Alternate daily cover is material used by landfills to cover daily deposits of waste to prevent blowing waste, odors, and attraction of birds or vermin.

Anaerobic Digestion (AD)

Anaerobic digestion is a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen. The digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide.

Most anaerobic digestion occurs within two temperature ranges: mesophilic (in the 95 to 100-degree F range), and thermophilic (in the 127 to 132-degree F range).  While mesophilic digestion does not destroy all pathogens, thermophilic digesters can achieve the same pathogen reduction as composting or heat drying.

Batch or Portion of Production

A unit of raw/whole digestate, separated liquid, or separated solids produced by an AD process that can be identified for the purposes of retreatment or disposal should monitoring checks or sample tests necessitate such actions. For AD facilities using a batch process, the unit will be a completed batch. For AD facilities using a continuous process, the unit may be a quantity that is representative of ongoing production.


Treated sewage sludge that meets the EPA pollutant and pathogen requirements for land application and surface disposal. See EPA’s Biosolids Program for more general information and documents.  Biosolids are nutrient rich and often used as a fertilizer or soil amendment in compliance with EPA 40 CFR Part 503.  See also Sewage Sludge.

Class A Biosolids

Biosolids where pathogenic bacteria, viruses, and parasites are reduced to non-detectable levels.  There are no site or harvesting restrictions under 40 CFR 503 for Class A biosolids.  Options for demonstrating Class A include the “Processes to Further Reduce Pathogens (PFRP)”:  e.g. composting at specified times and temperatures, heat drying to 90% solids, and pasteurization.  Other Class A alternatives include Alternative 1, a time/temperature equation typically used for thermophilic digestion; Alternative 2, a time/temperature/pH demonstration, and Alternatives 3 and 4, direct testing to show non-detectable levels of enteric viruses and viable helminth ova.  To demonstrate Class A, in addition to demonstrating one of these operational parameters or pathogen testing, it must be demonstrated that either fecal coliform levels are < 1,000 mpn/gram or salmonella are non-detects.  (See 40 CFR 503.32(a))

In addition to pathogen reduction, biosolids must meet one of 10 Vector Attraction Reduction Options: these include being stabilized by digestion, drying, or lime stabilization, or being incorporated into the soil within specified timeframes.  The Vector Attraction Reduction step must occur at the same time or after the pathogen reduction step.

Class B Biosolids

Biosolids where pathogens including pathogenic bacteria, viruses, and parasites have been reduced by several orders of magnitude but not completely eliminated.  They can be used for growing non-food crops with site and harvesting restrictions.  There are several options for demonstrating Class B pathogen reduction, including “Processes to Significantly Reduce Pathogens (PSRP)”: e.g. mesophilic anaerobic digestion, aerobic digestion, air-drying, or lime treatment at specified times and temperatures. Class B can also be demonstrated by direct testing of the indicator fecal coliform to show levels < 2,000,000 mpn/gram.  (See 40 CFR 503.32(b))

In addition to pathogen reduction, biosolids must meet one of 10 Vector Attraction Reduction Options: these include being stabilized by digestion, drying, or lime stabilization, or being incorporated into the soil within specified timeframes.  The Vector Attraction Reduction step must occur at the same time or after the pathogen reduction step.


A contaminant is a solid or liquid manmade material, which does not biologically decompose under anaerobic conditions and which detracts from the ultimate marketability and usefulness of the digestate, due to its visibility and/or its impact on soil quality. This includes, but is not limited to glass, metals, non-biodegradable plastics, elements or chemicals, etc.

Control Measure

An action or activity taken to prevent or eliminate a digestate safety hazard or reduce it to an acceptable level.

Corrective Action

Action taken when results of monitoring at a critical control point indicate a loss of control or when samples of a batch or portion of production of digestate indicate a failure to meet critical limits.

Critical Limit (CL)

A critical limit is a value at which a parameter transitions from being acceptable to unacceptable.

Critical Control Point (CCP)

A critical control point is the last step at which control can be applied and is essential to prevent or eliminate hazard or reduce it to an acceptable level of risk.  Critical control points are specific to the digestion process and facility design and should be clearly outlined in the operations manual for a facility. An example of a critical control point for reducing the level of pathogens to the maximum extent may be a holding tank in which liquid digestate is heated and held for 1 hour at greater than 70 degrees C.


