EPA/600/R-15/304 | October 2015 | www.epa.gov/research
United States
Environmental Protection
Aqencv
Anaerobic Digestion and its Applications
Office of Research and Development
NRMRL/LRPCD
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EPA/600/R-15/304
October 2015
Anaerobic Digestion and its Applications
by
Allison Costa, USEPA, AgSTAR, OAR
Charlotte Ely, USEPA, Region 9
Melissa Pennington, USEPA, Region 3
Steve Rock, USEPA, NRMRL, ORD
Carol Staniec, USEPA, Region 5
Jason Turgeon, USEPA, Region 1
Land Remediation and Pollution Control Division
National Risk Management Research Laboratory
Cincinnati, OH 45268
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Abstract
On September 16, 2015, U.S. EPA Administrator Gina McCarthy and U.S. Department of
Agriculture Secretary Tom Vilsack announced the United States' first-ever national food waste
reduction goal, calling for a 50-percent reduction by 2030. The U.S. Environmental Protection
Agency (U.S. EPA) seeks to prevent and reduce wasted food (and other organic materials)
that will otherwise be lost as a resource into landfills. "Let's feed people, not landfills. By
reducing wasted food in landfills, we cut harmful methane emissions that fuel climate change,
conserve our natural resources, and protect our planet for future generations" said EPA
Administrator Gina McCarthy. "Today's announcement presents a major environmental, social
and public health opportunity for the U.S., and we're proud to be part of a national effort to
reduce the food that goes into landfills."
This paper provides a brief overview of the science of anaerobic digestion (AD). It describes
how increased implementation of AD systems supports current EPA priorities and summarizes
current applications of AD systems to achieve various environmental goals. Information is
presented on the connection between AD systems and EPA's strategic goals and cross-
agency strategies.
The phrase anaerobic digestion refers to both a natural process and an engineered
technology. There are many configurations and combinations of parts that can be called an
AD system. The technology can be and is used for a range of goals. A discussion of the
components, possible products, and multiple uses of the various technologies is included.
Concepts and nomenclature are introduced to show the range of AD system applications and
describe the uses of AD products. Regulations are discussed but the document is not intended
as a handbook or a regulatory guide. The authors envision that the paper will serve as a
framework for continued discussions on how the use of AD systems can achieve EPA's
strategic and programmatic goals.
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Foreword
The U.S. Environmental Protection Agency (EPA) is charged by Congress with protecting the
Nation's land, air, and water resources. Under a mandate of national environmental laws, the
Agency strives to formulate and implement actions leading to a compatible balance between
human activities and the ability of natural systems to support and nurture life. To meet this
mandate, EPA's research program is providing data and technical support for solving
environmental problems today and building a science knowledge base necessary to manage
our ecological resources wisely, understand how pollutants affect our health, and prevent or
reduce environmental risks in the future.
The National Risk Management Research Laboratory (NRMRL) within the Office of Research
and Development (ORD) is the Agency's center for investigation of technological and
management approaches for preventing and reducing risks from pollution that threaten human
health and the environment. The focus of the Laboratory's research program is on methods
and their cost-effectiveness for prevention and control of pollution to air, land, water, and
subsurface resources; protection of water quality in public water systems; remediation of
contaminated sites, sediments and ground water; prevention and control of indoor air pollution;
and restoration of ecosystems. NRMRL collaborates with both public and private sector
partners to foster technologies that reduce the cost of compliance and to anticipate emerging
problems. NRMRL's research provides solutions to environmental problems by: developing
and promoting technologies that protect and improve the environment; advancing scientific and
engineering information to support regulatory and policy decisions; and providing the technical
support and information transfer to ensure implementation of environmental regulations and
strategies at the national, state, and community levels.
Cynthia Sonich-Mullin, Director
NRMRL, ORD
IV
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Table of Contents
Abstract iii
Foreword iv
Acknowledgments vi
Executive Summary 1
Scientific Overview 4
EPA's Interest in AD 5
Applications of AD 7
Current AD facilities in the US 12
References 13
Appendix 14
Glossary 17
Figures
Figure 1. Anaerobic Digestion System Components
Figure 2. The Chemical Reactions that occur during Anaerobic Digestion
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Acknowledgments
This paper was conceived, written, reviewed, and edited by EPA staff. The authors would
like to acknowledge the many people who contributed to this work and the careful
reviewers. Many Offices and Regions contributed time and energy to make this wide
collaboration successful.
