Combustion Portal: Environmental Compliance for Combustion Processes

Sewage Sludge Incinerators (SSI)

SSIA sewage sludge incinerator (SSI) is an incinerator or combustion device that is used to burn dewatered sewage sludge. SSI units are typically located at wastewater treatment facilities are of one of two types: multiple hearth furnace (MHF) and fluidized bed (FBI) incinerator.


Air Emissions Regulations

Sewage sludge incinerator emissions are currently regulated under 40 CFR Part 60, Subpart O and 40 CFR Part 61, Subparts C and E. Subpart O in Part 60 establishes a New Source Performance Standard for particulate matter. Subparts C and E of Part 61--National Emission Standards for Hazardous Air Pollutants (NESHAP)--establish emission limits for beryllium and mercury, respectively.

The U.S. Environmental Protection Agency (EPA), under section 129 of the Clean Air Act (CAA), is required to regulate emissions from certain categories non-hazardous solid waste incinerators. Section 129 requires EPA to set numerical emissions limitations of nine pollutants from Sewage Sludge Incineration (SSI) units, which although are stand alone regulations are a subset of the category, Other Solid Waste Incineration Units (OSWI). The nine pollutants are:

All standards established pursuant to CAA Section 129(a)(2) must reflect maximum achievable control technology (MACT). The MACT "floor," or minimum level of stringency set forth differing levels of minimum stringency that EPA’s standards must achieve, depending on whether they regulate new or existing sources.

The CAA allows EPA to subcategorize a source category based on differences in class, type, or size. The SSI regulation is subcategorized by unit type:

SSI units are located at waste water treatment facilities designed to treat domestic sewage sludge. SSI units are enclosed device(s) using a controlled flame combustion that burns sewage sludge for the purpose of reducing the volume of the sewage sludge by removing the combustible matter.

The enforcement authority is different for the NSPS and EG. The NSPS are directly enforceable federal regulations, and under CAA section 129 (f)(1), become effective 6 months after promulgation. The EG are not themselves directly enforceable. Rather, the EG are implemented and enforced through either an EPA-approved state plan or a promulgated federal plan. States are required to submit a plan to implement and enforce the EG to EPA for approval not later than 1 year after EPA promulgates the EG (CAA section 129 (b)(2)). The state plan must be "at least as protective as" the EG and must ensure compliance with all applicable requirements not later than 3 years after the state plan is approved by EPA, but not later than 5 years after the relevant EG are promulgated. If a state does not develop an approvable implementation plan, EPA will promulgate a federal plan that will apply to existing SSI units located in that state.

For additional information, fact sheets, and supporting documents on the SSI standards and guidelines please visit:

Solid Waste

The burning of waste in incinerators creates residual ash (fly ash and bottom ash), which can contain any of the elements that were originally present in the waste. Incinerators reduce the need for landfill capacity because disposal of ash requires less land area than unprocessed waste. However, because ash and other residues from incinerators may contain toxic materials such as metals, the combustion residuals wastes must be tested regularly to assure that the wastes are safely contained to prevent toxic substances from migrating into groundwater supplies. Under RCRA and state regulations, incinerator ash must be sampled and analyzed regularly to determine whether it is hazardous or not. Hazardous ash must be managed and disposed of as hazardous waste. Non-hazardous ash may be disposed of in a municipal solid waste landfill, an ash monofill or recycled.

More resources for solid waste:

Water Resources Protection

Incinerators generate wastewater associated with processes such as cooling tower blowdown, flue gas treatment, and washdowns. Discharges of wastewater are regulated under the Clean Water Act (CWA). Facilities which discharge indirectly through a Publically Owned Treatment Works (POTW) are regulated under the Pretreatment Program, which insures that facilities pretreat wastewater to remove pollutants which would affect the pollutant removal ability of the POTW. Facilities that discharge process or non-process wastewater directly streams, rivers, etc. are regulated under the National Permit Discharge Elimination System (NPDES) and must obtain coverage under a General Permit or an Individual Permit.

In addition to compliance with rules covering wastewater generated by incinerators, applicable facilities must be concerned with stormwater runoff.

Each of these topics is summarized below with links to related web pages and documents.

Indirect Discharge

Facilities with SSI incinerators that discharge wastewater into a sewer system that leads to a municipal treatment plant, also known as Publicly Owned Treatment Works (POTW) are indirect dischargers. The POTW typically is owned by the local municipality or a regional board or sewer authority.

