Case Study SA-JW2
MECHANICAL AND FINE BUBBLE AERATION IN ACTIVATED SLUDGE SYSTEMS
As results of sharp increases in the price of electricity and the need to conserve energy, Johannesburg Water
has studied the energy efficiency of different aeration systems by investigating the cost and energy
efficiency of various aeration systems.
Energy efficiency of mechanical and fine bubble aeration systems
Description of Process:
Four types of aeration systems were compared in terms of their energy efficiency and overall suitability
and NPV or life cycle costs. The basic model for oxygen transfer in a dispersed gas-liquid system was used:
rate of mass transfer per unit volume of liquid = volumetric mass transfer coefficient x driving force
 Mechanical surface aeration: The total installed aeration capacity is 1020 kW and average electrical
energy demand of 16 820 kWh/d, using 4-110 kW, 4-90kW, 4-55kW aerators. A standard oxygen
transfer rate of 1.80 kg O2/kWh has been adopted in determining aeration requirements.
 Fine bubble diffused aeration: Oxygen transfer occurs during forced bubble formation, bubble ascent
and at water surface, with equipment producing an optimal bubble size of 2.2 mm. System has capacity
of 508 kW and average electrical energy demand of 10 080 kWh/d.
 Aerated turbine aeration: Submerged open bladed turbine with air sparged under the turbine. 24-45
kW units with lowers giving 1080 kW and average electrical energy demand of 24 216 kWh/d.
 Jet aeration: Submersible pump, venturi educator and induction tube to draw air into the venture.
Blowers can also be used to supply air to the venture under pressure. SAE of 1 kgO2/kWh is less than
the 1.8 kgO2/kWh specific for mechanical surface aerations systems, hence system were discarded from
study.
Potential Interventions

To minimising power consumption and maximising performance in mechanical surface aeration
systems, the following are important: i) control of the present oxygen concentration in aerated liquid
within 1 – 2 mg/l, where control can be facilitate by providing for controllable outlet weirs on the
reactor; ii) provision of the minimum required aeration intensity; iii) inspection and service for
uninterrupted operation

Energy efficiency in fine bubble systems is critically dependent on matching component sizing and
flexibility with process demand. Energy is wasted when oxygen supply does not match the process
demand. Flexibility can be positively altered by: i) add or remove (capacity) multiple aeration basins; ii)
adjust diffuser aeration by controlling air flow rates (O2 turndown ability); iii) provide multiple lowers
with max to min air flow ration of 5:1; iv) control of air flow rate and present DO concentration.

