SIST EN ISO 19388:2026
(Main)Sludge recovery, recycling, treatment and disposal - Requirements and recommendations for the operation of anaerobic digestion facilities (ISO 19388:2023)
Sludge recovery, recycling, treatment and disposal - Requirements and recommendations for the operation of anaerobic digestion facilities (ISO 19388:2023)
This document establishes requirements and recommendations for the operation of the anaerobic digestion of sludge in order to support safe and sufficient operation of anaerobic digestion facilities to produce to produce sufficient biogas and control by-products qualities.
In particular, conditions to optimize mixing within the reactor and appropriate control systems management for safe and reliable operation are described in this document. Performance of the processes in terms of biogas and digestate production are presented depending on type of technologies available on the market. Blending sludge with waste (co-substrate) and mixing the sludge with organic wastes to increase digester loading are also considered.
This document is applicable to decision-makers and operators in charge of an anaerobic digestion system.
Schlammgewinnung, -verwertung, -behandlung und -beseitigung - Anleitung für den Betrieb anaerober Faulungsanlagen (ISO 19388:2023)
Valorisation, recyclage, traitement et élimination des boues - Exigences et recommandations pour l'exploitation des installations de digestion anaérobie (ISO 19388:2023)
Predelava, recikliranje, obdelava in odlaganje blata - Zahteve in priporočila za delovanje naprav za anaerobni razkroj (ISO 19388:2023)
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
oSIST prEN ISO 19388:2025
01-maj-2025
Predelava, recikliranje, obdelava in odlaganje blata - Zahteve in priporočila za
delovanje naprav za anaerobni razkroj (ISO 19388:2023)
Sludge recovery, recycling, treatment and disposal - Requirements and
recommendations for the operation of anaerobic digestion facilities (ISO 19388:2023)
Valorisation, recyclage, traitement et élimination des boues - Exigences et
recommandations pour l'exploitation des installations de digestion anaérobie (ISO
19388:2023)
Ta slovenski standard je istoveten z: prEN ISO 19388
ICS:
13.030.20 Tekoči odpadki. Blato Liquid wastes. Sludge
oSIST prEN ISO 19388:2025 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
oSIST prEN ISO 19388:2025
oSIST prEN ISO 19388:2025
INTERNATIONAL ISO
STANDARD 19388
First edition
2023-03
Sludge recovery, recycling, treatment
and disposal — Requirements and
recommendations for the operation of
anaerobic digestion facilities
Valorisation, recyclage, traitement et élimination des boues —
Exigences et recommandations pour l'exploitation des installations de
digestion anaérobie
Reference number
ISO 19388:2023(E)
oSIST prEN ISO 19388:2025
ISO 19388:2023(E)
© ISO 2023
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Published in Switzerland
ii
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ISO 19388:2023(E)
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and abbreviated terms . 1
3.1 Terms and definitions . 1
3.2 Abbreviated terms . 3
4 Fundamentals .3
4.1 Boundaries . 3
4.2 Principle . 4
4.3 Pre-treatment . 5
4.3.1 General . 5
4.3.2 Physical pre-treatment . 8
4.3.3 Chemical pre-treatment . 9
4.3.4 Enzymatic hydrolysis . 9
4.4 Digester . 9
4.4.1 Shape . 9
4.4.2 Configurations . 10
4.4.3 Mixing system . 11
4.4.4 Heating system . 14
4.4.5 Operating temperature . 14
4.4.6 Line description . 14
5 Digestion performance .15
5.1 Feedstock composition.15
5.2 Feeding characterization .15
5.3 E valuation of the potential production of methane . 16
5.4 A ssessment of foaming risks . 19
5.5 Rheological properties .20
5.6 Prediction of biogas quality . 20
6 Operating performance .21
6.1 Pre-treatment . 21
6.1.1 General . 21
6.1.2 Shock loading or digester over-loading .22
6.1.3 Inadequate or excessive heating . 22
6.1.4 Commissioning, start-up . 22
6.1.5 Mixing efficiency and hydraulic retention time . 24
6.1.6 Gas system . 25
6.1.7 Gas monitoring . 26
6.1.8 CH production. 26
6.1.9 Process monitoring . 26
6.1.10 Return liquors . 27
6.2 Digestate quality and characteristics . 27
6.2.1 Process efficiency . 27
6.2.2 Dewaterability .28
6.2.3 Biogas quality .28
6.2.4 Biogas quantity .28
6.2.5 Biogas conditioning .29
7 Process safety — Trouble shooting .30
7.1 Pressure control . 30
7.2 Stop of CHP machines . 30
7.3 Odour management . 30
7.4 Foaming . 30
iii
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7.5 Corrosion . 31
7.6 Struvite deposits . 31
7.7 Sand and grit removal . 31
Annex A (informative) Stabilization of sludge .32
+
Annex B (informative) Chemical parameters of ammonium — pKa values of NH /NH .35
3 4
Bibliography .36
iv
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Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 275, Sludge recovery, recycling, treatment
and disposal.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
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Introduction
Anaerobic digestion of sewage treatment plant sludge is an increasing market at world scale. It presents
advantages for sludge treatment in terms of sludge volume decrease, organic matter recycling and
energy recovery.
