Odour and Air Management Studies

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Odour and Air Management Studies
Presented by Wayne Wong, M.A.Sc, EIT

• Key Tool in Determining Effective Odour Control
  Solutions

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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 VA Company

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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

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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)

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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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

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Differential Pressure Monitoring
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Hydrogen Sulphide Monitoring
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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

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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.

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Case Study Highbury InterceptorMetro 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

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Case Study Highbury InterceptorMetro 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

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Case Study Highbury InterceptorMetro 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)

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Case Study Highbury InterceptorMetro 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

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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.

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Questions?

• Contact Information
• Wayne Wong, M.A.Sc., EIT
• Kerr Wood Leidal Associates Ltd.
• wwong_at_kwl.ca
• [phone]