Blowers for aerations systems are a major source of energy consumption. Proper selection can result in
high efficiencies. Proper maintenance of aeration diffusers is critical to preset head loss due to fouling
and to reduce energy costs. For dynamic compressors, turndown is best achieved using inlet throttling
using adjustable inlet guide vanes.
Range of potential Savings
Based on energy efficiency as only criteria, the fine bubble aeration system is a preferred system. Based on
overall suitability and total life cycle cost or NPV, mechanical surface aeration is the preferred system in
Johannesburg Water.
Case Study SA-JW2
Case Study SA-JW2
MECHANICAL AND FINE BUBBLE AERATION IN ACTIVATED SLUDGE SYSTEMS
Ref
Case Study SA-JW2
Location:
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Sector:
Johannesburg Water is situated in Central and Southern Gauteng
Province of South Africa. The areas are densely populated. Two of the
works are situated in the northern catchment and three in the
southern catchment.
Sewage – mixed liquid phase
Works Owner or Operator:
The plants are owned by the City of Johannesburg (Water Services
Authority) and operated and maintained by Johannesburg Water
(Water Services Provider). Department of Water Affairs is the
Regulator and Johannesburg Water need to comply with the water use
license specific to each of its 5 plants.
Size:
Some 3.5 million people are served in the northern catchment and 2.5
million people in the southern catchment.
Energy Provider:
Power is in form of electricity, provider by the national electricity
agency, ESKOM. Electricity tariffs are made up of: service and
admin charge (R722.28/day), network charges: demand and access
(R18.39 /kVA), energy charges and 2c/kWh Environmental
Electricity levy (17.422c /kWh).
Process:
Operation consists of oxygen transfer through various means, media
and equ9pment to the mixed liquid in the activated sludge basins. As
result of this physical intervention, both biological and chemical
reactions are affected.
Component:
This case study refers to the activated sludge and mixed suspended
solids phase only.
Motivation for the case study:
The cost of electricity is set to double over 13 years starting 2008 in
South Africa. With most cities and large towns employing aeration of
the mixed liquor suspended solids in the activated sludge processes,
the energy efficiency of different aeration types are becoming a key
consideration as part of the overall life cycle cost.
Process/Plant changes:
Comparison of energy efficiency, life cycle cost and NPV of different
fine bubble aeration systems, using mechanical mixing as benchmark.
Civil/Physical Changes:
Opting for continued use and installation of mechanical aeration as
preferred technology across all wastewater treatment plants.
Operational Changes:
Minimising power consumption and maximising performance in
mechanical surface aeration systems, by controlling present oxygen
concentration in aerated liquid within 1 – 2 mg/L, provide minimum
required aeration intensity, and monitor and service and
performance for uninterrupted operation. High skills levels need to
be maintained.
Risksand Dependencies:
Risk is in considering only energy efficiency as criteria when making
technology decisions. Decisions need to be based on multiple criteria
to include aeration efficiency, energy efficiency, performance ito legal
specification, and life cycle costs or NPV.
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Response information, description and remarks
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Implementation:
The project was implemented in multi-phase: evaluation of existing
systems, comparison with alternative aeration systems available in
South African market, procurement of preferred options will be
implemented over a 30 year life span using design/build models.
Energy Efficiency gains:
Mechanical system: 319 – 336 kWh/ML power used, 267 – 301 O2 kg
O2/d/ML demand
Fine bubble system: 320 kWh/ML power used, 317 O2 kg O2/d/ML
demand
Fine bubble aeration is 11-19% more efficient than mechanical surface
aeration, which translate to a higher long term energy efficiency of
11% compared to mechanical surface aeration. However, based on
multiple criteria evaluation, mechanical surface aeration the preferred
system for Johannesburg Water. Long term investment with average
real return on investment could indicate 4% - 6%.
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Cost/Benefit analysis:
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Project review:
Continued learning. With fine bubble aeration, the requirement for
standby facility and reactor and the decreased efficiency over time
(=increase energy demand) need to be considered.
Confidence grade:
High – case study based on material and data analysed by specialist a
group. Further work is continuing.
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The cost of electricity is a significant component of the overall cost of wastewater treatment. The aeration of