Standardization of conditions of operation is therefore a main issue to ensure an efficient development
of anaerobic digestion treatment. Anaerobic digestion process is subject to appropriate safety measures
because it can represent many risks. Safety parameters are included in risks analyses (e.g. HAZOP).
Therefore, the objectives of this document are:
— to reduce volatile solids, mitigate odours production and generate biogas;
— to obtain good process stability and performance;
— to maximize qualities of by-products: digestate quality, biogas quality for different uses (injection
of upgraded biogas into the gas grid, liquefied storage, fuel reuse, electricity and heat production);
— to perform safe and reliable operation: industrial safety for piping and automatism network and
biogas equipment is in particular an important issue;
— to reduce emission of greenhouse gasses, especially of methane.
Anaerobic stabilization does not mean sludge sanitization: pathogens reduction is limited to 1 log to 3
logs. Higher reduction can only be obtained with specific conditions of temperature and residence time
which are not discussed in this document.
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INTERNATIONAL STANDARD ISO 19388:2023(E)
Sludge recovery, recycling, treatment and disposal —
Requirements and recommendations for the operation of
anaerobic digestion facilities
1 Scope
This document establishes requirements and recommendations for the operation of the anaerobic
digestion of sludge in order to support safe and sufficient operation of anaerobic digestion facilities to
produce to produce sufficient biogas and control by-products qualities.
In particular, conditions to optimize mixing within the reactor and appropriate control systems
management for safe and reliable operation are described in this document. Performance of the
processes in terms of biogas and digestate production are presented depending on type of technologies
available on the market. Blending sludge with waste (co-substrate) and mixing the sludge with organic
wastes to increase digester loading are also considered.
This document is applicable to decision-makers and operators in charge of an anaerobic digestion
system.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and abbreviated terms
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1 Terms and definitions
3.1.1
acetoclastic methanogenic microorganism
anaerobic microorganism which use acetate as a main substrate
3.1.2
anaerobic digestion
anaerobic process which achieves two equally important functions, the anaerobic stabilization of
substrate and the production of energy through conversion of substrate into biogas
3.1.3
biochemical methane potential
BMP
volume of methane generated during the sample degradation referred to the mass of the sample of
biosolid and expressed in normal conditions of temperature (0 °C) and pressure (1 013 hPa)
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3.1.4
digestate
digested sludge
remaining effluent from the anaerobic digestion process including solid fraction and liquid fraction
[SOURCE: ISO 20675:2018, 3.19]
3.1.5
digester gas
biogas
gas mixture generated during anaerobic digestion consisting mainly of methane and carbon dioxide
3.1.6
feeding
process of adding substrate into an anaerobic digester
3.1.7
hydrolysis
biological, chemical, thermal or physical transformation of solid chemical oxygen demand into dissolved
chemical oxygen demand by reaction with water
3.1.8
phase
distinct metabolic pathways
EXAMPLE Two-phase digestion: hydrolysis/acidogenesis followed by acetogenic/methogenic.
3.1.9
readily degradable substance
substance which is easily and completely degradable by microorganisms
3.1.10
sludge age
solids retention time in a reactor
Note 1 to entry: The common unit is d.
