ISO 23138:2024

Biological equipment for treating air and other gases - General requirements

This document specifies the technology of biological exhaust air purification. The relevant requirements for a possible application are specified. The different variants of this technique are also presented. NOTE The process principles of this method are described in Clause 4.

Équipements biologiques pour le traitement de l'air et autres gaz — Exigences générales

General Information

• Status: Published
• Publication Date: 09-Jul-2024
• ICS: 91.140.30 - Ventilation and air-conditioning systems
• Technical Committee: ISO/TC 142 - Cleaning equipment for air and other gases
• Drafting Committee: ISO/TC 142/WG 13 - Biological equipment for waste gas treatment
• Current Stage: 6060 - International Standard published
• Start Date: 10-Jul-2024
• Due Date: 22-Jul-2024
• Completion Date: 10-Jul-2024

Relations

• Consolidated By: ISO 18397:2025 - Dentistry - Powered scalers
• Effective Date: 06-Jun-2022

Overview

ISO 23138:2024 - Biological equipment for treating air and other gases - General requirements specifies the technology, terminology and application requirements for biological exhaust‑air purification systems. The standard describes process principles (Clause 4), defines key terms (biofilter, biotrickling filter, bioscrubber, waste gas, elimination capacity, residence time, etc.), and presents the different technique variants and relevant requirements for their practical use.

Keywords: ISO 23138:2024, biological exhaust air purification, biofilter, biotrickling filter, bioscrubber, VOC treatment, waste gas cleaning, air purification standards.

Key topics and technical requirements

• Process principles (Clause 4):
  • Two‑step pollutant removal: physical absorption/mass transfer followed by biochemical conversion by microorganisms.
  • Mass transfer mechanisms (gas–liquid interface, absorption, diffusion) and reaction at microbial cells.
  • Requirements for successful biodegradation: good biodegradability, water solubility, suitable temperature range (mesophilic), and absence of toxic compounds.
• Technique classification and design parameters:
  • Descriptions and distinctions among biofilters, biotrickling filters, and bioscrubbers.
  • Design considerations such as empty bed residence time, pressure drop, support media, moisture content and water retention capacity.
• Component and operational requirements:
  • Filter media characteristics, nutrient and nutrient‑salt management (C:N:P considerations), moisture reservoirs, irrigation and clogging control.
  • Regeneration, replacement and safe disposal of filter media.
• Performance metrics and testing (Clause 6):
  • Definitions of efficiency (removal efficiency), elimination capacity and pollutant concentration metrics.
  • Guidance on measurement and testing to verify performance.
• Terminology and references: clear definitions to ensure consistent specification, procurement and commissioning.

Practical applications and users

ISO 23138:2024 is intended for organizations and professionals involved in design, selection, installation, operation and regulation of biological air‑treatment systems:

• Environmental and process engineers designing biofilters, biotrickling filters and bioscrubbers.
• Manufacturers of biological air‑cleaning equipment and media suppliers.
• Plant operators and maintenance teams at wastewater treatment plants, composting and waste treatment facilities, anaerobic digestion plants, industrial sites and paint shops.
• Environmental consultants and authorities assessing emissions control strategies for odours and volatile organic compounds (VOC). Practical use cases include odour control (H2S, NH3, sulfides), VOC removal from industrial exhausts and low‑energy, near‑CO‑neutral air cleaning solutions.

Related standards

• The document builds on established German VDI guidance (VDI 3477, VDI 3478 parts) and references ISO terminology resources (e.g., ISO Online Browsing Platform). See ISO/TC 142 (Cleaning equipment for air and other gases) for committee context.

For procurement or compliance, reference ISO 23138:2024 when specifying biological exhaust‑air treatment systems to ensure consistent terminology, validated design parameters and documented testing methods.