In particular the reviews of Phil Zahreddine (Office of Water (OW)), Linda Barr (Office of
Solid Waste and Emergency Response (OSWER)), Lana Suarez (OSWER), Virginia Till
(Region 8), and Pamela Franklin (Office of Air and Radiation (OAR)) were helpful. Bob
Bastian (OW), Bill Dunbar (Region 10), and Jay Bassett (Region 4) contributed extensive
useful comments as well.
All photos included were taken by the authors.
VI
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Executive Summary
Prior to January 2014, there were a number of EPA technical staff working individually
on various aspects of anaerobic digestion (AD) projects in the Regions, Office of Solid
Waste and Emergency Response (OSWER), Office of Air and Radiation (OAR), Office
of Research and Development (ORD) and Office of Water (OW). As AD applications
become more widespread and visible, the Agency began receiving an increase in the
number of internal and external inquiries surrounding AD systems.
As EPA integrates sustainability concepts into long and short term waste and
wastewater management frameworks, AD continues to be identified as a useful tool. As
the individual offices continued working with AD applications within their media
structure, different frames of reference regarding the benefits of AD processes
emerged.
Some of the benefits from effective use of AD systems can include:
nutrient management alternatives;
soil improvement opportunities;
methane emissions reduction;
production of renewable energy; and
diversion of organic wastes from less preferred disposal options.
AD systems are built by stakeholders for different reasons, including a waste treatment
step, a means to reduce odors, a source of additional revenues or a way to improve
public image. The individuals working with AD systems in their respective media(s)
became increasingly aware that AD systems are one of many tools that can support the
environmental goals and strategies of several different EPA offices. It also became
apparent that the Agency would benefit from an OneEPA approach to capitalizing on
the different media approaches and benefits that AD systems can offer.
In February 2014, a true Cross-Agency AD Team made up of individuals from different
EPA offices and Regions was formed. There are approximately 60 people on this team.
The Cross-Agency AD Team's purpose is to share information so members gain a
better understanding of the differing perspectives throughout EPA regarding AD
systems. The Cross-Agency AD Team strives to communicate effectively and therefore
avoid working at cross purposes.
Soon thereafter, in March 2014 a smaller group of individuals was formed to focus on
the technical aspects of AD and its applications. This team, the Core AD Technical
Team, or Core Team, is composed of technical individuals from EPA offices and regions
that are working on AD applications and systems. The mission of the Core Team is to
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synthesize the Agency's knowledge, reach consensus regarding technical and scientific
facts surrounding AD systems, and develop consistent and effective communications
about anaerobic digestion. This Core Team also solidifies effective communication
across EPA Regions, OSWER, OAR, OW and ORD.
Core AD Technical Team Members (in alphabetical order):
Jay Bassett, Region 4
Bob Bastian, OW
Allison Costa, AgSTAR, OAR
Bill Dunbar, Region 10
Charlotte Ely, Region 9
Amanda Hong, Region 9
Melissa Pennington, Region 3
Steve Rock, NRMRL, ORD
Carol Staniec, Region 5
Jason Turgeon, Region 1
Intent of the Authors:
This paper is the first of many collaborative efforts of the Core AD Technical Team. The
mission of the Core Team is to synthesize the Agency's knowledge, reach consensus
regarding technical and scientific facts surrounding AD and develop consistent and
effective mechanisms for messaging about anaerobic digestion. This document was
peer reviewed by representatives of OSWER, OAR and OW in accordance with ORD's
peer review protocol.
This document will help us achieve the mission of the Core AD Team by:
1. Presenting basic technical information and indisputable facts regarding the
anaerobic digestion process to anyone interested in learning about the
technology.
2. Describing the facts regarding the ability of AD systems to achieve OneEPA
goals as well as regional initiatives i.e., the Great Waters Program.
3. Communicating information on AD systems as OneEPA.
4. Capturing the perspectives of and emphasizing agreement reached by all three
media offices (OSWER, OW and OAR) and the EPA Regions.
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5. Clarifying the definition of terms associated with AD and AD systems to promote
consistent language and messaging generated by EPA.
6. Highlighting how the use of AD systems to meet program goals aligns with EPA's
FY2014-2018 Strategic Plan and Cross-agency Strategies.