3.1.11
stabilization
process in which organic substances are converted to materials that are not biodegradable or are slowly
biodegradable
3.1.12
stage
consecutive part of a process
EXAMPLE Two-stage digester, i.e. a primary digester followed by secondary digester for completing
processes.
3.1.13
substrate
feedstock containing degradable organic components
3.1.14
volumetric organic load
mass of substrate, measured as total solids, volatile solids, biochemical oxygen demand or chemical
oxygen demand, fed per digester volume and day
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3.2 Abbreviated terms
ATU allyltiourea assay
BMP biochemical methane potential
BOD biochemical oxygen demand
CAPEX capital expenditure
CHP combined heat and power
COD chemical oxygen demand
ECP extracellular polymer
FOG fats, oils and greases
HRT hydraulic retention time
ITHP intermediate thermal hydrolysis process
OUR oxygen uptake rate
OPEX operational expenditure
SOUR specific oxygen uptake rate
SRT solids retention time
TS total solids
VFA volatile fatty acids
VS volatile solids
4 Fundamentals
4.1 Boundaries
Figure 1 describes the system configuration of the anaerobic digestion. In this document, the focus is
oriented on anaerobic digester operation and pre-treatments.
Figure 1 — Typical system configuration of anaerobic digestion
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4.2 Principle
Anaerobic digestion is a cornerstone process of wastewater treatment operations, wherein solid waste
streams can be effectively treated and resources recovered. Anaerobic digestion usually requires
primary sludge. Most of the digester gas is generated from primary sludge. Secondary or tertiary sludge
can also be stabilized, but they should be (usually mechanically) thickened prior to anaerobic digestion.
The pervasive rollout of activated sludge-based wastewater treatment processes in particular
throughout the 20th century, necessitated digestion processes to effectively stabilize the large volumes
of generated waste activated sludge. The key purpose of anaerobic digestion during wastewater
treatment is to achieve disintegration and destruction of the degradable sludge solids fraction in order
to reduce this fraction and to reduce the mass and volume of the sludge material after dewatering or
drying.
This treatment recovers useful resources such as combustible digester gas (methane) and nutrients in
the digester sludge. Anaerobic digestion involves microbial decomposition of the organic constituents
present in wastewater sludge (i.e. proteins, carbohydrates and lipids) in the absence of dissolved
oxygen. Microorganisms involved in anaerobic digestion comprise a complex consortium of microbes,
with different metabolic properties and physicochemical requirements. The key products of anaerobic
digestion, apart from digested and stabilized solids rich in phosphorus include water containing high
levels of ammonia and alkalinity, and a biogas which comprises principally methane (typically 60 %
v/v to 70 % v/v) and carbon dioxide (typically a volume fraction of 30 % to 40 %), with other minor
constituents including hydrogen, nitrogen, hydrogen sulfide and siloxanes. Digester gas composition
depends on substrate quality. It can be different for industrial sludge or where co-substrates are added.
Anaerobic digestion transfers energy from solids to digester gas (methane). Only a very small amount
of energy is used for the production of biomass. Theoretical calculation gives 0,35 m methane with an
energy content of 3,5 kWh per kg of COD removed.
Anaerobic digestion is performed through four distinct biological steps, namely hydrolysis, acidogenesis,
acetogenesis and methanogenesis; an additional pre-treatment stage may be added prior to hydrolysis
for feedstocks containing solid particles in order to breakdown solids to smaller particles which are
[50]
more amenable to hydrolysis.
— Hydrolysis: Hydrolysis generates soluble organic components (e.g. sugar) from volatile solids which
microorganisms can absorb through their cell membranes. Hydrolysis is usually the rate-limiting
step during the digesting process.
— Acidogenesis: Hydrolyzed compounds formed during the hydrolysis step are further converted to
a mixture of short-chain volatile fatty acids (e.g. acetic acid, propionic, butyric and valeric acids),
alcohols, esters, sugars and other simple organic compounds (e.g. carbonic acid) by a diverse array
of microorganisms called acidogens. The relative proportion of the different metabolic co-products
(H and CO ) depends on the substrate quality as well as the operating conditions.
