In designing or upgrading your facility, examine the full suite of applicable codes and consult with your local fire and building officials and state permitting authorities early and often to assure that you satisfy all of their requirements.
By David Nightingale

Solid waste transfer stations often incorporate misting systems to mitigate dust. Compost operations use biofilters, timely turning, diligent monitoring of moisture and temperature, and air controls to avoid fugitive offsite odors. The corollary to these at an HHW collection facility is exhaust ventilation. In all three cases, the need is obvious, but implementing effective and efficient operations can be elusive.

Designers often consult established standards to guide their work. Primary among these resources for design professionals is the system of International Codes® adopted across the U.S. This system of codes includes the International Building Code®(IBC®), International Fire Code® (IFC®), and International Mechanical Code®(IMC®), among others. Without a deep dive into the codes and referenced ventilation standards, an HHW facility exhaust ventilation system is unlikely to be designed adequately or efficiently. Unfortunately, this has been the result at the vast majority of more than 140 HHW collection facilities I have visited. Before we dive into the ventilation part of the codes, a bit of context for how HHW operations fit into the codes is needed.

When flammable liquids are consolidated into drums, as is common at HHW facilities, this is called either “dispensing” or “use” by the International Codes®. With the typical volumes of flammable liquids consolidated and stored at HHW collection facilities, the IBC® requires that all or part of the building be a high-hazard “H” occupancy. The consolidation activity normally calls for an H-2 occupancy designation for that area. An H-2 occupancy also allows large quantities of flammable liquids to be stored before shipment. This occupancy designation drives a lot of code requirements.

Figure 1: Flammable liquids consolidation exhaus t hood at Thurston County, WA.1

H occupancy exhaust ventilation requirements are found largely in Chapter 5 of the IMC®. Subsection 502.8.1.1 of the IMC® applies generally to all types of H occupancies, not just H-2. It specifies a minimum ventilation rate of 1 cubic foot of air per minute for every square foot of floor area (cfm/sf). So, if any H occupancy is a room 10 feet by 30 feet you would be required to have a minimum of 300 cubic feet per minute (cfm) of exhaust air ventilation. This IMC® subsection also requires that the ventilation system operate continuously (24/7), have an emergency shut off switch outside the room, consider the location of the exhaust vents based on the vapor density of the materials consolidated, provide air movement that sweeps the floor and prohibits recirculation of the exhaust air back into the building. This seems comprehensive and complete, but it is not. There are additionally more specific requirements for H-2 occupancies deeper in the IMC®.

More specific requirements for areas where flammable liquids are dispensed or used, in H-2 occupancies, is at IMC® 502.9.5.4. Makeup air must be provided in any pit or sumps and local or spot ventilation must be provided to prevent accumulation of hazardous vapors. This additional set of requirements is often not well addressed by designers. Often only the 1 cfm/sf minimum flow rate, continuous ventilation and other general H occupancy requirements are addressed in the design drawings and specifications.

At a flammable liquids consolidation work station, the vapors are generated at their maximum concentration, as the liquid evaporates and contaminates the air and potentially enters the breathing zone of the worker. Removing vapors at the point of generation uses the least amount of ventilation air and fan power. Consequently, providing local/spot ventilation as close as possible to the point of generation is the most effective and energy-efficient way to remove those vapors. Not surprisingly, IMC® 502.1.1 requires that “an exhaust system shall be located in the area of highest concentration of contaminants”.

Figure 1 shows an image of a custom exhaust hood designed for an HHW flammable liquids bulking station with a design exhaust flow of 600 cfm, twice that of the minimum 300 cfm calculated above for a 300 square foot room. The 300 cfm general H occupancy design minimum implemented as general area ventilation would clearly be inadequate.

At IMC® 502.8.1.1(5) “for fumes or vapors that are heavier than air, exhaust shall be taken from a point within 12 inches of the floor”. So, an additional exhaust grate should be located near floor level and in the containment sump, if the floor has one. The inlet (supply) and exhaust must be oriented to sweep across the floor to prevent high concentrations of flammable vapors from accumulating anywhere in the room, per IMC® 502.8.1.1(6).

Figure 2: Lab packing drum exhaust hoods at Mesa County, AZ. Images courtesy of Special Waste Associates.

Key Requirements
Some additional key ventilation requirements sometimes missed include:
• Ventilation of flammable vapors must be independent from other HVAC systems, IMC® 510.4
• Makeup air must be provided at a rate approximately equal to the air exhausted, IMC® 510.6.5
• The exhaust duct needs to discharge directly to the exterior of the building, IMC® 510.6.7
• Exhaust ducts larger than 10 inch diameter may require a fire suppression system, IMC® 510.8
• All parts of the duct system, including switches, fans, wiring, and motors, must be rated to operate safely where flammable vapors are present

There are additional requirements in the IMC® and some state rules add to those. For instance, the Washington State Department of Ecology (Ecology) HHW collection facility rules call for “sufficient ventilation to remove toxic vapors and dust from the breathing zone of workers”.2 This is aimed at locations where packing absorbents with fine particles may become airborne or toxic materials may release hazardous vapors, such as when packing pesticide containers into lab pack drums. Figure 2 shows a row of lab pack drums with local ventilation at each to capture dust and dangerous vapors.

The citations and examples above illustrate a more comprehensive design path for HHW facility ventilation based mostly on existing code provisions. However, this brief overview is neither complete nor exhaustive. In designing or upgrading your facility, examine the full suite of applicable codes and consult with your local fire and building officials and state permitting authorities early and often to assure that you satisfy all of their requirements. Including a mechanical engineering firm with industrial ventilation experience on your design team is also a good practice to avoid an inefficient and ineffective HHW facility ventilation design.| WA
Next month, we will focus on the need for and requirements of flammable liquid consolidation and storage areas.

David Nightingale, CHMM, S.C., is Principal at Special Waste Associates (Olympia, WA), a company that assists communities in developing or improving HHW and VSQG collection infrastructure and operations. They have visited more than 140 operating HHW collection facilities in North America. As a specialty consulting firm, Special Waste Associates works directly for program sponsors providing independent design review for new or upgrading facilities—from concept through final drawings to create safer, more efficient and cost-effective collection infrastructures. Special Waste Associates also published the book, HHW Collection Facility Design Guide. David can be reached at (360) 491-2190, e-mail [email protected] or visit

Washington Administrative Code 173-350-360(4)(a)(vii).
This effective bulking hood design was originally developed for the Boulder County, Colorado HHW facility and has been adopted by other facilities in California and Washington.