Figure 1. Selected buildings and stacks for an anaerobic digester

October 4, 2017 | General

Emission Control For Anaerobic Digesters

An emission control evaluation, air quality modeling, permit application preparation, and post permit compliance activities are significant components of the air quality permitting process. Part II

John Hinckley
BioCycle October 2017

Anaerobic digesters, like composting facilities, must comply with air quality regulations established under the federal Clean Air Act. Part I, “Air Quality Permitting For Anaerobic Digesters” (September 2017), outlined six sequential steps in the air permitting process. Steps 1 and 2, site evaluation and emission estimation, were covered in Part I. In Part II, the remaining four steps are explained. These include Emission control evaluation; Air quality modeling; Permit application preparation; and Permit compliance.

Emission Control Evaluation

An emission control evaluation involves identifying management practices and/or equipment (devices) that can reduce the quantity of air pollutants emitted by a facility. Many different types of management practices and emission control equipment are available. Pertinent examples include:
• Removing particulate matter from process or combustion exhaust with cyclones, fabric filters, and electrostatic precipitators (ESPs).
• Cleaning the biogas with scrubbers prior to combustion. For example, scrubbers can remove odorous sulfur-based compounds from biogas to prevent them from being converted to sulfur dioxide (SO2), an acid rain precursor.
• Converting pollutant compounds to a different, unharmful and/or unregulated compound. For example, an oxidation catalyst can convert carbon monoxide (CO) and hazardous air pollutants (HAPs) such as formaldehyde to carbon dioxide (a greenhouse gas, but not a pollutant with immediate harmful effects). Selective Catalytic Reduction (SCR) can convert nitrogen oxides to atmospheric nitrogen (N2).
There are pros and cons to emission control devices. On the one hand, they may have substantial capital and operating costs and require time for operation and maintenance. On the other hand, they can help simplify the air permitting process. For example, an emission control device could reduce a facility’s emissions below major source and air dispersion modeling thresholds, reduce the required stack height needed to meet air quality standards, reduce odors, minimize risk of public opposition, and have an overall positive environmental impact.
Another frequently overlooked benefit of emission control devices is reduction of the annual emission fee, also called a “registration fee,” that is charged by many states. This is the fee for the actual emissions produced in a calendar year (like an emissions tax). These fees can vary from a few thousand to tens of thousands of dollars annually for a given facility. A well-designed emission control device can literally pay for itself in 5 to 15 years (depending on the type of device) just by reducing the registration fee.
The type of emission control to install is frequently determined by what is called a Best Available Control Technology (BACT) study. A BACT analysis is a formal pollution control technology study where technical feasibility, degree of emission control, cost, and economic, energy, and environmental impacts are considered to identify the appropriate emission control technology. BACT studies are required by all states for major sources (source of air pollution that emits criteria and HAPs above major source thresholds). Not all states require BACT for minor sources (below the major source thresholds), so in many states, a formal BACT analysis may not be required. For example, the BACT determination for an organics recycling and anaerobic digestion facility in Connecticut was for selective catalytic reduction (SCR) to control NOx and an oxidation catalyst to control CO. Similarly, the BACT determination for an anaerobic digestion facility in Rhode Island was for the same technology.

Figure 1. Selected buildings and stacks for an anaerobic digester

Figure 1. Selected buildings and stacks for an anaerobic digester

Air Quality Modeling

Air quality modeling, also referred to as “air dispersion modeling,” is used to model how the air pollutant emissions (including odorous compounds) from a proposed facility will disperse in the atmosphere around the facility and to determine if it can meet ambient air quality standards. The model takes into account meteorology (wind speed, wind direction, etc.), terrain, land use, building geometry, stack parameters (exhaust flow rate, exhaust temperature, stack height, stack diameter), and emission rates to estimate pollutant concentrations in ambient air at defined locations (called receptors). The most widely used model for renewable energy projects, “AERMOD,” was developed, tested and is recommended by the U.S. EPA. Figure 1 shows a sample 3D rendering  for a proposed AD facility.
Air quality modeling is not always required in the permitting process. Sometimes estimated emission levels do not exceed a particular state’s modeling threshold. However, modeling is still frequently performed even if it is not required as part of due diligence to ensure human health will not be adversely affected by the operation. Modeling results also can be beneficial background information for public education and outreach programs about a proposed facility prior to permitting.
Air quality models can be run to reflect differences in stack height, stack location, degrees of emission control, and building configurations. These “what if” scenarios (or sensitivity analyses) early in the planning process help determine the critical design parameters, which can affect capital and operating costs.
Modeling adds some time and cost to the permitting process. For example, if modeling is required by a regulatory agency, a modeling protocol must be submitted in advance of doing the work. This document explains how the modeling will be performed, and must be approved by the agency prior to completion. The review of the modeling protocol and final modeling can add anywhere from a few weeks to a few months to the permitting process. The time required to complete and evaluate modeling depends on the types of pollutants modeled, the relative stringency of their corresponding air quality standards, the magnitude of emissions, and the degree of public opposition to the project.

Application Preparation

Air pollution control permit applications are submitted to the state regulatory agency when a permit is required. The size and content of the air permit application is a function of the quantity/type of proposed emissions, the respective state’s application requirements, and applicable federal requirements. Permit applications are typically produced in a report format including a narrative explaining the project, relevant forms, equipment information (cut sheets), site plans, and emission calculations. A modeling report is also included within the application or as a separate stand-alone report. In some cases, a pollution control technology assessment report (BACT analysis) is submitted.
State agencies typically take 3 to 6 months to conduct a technical review and issue a draft permit for a minor source and 6 to 12 months for major sources. The review time can be substantially longer if there is public opposition. A 30-day public comment period follows issuance of the draft permit. During this time, the project typically puts a notice in a local paper and the public can submit comments. A final permit is issued at the end of the comment period barring submission of any substantial comments.

Permit Compliance

Air permits include permit conditions that stipulate numerous compliance activities in the form of monitoring, and emission testing (e.g. requiring stack testing to confirm permitted emission limits will be met). Taken together, these requirements can generate compliance activities on a daily, weekly, monthly, annual, and continuous (for monitoring) frequency. Staff time, equipment, and third party emission testers are needed to satisfy these requirements.
Costs for this labor can amount to a few thousand dollars per year for a minor source to $10,000/year for a major source. Failure to complete compliance activities can result in expensive fines. Hence, it is important to develop an understanding of what will be required by an air permit as early in the permitting process as possible.

John Hinckley, QEP is an air quality permitting expert and a Director at RSG in White River Junction, Vermont (

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