Anaerobic Digester System
Wasted Food
Sewage Sludge and
High Strength
Wastewater
Impurity
Removal
Post-Processing
of Solid/Liquid
Effluent
Thermal
(e.g., direct use)
Fuel
(e^., pipeline,
vehicle fuel)
Solid Products Liquid Products
(e.g., soil amendment, (e.g., organic fertilizer)
animal bedding)
Figure 1. Anaerobic System Components
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Scientific Overview
Anaerobic digestion is a natural biological process. The initials "AD" may refer to the
process of anaerobic digestion, or the built systems of anaerobic digesters. While there
are many kinds of digesters, the biology is basically the same for all. Anaerobic
digesters are built systems that deliberately harness the natural process. AD systems
can minimize odors and vector attraction, reduce pathogens, produce gas, produce
liquid and solid digestate, and reduce waste volumes.
Anaerobically digesting organic carbon involves naturally occurring bacteria. Digestion
takes place when organic materials decompose in an oxygen-free environment. Some
digester systems differentiate between "wet" and "dry" digesters, or low-solid and high-
solid systems, and sometimes the process is called fermentation. The different
language used to describe the same processes reflect the varied historical uses and
development of AD.
During digestion, various microbes use the organic matter such as animal manure,
sewage sludge, wasted food and other organics in the absence of oxygen. The process
can be controlled and enhanced through chemistry and engineering.
The chemical reactions that occur in stages during anaerobic digestion are hydrolysis,
fermentation, also called acidogenesis (the formation of soluble organic compounds and
short-chain organic acids), and methanogenesis (the bacterial conversion of organic
acids into methane and carbon dioxide) (Metcalf & Eddy, 2003).
complex organic matter
carbohydrates, proteins, fats
(?) hydrolysis
(2) fermentation
(f) acetogenesis
(4) methanogenesis
X*
soluble organic molecules
sugars, ammo acids, fatty acids
Figure 2. The chemical reactions that occur during anaerobic digestion
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In the methanogenesis step, acetic acid, carbon dioxide, and hydrogen are converted to
biogas by methanogens. Biogas consists mainly of methane and carbon dioxide and
can be used as a renewable energy fuel in a variety of applications.
BIOGAS
EPA's Interest in AD
AD systems are installed by stakeholders for many different purposes, such as a waste
treatment step, a means to reduce odors, a source of additional revenues, or a way to
improve public image.
AD systems can impact several environmental sectors, particularly methane control,
production of renewable energy, and integrated waste management. This illustrates
EPA's views on AD in relation to its crosscutting strategies to achieve Agency priorities
in Air, Water, Waste, and Climate Change:
Biogas production was recognized as a key component of the President's
Climate Action Plan: The USDA (along with EPA and DOE) was tasked with developing
a Biogas Road Map as part of the directives in the President's Climate Action Plan:
Strategy to Reduce Methane Emissions (Climate Action Plan).
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AD can help EPA support the following strategic goals:
A) Taking action on climate change: AD projects can minimize the threats posed by
climate change in a number of significant ways:
AD projects reduce greenhouse gas emissions by capturing and combusting
methane; generating renewable energy (thereby reducing dependence on fossil
fuels); sequestering carbon by land applying nutrient rich digestate; and diverting
organics from landfills (thereby decreasing methane production and release).
AD projects help communities become more sustainable by producing renewable
energy thereby offsetting the need for purchased energy.
B) Protecting America's waters: Use of AD systems protects America's waters by
providing a sewage sludge treatment step and facilitating subsequent nutrient
recovery.
The anaerobic digestion of sewage sludge from wastewater treatment facilities plays
an important role in cost-effective wastewater solids treatment and management at
thousands of facilities due to the significant reduction in solids volume and their
stabilization. If energy recovery systems are added, AD provides the opportunity for
many of these facilities to offset their energy use.
Anaerobic digestion concentrates nutrients such as nitrogen and phosphorus, which,
if discharged in excess quantities, can cause algae growth and eutrophic conditions
in water bodies. These concentrated nutrients can then be recovered and
beneficially used. With proper post-digestion nutrient management, AD systems
improve water quality.
C) Advancing sustainable development:
AD projects support sustainable, resilient, and livable communities by reducing fossil
fuel consumption; producing a local source of renewable heat, electricity and/or fuel;
lessening odors; and strengthening the local economy by reducing energy costs and
generating revenue.
AD projects may conserve resources and increase sustainable materials
management by reducing the volume of wasted food disposed of in landfills, since
food is a large component of landfilled municipal solid waste (MSW).
AD may produce liquid and solid products that can be used as fertilizer or soil
amendment, replacing some conventional fertilizer.
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D) Preventing pollution:
AD may prevent pollution by diverting organics from landfills and agricultural sites,
reducing nutrient runoff, and hauling costs.