2 2
— Acetogenesis: Products of acidogenesis are further transformed to acetic acid, CO and H by
2 2
acetogenic microorganisms. Acetogenic microorganisms are relatively slow growing compared to
the acidogens, such that careful process control and stable digester operation is required to avoid
excessive acid accumulation and concomitant pH drop which can lead to digester upsets or process
failure. Nevertheless, the slowest growing microorganisms are the methanogens.
Some minor route such as syntrophic acetate oxidation performed by methane microorganisms
(oxidation of acetate into H and CO ) can occur and be prevalent when stressful conditions are
2 2
encountered (e.g. high concentration of ammonia resulting from high ammonium concentration,
high pH and high temperature).
— Methanogenesis: This final stage generates methane from either acetate or hydrogen by
methanogenic microorganisms (Archae). Usually acetate is the main source for the production
of methane (approximately 70 %) via so-called acetoclastic methanogens, with the remaining
approximately 30 % of generated methane being generated from hydrogen-utilizing methanogens.
The balance between methane generation from acetate and from hydrogen is variable depending
on operating conditions and substrate characteristics. Methanogens are slower growing than both
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the acidogens and acetogens, and are also susceptible to environmental stresses in the form of pH
and temperature imbalance, toxic or inhibitory substances such as free oxygen, or disruptions of
nutrient supply.
This succession of steps shows two points of attention in order to optimize anaerobic digestion
operation.
a) Methanogenic microorganisms (mainly acetoclastic methanogens) are the slowest to grow because
their substrate and their end products have a small energy difference (i.e. they gain little energy).
In addition, they are most sensitive to inhibition.
−1
Methanosarcina have a maximum growth rate of 0,3 d and Methanothrix have a maximum growth
−1 [8]
rate of 0,1 d . According to the Monod equation, the growth rate depends on the substrate
concentration. The Monod equation is given in Formula (1):
μ ×C K
max s
i
μ= × (1)
KC+ KC+
ss ii
where
−1
μ is the growth rate, in d ;
−1
μ is the maximum growth rate at unlimited substrate concentration, in d ;
max
K is a constant, in g/l, depending on the kind of microorganism and its substrate; if C = K , then
S S S
μ = 1/2 × μ ;
max
K is a constant, in g/l, depending on the kind of microorganism and its inhibitor; if C = K , then
i i i
μ = 1/2 × μ ;
max
C is the inhibitor concentration, in g/l;
i
C is the substrate concentration, in g/l.
s
b) Hydrolysis is the velocity-limiting process step during the digesting process. In preferably heated
raw sludge storage tanks some biological hydrolysis takes place. Particulate COD is turned into
dissolved and easily degradable COD.
Two stage digestion occurs in a highly loaded first-stage digester followed by a less loaded second-
stage digester; the microorganism in both stages can be the same. A two-stage digestion (mesophilic
and mesophilic) is more efficient than a single mesophilic reactor because the distribution around
the mean retention time is tighter.
4.3 Pre-treatment
4.3.1 General
Substrate thickening is usually the first pre-treatment process. The fed substrate should have a solids
concentration (30 g/l to 80 g/l) in accordance with the anaerobic digester operating conditions. This
concentration should reach 150 g/l to 250 g/l in case of co-digestion of sludge and other organic waste.
Concentration of either sludge or organic waste, or both, shall be performed by gravity or mechanical
thickening.
Additional pre-treatments improve anaerobic digestion performance leading to either an increase of
organic volumetric organic load or an increase of gas yield, or both. These pre-treatments are preferably
designed to enhance sludge hydrolysis which is the velocity limiting step of anaerobic digestion. Fine
screening of all fed substrates is generally recommended to remove coarse material, such as hygienic
and cosmetic products and plastic matter. Removal of sand and grit reduces abrasion and wear of
mechanical equipment and deposit formation in pipelines and channels, and accumulation of grit in
anaerobic digesters.
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Advantages and drawbacks for different types of treatment before anaerobic digestion are presented in
Table 1.
The full-scale estimations represented in Table 1 are average values which depends on the process
characteristics.
