plant endures becomes
extremely expensive for those choosing to select knockoff equipment. Original
equipment was designed to operate with very little maintenance and few parts
replacement for extended periods of time. Typical knockoff equipment is lucky if it
ever reaches a five-year lifetime. Although the knockoff equipment might be as little
as 50 per cent the cost of the original product and attractive to the customer/contractor
during the bidding process, when factor in its failure ratio, frequent parts failure and
its inability to perform, the decision is quite easy.
36
This high cost of operation endured by selecting the knockoffs ends up causing
the client to pay sometimes over double the amount in operation expenditures over a
10 year period of time. The decision to choose the cheapest knockoff to save a small
portion of investment usually ended up creating numerous headaches and costing
them (and the public taxpayers) a heap of cash.
Estimated capital cost of each and every aeration system in this study is stated
in below table. It may vary from time to time due to financial stability. Although it
appears that diffusers are the cheapest among the devices but this is only the
cost of blower. The costing of diffusers is not included because each aeration tank
that uses this device are different size thus making the costing vary. Each diffusers
cost are around RM 100.00. Usually an aeration tank would be using around 50
numbers of diffusers. Surface aerators would be costing around RM 15,000.00 (3.7
kw) to RM 25,000.00 (7.0 kw). Aspirators will be costing slightly cheaper compare to
tornado. Aspirators costing are from RM 25,000.00 (3.7 kw) to RM 45,000.00 (7.0
kw). Tornado costing are from RM 30,000.00 (3.7 kw) to RM 50,000.00 (7.0
kw).There are no costing on brush aerators because it is an obsolete product which
is no more in the market. Even-though the costing varies, but in terms of operational
and maintenance, diffusers is the most difficult to be maintained due to frequent
choking.
Aspirators are the cheapest and the easiest to maintain compared to others as
shown in Table 4.7 and Table 4.8. This is due to aspirators are the latest product in the
market.
37
4.3.
ELECTRICITY CONSUMPTION
Summary of TNB bill for each STP for a period of six months are stated in the table
4.9 and the comparison of electricity consumption in Figure 4.4.
TABLE 4.9: SUMMARY OF ELECTRICITY CONSUMPTION FROM NOV'06 - APRIL'07
MONTH
TAMAN
NOV'06
DEC'06
JAN'07
FEB'07
MAC'07
APR'07
Average
kw
RM
kw
RM
kw
RM
kw
RM
kw
RM
kw
RM
kw
2,338
678
2,050
594
2,066
599
1,736
503
1,003
290
na
na
1,839
533
TMN GERMUDA (BRUSH
AERATOR)
590
171
1,295
375
na
na
728
211
1,505
436
437
126
911
264
TMN DESA KEBUDAYAAN
(SURFACE AERATOR)
2,753
798
588
170
577
2,730
731
1,865
540
2,423
702
TMN ANDA (TORNADO)
4,808 1,394 4,060 1,177 3,302
850
3,465
1,004
3,306
958
3,646 1,056
MEDAN PENGKALAN IMPIANA
(DIFFUSER WITH BLOWER)
3,884 1,126 3,980 1,154 4,870 1,412 4,948 1,434 4,364
1,265
3,872 1,122
TMN PAKATAN JAYA
(ASPIRATOR)
4,612 1,337 1,990
957
2,932
COSTING (RM)
SUMMARY OF ELECTRICITY CONSUMPTION
1,300
1,200
1,100
1,000
900
800
700
600
500
400
300
200
100
TMN PAKATAN
TMN GERMUDA
JAYA (ASPIRATOR) (BRUSH AERATOR)
Average
TMN DESA
KEBUDAYAAN
(SURFACE
AERATOR)
TMN ANDA
(TORNADO)
TAMAN
FIGURE 4.4 : GRAPH SUMMARY OF ELECTRICAL CONSUMPTION
MEDAN
PENGKALAN
IMPIANA (DIFFUSER
WITH BLOWER)
RM
4,320 1,252
38
The operational requirement for wastewater system differs according to
variation flow and strength of wastewater. The peak energy demand usually occurred
from midday to early evening hours when other peak demands for electricity occurs in
the community. As the wastewater load changes during the course of the day the
requirements for aeration, pumping and solid processing change accordingly. Some
sewerage treatment plants modified schedules for equipment operation to meet the
load condition while others operate their devices such as aeration devices
continuously at full capacity regardless of the electricity consumption. Sewerage
treatment plant that has biological treatment for nutrient removal used 30 to 50
percent more electricity for aeration, pumping and solid processing. With introduction
of new technologies for wastewater treatment, the energy requirements will change.
The impacts can either reduce in electricity or increase due to higher level of
treatment. Refer to the electricity data collected for the five different aeration system
if appears that Taman Pakatan Jaya (aspirator) needed an average of 1,839kw per
month while meeting the biological oxygen demand of Standard B requirement.
Although Taman Germuda (Brush Aerator) recorded a mere 911kw per
month, it was found that the brush aerator was unable to function efficiently due some
mechanical problem that continued for three months. Apparently this caused the
electricity consumption to be very low compared to the others which recorded
between 2423 kw till 4320 kw per month.