Standards Content (Sample)

International
Standard
ISO 23138
First edition
Biological equipment for treating
2024-07
air and other gases — General
requirements
Équipements biologiques pour le traitement de l'air et autres
gaz — Exigences générales
Reference number
© ISO 2024
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Process principles . 4
4.1 General fundamentals .4
4.2 Steps involved in pollutant elimination .4
4.2.1 General .4
4.2.2 Mass transfer .4
4.2.3 Biochemical conversion by microorganisms (enzyme catalysed reaction) .4
4.3 Classification of techniques .7
4.4 Design parameter .8
4.5 Biofilter .8
4.5.1 General description . .8
4.5.2 General description of the procedure .9
4.5.3 Filter media .10
4.5.4 Nutrient and nutrient salt source .10
4.5.5 Moisture reservoir .11
4.5.6 Regeneration, replacement and disposal of filter media .11
4.6 Biotrickling filter .11
4.6.1 General description . .11
4.6.2 General description of the procedure . 12
4.6.3 Filter media . 13
4.6.4 Clogging in biotrickling filters for the purification of organic compounds (VOC) . 13
4.6.5 Nutrients and irrigation .14
4.6.6 Regeneration, replacement and disposal of filter media .14
4.7 Bioscrubber .14
4.7.1 General description . .14
4.7.2 General description of the procedure . 15
4.7.3 Nutrients and irrigation .16
5 Application .16
6 Measurement and testing .18
Bibliography . 19

iii
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 through
ISO technical committees. Each member body interested in a subject for which a technical committee
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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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
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This document was prepared by Technical Committee ISO/TC 142, Cleaning equipment for air and other gases.
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.

iv
Introduction
The biological exhaust air purification has experienced a very rapid spread in recent years with very positive
effects in various applications.
The most important advantage is the fact that the cleaning process is natural and carried out by
microorganisms. It is currently by far the most environmentally friendly exhaust air purification technology.
The main advantages of this technology are as follows:
— it is a natural process at ambient pressure and ambient temperature;
— the principle is comparable with the wastewater treatment technology which is also well-established
for years;
— there is no need of additional energy in the form of natural gas or oil;
— it is a nearly CO neutral air cleaning technology;
— it has low operation costs;
— it has low investment costs.
Long lasting experiences have shown that biological systems especially can be useful for the treatment of:
— odorous air from waste water treatment plants (e.g. H S, sulfides);
— odorous air from waste treatment plants as composting plants, anaerobic digestion plants (e.g. H S, NH ,
2 3
organic compounds);
— odorous air from industrial processes;
[9]
— waste air from paint houses and other industrial processes containing volatile organic compounds (VOC) .
[1]
Some parts of this document are based on the German Standards VDI 3477 (Biofilter, first published
[2] [3]
in 1984), VDI 3478-1 (Bioscrubber, first published in 1985) and VDI 3478-2 (Biotrickling filter, first
published in 1985). (With permission of the Association of German Engineers VDI).

v
International Standard ISO 23138:2024(en)
Biological equipment for treating air and other gases —
General requirements
1 Scope
This document specifies the technology of biological exhaust air purification. The relevant requirements for
a possible application are specified. The different variants of this technique are also presented.
NOTE The process principles of this method are described in Clause 4.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
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
biofilter
bioreactor treating waste gas with the aid of biofilm attached to the packing media which moisture is
maintained by a prepositive humidifier or intermittent water feeding to the filter bed
Note 1 to entry: Organic materials are usually used as carrier materials. However, inorganic materials with a large
inner surface area and corresponding microorganism population are also used. The materials used are usually
arranged as bulk layers through which the exhaust gases flow.
[SOURCE: ISO 29464:2024, 3.5.10, modified — Note to entry has been added.]
3.2
biotrickling filter
bioreactor treating waste gas with free moving liquid layers on the surface of inert packing media to supply
nutrients, take away metabolites or control pH for the biofilm attached to the packing media
3.3
bioscrubber
absorber transferring contaminants from waste gas to liquid absorbent, and removing the dissolved
contaminants by suspended-growth microorganisms in a supplementary space
3.4
waste gas
odorant and pollutant-laden gas streams from industrial and agricultural processes and exhaust ventilation
streams from tanks and rooms unsuited for being permanently occupied by humans
3.5
acclimation
adaptation of microorganisms to the substrate volume and composition as well as other environmental factors