Failure to meet a critical limit.

Digestate, Whole Digestate, Separated Liquid Digestate, Separated Solid Digestate

Digestate is the solid and liquid product resulting from the anaerobic digestion of organic materials. Digestate contains a combination of remaining undigested organic and inorganic materials, as well as the biomass and micronutrients created by the microbial process. Whole Digestate denotes the material as removed from the digestion vessel without any additional physical or chemical processing. Separated Liquid Digestate and Separated Solid Digestate denote the materials created from a physical separation of whole digestate, with or without the use of dewatering polymers or heat, but without the use of any additional chemical processing.

Digestate-Derived Product

A new product that is produced using digestate and employing one or more post-treatment processes other than simple physical separation into solid and liquid digestate.


Feedstock is the umbrella term for a wide variety of organic materials that may be introduced into an anaerobic digester. Feedstocks may also contain varying levels of non-digestible organics (e.g. woody materials) or contaminants (e.g. glass, metal, plastic, sharps) depending on the source of feedstock, feedstock collection procedures, feedstock pretreatment procedures, and type of digester. Feedstocks may be either liquid, semi-solid or solid, and may originate from agricultural, residential, commercial, industrial, or institutional sources. Feedstocks may be residuals of other processes or a fraction of waste stream such as sewage sludge or biosolids, source-separated organics (SSO), organic fraction of municipal solids waste (OFMSW) , or purpose-grown energy crops.

Fit for Purpose

A material is fit for purpose if it does not have any properties or characteristics that prevent it from being suitable for its intended use(s) and it meets all applicable laws and regulations for that use. As applied to input material to an AD process, it means that such material has no physical or chemical properties that would prevent the digestate from being fit for purpose.  As applied to digestate, it means that the whole or separated digestate has no physical or chemical properties that would cause harm to the environment or downstream process.

Generally Unrestricted

The use of digestate is deemed generally unrestricted when the digestate shows no physical or chemical properties that would cause harm to the environment or downstream process. As applied to land application, it means that digestate can be land applied consistent with nutrient management plans and any other local regulations pertaining to land application or beneficial use of organic residuals.

Hydraulic Retention Time (HRT)

Average time that material stays in the digester vessel for treatment. For batch digesters, where percolate is recycled, the batch cycle time for the solid fraction will be the HRT for the solid fraction, while the HRT for the circulating liquid fraction is separately calculated, based on the design of the system. For continuous digesters with a plug flow configuration, the HRT is the time that it takes for influent to travel from the entry point to the exit point. For completely mixed continuous digesters, the HRT is calculated based on the annual input and digester capacity (i.e. 30,000,000 gallons input per year to a 1-million-gallon digester yields an HRT of 12.2 days.)


Descriptive of digestate (whole, solid, or liquid) that exhibits a very low rate of residual biogas potential, and which is unlikely to be phytotoxic when used as directed.  See also the definition of “stable,” below.

Mesophilic and Mesophilic Bacteria

Organisms for which optimum growth temperatures are within the temperature range of 97 degrees to 104 degrees Fahrenheit and that are tolerant of a range of 83 degrees to 107 degrees Fahrenheit.

Organic Fraction of MSW or OFMSW

The organic fraction of MSW or OFMSW is the separated organic fraction derived from the mechanical treatment of mixed municipal waste at a processing facility.

Organic Loading Rate (OLR)

Weight of organic matter fed to a unit volume of the digester per unit time.  Normally expressed as OLR = kg COD/m3/day or kg VS/m3/day, where COD is chemical oxygen demand and VS is volatile solids. A similar way to describe OLR is weight of organic dry matter added per day (kg VS d-1) divided by digester volume (m3).

Organic Residuals 

By-products of human activities that contain carbon and are putrescible (will decompose fairly quickly), including food scraps, food processing wastes, agricultural wastes, pulp and paper mill sludges, industrial process wastes (e.g. gelatin from film production), animal manures, biosolids, and septage.


A process step during which the numbers of pathogenic bacteria, viruses, and other harmful organisms are significantly reduced or eliminated by heating the material to a critical temperature for a minimum, specified time.


