biological odor control media

Base Layers

Use of Crater-Max® and Cell-Max™ Plus for Media Support and Preliminary Treatment in Biofilters

Crater-Max and Cell-Max Plus media

Rigid, breakdown-resistant media can provide a stable transition between the air-distribution floor and the primary treatment bed.

Gas-phase biofilters remove hydrogen sulfide, reduced sulfur compounds, volatile organic compounds, and other odor-causing contaminants by passing foul air through a porous media bed. Contaminants transfer from the air into a moisture film on the media and are subsequently oxidized by attached microorganisms.

Successful treatment depends on more than the biological properties of the upper media. The system must also maintain:

Biofilter references have historically described upper organic media placed over a coarser support layer, including bark, wood chips, gravel, or porous lava rock. The support layer helps prevent finer media from entering the air-distribution system and provides a more open transition for airflow and drainage.

Crater-Max® and Cell-Max™ Plus (CMP) can perform this support function while also providing usable biological treatment capacity.

Purpose of the Base Support Layer

A properly designed base layer should perform several related functions.

Protect the Air-Distribution System

The base layer separates the primary treatment media from the underdrain, aeration floor, support grating, or air-distribution plenum. This reduces the potential for small media particles to fall into floor openings, ductwork, or drainage channels.

Improve Airflow Transition

Air entering a biofilter through floor openings or laterals initially has localized velocity. An open base layer provides a transition zone in which the air can spread laterally before entering the denser upper media.

The base layer does not correct a poorly designed floor or undersized air-distribution system. However, when combined with a properly engineered floor, it can help reduce localized high-velocity zones and support more uniform airflow through the treatment bed.

Promote Drainage

Irrigation water, condensation, and acidic biological byproducts must drain freely through the media. A coarse, noncompacting lower layer helps prevent water from collecting at the bottom of the bed.

Standing water can restrict airflow, increase differential pressure, create anaerobic areas, and reduce available treatment volume.

Support the Upper Media

The bottom layer distributes the weight of the upper treatment media and provides a stable working platform. A rigid base layer can reduce settlement and help maintain the specified treatment depth over the life of the system.

Provide Supplemental Biological Treatment

Unlike a nonporous gravel support layer, Crater-Max® and CMP provide surfaces on which biological films can develop. The lower layer can therefore contribute to contaminant removal rather than functioning only as inert structural fill.

Crater-Max® as a Base Layer

Crater-Max® is a porous mineral media intended for biological odor-control applications. Its irregular shape and internal porosity provide attachment surfaces for sulfur-oxidizing microorganisms.

Advantages

Crater-Max® offers several potential benefits when used beneath an upper biofilter media bed:

Long-term wastewater odor-control studies have demonstrated that lava-rock-based biofilters can maintain biological odor treatment over extended operating periods. Porous lava rock has also historically been used as a support or treatment medium in municipal biofilter systems.

Limitations

Crater-Max® has a greater bulk density than CMP and most organic media. The increased dead load must be considered when evaluating:

Its irregular particles may also require a separation screen when the upper media has a substantially smaller particle size.

Recommended Applications

Crater-Max® is particularly appropriate where:

Cell-Max™ Plus as a Base Layer

CMP is an engineered porous media manufactured from recycled glass. It is lightweight, chemically inert, acid resistant, and designed to provide a stable structure for biological growth.

Typical CMP physical characteristics include:

Advantages

The principal advantage of CMP as a base layer is its combination of low weight and structural stability.

Compared with conventional mineral aggregate, CMP can substantially reduce:

CMP also remains biologically active. When supplied with adequate moisture, nutrients, oxygen, and foul air, sulfur-oxidizing microorganisms can colonize the media and begin treating hydrogen sulfide in the lower portion of the bed.

Engineered glass media are used commercially in biological odor-control systems because they are inert, resistant to decomposition, and capable of supporting biological films.

Limitations

CMP is not intended to compensate for an inadequate support floor or an improperly designed airflow system. The supporting floor, grating, or screen must be compatible with the CMP particle size and must prevent the media from entering the air plenum.

CMP is also an inert medium. It does not inherently supply all nutrients required for long-term microbial activity. Where nutrient loading from the foul-air source is insufficient, supplemental nutrient addition may be required through the irrigation system.

