First step to effective odor control

The key to controlling nuisance odors may not be as straightforward as expected. Yes, the technology to treat nuisance odors that are released from municipal and industrial wastewater treatment plants is available. The challenge comes in selecting the most cost-effective technology. But the best technology in the world or the most costly odor control system can only be effective in reducing odorous emissions if the odors reach the odor control device. If the containment and ventilation systems are not able to keep the nuisance odors from escaping, then the money spent on the odor control has been wasted.

Flat aluminum cover for odor control

A critical element of any odor control evaluation and system, which is often overlooked, is the design of odor containment and ventilation systems. These systems dictate the size of the odor control system and also keep the nuisance odors from escaping before they can be treated by the odor control devices.

In recent years, the focus of many papers has been either the procedures to follow when conducting an odor assessment study or the various technologies available to control odors after they have been identified and collected. Very little has been presented regarding how to contain the odors at the source, minimize the volume of air that has to be treated, and ventilate the enclosed area so that the odors can be transported to the selected odor control device for treatment. These tasks have been taken for granted when they should actually be a part of the critical first steps in the odor control process design.

Odor containment system

This story begins with a wastewater treatment facility that is trying to be a good neighbor and act responsibly by implementing odor control actions. When the first level of odor control actions (measures including operational adjustments, chemical addition, and air dilution) fails to achieve the goal of reducing off-site neighborhood odors, the second and more comprehensive level is required. The second level consists of:

• Odor containment by covering open channel and tank surfaces
• Odor transport by providing negative pressure ventilation under the covering system
• Odor treatment in the appropriate control device
• Ventilation discharge with adequate dispersion to minimize odor and visual effects on the surrounding area.

Once the source of the odors has been identified through field assessments and the control system identified, the odor containment system needs to be selected. A full range of alternatives is available today. The appropriate containment system depends on several factors, including:

• Site-specific unit processes that need to be controlled
• Climate
• Worker safety
• Ease of construction
• Staff needs
• Aesthetics
• Effectiveness
• Durability

Cover or enclosure system for primary sedimentation tanks

A topic at wastewater treatment plants that is receiving much attention lately is the need for a cover or enclosure system for primary sedimentation tanks because these tanks release nuisance odors that are being detected by the neighbors. Many plants around the country have either already covered these tanks or are in the process of doing so.

The evaluation and selection of a functional covering or enclosure system should consider several important factors, including:

Operation and maintenance
The operators must be able to gain access to critical system components for daily inspections and routine maintenance. Critical system components could include:

• Bottom sludge and surface scum collector drive equipment
• Water surface observation for collector system inspection and wastewater color
• Effluent weirs.

Minimization of cost
By designing a system that requires the smallest volume of air necessary for thorough odor control, smaller and less costly equipment will be needed.

Durability
The design should use corrosion resistant and durable materials of construction.

Architectural conformity of new structures
New structures should be consistent with the existing architecture to maintain aesthetic harmony.

Structural integrity
The design should include structural considerations and provide necessary modifications if additional support is required.

Alternative covers or enclosures

Flat covers. Flat covers minimize the air space between the cover and the water surface and generally require limited external structural support, although external structural support depends on the size of the opening that needs to be covered. Therefore, flat covers could be the least costly for covering and treatment. The major disadvantage of flat covers is that they limit operator accessibility. The covers have to be removed or displaced for routine daily inspections unless access hatches are installed. As a result, flat covers are often the cover of choice for areas that do not require frequent and extensive operator accessibility.

Flat aluminum cover

Barrel arch covers. Barrel arch covers are appropriate for relatively narrow tanks or channels and not typically used for extensive open surface areas. Semi-circular in nature when viewed along their longitudinal axis, the height of the top of the arch above the water surface is variable. The height selected is often chosen more for aesthetics and visibility of the water than any other function. View windows and inspection or access hatches provide the operator with an extended and uninterrupted view of the water surface.

Barrel arch cover

Barrel arches can be equipped with a roll back feature at either end for greater access to critical tank areas. For example, critical areas for sedimentation tanks include the influent (where surface scum can accumulate and workers need access to remove it) and effluent ends (where effluent weirs need to be maintained). Installing high intensity lights improves tank visibility for both day and night inspections.

Movable barrel arch cover

While providing greater operator access and visibility, the barrel arches have a pronounced exterior profile. The visual effect of the profile can be minimized by reducing the height of the arch above the water surface. A balance between exterior profile and operator access and visibility, however, is needed and dictates the final selection.

The volume of air and thus the flow of odorous air to be treated is greater with the barrel arch cover than with flat covers. The relatively low rise barrel arch cover can, however, also minimize the air volume requiring treatment. Barrel arch covers are applicable for long distributed areas requiring routine but not full operator access. The rollback arch cover option allows regular operator access.

Pitched covers. Pitched covers are similar to barrel arches in that they are appropriate for long distributed areas with generally narrow width. The visual effect of their exterior profile is also similar. The pitched cover slopes up evenly on both sides to a peak, the way the roof on a house does.

One disadvantage of the pitched cover is that it requires an elaborate truss system to support the panel covers. This truss limits interior visibility and removes the option of rolling back the covers. Pitched covers, therefore, are most applicable to areas requiring limited visibility, and minimal worker access.