3.6
absorbent
liquid suitable for collecting gas components
3.7
absorber
device in which specific substances are absorbed into an absorption liquid
3.8
absorption
selective separation of one or more components from gas mixtures by scrubbing with a scrubbing medium
(typically water)
Note 1 to entry: A distinction is made between physical absorption, for the assessment of which the physical equilibrium
curve is used as a basis, and chemical or biochemical absorption, during which the absorptive and absorbent enter a
chemical reaction with each other and substances are converted.
3.9
absorptive
substance destined for absorption
3.10
microbial activity
biological conversion/elimination of waste gas components per unit time
3.11
support media
slatted floor or grating with sufficiently small openings to support a bed of solid particles (e.g. filter media)
3.12
C:N:P ratio
carbon :nitrogen :phosphorus ratio
ratio of biologically available carbon in the exhaust air to nitrogen and phosphorus in the filter media
3.13
pressure drop
Δp
difference between the static pressures at the inlet and outlet of a bounded flow system which is used as a
measure for the energy loss caused by the flow within the system
Note 1 to entry: The pressure drop across a biofilter system is the total of the flow resistances offered by the piping,
[4]
dampers, bed of media, etc .
Note 2 to entry: Endotoxins are released on lysis or death of the bacterial cells and can be, due to its low vapour
pressure, inhaled after aerosol formation only.
3.14
moisture content
mass fraction of water related to the moist mass of the filter media
Note 1 to entry: This definition is different from that of the European odour unit, in that only the latter is traceable to
[5]
a known odorant mass, defined as the EROM .
3.15
nutrient salt
2+ 2+ + +
nitrogen- and phosphorus-containing salt of inorganic ions such as Ca , Mg , Na , K which is required in
major amounts to maintain the cell function

3.16
relative humidity
U
ratio of the partial water vapour pressure to the saturation water vapour pressure at a given temperature:
p
D
U=
p
DS
T
3.17
pollutant concentration
ratio of pollutant mass to the waste gas volume at standard temperature and pressure conditions (0 °C,
1 013 hPa), dry basis
Note 1 to entry: The unit ppm (parts per million) (volume fraction or mass fraction) denotes the volume or mass
fraction.
3.18
sorption
process by which a substance is selectively sorbed on or attached to another substance with which it is
contacted
3.19
service life
duration over which the filter media retains its function at a sufficient efficiency
3.20
substrate
substances on which the microorganisms both feed and colonise, and which are suitable for synthesising
cell mass or supplying energy for metabolic action
3.21
residence time
empty bed residence time
ratio of the filter volume to the volumetric flow of the fluid
3.22
water retention capacity
water storage capacity
maximum mass concentration of water that can be retained by the filter media for prolonged periods,
expressed as per cent moist filter media mass
3.23
efficiency
removal efficiency, η
difference between the biofilter inlet and outlet concentrations of one or several defined waste gas
constituents related to their concentrations in the raw gas:
G G
ρρ−
crude clean
η=
G
ρ
crude
3.24
elimination capacity
Ev
3 -3 -1
microbially converted VOC freight per m of packing or aeration tank volume in g C m h .
G G G
ρρ− ηρ⋅
crude clean G crude G