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Table 1 — Advantages and drawbacks for different types of treatment before anaerobic digestion
Pre-treatment technique Type of sludge Hydrolysis Biodegradation VS removal Dewaterability of Return load Biogas OPEX CAPEX Reference
rate (khyd) digestate (NH4-N) yied
Physical pre-treatment
Thermal < 100 °C, time
Mixed sludge + ± ± ± –- ± ++ ++ [44]
application < 24 h(pasteurization)
Mixed sludge [25];[26];[28];[29];[30];[34];
+ –
Thermal > 100 °C and ++ ++ ++ ++ ++ +++ [35];[36];[37];[38];
– polymer demand –- COD
excess sludge [39];[41]
Mechanical (pressure homogeniz-
±
[47];[45];[40];[46];[26];
er, ultrasonic and mechanical Mixed sludge +/++ + +/++ - + +/++ +/++
[33];[32]
– polymer demand
disintegration)
Chemical pre-treatment
Mixed sludge
-
Alkaline hydrolysis (industrial and ++ + ++ – ± ++ ± [7]
– polymer demand
municipal)
+ -
Oxidation (O ) Mixed sludge ++ + ++ + ++ ++ [45]
- polymer demand – COD
Biological pre-treatment
Enzymes Mixed sludge + ± ± + - ± +/++ ± [31];[43];[26]
Key
+ : improvement (+ low, ++ medium, +++ high)
- : degradation (- low, – medium, –- high)
±: no significative change
The signs (±) used in this table derive from full-scale anaerobic digestion data with or without pre-treatments.
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A new excess biological sludge treatment configuration has been developed which consists of an
ITHP. The potential benefits of this new configuration are volatile matter reduction, enhanced biogas
[27]
production and a significant increase of the dewatered digested sludge dryness .
4.3.2 Physical pre-treatment
4.3.2.1 Thermal hydrolysis
As hydrolysis is the rate-limiting process step, pre-hydrolysis of thickened raw sludge in a heated and
agitated blending and storage tank improve the overall process performance. Hydrolysis can also be
achieved through high-thermal pre-treatment (between 80 °C and 200 °C), with or without the addition
of chemicals (acidic or caustic). Degradation and digester gas production is increased and dewaterability
of the digestate is improved.
High thermal hydrolysis processes can employ direct steam injection into the sludge hydrolysis tank in
order to reach temperatures in the range of 140 °C to 170 °C (higher temperatures shall be avoided in
1)
order to limit production of refractory COD). Average pressure is around 4 bars to 6 bars and the usual
retention time under these conditions of temperature and pressure is 30 min. Heat recovery commonly
include two or three reactors: one for preheating and homogenization, one for reaction, and one for
cooling and heat recovery, respectively.
Hydrolysis can also be enhanced with a combination of thermal and alkaline processes. They require
often lower temperature and addition of chemicals. The temperature is usually around 70 °C and pH
varies from 10 to 12. In comparison to sole thermal hydrolysis alone, the operating temperature and
pressure are lower, which is the main interests of this configuration.
4.3.2.2 Sanitization
Pasteurization is the most common technology in case of added organic substrate which needs to
be sanitized (e.g. feedstock originating from animal waste). The feedstock is quickly heated at a
temperature of minimum 70 °C for a minimum of 1 h. In case of feedstock originating from animal waste,
the particle size should not exceed 12 mm to ensure that the whole matrix is properly hygienized. Heat
recovery is commonly included; usually three reactors are provided: one for heating, one for reaction
time and one for cooling and heat recovery.
4.3.2.3 Compression or decompression — Pressure homogenizer
The use of rapid vacuum conditions of up to 200 bars (e.g. 20 MPa) in a dedicated reactor (e.g. cavitation
chamber) to facilitate the destruction of microorganisms and polymers structures by combining large
pressure drop, turbulent eddies and shear forces, and make the sludge more feasible to the anaerobic
biodegradation.
4.3.2.4 Ultrasounds pre-treatment
Ultrasound treatment operates through cavitation phenomenon inducing local spots of high pressure
and temperature. The use of high intensity ultrasounds, at a frequency of approximately 20 kHz breaks
down cell walls, releasing additional structural polymers and intracellular organic contents and
intracellular organic content by fractionating the organic material to make it readily biodegradable. A
fraction of sludge, generally thickened or dewatered from 6 % to 12 % of dryness, is introduced into the
ultrasonic vessel, this fraction is usually around 30 % and affects mainly biological sludge. The other
interest of ultrasonic treatment is the destruction of filamentous bacteria in the upstream digester.