Prevention of pollution is also achieved at the end of the process, through the
production of natural soil amendments. Use of soil amendments can reduce the
need for synthetic fertilizers (which require significant energy to produce), reduce the
need for pesticides, and/or increase water retention properties when applied to
farmlands.
AD can also help EPA pursue the following cross-agency strategies:
A) Working towards a Sustainable Future:
Sustainable Materials Management There is an increased focus on the benefits
of diversion of organics from landfills. Anaerobic digestion of wasted food is a
materials management option that can generate energy, reduce GHG emissions
and create nutrient rich soil amendments. EPA is engaging and empowering
stakeholders by sharing programs, tools, and resources that support AD projects
for wasted food.
B) Working to Make a Visible Difference in Communities:
Strengthening EPA's community of practice by leveraging opportunities to
coordinate with other federal agencies to provide tools and resources to support
AD projects. EPA is actively engaging with USDA and DOE on the President's
Methane Reduction Strategy, a key component of which is the development of
the Biogas Roadmap.
Applications of AD
Generalizations about anaerobic digester systems often overlook variations. There are
many sizes, styles, and applications of digesters. AD systems can be house-sized or
town-sized. They can be used primarily for waste processing or energy generation.
Anaerobic digester systems can be designed to optimize mixing, volume reduction,
biogas production, pathogen destruction, vector attraction reduction, and odor control.
Systems can be designed as batch or continuous flow systems, within a sealed vessel
or holding tank, or with a series of vessels (see Appendix A).
Anaerobic digestion processes come in different configurations. Low rate anaerobic
digesters are usually used for small systems (under 1 million gallons per day), usually
contain no auxiliary mixing, and are operated at long sludge retention times (SRTs) in
the 30-60 day range. High rate systems are more commonly used and are
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characterized by supplemental heating, auxiliary mixing, uniform feeding rates, and
sludge thickening before digestion (WEF, 1998). They are designed for mesophilic
temperatures (86-100° F), the most common configuration in North America, at SRTs in
the 12-25 day range; or thermophilic temperatures (122-140°F) at SRTs in the 10-12
day range.
Two-stage anaerobic digester systems include a first stage (mesophilic or thermophilic),
where most of the gas is produced, and a second stage used for solid-liquid separation
or as a holding tank before dewatering. Temperature-Phased anaerobic digestion
configurations combine in one system both mesophilic and thermophilic digestion
stages connected in series and can offer significant advantages including significantly
improved volatile solids reduction and biogas production. Two-phase AD systems are
also available with the first stage being an acid phase reactor and the second phase
being a methanogenic reactor. Three and multi-stage configurations are also available.
Each AD alternative has advantages and disadvantages (Kalogo and Monteith, 2008).
Digesters can handle a variety of feedstocks. Some digesters are designed for one
feedstock but may be adapted to other feedstocks or a combination of them. Co-
digestion of sewage sludge with other feedstocks (e.g. fats, oils & grease (FOG),
wasted food, cheese or wine wastes, manure) can increase biogas production.
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Co-digestion can significantly increase biogas production and possibly volatile solids
reduction depending on the type of organic feedstock added and other factors (Parry,
2014).
Economic feasibility of co-digestion is strongly dependent upon waste characteristics,
regional energy costs, and biosolids residual management costs. Most waste streams
(perhaps with the exception of FOG) require a tipping fee paid to the digester owner to
achieve economic feasibility. Facilities considering co-digestion should consider utilizing
existing process capacity prior to exploring construction of additional capacity for co-
digestion (Parry, 2014).
Feedstocks are converted to biogas and digested material, which reduces their volume.
The volume reduction and gas production is dependent on the specific feedstock and
process.
In addition to the digester there may be additional equipment/technologies needed,
either upstream for particle size reduction, de-packaging, screening or moisture
adjustment. Downstream of the digester further processes may be needed to clean the
gas or modify the digestate into value added products to achieve the desired results in
any particular location.
Anaerobic digestion systems may be enhanced by using pre-treatment of the
feedstocks or by using different modifications or configurations of anaerobic digestion.
Pretreatment methods include thermal, mechanical, chemical, biological, ultrasonic, and
combinations of these methods. These pretreatments make the feedstock more
accessible to anaerobic microorganisms. No single pretreatment technology is suitable
for all anaerobic digestion systems and feedstocks.
Digesters produce biogas:
Biogas is the gaseous product of AD.