Ev= ⋅=V ⋅V
V V
filter filter
4 Process principles
4.1 General fundamentals
Biological air treatment systems are particularly suited to all waste gas cleaning applications involving air
[10],[11]
pollutants that are readily biodegradable . Biodegradation of the air pollutants is accomplished under
aerobic conditions by microorganisms colonizing on solid support media or existing as activated sludge in
washing liquid.
The requirements for a possible application of biological air treatment techniques are:
— Good biodegradability of the pollutants so that the biochemical conversion will take in a few seconds.
Otherwise, the residence time in the filter chamber or in the scrubber would be too long and this would
lead to very large filter volumes. If the degradation rate of the substances is not known pilot tests are
recommended.
— Good water solubility of the pollutants, as nearly all biochemical reactions take place in an aqueous
environment.
— A temperature in the mesophilic range (under special conditions also in thermophilic range possible).
— Absence of toxic compounds.
4.2 Steps involved in pollutant elimination
4.2.1 General
Biological waste gas purification is based on two steps.
A physical absorption followed by biochemical conversion of pollutants by microorganisms (enzyme
catalysed reactions).
The bacteria are mainly suspended in water (bioscrubbers) or fixed on supporting elements
(biotricklingfilters, biofilters).
4.2.2 Mass transfer
Mass transfer from the gas phase to the liquid phase (absorption and diffusion) is described by the simplified
[13]
two-film theory .
In detail, the following steps take place:
— mass transfer to the gas/liquid interface;
— absorption into the liquid phase;
— mass transfer through the liquid phase to the bacterial cell;
— sorption and degradation by the cell.
The driving force is the concentration gradient between the pollutant concentration in the gas phase and the
concentration in the liquid phase.
4.2.3 Biochemical conversion by microorganisms (enzyme catalysed reaction)
As in all biological purification processes (waste water/waste air cleaning, soil remediation), the substance
being cleaned is biochemically processed by the microorganisms after absorption in the microorganism cell
and gradually converted to yield energy for the cell and biomass. Waste air purification so far has always
been an aerobic process, i.e. the substances are broken down by oxidation – ideally to yield the end products
of complete oxidation, CO and H O as well as sulphate and nitrate in the case of sulphur and nitrogen
2 2
components in the raw gas.
As the waste gas passes through the filter bed or package material, the pollutants are sorbed onto the surface
of the filter media where they are degraded by microorganisms in the liquid film surrounding the media.
The microorganisms involved are usually ubiquitous, i.e. microorganisms occurring everywhere in the
environment. It is recommended to further increase the biodiversity of the microbial community by adding
biomass, e.g. from sewage sludge or a soil slurry. The degradation of waste gas is dominated by a group of
bacteria. All involved microorganisms have the capability to adapt to major changes in the nutrient supply,
although sometimes this can take some time. Organic carbon compounds, for instance, are mineralized to
innocuous products, ideally to carbon dioxide and water, by bio-oxidation with air oxygen. Normally, many
different microorganism species are involved in the biodegradation processes.
Factors governing the rate of reaction (degradation rate) include:
— solubility of the waste gas components;
— biodegradability of the waste gas components;
— concentration and structure of waste gas components;
— type, number and activity of the microorganisms colonizing on the respective filter media;
— temperature;
— pH-value;
— moisture content of the waste gas and the filter media;
— type and concentration of reaction products accumulating in the filter media.
An increase in the temperature initially accelerates the enzymatic and non-enzymatic reactions (RRT rule).
If the temperature is further increased, the rate of acceleration slows down. At elevated temperatures,
the enzymes start to denature and alter their shape, preventing them to perform their function. This
counteracting effect reduces the overall rate of reaction, resulting in the typical profile of enzyme-catalysed
reactions with a temperature optimum (see Figure 1).
Key
X temperature (°C) 2 overall rate
Y rate of reaction 3 heat denaturation of enzymes
1 RRT function 4 optimum temperature range 20 °C to 35 °C
[1]
Figure 1 — Dependence of the reaction on the temperature
A changed substrate supply leads to changes in the composition of the microbial population without affecting
the performance of the system. An important factor for a biological degradation is a quite constant pH value
between 5,5 and 9. For special applications, for example, hydrogen sulphide elimination, a pH range from