4.3.2.5 Mechanical disintegration
Mechanical pre-treatment is used to destroy cells of the biological sludge. The content of the cells
becomes easily degradable. Disintegration increases biogas generation. Common systems use,
1) 1 bar = 0,1 MPa = 105 Pa; 1 MPa = 1 N/mm .
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high pressure nozzles or milling. Only (ball) mills are systems for mechanical pre-treatment unless
decompression is also taken as a mechanical treatment.
4.3.3 Chemical pre-treatment
4.3.3.1 Oxidation by ozone or hydrogen peroxide
Chemical treatment with oxidation agents mainly aims on the destruction of unsaturated fatty
acids in the cell membrane of microorganisms. Solubilisation of organic complex matters including
microorganisms by strong oxidation is carried out in a separate and dedicated tank. Literature gives a
dose of ozone injected around 0,1 kg O /kg TS.
4.3.3.2 pH variation
Acid or alkaline chemicals are added to the sludge in a chemical conditioning tank in order to enhance
degradability of volatile solids. pH modification can be coupled with thermal or mechanical pre-
treatments (see 4.3.2). Alkaline chemicals are more commonly used. The hydrolysis rate increases
+ −
proportionally with the concentration of H or OH ions. It causes the cleavage of carbohydrates,
proteins and other biopolymers and thus dissolution of the cell wall.
4.3.4 Enzymatic hydrolysis
Selected enzymes are mixed with sludge in order to accelerate hydrolysis of slowly biodegradable
organic compounds into small compounds easy to convert into methane. Enzymes act as a catalyst of
hydrolysis reaction and are not consumed in the anaerobic digestion process but washed out with the
digestate without impact on environment. They are biologically degradable and therefore not harmful
to the environment. Main types of enzymes are cellulase, hemicellulase, protease, lipase and DNase.
Operating conditions are strictly dependant on the enzyme used. Usually the mixture of enzymes is
added either directly into the digester or via a pump into the feed line.
Enzymes can be in situ produced in a dedicated hydrolysis reactor with specific conditions of
temperature and pH [see 4.2 b)]. These exo-enzymes are produced by hydrolysing bacteria.
Temporary addition of enzymes may be useful if a digester is upset. Continuous addition of enzymes is
not economical because they are washed out with the digested sludge.
4.4 Digester
4.4.1 Shape
Sludge is usually digested in a more or less continuously fed and mixed reactor (CFMR) in one or more
steps. The main features of these continuous stirred tank reactors (CSTR) include the tanks, mixers,
covers and heating systems.
There are different types of anaerobic digesters and configurations. There are wide variations of
digester shapes, from shallow cylindrical digesters (so called pancake digesters) with or without
floating cover to egg-shaped digesters. Digester equipment, in particular their mixing systems, depends
on digester geometry.
There are single-stage and two-stage digester systems (e.g. mesophilic and mesophilic), the latter
offering a higher degradation rate per unit volume. There are additional processes used in combination
with anaerobic digestion, for example, disinfection, disintegration or hydrolysis processes (see
4.3). There are processes to remove phosphate from anaerobic digesters in the form of digesters
(magnesium-ammonium-phosphate) in order to avoid operational problems in the digester and
downstream equipment and to recover phosphorous. Operational requirements depend on the type of
digester, process configuration and coordination.
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Characteristics of the feedstock can influence decisions on the kind of system used, in particular solids
concentration.
There are additional operational considerations: anaerobic digestion at wastewater treatment plants
needs to be very reliable because raw sludge is generated continuously.
