Anaerobic Digestion Fundamentals

Anaerobic digestion is a solids stabilization process commonly used at municipal and industrial water resource recovery

facilities (WRRFs). The primary objective of anaerobic digestion is to convert the microbiological cells and other solids

generated during the treatment process to a stable end product. Other solids stabilization processes include aerobic

digestion, composting, thermal drying, thermal oxidation, and alkaline stabilization.

Anaerobic digesters typically consist of concrete or steel tanks and receive solids from separation processes in the liquid

treatment train. Solids may be comprised of settled material from primary clarification and secondary clarification, as

well as scum and grease. Fats, oils, and grease (FOG) or food waste from sources outside of the WRRF also may be fed

directly to digesters in some applications.

Similar to other stabilization processes, anaerobic digestion reduces odors and pathogens in the solids stream. Following

stabilization, the digested product, referred to as biosolids, may be removed from site for disposal or processed further to

reduce water content in preparation for disposal or beneficial reuse.

The primary benefit of anaerobic digestion compared to other forms of solids stabilization is the energy recovery

potential. Unlike the other commonly used stabilization processes, anaerobic digestion generates a biogas comprised

primarily of methane and carbon dioxide that may be recovered for beneficial use such as heat or power generation.

The potential for resource recovery has led to increased use of anaerobic digestion in recent decades.

PROCESS DESCRIPTION

Anaerobic digesters may receive solids from upstream processes in the liquid treatment train or FOG and food waste

from outside sources such as restaurants and other commercial or industrial facilities.

The solids from upstream processes typically are comprised of primary sludge, secondary sludge, scum, grease, and/or

other solids and liquids that may enter the solids collection system (including grit). Solids handling pumps like progressive

cavity or rotary lobe pumps typically convey solids to the digester tanks.

Intermediate steps may be implemented prior to digestion to improve the process efficiency or prevent excessive

maintenance on tanks or equipment. Grinding helps prevent maintenance issues by shredding large or stringy material

into smaller components. Screening also prevents maintenance issues by removing large or stringy materials. Degritting

improves process efficiency by preventing accumulation of grit inside digester tanks.

Grit is particularly concerning. The accumulation of settled grit and other inert solids within digester tanks reduces the

effective treatment or digestion volume. This digestion capacity reduction hinders the stabilization performance, reduces

gas production, and increases the frequency of tank maintenance and cleaning.

Solids blending (combining separate streams such as primary sludge and waste activated sludge into one feedstock)

and temporary storage in a holding tank also may be implemented upstream of the anaerobic digestion process.

Blending and storage produces a more homogeneous loading and minimizes digester feed flow variability. Feed

variability (in quality or flow) may cause digester foaming issues and require more maintenance.

Anaerobic Digestion Fundamentals

Anaerobic digestion is a sustainability staple at resource recovery facilities. In addition to performing vital solids treatment

processes such as stabilization and volatile solids reduction, anaerobic digestion also generates biogas that can be used

at the resource recovery facility to generate heat and power.

FACT SHEET

WSEC-2017-FS-002—Municipal Resource Recovery Design Committee—Anaerobic Digestion Fundamentals

An additional intermediate step prior to digestion may include solids thickening. In this process, the water content of

primary and/or waste activated sludge is reduced. Process examples include gravity thickening, rotary drum thickening,

and gravity belt thickening. By reducing the overall volume of digester feed sludge, thickening may reduce the

equipment and tankage capacity required for digestion, conveyance, or storage, as well as the energy required for

digester heating, and chemical use for additional conditioning (if required).

Following any intermediate processing or thickening, solids are conveyed to digesters. Many different digester

configurations, shapes, and flow patterns are available depending on the quality of the digester feed and the primary

process objectives. These process objectives may be based on the degree of volatile solids reduction, gas production, or

pathogen destruction required. Table 1 (p. 2) provides a brief introduction into some of the digester process options.