Process to Further Reduce Pathogens.  A treatment process that is able to consistently reduce pathogens (i.e., enteric viruses, viable helminth ova, fecal coliforms, and Salmonella spp.) to below detectable levels at the time the treated material is used or disposed.  See 40 CFR Part 503, Appendix B(B).

Physical Contamination

The level of visible, manmade materials that do not biologically decompose under anaerobic conditions and which detract from the ultimate marketability and usefulness of a digestate due to either its visibility or to its impact on soil quality. This includes but is not limited to glass, metals, non-biodegradable plastics, etc. Physical Contamination is determined by screening digestate over a screen or sieve to calculate the dry weight fraction of these materials at different size increments.


A substance that is toxic to plants. Toxic effects may include delayed seed germination or inhibited development and rate of growth.

Post-Treatment of Digestate

Post-treatment or processing of the digestate may include physical, mechanical, biological, or chemical steps to remove contaminants, further stabilize the digestate, further reduce pathogens, sequester or concentrate nutrients, or a combination of steps to produce a digestate-derived product.

Potentially Toxic Elements

A chemical element that has potential to have toxic effects on humans, flora, or fauna.  Examples include quaternary ammonia, lead, cadmium, chromium, mercury, copper, zinc, nickel, etc.

Pre-Treatment of Feedstocks

Pretreatment of feedstocks may include physical, mechanical, or biological steps to remove contaminants or materials not compatible with a given digester design, reduce the size of feedstocks prior to digestion, or enhance the rate or extent of degradation in the digester. Such pretreatment steps may affect the composition of the digestate.


Process to Significantly Reduce  Pathogens.  A process that consistently reduces the density of pathogenic bacteria, viruses, and/or parasites (number/gram of organic material on a dry weight basis) in organic materials by equal to or greater than a factor of 10.  See 40 CFR Part 503, Appendix B(A).

Quality Management System (QMS)

A system for planning, achieving, and demonstrating effective control of all operations and associated quality management activities necessary to achieve production of digestates that are fit for purpose

Refuse Derived Fuel (RDF)

Refuse derived fuel or RDF is a high BTU fraction separated from municipal or other waste streams, generally comprised of non-recyclable plastics, synthetic textiles, and coated papers.

Restricted Use Digestate

Restricted use digestate is a digestate that has been tested in this Program and has been shown to have physical or chemical properties that, unless properly managed, may cause harm to the environment or process into which it is introduced.

Sewage Sludge

The solids separated during the treatment of municipal wastewater.  See also biosolids.

Source Separated Organics (SSO)

Source separated organics (SSO) are organic materials separated at the point of generation of the waste – either at households, at businesses, or at industrial sites – and placed in a designated SSO bin for separate collection. The materials to be separated and included in the SSO bin are normally specified by the municipality or waste hauler, and generally include food waste, yard waste, soiled non-recyclable paper, and other compostable products and packaging.


The point at which the rate of biological activity has slowed to an acceptably low and consistent level and will not significantly increase under altered conditions such as transport or land application.  Stable digestate is not attractive to vermin or wild animals nor so odorous that its storage or use causes nuisance to humans. In a stable but immature state, digestate might still contain insufficiently biodegraded natural or manmade substances that exert phytotoxic effects in some applications.

Significant Feedstock Changes

A change of more than 30 percent in the mix of feedstocks fed into a digester, or a 30 percent change in Volatile Solids (VS) content from steady state operations in a digester, as documented in Program participant operations logs.

Steady State Operations

No more than a 30 percent variance in feedstock composition and no more than 30 percent change in VS content over a rolling 6-month average, as recorded in Program participant operations logs.

Thermophilic Bacteria

Organisms for which optimum growth temperatures are within the temperature range of 125 degrees to 131 degrees Fahrenheit, and which are tolerant of a range of 113 degrees to 140 degrees Fahrenheit, provided excessive levels of dissolved ammonia are not present.

Volatile Fatty Acids (VFA)

Fatty acids, or organic acids, with a carbon chain of six carbons or fewer. VFAs are a family of intermediary chemicals produced during the digestion process, and the ratio between them is a leading indicator of inhibitory effects on the microbial community.

Volatile Solids (VS)

The solids in a sample of material that are lost on ignition of the dry solids at 550 degrees Celsius/1022 degrees Fahrenheit. Bio-digested volatile solids are a subset of total volatile solids and include only those solids consumed by anaerobes.