Biogas tends to be about 60%
methane. Directly out of the digester
it may contain water, hydrogen
sulfide, carbon dioxide, and other
gases.
Biogas can be:
o burned to generate electricity,
o burned to produce heat,
o compressed for vehicle fuel,
o added to natural gas pipelines, or sometimes
o a combination of those uses.
Biogas may require cleaning, drying, or other processing to meet a specific use.
Some generators of the biogas may flare (waste) the gas.
The amount of biogas production will vary based on feedstock, operation, and
process design.
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Digesters produce digestate:
Digestate refers to any non-gas products coming from a digester, which in some
cases is separated into liquid and solid streams.
The end uses of digestate are usually chosen based on its quality (e.g. nutrients,
degree of pathogen reduction), and local conditions including market demand.
Digestate can be rich in nitrogen (particularly ammonia) and phosphorus.
Technologies are available to recover ammonia and phosphorus and produce
fertilizer products.
Digestate uses include:
o soil amendment, direct land application;
o soil amendment, processed, bagged, and sold;
o animal bedding; and
o alternative daily cover for landfills.
Some digestate needs further processing before it can be used for certain
purposes. This processing can include drying or composting.
Except for digestate made
using sewage sludge, there
are currently no national
standards for the
classification of digestate
products. Code of federal
regulations 40 CFR Part 503
governs the standards for
final use and disposal of
sewage sludge and derived
products.
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Factors influencing AD Applications
Anaerobic digestion is one of
several integrated waste and
organic materials management
choices. Other choices include
landfilling, composting and
thermal processes. Landfilling
produces gas as anaerobic
digestion takes place within the
landfill, but as this AD is largely
uncontrolled, it is very inefficient
and often leads to unintended
releases of methane to the
atmosphere.
The following factors have been observed to affect the economics and operations of
anaerobic digestion systems:
The economic viability of an anaerobic digestion project depends on the type and
availability of feedstock, regional price of energy, the cost and type of
transportation, amount of biogas produced, local air quality standards, tipping
fees received for co-substrates, availability of GHG reduction or other credits,
incentives and subsidies, and the quality and local demand or options for
utilization for resulting products.
Seasonality of operations (i.e. wet vs dry or hot vs cold).
Geography (i.e. temperate vs arid climate, or proximity to food processing
manufacturers, population centers or farmland).
Regulations/Legislation
Other market forces (e.g. competition with low
tipping fees at a nearby landfill).
Comparisons between AD systems are complicated,
difficult, and dependent on local variables. There are
often multiple acceptable choices depending on timing,
population, and area of the country. Instead of
choosing one system over another, the optimal solution
may involve the integration of multiple system types.
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Current AD facilities in the US
As of August 2015, www.biogasdata.org estimates there are over 1270 sewage
treatment facilities producing
biogas.1
As of January 2015, EPA's
AgSTAR program estimates
that there are approximately
247 anaerobic digester
systems operating at
commercial livestock farms
in the United States.2
EPA tracks anaerobic digestion
facilities that process food-
based materials3. As of
September 2015 there are 98
such facilities in operation.
Included in this facility count
are municipal food digesters,
single source industrial
digesters, codigesters on farms
and ADs at WWTPs.
1 Numbers of AD systems at WWTPs are from 2013 EPA and WERF documents
(http://www.biogasdata.org/)
2 Number of Agricultural AD systems reported by the EPA's AgSTAR program.
http://www.epa.gov/agstar/proiects/index.html
3 Food-based materials include, but are not limited to: Food scraps that have been
separated and collected by municipalities from residential sources; food scraps that have been
separated and collected from institutions or venues (prisons, hospitals, stadiums, etc.); food
scraps from food preparation at restaurants, cafeterias, and other food services; plate scrapings
from restaurants, cafeterias, and other food services; Fats, oils and greases; unused food
collected from grocery stores (bakery items, bruised fruit, items passed shelf life, etc.); and pre-
consumer by-products of the food and beverage processing industries.
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References
Metcalf & Eddy., Tchobanoglous, G., Burton, F. L. 1., & Stensel, H. D. (2003). Wastewater
Engineering: Treatment and Reuse (4th ed.). Boston: McGraw-Hill.
Water Environment Federation - WEF - (1998). Design of Municipal Wastewater Treatment
Plants, 4th Edition, Manual of practice No. 8: Alexandria, VA, U.S.A.; American Society of Civil
Engineers, Manual on Engineering Practice No. 76: New York, U.S.A.