Table 1. Common digester configurations

Mesophilic digestion is by far the most common method of digestion, followed by thermophilic. Other methods, such as

temperature-phased anaerobic digestion (TPAD) and acid/gas-phased anaerobic digestion (AGAD), are far less common

and are less rigidly defined compared to mesophilic and thermophilic digestion methods. For example, the TPAD

configuration may include an initial acidic stage, and the AGAD configuration may include an initial thermophilic stage.

Regardless of the configuration, the objective of digester operation is to create an environment that promotes organic

decomposition and reduction of inorganic matter. This is accomplished primarily by maintaining a certain temperature

(depending on the digestion method) and ensuring sufficient mixing.

External heat exchangers typically are used to maintain sufficient temperature within the digester. Spiral type heat

exchangers and water bath type heat exchangers increase the temperature of solids by exposing a pipe containing solids

to high temperature water. As the solids pass through the heat exchanger piping, energy (heat) is transferred from the hot

water to the solids. Solids (at an elevated temperature) then return to the digester to increase or maintain the temperature

of the contents.

Numerous methods are available to maintain digester contents in suspension, including pumping from one location within

the tank to another. This uses an external draft tube or an external centrifugal pump. Mechanical mixing via an impeller or

linear motion mechanism located within the tank or gas injection are other mixing methods.

The biogas that forms during anaerobic digestion is a result of the biological decomposition of organic matter taking

place in the absence of oxygen. Methane (CH4) makes up 65% to 70% of biogas, while carbon dioxide (CO2) comprises

about 25% to 30%. Trace quantities of nitrogen (N-2), hydrogen (H2), and hydrogen sulfide (H2S), water vapor, and other

gases make up the difference.

The energy potential of methane makes biogas a valuable resource as mentioned above. The heat produced by burning

biogas fuel in a boiler may be utilized to heat water for use in a heat exchanger or for the facility’s building heating

system. Cogeneration or combined heat and power (CHP) systems are used to produce heat and to convert the energy

contained in biogas to useable electrical energy. Microturbines, gas turbines, or internal combustion engines connected

to generators are used to produce electricity, which may be used onsite or exported to an external power grid.

WSEC-2017-FS-002—Municipal Resource Recovery Design Committee—Anaerobic Digestion Fundamentals

Anaerobic digestion

method

Operation Advantages

Mesophilic

Temperatures may range from 35°C to

39°C

 most common digestion method

 produces biosolids and biogas for

beneficial use

Thermophilic

Temperature may range from 50°C to

57°C

 improved pathogen destruction

 higher quality biosolids produced

 improved gas production

 reduced volume requirements (due

to increased reaction rate)

Temperature-phased

Mesophilic and Thermophilic operation

 reduced thermophilic heating vol-

ume

 improved volatile solids destruction

 increased gas production

Acid/gas phased

Multiple phases including acid stage

(with pH conditions below 6.0) and gas

stage (with neutral pH conditions)

 improved volatile solids destruction

 foaming control

Higher-value fuels such as compressed natural gas (CNG),

liquid natural gas (LNG), or methanol may also be produced

from biogas. While these fuels are considered higher-value,

the processes for producing and using CNG, LNG, and

methanol are more complex in comparison to using biogas

for heat and power generation. As technology advances,

the use of biogas to produce these higher-value fuels may

become more common.

Prior to use, biogas must be pretreated to remove impurities

and other substances which may cause maintenance and

process efficiency issues. The water vapor present in biogas

reduces process effectiveness, and may be removed by

sloping biogas piping toward sediment/drip traps that

collect condensed water vapor for disposal.

Hydrogen sulfide, when combined with water vapor, forms a

weak acid that may damage biogas piping or equipment.

Passing biogas through wood chips impregnated with iron

sponge, biological scrubbers, or activated carbon help

remove hydrogen sulfide from biogas. Liquid phase

oxidation is an alternative removal method.

Siloxanes present in biogas convert to silicon dioxide

particles or sand when heated (for example, in a boiler,

engine, or turbine). These particles may cause damage and

reduce equipment life. Adsorption using activated carbon

or condensation may be employed to remove siloxanes.