The common digester shapes include but are not limited to the following list:
a) Shallow, cylindrical, flat bottom tanks with a height to diameter ratio below 1:2: Such tanks are, for
example, common in the US. They can be equipped with a floating cover to retain the biogas, or a
fixed cover. Mixing is usually done with external recirculation (e.g. jet mixing), multiple draft tube
mixers or multiple injection point gas mixing. Removal of deposits is required every few years.
b) Cylindrical, flat bottom tanks with a height to diameter ratio of at least 1:1: Such tanks are common
in Europe, for example, in France and Germany. Digester mixing is usually achieved with mechanical
mixers or multiple point biogas injection.
c) Cylindrical tanks with a top dome and bottom cone with a ratio of cylinder height to diameter
of about 1:1: The top dome serves to concentrate scum on a small surface from where it can be
removed. Since fine screens have become common in preliminary works, this design is less
common in some countries. Digester mixing is done through external sludge recirculation (for
small digesters), a central mechanical draft tube mixer or peripheral biogas injection.
d) Egg-shaped digesters were developed in Germany and are used in other countries too: They are
used at medium to large plants. They are mixed with either a central mechanical draft tube mixer
or through peripheral gas mixing, or both.
4.4.2 Configurations
Conventional digesters are designed to have a SRT equal to the HRT. The extended SRT digestion process
which has greater SRT than HRT done by thickening the digestate and then recycling it back to the
digester. But that is not well adapted to highly concentrated sludge due to the difficulty to have adapted
thickening equipment and risk to lose anaerobic conditions. Anaerobic digestion can be designed as
either single, two stages or even multi-stages digesters.
Two-stage digestion is usually limited to plants equal or higher than a total population of 20 000
inhabitants. The two digesters can be operated under the same mode or the second one can be operated
as a gravity thickener and sludge returns from the bottom of the second digester to the first digester in
order to increase SRT over HRT.
Another configuration called two-phase digestion is designed and operated in order to separate and
optimize the main biological steps, for example, hydrolysis and partial acidogenesis occurring in the
first reactor and methanogenesis in the second. Main advantage is to apply optimal operating conditions
for each phase, for example, thermophilic condition for enhancing hydrolysis or increasing pathogen
inactivation in the first phase and mesophilic in the second phase. SRT and mixing conditions shall be
specific and adapted for each phase.
Volume of single digester is calculated to provide usually an HRT or SRT in the range of 16 d to 30 d. The
overall SRT or HRT of two-stage digestion is usually in the range of 15 d to 22 d and for the two-phase
process, volume can be reduced to 2 d or 4 d of HRT depending on the size of the plant and the quality
of the sludge for the first phase and around 10 d or 15 d for the second one. In case of mixture with
organic waste, the recommended TSH / TSS can be around 35 d to 45 d, even more (up to 60 d when
mixed with, for example, manure with high straw content or cereal residues).
The advantages and drawbacks of each common digester configuration is presented in Table 2.
oSIST prEN ISO 19388:2025
ISO 19388:2023(E)
Table 2 — Main advantages and drawbacks for three digester configurations
Advantages Drawbacks
low capital expenditures; easy to operate;
Single digester high HRT; big volume
low sensitivity to feedstock variations
great biological stability in the second stage;
Two-stage digesters
low SRT and HRT; great stability regarding
feedstock variations; decreased anaerobic high CAPEX
(primary followed by
digester total volume; increased VS degrada-
secondary digester)
tion and digester gas production
short SRT/HRT; great biological stability;
Two-phase digesters
great stability regarding feedstock varia- less easy to operate than the single
tions; possible sanitization in case the first digester; high CAPEX; large footprint;
(two distinct metabolic
phase is thermophilic with a certain mini- large risk zone around the digesters
pathways)
mum batch period
Digesters are usually made of concrete or steel. The choice of material depends on the size and the shape
of the digester. Above the minimum sludge level, concrete or steel shall be protected against corrosion.
Efficiency of digestion systems can be enhanced by additional pre-treatments (see 4.3).
4.4.3 Mixing system
4.4.3.1 General
Even if natural mixing occurs in anaerobic digesters thanks to gas bubble rising and thermal convection,
additional mixing is required. Mixing prevents stratification within digesters and it also ensures
uniform conditions (temperature, concentration of substrate, etc.) throughout the entire volume.
Adequate mixing permits treatment of raw sludge with a solids concentration of up to 10 %. The higher
the sludge concentration, the higher is its viscosity and the higher is the required mixing energy. In case
of mixing with biowaste or for dewatered sludge, the dryness at the feed can reach 20 % or slightly
more.
Digester mixing has the following objectives:
— fast and even distribution of the raw sludge (substrate) into the bulk of the digested sludge within
the digester;
— avoiding of concentration and temperature gradients within the digester;
— acce
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