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Odour and Air Management Studies Key Tool in Determining Effective Odour Control Solutions
Presented by:
Wayne Wong, M.A.Sc, EIT
September 28, 2011
2 Project Team
Yuko Suda, P.Eng. – Kerr Wood Leidal Associates Ltd.
Ted Steele, P.Eng. – Kerr Wood Leidal Associates Ltd.
Karl Mueller, P.Eng. – Kerr Wood Leidal Associates Ltd.
Chris Hunniford, PE – OCTC, a V&A Company
September 28, 2011
3 Introduction to Sanitary Odours
Sewer odours can be found everywhere!
• Gravity sewers
• Air vents
• Pump stations
• Forcemains
• Manholes
• Wastewater treatment plants
Operational headache and nuisance
Costly problem for municipalities
September 28, 2011
4 Source of Odours in Sanitary Systems
Odour Generation + Odour Release = Problem
Odour Generation
Combination of organic waste material and bacteria in the sewer generates hydrogen sulfide (H2S) and volatile organic compounds (VOCs)
Especially problematic in collection systems with large detention times
Odour Release
Local pressurization of the air space above sewage results in odourous air being released from a contained space (sewer, manhole, tanks)
September 28, 2011
5 Odour Generation
Occurs where there are large concentrations of organic
waste materials and bacteria in the collection system
Odour generation is accelerated under the right
environmental conditions
- Anaerobic conditions (typical in long forcemains or in
collection systems with long hydraulic detention times)
Odour generation is generally difficult to avoid
September 28, 2011
6 Odour Release
Odourous compounds generally exist within the collection system
headspace.
These odourous compounds become a nuisance when there is
pressurization and air movement which transport these odourous
compounds from the headspace and released into the environment
There are a number of physical mechanisms that result in
pressurization. The primary mechanism is air movement in the
sewer due to the effects of friction drag
September 28, 2011
7 Air Movement in Sewers
The primary force for air movement in
gravity systems is the friction between
the sewer headspace air and the
moving wastewater below.
Resistance to air movement due to
friction between air and pipe wall
An idealized velocity gradient can be
developed based on these simple
boundary conditions
September 28, 2011
8 Air Movement in Sewers
Friction factor between water and air varies depending on factors such
as turbulence and rough water surfaces (high friction factor)
Slower moving, quiescent water surfaces will generally result in a lower
friction factor.
The flow rate of air that is conveyed is proportional to the air velocity in
the headspace and the cross sectional area of the headspace
September 28, 2011
9 Pressurization in Sewer Headspace
Occurs when there are abrupt changes in rate of air flow in the sewer
High flow rate of air from one section colliding with air in a downstream
section that has a lower air flow rate
Change in air flow rates can be caused by changes in pipe slope and/or
restrictions in the sewer headspace
When an area of pressurization coincides with a vent or manhole,
sewer air will be expelled at that point
September 28, 2011
10
Other Factors That Can Contribute to
Odour Ventilation Problems
Change in atmospheric (barometric) pressure or ambient temperature
(air density change) can cause air movement in/out of the collection
system
Strong surface winds can draw air out of the collection system via
eduction
Decrease in pipe diameter in downstream pipe sections
Opposing or perpendicular flows entering a junction can cause a
temporary backup of air
September 28, 2011
11
Other Factors That Can Contribute to
Odour Ventilation Problems
Ventilation effects are more pronounced in collection
systems with fewer service connections, vents and
manholes, where there are fewer relief points for
expelling air
Ventilation effects can be most severe at inverted siphons,
full-flowing or surcharged sewers and pump stations
where airflow can be stopped completely, creating high
pressures in the sewers
September 28, 2011
12 Developing Solutions
Conventional solution has been to seal manholes or install
carbon scrubbers
This results in increased air pressurization of the overall
collection system and causes air to be expelled
elsewhere.
This is a reactive approach which shifts the problem to
another location rather than solving the problem
September 28, 2011
13 Developing Solutions
Completing an odour and air management study to
determine the most effective mitigation strategy is a
more proactive approach
A comprehensive odour and air management study can be
used to:
• Determine the root cause of odour emissions
• Develop options to eliminate problems while minimizing
capital and operating costs.
September 28, 2011
14 Odour and Air Management Study
An effective odour control and air management study includes:
• Monitoring program (H2S, VOCs, differential air pressure)
• Ventilation modeling (areas of pressurization, air flow rates,
ventilation dynamics, release points)
• Hydraulic modeling (displacement effects)
Based on the above study, an evaluation can be conducted
to develop the most effective strategy for mitigating
odour
September 28, 2011
15 Differential Pressure Monitoring
September 28, 2011
16 Hydrogen Sulphide Monitoring
September 28, 2011
17 Odour and Air Management Study
An evaluation can be conducted to develop the most effective strategy
for mitigating odour emissions
Key Considerations:
•Cost
•Feasibility of implementation
•Environmental impacts (hazardous chemicals, noise, etc.)
•Overall treatment effectiveness
September 28, 2011
18 Active Odour Control Facilities
An active odour control facility draws air from the sewer using a fan,
treats it, and releases it to the atmosphere.
Types of treatment include biofilters, activated carbon adsorbers, and
chemical scrubbers etc.
Drawing air in from the sewer with a blower and treating it creates an
area of negative pressure (zone of influence) in the vicinity of the
collection system within the sewer.
September 28, 2011
19 Case Study – Highbury Interceptor Metro Vancouver
The Highbury Interceptor is owned and operated by Metro Vancouver.
Combined sewer (expected to be fully separated by 2050)
Total length = 6 km
Pipe diameter – 2,900 mm
Significant odour complaints and headspace pressurization issues
Noise issues during winter storms in which large volumes of air are
expelled from vents
Manhole lids have been observed to be blown off
September 28, 2011
20 Case Study – Highbury Interceptor Metro Vancouver
Monitoring of differential pressure, H2S, and VOCs were carried out
during both wet and dry weather periods
Differential pressure indicated significant positive pressure occurs
throughout the Highbury Interceptor
Ventilation modeling estimated the drag airflow at 10,000 cfm
Downstream end of the interceptor is a siphon, and no air can be
conveyed beyond this point, creating an area of high pressurization
September 28, 2011
21 Case Study – Highbury Interceptor Metro Vancouver
Hydraulic modeling indicated that sections of sewer becomes
completely isolated from upstream, downstream, and tributary sewers
during high flow (backwatering)
The ventilation model estimated that a typical storm could displace up
to 7,000 cfm
Model indicated that as sewage level increase, a large amount of air
can only be displaced at a few small vents (high pressure and high air
discharge velocity)
September 28, 2011
22 Case Study – Highbury Interceptor Metro Vancouver
KWL/OCTC project team proposed three active odour control facilities
along the interceptor sewer
The main active odour control facility, with a design treatment capacity
of 10,000 cfm, would have a zone of influence of approximately 4.6 km
September 28, 2011
23 Conclusion
An odour control and air management study that includes monitoring,
ventilation modeling, and hydraulic modeling is key to determining the
root cause of odour complaints and can be used to develop a cost
effective solution for controlling odour in a sanitary collection system.
September 28, 2011
24 Questions?
Contact Information
Wayne Wong, M.A.Sc., EIT
Kerr Wood Leidal Associates Ltd.
(604) 293-3274
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