What makes an effective odor control cover system design?

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Courtesy of Geomembrane Technologies Inc. (GTI)

Background

A common side effect of wastewater treatment processes is the unpleasant odor that can linger near the plant, annoying neighbors, who then complain.

Often, the wastewater treatment plant was built many years ago in an isolated part of town.  Over the years, the town’s population grew until subdivisions were developed next to the treatment plant.   The new neighbors, offended by the odors, expect a resolution and know how to have their complaints taken seriously.   Odor complaints become a political problem that must be solved.  Budgets are allocated to study and solve the odor problem.  Professional consultants are retained.  Odor sources are identified and odor levels are quantified.  Solutions are explored.

Potential solutions that are often considered include:

  • Manage the neighbors’ complaints. Explain to the public that the treatment plant is a necessary part of the town and is helping to fuel the local economy.
  • Stop the foul air production by introducing new treatment technologies, fine-tuning the treatment process, or adding chemicals to the wastewater.
  • Stop foul air from being conveyed to neighbors by intercepting and treating/masking odors with fogging/deodorizing systems, diluting odors by adding buffers, containing odors with covers, or capturing odors with covers and then withdrawing and treating the captured foul air.

Potential solutions are evaluated in light of certain basic realities:

  • The treatment plant cannot be moved.
  • Utilities do not have unlimited resources; there are many competing demands for budget allocations.
  • The treatment plant is a complex system that must be maintained so that it can continue to function.  Any odor control solution must allow plant workers to continue to do their jobs efficiently, effectively, and safely.  Often, periodic access to the tanks for inspection and maintenance is an important consideration.

The right solution is identified.  For some applications the right solution is to ‘capture and treat’ odorous air (i.e., foul air is captured under a gastight cover and is then withdrawn and treated).  This white paper presents considerations in the design of odor control cover systems.

General Design Components

The basic concept for an odor control cover system is straightforward: Cover a wastewater tank to capture odorous offgas, then remove and treat the odors.

For the purposes of this white paper, an odor control cover system is considered to have the following components: a fresh air inlet point, a gastight cover, a foul air withdrawal point, a blower with ductwork, and an odor treatment system.  These are discussed in more detail below.

Fresh air inlet point:
This is an element that is often initially overlooked.  The general design concept is that fresh air is drawn under the cover, mixes with foul air, and this mixture flows along under the cover until it is withdrawn.  A fresh air inlet is therefore required.  This is typically located at one end of a cover system, and the foul air withdrawal point is located at the other end.

Gastight cover:
From the designer’s point of view, the desire is to have a cover which is sufficiently gastight that:

  • Foul air does not escape through leak points and lead to odor complaints.
  • Excessive fresh air does not infiltrate through leak points along the cover, creating relative ‘dead spots’ upstream where offgas could collect in concentrations that could potentially damage concrete or equipment under the cover.

The gastightness of a cover is important because less leakage means that foul air can be removed using a smaller blower and ductwork, thus reducing both capital and operating costs.From the treatment plant staff’s point of view, the desire is to have a functional system that controls odors while allowing workers to do their jobs operating and maintaining the plant.  This often involves full access below the cover for inspection and maintenance.

Foul air withdrawal point:
The foul air withdrawal point is the point where ductwork connects to cover system.  The connection detail could be a duct stub, flanged duct or flexible coupler.

Blower and ductwork:
As noted above, the gastightness of a cover is important because less leakage means that foul air can be removed using a smaller blower and ductwork, thus reducing both capital and operating costs.

Odor treatment system:
Once foul air is captured and conveyed in a ductwork system, it can be treated by a variety of technologies such as a: biofilter, wet scrubber, carbon filter, proprietary media, etc.

Design Considerations

Gastightness:
The gastightness of a cover is crucial to effective odor control. Most covers can be made sufficiently gastight through the use of ample caulking, gaskets, and anchors, but it is important to note that there can be a trade-off between gastightness and ease of access.

Access:
Full or partial access below the cover for inspection or maintenance is often desirable.  With some covers, achieving the full access required in some applications means that the cover has to be either essentially disassembled and removed in pieces, or lifted off as one unit using a crane.  This requires resources and assumes that the cover removal is scheduled in advance as a planned maintenance activity. Full access in an emergency can be difficult.  However, some covers are designed to provide both quick and easy full access and gastightness.  Ultimately, the choice of cover style depends on access requirements of the application, but choosing a cover designed to provide both ease of access and gastightness can be beneficial in terms of both design and operation.

Safety during access:
Worker safety is always important at treatment plants and this certainly applies when covers are required to be opened for maintenance.  If a cover is being manually disassembled and removed, the removal work necessarily creates openings through which workers could fall. This safety issue should be assessed and managed through the cover system design.

Cover height:
The height of the cover profile is an important consideration in choosing a cover system.  Some cover systems have a flat profile, some have a low profile arched shape, and some have a high profile arched shape.  A flat or low profile results in less air space below the cover and allows use of a smaller mechanical system if the foul air withdrawal design is based on ‘number of air changes per hour’.  A higher profile cover accommodates equipment protruding from the tank and may allow workers to walk under the cover if the environment under the cover is safe for them to work in, but it can require a larger mechanical ventilation system, which is more costly to install and operate.

Penetrations:
Typically, covered tanks will have a variety of penetrations through the cover.  This can include pipes (discharge, aeration, water, etc.), valve operators, instrumentation, etc.  All penetrations should be identified and coordinated with the cover manufacturer.

Site-specific customization:
Almost every cover includes custom-designed elements.  Factors that might influence a site-specific cover design include:

  • A tank opening with an irregular shape.
  • The presence of equipment that protrudes into the area to be covered.
  • Varying wall heights around a tank perimeter.
  • Specific locations where inspection access is required.

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