Beside biogas, digestion also produces biosolids. Biosolids

may be conveyed to a secondary digester (if present) or

the mechanical dewatering process. Alternatively, — and

depending on regulations and WRRF operation — the

biosolids may be collected and hauled to an offsite location

for final disposal or further processing.

Secondary digester tanks may be employed to enable

liquid–solid separation of biosolids. This separation produces

a liquid called supernatant as well as thickened biosolids.

Supernatant is returned to the head of the WRRF (usually the

headworks), while the thickened biosolids may be

processed further or hauled offsite for final disposal.

Dewatering is the most common form of biosolids processing

post-digestion. The objective of dewatering is to reduce the

water content of biosolids; this reduces hauling costs. The

dewatered “cake” that is generated from dewatering may

be landfilled or, depending on the quality of the cake,

added to soil as an amendment. Numerous technologies

are available to mechanically dewater solids; common

equipment types include centrifuges, screw presses, belt

filter presses, and rotary presses. Solids drying beds are a

more passive dewatering approach and require more

space and additional labor to produce dewatered cake.

The liquid removed from solids during dewatering typically is

returned to the head of the WRRF.

Incineration is an alternative form of biosolids processing

and may be accomplished using multiple hearth reactors or

fluidized bed reactors. End products include carbon dioxide,

water, and ash. The objectives of incineration include

volume reduction in preparation for final disposal and

energy recovery.

DESIGN

There are three main stages of anaerobic digestion:

hydrolysis, fermentation, and methanogensis. All three occur

simultaneously and in the same vessel.

During hydrolysis, cells that were instrumental in biological

treatment in the liquid train are broken down into a soluble

form. Fermentation follows hydrolysis. During this stage, the

soluble products formed during hydrolysis are converted to

a mixture of volatile fatty acids (a process called

acidogensis) and, then, the mixture of volatile fatty acids is

converted primarily to acetic acid, carbon dioxide, and

hydrogen (a process called acetogenisis). The final stage is

methanogensis, which involves the conversion of acids and

hydrogen (formed during fermentation) to methane and

carbon dioxide. The proper design of anaerobic digesters

helps maximize the effect and success of each of these

three stages.

While there are numerous styles and operational strategies,

most anaerobic digesters are operated as single stage

mesophilic reactors. Table 2 (p. 3) provides typical design

criteria for high rate mesophilic digesters.

Table 2. Typical high rate mesophilic digester design criteria

Numerous factors greatly affect the construction and

operation of anaerobic digesters and must be considered

during design. The following section summarizes a few of

these factors.

WSEC-2017-FS-002—Municipal Resource Recovery Design Committee—Anaerobic Digestion Fundamentals

Table adapted from Design of Municipal Wastewater Treatment Plants

(Manual of Practice No. 8), published by the Water Environment

Parameter

Typical target range

Temperature

95°F to102°F

Volatile solids loading rate

0.12 to 0.16 lb/ft

•day

Feed percent solids

4% to 7%

Solids retention time

15 to 20 days

WEF Photos

Tank volume and retention time

When sizing an anaerobic digester and selecting a design solids retention time (SRT), the engineer must consider such

factors as biochemistry, microbiology, industrial contributions, and regulatory requirements. Simplified empirical methods

are available for use during design. High rate mesophilic digesters may employ an SRT between 15 and 30 days. In

general, the longer the SRT, the more volatile solids reduction.

Volatile solids reduction

Volatile solids reduction is a common metric used to evaluate digestion effectiveness, but is difficult to quantify for design

as it is greatly affected by both operating conditions and solids feed variation. The volatile solids loading rate is a design

parameter that may be used to size digester tanks.