Recommended Applications

CMP is particularly suitable where:

Comparison of Base-Layer Options

Characteristic Crater-Max® Cell-Max™ Plus Organic Wood or Bark Conventional Gravel
Resistance to decomposition Excellent Excellent Limited Excellent
Relative weight High Low Low to moderate High
Biological attachment potential High High High Low to moderate
Acid resistance Excellent Excellent Variable Variable
Settlement potential Low Low Moderate to high Low
Moisture retention Moderate Moderate to high High Low
Airflow and drainage Good Good Variable over time Good
Supplemental treatment value Yes Yes Yes Limited
Replacement frequency Low Low Higher Low

Recommended Layer Configurations

The final configuration should be based on airflow, contaminant loading, media depth, floor design, allowable structural loading, and required empty-bed residence time.

CMP Beneath Engineered Biofilter Media

A typical arrangement may include:

This arrangement provides a lightweight, stable support layer that also contributes to hydrogen sulfide treatment.

Crater-Max® Beneath Organic Media

A typical arrangement may include:

This configuration can be beneficial when the upper organic media requires a rigid, freely draining foundation.

Combined Crater-Max® and CMP Base

For selected applications, the base section may use both products:

This approach should only be used when the additional interface and media depth provide a clear process or structural benefit.

Design Considerations

Empty-Bed Residence Time (EBRT)

All media layers that receive foul air contribute to total bed volume and empty-bed residence time. However, the designer should not automatically assign the same treatment performance to every layer.

The expected removal contribution of the base layer depends on:

The primary treatment layer should therefore be sized to achieve the required performance without relying entirely on the base layer unless the complete layered system has been specifically evaluated.

Differential Pressure

Pressure loss through the clean media bed should be calculated using representative media data and the proposed face velocity. The design should also include allowance for:

Physical characterization research has shown that changes in media structure, compaction, moisture, and retained solids can significantly increase pressure drop in gas-phase biofilters.

Differential-pressure monitoring ports should be provided across the complete bed and, where practical, across individual media layers.

Air-Distribution Floor

Neither Crater-Max® nor CMP should be installed directly over floor openings that are too large to retain the media.

The support system may consist of:

The floor must be designed for the wet operating load of the entire media bed, maintenance personnel, and any applicable safety factor.

Separation Screen

A separation screen is recommended when:

The screen must have sufficient open area to avoid becoming the controlling pressure-loss component.

Moisture and Drainage

The irrigation system should distribute water uniformly across the entire surface. Irrigation should maintain the biological moisture film without creating continuous flooding at the bottom of the bed.

Drainage provisions should allow removal of:

Installation

Recommended installation practices include:

Performance Verification

A layered biofilter should be commissioned using a combination of physical and biological testing.

Recommended verification includes:

  1. Inspection of the support floor and drainage system before media installation.
  2. Smoke testing of the exposed air-distribution floor.
  3. Smoke testing after placement of the base layer.
  4. Confirmation of final media depths.
  5. Measurement of initial differential pressure.
  6. Verification of surface irrigation coverage.
  7. Biological acclimation using continuous foul air.
  8. Inlet and outlet odor-compound testing after the required startup period.

Smoke testing after placement of the base layer is particularly useful because it allows airflow deficiencies to be identified before the upper treatment media is installed.

Benefits of Using an Engineered Base Layer

When properly designed, Crater-Max® or CMP can provide the following advantages over conventional base materials:

CMP provides the additional benefit of low structural weight, while Crater-Max® provides the rigidity and field history associated with porous mineral media.

Limitations and Engineering Cautions

Crater-Max® and CMP should not be represented as universal solutions for all biofilter systems.

Project-specific engineering is required when:

The use of a durable base layer can improve system reliability, but biological performance still depends on appropriate residence time, loading, moisture, pH, nutrients, temperature, and airflow distribution.

Conclusion

Crater-Max® and Cell-Max™ Plus can serve as effective lower support and preliminary treatment layers in municipal and industrial biofilters.

Both products offer advantages over decomposable organic support layers and nonbiologically active conventional aggregate. When combined with a properly designed air-distribution floor, separation screen, upper treatment media, drainage system, and irrigation system, these media can improve the structural and operational reliability of a layered biofilter.

Final media selection and depth should be based on project-specific contaminant loading, airflow, required treatment efficiency, structural limitations, and lifecycle objectives.

 

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