the mixed liquor suspended solids in the activated sludge process constitutes a major portion of the
electricity consumption on a plant. The majority of plants in major cities and large towns in South Africa use
mechanical surface aerators for aeration, with some variations in fine bubble diffused aerations systems. As
results of sharp increases in the price of electricity and the need to conserve energy, Johannesburg Water
has studied the energy efficiency of different aeration systems through the Johannesburg Wastewater
Partnership by investigating various fine bubble aeration systems and their performance and potential cost
savings against the existing surface aeration systems.
Description of Process
The performance of existing aeration systems based on existing influent sewage and treated effluent
characteristic and historic energy consumption of the aeration systems has been evaluated. The following
sites with their respective design parameters were evaluated:
Plant name
Northern
ADWF
design
capacity &
technology
type
Oxygen
demand
(kgO2.d) average
Effluent quality
COD
(mg/L)
SS
(mg/L)
NH3
(mg/L)
PO4
(mg/L)
COD
(mg/L)
SS
(mg/L)
NH3
PO4
(mg/L)
(mg/L)
13 397
361
77
26.1
7.2
46
4
0.6
0.7
15 067
399
169
28.6
8.6
27
1
0.4
0.5
15 872
490
92
21.8
7.6
74
-
-
-
of summer (22oC)
& winter (14oC) OD
450 ML/d
Influent quality
mechanical
surface
aeration
Olifantsvlei
200 ML/d
mechanical
surface
aeration
Bushkoppies
200 ML/d
fine bubble
aeration
The basic model for oxygen transfer in a dispersed gas-liquid system was used:
rate of mass transfer per unit volume of liquid = volumetric mass transfer coefficient x driving force
Expressed in terms of the overall liquid phase volumetric mass transfer coefficient the above relationship
becomes:
W = KL a* (*C* - C)where:
KL a* = overall volumetric mass transfer coefficient in clean water based on liquid resistance
C* = dissolved oxygen saturation coefficient
C = dissolved oxygen concentration
The total transfer rate expressed ito the apparent average volumetric mass transfer coefficient and oxygen
saturation coefficient is:
OTR = KL a V (C* - C)where:
OTR = oxygen transfer rate occurring in the liquid V
KL a* = apparent spatial average volumetric mass transfer coefficient in clean water
C* = spatial average dissolved oxygen saturation coefficient
C =spatial dissolved oxygen concentration
The following correction factors were applied to correct for the effect of variables on the oxygen transfer
coefficient and saturation values, i.e.
 alpha (process water characteristics),
 beta (process water characteristics),
 theta (temperature),
 tau (temperature) and
 omega (pressure) correction factors.
Using mechanical aeration as benchmark (plants = 1550 m above sea level), four types of aeration systems
were compared in terms of their energy efficiency and overall suitability and NPV or life cycle costs:
 Mechanical surface aeration: The total installed aeration capacity is 1020 kW and average electrical
energy demand of 16 820 kWh/d, using 4-110 kW, 4-90kW, 4-55kW aerators. A standard oxygen transfer
rate of 1.80 kg O2/kWh has been adopted in determining aeration requirements.
 Fine bubble diffused aeration: Oxygen transfer occurs during forced bubble formation, bubble ascent and
at water surface, with equipment producing an optimal bubble size of 2.2 mm. System has capacity of
508 kW and average electrical energy demand of 10 080 kWh/d.
 Aerated turbine aeration: Submerged open bladed turbine with air sparged under the turbine. 24-45 kW
units with lowers giving 1080 kW and average electrical energy demand of 24 216 kWh/d.
 Jet aeration: Submersible pump, venturi educator and induction tube to draw air into the venture.
Blowers can also be used to supply air to the venture under pressure. SAE of 1 kgO2/kWh is less than the
1.8 kgO2/kWh specific for mechanical surface aerations systems, hence system were discarded from
study.
Results
Oxygen transfer and energy efficiency:
The actual oxygen transfer rates for the mechanical aeration system are 16 – 28% less that the theoretical
optimum transfer rates. It is likely that the aerators are not operating under optimum conditions because of
varying flows and loads on the plant, and because the system have been designed to handle peak loads
under optimum conditions rather than average loads.
The Olifantsvlei plant have higher transfer efficiency than the Northern plant, attributed to the latter plant
operating under less favourable conditions than the former plant. Northern work unit 4 receives flow in
excess of its design ADWF.
Oxygen transfer at the Bushkoppie plant (fine bubble) is 10.8 and 18.5% more efficient than the 2
mechanical aerated systems, but less efficient than its theoretical oxygen transfer efficiency.
The following factors are to be considered in addition to energy efficiency when opting for different aeration
systems:


Mechanical and aerated turbine systems are robust systems from operations point of view, as
failure of one of the units may require a small reduction in the load whilst the unit is repaired or
replaced. The plant does not have to be withdrawn from service. These plant therefore do not
need standby treatment capacity
Fine bubble aeration systems will result in the whole plant being withdrawn from service if the
diffuser system fails. Standby capacity can be provided in various ways: provide for multiple


smaller units and run those under capacity; divert flow to another plant with spare capacity;
work on system to be deferred until capacity expansion occurs; and design aeration basin as
modular units within reactor to allow for sections to be taken out of service.
The aeration efficiency of fine bubble systems decrease over time (up to 50%). Research points
towards rapid decline in first 24 months, followed by stabilisation at a lower value. The
performance decline is mostly as result of fouling and scaling depending on process conditions,
water quality and diffuser type. Manufacturers recognise this and incorporate measures to
mitigate the decrease in efficiency. i.e. automatic monitoring and cleaning measures (acid
washing and injection, etc).
Oxygen transfer efficiency of mechanical surface aeration and aerated turbine systems will not
decrease over time.
Potential Interventions

To minimising power consumption and maximising performance in mechanical surface aeration systems,
the following are important: i) control of the present oxygen concentration in aerated liquid within 1 – 2
mg/L, where control can be facilitate by providing for controllable outlet weirs on the reactor; ii)
provision of the minimum required aeration intensity; iii) inspection and service for uninterrupted
operation.

Energy efficiency in fine bubble systems is critically dependent on matching component sizing and
flexibility with process demand. Energy is wasted when oxygen supply does not match the process
demand. Flexibility can be positively altered by: i) add or remove (capacity) multiple aeration basins; ii)
adjust diffuser aeration by controlling air flow rates (O2 turndown ability); iii) provide multiple lowers
with max to min air flow ration of 5:1; iv) control of air flow rate and present DO concentration.

Blowers for aerations systems are a major source of energy consumption. Proper selection can result in
high efficiencies. Proper maintenance of aeration diffusers is critical to preset had loss due to fouling and
to reduce energy costs. For dynamic compressors, turndown is best achieved using inlet throttling using
adjustable inlet guide vanes.
Cost Saving
In terms of equipment costs, energy requirements/cost and life cycle costs (NPV) for each system, the
following results are evident:

Mechanical surface aeration has the lowest capital and fine bubble the highest capital cost

Fine bubble has the lowest energy cost

Fine bubble has the lowest NPV for a discount rateof 4%, but the cost does not include standby
treatment capacity, reduced aeration efficiency and increased energy cost, and maintenance costs
associated with cleaning of the diffuser system. Hence, mechanical aeration is likely to have a lower NPV
than fine bubble aeration.
Note: if the decrease in aeration efficiency results in an average increase of 20% in electricity use, the life
cycle cost advantage of fine bubble aeration over mechanical surface aeration would be eliminated.

Based on a multi-criteria analysis (see comparative table hereunder), mechanical aeration has the
highest ranking and is the preferred system in the cases of Johannesburg Water.
#
Criterion
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2
3
4
5
6
7
8
9
10
Initial capital cost
Energy costs
NPV life cycle costs
Maintenance costs
Equipment replacement costs
System robustness
Energy efficiency
Ease of maintenance
Standby capacity
Environmental factors
Rating
Mechanical
surface aeration
1
2
2
1
2
1
2
2
1
3
17
Aeration System
Aerated
turbine
2
3
3
2
3
2
3
1
1
1
21
Fine bubble
aeration
3
1
1
3
1
3
1
3
3
1
20
In conclusion, Johannesburg Water considers the overall suitability of an aeration system when making
future investment and decisions. In accordance with this case study based on the life cycle cost and the
energy efficiency, the mechanical surface aeration will be adopted as preferential technology in
Johannesburg Water.
References:
i.
ii.
iii.
Interview: Shaun Deacon, Process Specialist, Johannesburg Water, 19 December 2011
Draft Report on the investigation of various fine bubble aeration systems, Johannesburg JW
Wastewater Partnership, PS702(E A-303-00141=Rep-200905019), May 2009
Wastewater Engineering: Treatment and Reuse, Metcalf and Eddy, 4th Edition, 2003, McGrawHill Eds.