Gas production and collection

Typically, gas is produced at a rate of about 13 to 18 ft3/lb volatile solids reduced. Methane may account for up to about

70% of the total volume of gas produced during digestion. Gas produced during anaerobic digestion is collected under

the digester tank cover, where it is temporarily stored prior to its ultimate use. Numerous cover styles and types are

available, including fixed (immobile), floating, and membrane covers. During cover selection and design, the engineer

should consider gas production, thermal requirements, and odors, as well as foam and scum control.

Tank shape

The three most common shapes are cylindrical (with slightly sloped tank floor), egg-shaped, and the “German style”,

which features a cylindrical tank with more steeply sloped bottom and top sections. Most anaerobic digester designs in

the U.S. feature conventional cylindrical tanks.

Mixing

Multiple mixing options are available. Common examples include external or internal draft tubes, which pump contents

from one section of the digester to another via tubes located, respectively, on the tank exterior or interior; pumps, which

receive contents from one section of the digester and pump to another section; and submersible mixers, which stir

contents within a digester via impellers.

Biosolids classification

According to the U.S. Environmental Protection Agency’s regulation at 40 CFR Part 503, there are two levels of pathogen

reduction: Class A and Class B. These designations and the resulting processes to achieve them affect the quality and

ultimate use of biosolids. The biosolids that result from Class A pathogen reduction may be directly applied to land for

beneficial reuse. Reaching Class A requires additional treatment during digestion, for example exposing digester contents

to higher temperature. More regulations exist for the end use of biosolids that result from Class B pathogen reduction. To

meet Class B pathogen reduction, anaerobic digesters must be operated for a minimum SRT of 15 days and a

temperature between 95°F and 131°F.

OPERATION AND MAINTENANCE

Table 3 (p. 4) provides typical operating parameters for mesophilic anaerobic digesters.

Table 3. Typical mesophilic digester operating parameters

WSEC-2017-FS-002—Municipal Resource Recovery Design Committee—Anaerobic Digestion Fundamentals

Parameter

Value

VSS destruction

45% to 55%

6.8 to 7.2

Alkalinity

2500 to 5000 mg/L as CaCO

Methane content of biogas

60% to 65% (by volume)

Carbon dioxide content of biogas

35% to 40% (by volume)

Volatile acids (VA)

50 to 300 mg/L as VA

Volatile acid: alkalinity ratio

<0.3 mg CaCO

/ mg VA

Ammonia

800 to 2000 mg/L as N

Table adapted from Design of Municipal Wastewater Treatment Plants (Manual of Practice No. 8), published by the Water Environment

Federation and McGraw Hill

The operations and maintenance plan for anaerobic digesters focuses on maintaining conditions that promote the

digestion process, including pH, temperature, and alkalinity. Tank foaming and odor must be considered and addressed

to prevent additional maintenance or nuisance issues. And, in general, tanks should be emptied periodically to enable

operators to check mechanical equipment and clean tank interiors. The table below provides additional information on

O&M typical of conventional mesophilic digestion.

Table 4. O&M Typical of Conventional Mesophilic Digestion

WSEC-2017-FS-002—Municipal Resource Recovery Design Committee—Anaerobic Digestion Fundamentals

O&M activity

Effect

Maintaining pH between 6.8

and 7.5

Methanogens are sensitive to variations in pH. Operating outside of this range

may reduce methane generation and impair overall digestion performance.

Maintaining Temperature

between 95°F and 102°F

Methanogens are sensitive to variations in temperature. Operating outside of

this range or even changing digester temperature more than about 2°F per

day may reduce methane formation, increase foaming, and impair overall

digestion performance.

Maintaining sufficient

alkalinity

Sources of alkalinity like ammonia and bicarbonate are produced during

digestion and help maintain pH. A well-performing digester should not require

alkalinity supplementation. The need for alkalinity supplementation with

chemicals such as sodium bicarbonate and lime to stabilize pH is indicative of

overall system imbalance. Further evaluation may be necessary.