Building extension or enclosure An alternative to covering the tanks is enclosing the area to be contained within a building or structure. This can entail the extension of existing superstructures or enclosing the area in a new building, penthouse, or dome. This alternative provides the operator with full tank or section accessibility, but it also requires the maximum amount of air volume to be treated, a result of the extended area and higher ventilation requirements for worker space environments. This alternative is most beneficial for areas that require complete access for frequent maintenance or repairs.

Enclosure for odor control

Although buildings and enclosures can be placed over any shape basin or unit process odor source, the dome has generally been used to cover circular basins. Like the barrel arch, the dome structure has an open interior volume (no trusses are necessary for support) and are variable in their height of rise. The greater the height of rise, the greater the volume of odorous air needing treatment. Accordingly, minimizing the height of rise of the dome corresponds to a more economical odor control system.

Aluminum dome enclosure

A standard dome feature includes a single door for access to the dome interior to inspect and perform maintenance. This means that the dome is a worker space area and, as such, the full volume of air within the dome has to conform to worker safety standards. The volume of air vented from a full dome which needs odor control is significant.

Modifications to the dome structure now provide options to this standard dome feature so that less air is needed to ventilate the open interior air space. Some of the options include:

• Providing a fresh air supply directly overhead of the areas where the worker spaces are located
• Avoiding the need for the worker to enter the confined environment of the dome. Independent access ways are now available in a dome structure. These access ways are separate and independent from the environment of the internal dome.

Materials of construction

The materials of construction for the cover structure depend on the type of cover selected and the characteristics of the environment to be served. In general, the materials of construction should be selected to provide durability, ease of maintenance, good performance, corrosion resistance, and low cost. The three most common materials used at wastewater treatment plants for the containment of odors are:

• Concrete (precast of cast in place)
• Aluminum
• Fiberglass reinforced plastic.

Web extra: materials of construction

Ventilation rate

The ventilation rate required to contain odors, reduce corrosion, provide for a safe and comfortable worker access, and minimize the volume of air to be treated, can be selected based on experience with designing similar systems but must conform to the standards set by local, state, and federal building codes. In addition, certain groups provide information on recommended practices for ventilating specific areas.

Check the National Fire Protection Association (NFPA,
 . . . and FM Global (formerly Factory Mutual

For example, NFPA 820, "Standard for Fire Protection in Wastewater Treatment and Collection Facilities" (1995 edition) discusses ventilation rates for areas throughout a wastewater treatment plant. Although the information provided in NFPA 820 is not binding in itself, it is widely accepted and many municipalities and authorities have adopted the recommendations in their standards.

This is a rule of thumb for ventilation rates from covered or enclosed process units at wastewater treatment plants:

Nonworker space areas 4 to 6 air changes/hr (flat covers, barrel arch or pitched covers, low rise domes)
Worker space areas 12 air changes/hr (enclosures, domes).

These rates depend on the effectiveness of the cover or enclosure system design to be tight (no leaks). Accordingly, the final selection also depends on the type of cover or enclosure system, since some are more leak resistant.

Not all the basins to be odor-controlled are quiescent. Some are aerated. The ventilation rate for aerated basins is the sum of the appropriate ventilation rate presented above and the design aeration rate. This summed ventilation rate provides the capacity to contain the supplied diffused air while maintaining negative pressure under the cover.

Ventilation rates can also increase as a result of the presence of odor-causing constituents, such as hydrogen sulfide, which is present at levels that would be considered either dangerous for worker space enclosures or that would exceed the maximum design influent concentration of the odor control device. In this event, the ventilation rate should be increased to dilute the concentration to acceptable levels. The ventilation rate increase would be based on site-specific information.

Web extra: details on ventilation system layout

Case histories

The New York City Department of Environmental Protection (DEP) owns and operates fourteen wastewater treatment plants in the five boroughs of New York City. Recently, Malcolm Pirnie, Inc. prepared cover designs for the primary sedimentation tanks at the Coney Island and North River plants.

Coney Island

At Coney Island, the influent channel and effluent launders of the rectangular primary sedimentation tanks (PSTs) were key odor sources. More than twenty alternatives were evaluated, ranging from all flat covers to barrel arches to enclosures and combinations. After discussions with the plant staff regarding their needs for safety and access to the process, the list of alternatives was reduced to three.

Flat covers on influent channel
• Movable barrel arch covers on influent and effluent tank ends - Flat covers on remaining tank areas
• Access hatches on effluent launders.

Advantages: Easy operator access and visibility at either end of tanks, relatively low air treatment volume of 22,000 cubic feet per minute (cfm)

2. Flat covers on influent channel
• Barrel arch covers over entire tanks with movable sections on both ends and at center of tank
• Access hatches on effluent launders.

Advantages: Easy operator access and visibility throughout the tank, moderate air treatment volume of 30,000 cfm.

3. Flat covers on influent channel
• Barrel arch covers over entire tanks with movable sections at influent end and center of tank
• Extend skimmings enclosure building tank 20 ft
• Access hatches on effluent launders.

Advantages: Full operator accessibility at the effluent end, easy operator visibility throughout the tank.

Differences between cover types and materials are compared (Table 1).