39
To further prove that aspirator performed well by providing effective aeration
Auburn University in United Kingdom conducted an experiment on mixing capability
and flow pattern. The results were as below:(1)
(2)
(3)
Aspirator: Utilizing units, a circular horizontal flow pattern was created,
covering the whole lagoon area, thus preventing short circuiting and
maximizing lagoon volume. The results was a highly oxygenated flow pattern
that provides complete mixing of the lagoon, keeps solids suspended in any
climate and maintain optimum temperatures year round.
Surface aerators: The area of influence for surface splashers was limited and
has the additional negative affect of cooling the aeration basin through
evaporation.
Blower/Diffusers System: As seen in the photograph, the diameter of
influence for a diffuser system was very limited, requiring a large quantity of
diffusers to cover the area needed. Much of the area was still snow covered
from lack of aeration and mixing, giving the lagoon a pincushion looks.
FIGURE 4.5 : RESULTS OF MIXING CAPABILITIES
40
CHAPTER V
CONCLUSION AND SUGGESTION
5.1.
CONCLUSION
When selecting aeration equipment to use for a specific application, issues it
address include reliability, serviceability, capital cost, system appurtenances and cost
of operation and maintenance. Another important consideration is oxygen transfer rate
using equipment with high oxygen transfer rate values would obviously increase the
electricity consumption.
As conclusion an effective aeration system designed for wastewater treatment
process must be adequate and comply with the biochemical oxygen demand (BOD)
Standard required by the regulators satisfy the oxygen demand of nitrification,
provide adequate mixing, maintain a minimum dissolved oxygen (1 to 3 mg/l)
throughout the aeration basin and efficient in energy saving.
Thus, from the experiment and evaluation conducted from this study case,
aspirator performed extremely well in order to provide the most effective aeration in
wastewater. It was also a very cost effective device compared to other aeration
devices.
41
5.2.
SUGGESTION
Since this study only concentrated on an average Population Equivalent
of 2500, further studies or experiments should be carried out on higher Population
Equivalent with different kind of STP (sewerage treatment plants) such as sequence
batch reactors, rotating biological contacted, activated sludge and
others . This
would be an indicator whether different population equivalent plants reacts
differently in terms of oxygen transfer rate.
It should concentrate on different age of plants because it may produce
different results. New plants or aeration devices most probably would perform
without any hiccups compared to the aged equipment or devices.
Another factor that should be taken into consideration is the location of
the sewerage treatment plant whether it is on highland or lowland such as
Cameron Highland because low temperature would produce different results
compared to high temperature.
42
REFERENCES
Indah Water Konsortium. 2002. Operation and Maintenance . Kuala Lumpur,
Tim Hobson. 1992. Activated sludge, Evaluating and Controlling Process, 2nd Ed.
Mckenna
Metcalf and Eddy. 1991. Wastewater Engineering. 3rd Ed. McGraw Hill,
S.K.Gang. 2004. Sewage Disposal and Air Pollution Engineering. Khanna Publishers,
Mark J.Hammer, Mark J.Hammer,Jr. 2004. Water and Waste Water Technology 5th
Ed. Pearson Prentice Hall
D.Lal.A.K. Upadhyay. 2004. Water Supply and Waste Water Engineering. Revised
Edition. Sanjeer Kataria.
Sewerage Services Department (1999). Guidelines for Developers, Sewerage Policy
for New Developments. Malaysia, Volume I..
Sewerage Services Department (1998). Guidelines for Developers, Sewerage
Treatment Plants. Malaysia, Volume IV.
43
APPENDIX A
SUMMARY OF SAMPLING RESULTS FOR SEWERAGE PLANTS
SAMPLING
INDAH
RESULTS
LOCATION
SAMPLING
EFF.
SPL
PURPOSE
BOD
COD
NH3
NO3
DATE
STD
TYPE
CODE
mg/l
mg/l
mg/l
mg/l
IPH240
TMN PAKATAN
JAYA, FASA 1
28-Feb-07
B
FE
O
20
62
6
n.a
7.1
IPH052
TMN ANDA
5-Mac-07
B
FE
O
26
77
26
1
IPH145
TMN DESA
KEBUDAYAAN
21-Mac-07
B
FE
O
57
155
3
IPH005
TMN GERMUDA
8-Mac-07
B
FE
O
21
66
IPH346
MEDAN
PENGKALAN
IMPIAN
23-Mac-07
B
FE
O
28
66
REF
pH
O&G
SS
mg/l
mg/l
1
47
8.5
2
46
n.a
8.8
n.a
81
10
n.a
7.2
2
52
21
n.a
7.4
n.a
22
44
APPENDIX B
PHOTOGRAPH OF VARIOUS AERATION DEVICE
TAMAN ANDA
( TORNADO)
TAMAN PAKATAN JAYA
(ASPIRATOR)
45
MEDAN PENGKALAN IMPIAN
(DIFFUSERS AND BLOWERS)
TAMAN GERMUDA
(BRUSH AERATOR)
TAMAN DESA KEBUDAYAAN
(SURFACE AERATOR)