Minimizing tank foaming

Foaming impairs performance by reducing the active digestion volume; this

may lead to lower volatile solids reduction and biogas production, short

circuiting of pathogens, mechanical equipment damage, and foam overflows

or spills. Foaming may result from the presence of chemical surfactants,

biological surfactants, or filamentous organisms. Foaming may be

exacerbated by unstable operations such as highly variable loading rates or

mixing. Maintaining constant digester feeding (rather than loading in batches)

helps limit tank foaming issues.

Minimizing odor

Odorous compounds such as hydrogen sulfide and ammonia are produced

during digestion. The installation of digester tank covers limits the effect of

nuisance odors to the surrounding environment.

Tank cleaning

Digester tanks should be removed from service periodically for cleaning and

inspection. While offline, operators can check or repair any mechanical

equipment installed within the tank and inspect the tank itself for structural

deterioration. Additionally, grit and scum, accumulates within digestion vessels

and limits effective/treatment volume, and should be removed while the tank

is offline.

Maintaining safe work spaces

Biogas is a flammable substance. The lower explosive limit (LEL) for methane in

the air is 5%. Furthermore, empty digesters are classified as confined spaces.

The immediately dangerous to life or health (IDLH) limit for methane in the air is

0.5%. To ensure safety and minimize risk, air monitors should be installed where

appropriate and operators should follow all safety precautions when working

around digesters and related equipment.

WSEC-2017-FS-002—Municipal Resource Recovery Design Committee—Anaerobic Digestion Fundamentals

Acknowledgments

WEF Municipal Resource Recovery Design

Committee

Contributing authors:

Jeff Coyne (Primary Author)

Chris Wilson

Matt Scarborough

Art Umble

Resource

Description

Solids Process Design and Management (2012)

This publication is intended for use by professionals engaged in the

design, approval, and operation of municipal solids treatment and

disposal systems. This publication includes material on planning, solids

production and characterization, conveyance, conditioning,

thickening, waste minimization, anaerobic and aerobic digestion,

dewatering, composting, alkaline treatment, disinfection and

stabilization, thermal drying, thermal oxidation, pyrolysis and

gasification, transport and storage, odor management, sidestreams,

instrumentation and monitoring, land application and product

distribution, landfill management, emerging technologies, and

treatment and utilization of green gases.

Biogas Production and Use at Water Resource

Recovery Facilities in the United States (2013)

This report highlights existing anaerobic digestion systems at U.S.

Water Resource Recovery Facilities, as well as current uses of, and

potential future opportunities for, using biogas produced by these

facilities.

Biogas Utilization: A Regional Snapshot in

Understanding Factors that Affect Water Resource

Recovery Facilities (2015)

The goal of this report is to summarize “sprint” data collection

activities, which took place during 2014 as part of a larger collection

effort aimed at determining the beneficial use of biogas within the

water environment industry. As data continues to be supplemented,

additional regional reports will be released. The site-specific data can

be found at www.biogasdata.org.

Technical Practice Update: Direct addition of High

Strength Organic Waste to Municipal Wastewater

Anaerobic Digesters (2010)

The main purpose of this Technical Practice Update (TPU) is to provide

a high-level overview of some of the potential benefits and

challenges of direct co-digestion of high-strength organic wastes with

municipal wastewater sludge.

Moving Toward Resource Recovery Facilities (2014)

This book provides an overview of the fundamental drivers for

resource recovery from wastewater and presents the basic case for

resource recovery. It also provides an overview of state-of-the-art

technological approaches to resources recovery and provides

general guidance on the applicability of recovery technologies for

the cross section of facility types. This allows facilities to take steps

toward recycling a greater number of otherwise lost resources.

References

Additional Resources WEF Resources

1. Tchobanoglous, George, et al. Wastewater Engineering

Treatment and Resource Recovery, Metcalf and Eddy/AECOM

ed. New York: McGraw-Hill Education, 2014.

2. Grady Jr., C. P. Leslie, et al. Biological Wastewater Treatment,

ed. Boca Raton: CRC. 2011.

3. Design of Municipal Wastewater Treatment Plants, 5

ed. New

York: McGraw-Hill Education, 2010