In response to potential problems caused by industrial wastewater being discharged into POTW’s, federal pretreatment regulations were developed. These regulations apply to all municipal, industrial and commercial facilities. Local POTW’s with approved pretreatment programs have responsibility for enforcing pretreatment requirements. Otherwise, the rules are enforced by the state or EPA regional authority.

All indirect dischargers must meet national General Pretreatment Regulations (40 CFR 403). Additionally, certain types of facilities must also meet applicable categorical pretreatment standards. When a pollutant, discharged by an indirect discharging industry is not specifically limited by pretreatment standards, it is up to the state or local regulatory agency to develop local limits or to determine other appropriate means to control its discharge.

More information on discharges to POTW's:

Direct Discharge

Facilities with SSI incinerators that discharge process wastewater or cooling water to surface waters are direct dischargers. Direct dischargers must obtain a permit under EPA’s National Pollutant Discharge Elimination System (NPDES) program. A NPDES permit sets limits on the amount of specific pollutants that can be discharged to surface waters.

Some states offer general permits for non-contact process water (e.g., cooling water). The purpose of the general permit is to provide a streamlined NPDES permitting process for certain classes or categories of industrial point source discharges. Coverage under a NPDES general permit is unique in that a facility operates and discharges under the requirements of the applicable general permit rule rather than the requirements of an individual permit. Check with your state environmental agency to determine if a general permit is applicable to your facility.

More information on direct discharge NPDES permits:


Leachate from waste unloading and storage operations caused by exposure to precipitation and from residual liquids in the waste itself may contain organic matter, nutrients, metals, pathogens, and hazardous chemicals. Stormwater regulations promulgated under the Clean Water Act help prevent these materials from polluting nearby streams and other water courses. Operations such as CISWI incinerators must develop a stormwater pollution prevention plan (SWPPP), obtain coverage under a NPDES stormwater permit, and implement methods of controlling stormwater pollution, including best management practices. For more information about the Stormwater program, visit the Stormwater Basic Information page.

Throughout most of the nation, U.S. EPA has delegated the stormwater program to the states to administer as they see fit, so long as minimum federal requirements are met. For more information on state stormwater rules see the Industrial Stormwater State Resource Locator. The locator will help you find state-specific information on permitting, technical resources and points of contact.

More information on Stormwater:

Spill Prevention, Control and Counter Measures

EPA promulgated the Spill Prevention, Control and Counter Measures (SPCC) rule to reduce the risk of damaging our waterways from oil spills. These rules are applicable to a very wide rage of facilities and operations, including fuel oil storage tanks for incinerators. The rule requires specific facilities to prepare, amend, and implement SPCC Plans. The SPCC rule is part of the Oil Pollution Prevention regulation, which also includes the Facility Response Plan (FRP) rule.

An SPCC Plan is required for facilities which due to their location, could reasonably be expected to discharge oil to surface water or adjoining shorelines and have:

The SPCC regulations require the facility owner/operator to prepare and implement an SPCC plan for their facility. This plan must be well thought out and prepared in accordance with good engineering practices. It must document the location of storage vessels, types of containment, dangers associated with a major release of material from the tanks, types of emergency equipment available at each site, and procedures for notifying the appropriate regulatory and emergency agencies.

To assist facility owners and operators with SPCC compliance, EPA has published a useful document: Spill Prevention, Control, and Countermeasure (SPCC) Regulation.

More resources on SPCC:

Applicable rules: 40 CFR 112.

Pollution Prevention

An alternative to incineration of sewage sludge is use as fertilizer. When treated and processed, sewage sludge becomes biosolids which can be safely recycled and applied as fertilizer to sustainably improve and maintain productive soils and stimulate plant growth.

Only biosolids that meet the most stringent standards spelled out in the Federal and state rules can be approved for use as a fertilizer. Now, through a Voluntary Environmental Management System (EMS), being developed for biosolids by the National Biosolids Partnership (NBP), community-friendly practices will also be followed. More information.

More resources on Pollution Prevention:

Land Application of Biosolids

Use of Composting for Biosolids Management

More SSI Resources

Identification of Nonhazardous Secondary Materials That Are Solid Waste Wastewater Treatment Sludge (March 18, 2010). Materials Characterization Paper In Support of Proposed Rulemaking.

AP 42 Compilation of Air Pollutant Emission Factors Section 2.2 Sewage Sludge Incineration. AP-42, Compilation of Air Pollutant Emission Factors, has been published since 1972 as the primary compilation of EPA's emission factor information.

Use and Disposal of Biosolids (Sewage Sludge). Includes standards for the use or disposal of sewage sludge, biennial reviews, targeted national sewage sludge survey report, notice on dioxin, response to NRC/NAS report, and relevant documents and links.


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