Kalogo, Y, and Monteith, H.P., Water Environment Research Foundation - WERF - (2008).
State of Science Report: Energy and Resource Recovery from Sludge. Jointly funded by
WERF, UK Water Industry Research Limited, STOWA and sponsored by the Global Water
Research Coalition (GWRC). Alexandria, VA, U.S.A.
Parry, D.L., Water Environment Research Foundation -WERF - (2014). Co-Digestion of
Organic Waste Products with Wastewater Solids - Final Report with Economic Model.
Alexandria, VA, U.S.A.
Jones, R. et.al, Characterization of Sludges for Predicting Anaerobic Digester Performance.
Water Science & TechnologyWST Vol 57 No 5 pp 721-726 © IWA Publishing 2008.
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Appendix A: System Diagrams
Batch System with a Single Vessel
Biogas is produced continuously
throughout the retention time
o
Organic material is
loaded into the AD
vessel as a batch
f
Biogas is treated for use
The AD vessel is sealed for the
duration of the digestion process
After a specific
retention time,
the AD vessel is
manually emptied
and reloaded
Digestate (solids and liquids) can
be treated for use
Batch System with Multiple Vessels
O
Biogas is produced continuously
throughout the retention time
Organic material is
loaded into each
AD vessel as a
batch
Biogas is treated for use
= 7
1 1 1 _
Each AD vessel is sealed for the
duration of the digestion process
After a specific
retention time,
each AD vessel is
manually emptied
and reloaded
Digestate (solids and
liquids) can be treated for
use
14
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Continuous Flow System with a Single Vessel
Organic Waste
i^M
Organic material is
regularly fed into
the AD vessel
Biogas is treated for use
Digested material is
continuously removed as
new organic material is
added to the AD vessel
Digestate (solids and
liquids) can be treated for
use
Continuous Flow System with Multiple Vessels
Biogas is continuously collected
Organic
material is
regularly fed
into each AD
vessel
Biogas is treated for use
Digested material is
continuously removed
from each AD vessel as
new organic material is
added to each AD vessel
Qieestate
Storage
Digestate (solids and
liquids) can be treated for
15
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Phased Continuous Flow System with a Series of Vessels
Organic Waste
Biogas produced in the
first AD vessel is
collected for treatment
Hydrolysis products and acids
are pumped to the second AD
vessel to optimize the
digestion process
Biog,
iogas is treated for use
Biogas produced in the
second AD vessel is
collected for treatment
Organic material
is regularly fed
into the first AD
vessel
Digested solids are
continuously removed
from the first AD vessel
as new organic material
is added to the system
Effluent is continuously
removed from the
second AD vessel as
new material is added
Digestate (solids and
liquids) can be treated for
use
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Glossary
acetogenesis: The third of four biological steps in anaerobic digestion. The process by which
volatile fatty acids are converted into acetic acid, carbon dioxide, and hydrogen.
acidogenesis: The second of four biological steps in anaerobic digestion. The process by
which simple monomers are converted into volatile fatty acids. Also referred to as a
fermentation step.
aerobic digestion: is the biochemical decomposition of organic matter into carbon
dioxide and water by microorganisms in the presence of air.4
anaerobic digester: an anaerobic digester is a built system for excluding oxygen from
organic material and producing biogas.
anaerobic digestion: is the biochemical decomposition of organic matter into methane
gas and carbon dioxide by microorganisms in the absence of air.5
biogas: is a mixture of approximately 60% methane (CH4) and 40% carbon dioxide (C02)
together with trace levels of other gases. Biogas is produced when organic material is broken
down by microorganisms in an oxygen free, or anaerobic, environment.
codigestion: The digestion of two or more feedstocks in a single anaerobic digestion
system.
composting: composting is an aerobic process of using microbial communities to degrade
organic material.
feedstocks: any material used in an AD system can be considered a feedstock.
Typically AD feedstocks include manure, silage, sewage sludge, wasted food, yard waste,
FOG, and industrial organic byproducts, waste
high solid AD systems: greater than 15% solids (by volume) content.
hydrolysis: The first of four biological steps in anaerobic digestion. The decomposition of
organic compounds by interaction with water.
low solid AD systems: less than 15% (by volume) solids content.
methanogenesis: The process by which acetate is converted into methane and carbon
dioxide, while hydrogen is consumed.
Definition cited from 40 CFR 503.31 (a)
Definition cited from 40 CFR 503.31 (b)
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United States
Environmental Protection
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PERMIT NO. G-35
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