2 to 3 is applied. The optimum range for a stable biological mass transfer shall be defined when designing
the system. In the event of deviations from the design pH values, the activity of the microorganisms can
decrease.
Sufficient water supply is essential to the function of the biodegradation processes. Moreover, the presence
of water as a mass transfer agent is a basic prerequisite for all biodegradation processes. For this reason, the
water balance/mass transfer in the filter media is of key importance.
The preferred operating temperature range of biofilters is the so-called mesophilic temperature range
(roughly 20 °C to 35 °C).
The processes involved in the microbial degradation of organic carbon compounds occur in the neutral
to weakly acidic pH range. The effect of the end and intermediate products of biodegradation is normally
buffered by the filter media.
Biological systems also rely on a balanced nutrient supply for maximum performance. As well as carbon,
microorganisms require nitrogen, phosphate, sulphur and trace elements for their metabolism.
Consistent optimum performance of the microorganisms is only ensured if the environmental conditions
in the filter bed in terms of the above factors are controlled within narrow limits. As microorganisms are
affected by changes in their environment, they can require some time for acclimation before they develop
their full activity after biofilter startup or changes in the operating conditions.
The net conversion of pollutants in the filter bed is determined by the rate of reaction, the empty bed
residence time of the gas in the biofilter and the concentration of the pollutant in the raw gas. If the biological
reactions proceed relatively fast and the pollutants to be removed are sparingly soluble, transport processes
of the reactants from the gas phase to the inner surface of the filter media can become rate-limiting.
The rate of the biological reactions and the reactant transport processes are temperature-dependent. In the
temperature range of approximately 5 °C to approximately 40 °C, the rate of reaction generally increases
with rising temperature. Above the so-called optimum temperature, however, it can decline sharply due to
enzyme inactivation.
There are also microorganisms that thrive at higher temperatures (e.g. thermophilic bacteria). Such systems
shall be carefully designed. Switching between mesophilic and thermophilic conditions should be avoided
by all means.
Factors influencing the residence time are the volumetric gas flow, the bulk volume and the void volume
of the filter bed. Because physical and chemical data such as kinetic constants and effective diffusion
coefficients are only available for very few combinations of filter media and pollutants, biofilter systems are
sized and designed on the basis of pilot tests run in adequately sized pilot plants.
Excessive pollutant mass flow rates and/or oxygen shortage lead to incomplete pollutant degradation and
the accumulation of intermediates (e.g. formation of acids on conversion of alcohols or aldehydes).
The pilot tests should be run for a sufficient period of time to make sure that the waste gas does not contain
any substances likely to inhibit the microorganisms, e.g. SO . Depending on the application, such substances
shall be removed from the inlet gas stream, for instance, in a separate upstream treatment stage, before it
enters the biological waste gas treatment. The same applies to aerosols, dust and fats which tend to plug or
foul the filter material.
Supplemental nutrient addition during shutdown periods - even in the case of prolonged shutdowns - is not
required as long as the necessary nutrients can be obtained by the microorganisms from the support media.
Inversely, proper attention shall be given to the C:N:P ratio in the case of high carbon loads of the waste gas
to be treated and/or low-nutrient content of the filter media.
The microorganisms used for waste gas cleaning are so-called organo-heterotrophic microorganisms.
Concerning the biochemical reaction, it can be assumed that depending on the type of compounds the
degradation rate can be described either by a zero order reaction or a first order reaction.

4.3 Classification of techniques
Depending on different operation principles, there are currently three different types of biological air
treatment systems (see Figure 2). The choice of the plant type depends on several parameters such as the
type of the pollutants, the water solubility and degradation rate of the substances, space requirement, etc.
Key
1 bioscrubber 23 package material (mainly plastic)
2 biotrickling filter 24 package material (mainly plastic)
3 biofilter 25 circulation liquid
4 clean gas 26 package material (mainly organic)
5 height of the package material 1,0 m to 1,5 m 27 clean water
6 crude gas 28 nutrients
7 height of the package material 1,5 m to 1,8 m 29 acid/alcaline
8 effluent 30 additives (optional)
9 pump 31 wet
10 circulation pump 32 humidity
11 aeration 33 dry
12 loaded scrubber liquid 34 polar

13 swamp 35 VOC solubility
14 absorber unit 36 lipophilic
15 regeneration unit 37 suspended
16 combined absorber and regeneration unit 38 biomass fixation
17 pre-humidifier 39 biofilm
18 droplet separator injection nozzle 40 high
19 package material (plastic) 41 wastewater formation
20 injection nozzle 42 low
21 package material (plastic) 43 additive demand
22 regenerated scrubber liquid 44 risk of clogging
Figure 2 — Types of biological air treatment systems
The main distinguishing feature is whether the biodegradation is performed by suspended microorganisms
(bioscrubber) or ones fixed to surfaces (biofilters and biotrickling filters).
The two latter techniques differ by either permanent sprinkling in the case of biotrickling filters or
occasional sprinkling in the case of biofilters.
In the following, the three different types are more precisely described with their advantages and
disadvantages.
4.4 Design parameter
The VOC concentration shall be reduced by biological processes to at least national emission limits. In the
case of VOC elimination, the design of the plants is usually based on the elimination capacity and is in the
-3 -1 -3 -1 -3 -1
range of 10 g C m h up to 100 g C m h in the case of biotrickling filter and biofilter, and 20 g C m h up
-3 -1
to 200 g C m h in the case of bioscrubbers.
4.5 Biofilter
4.5.1 General description
[8],[27]
This technology, which is well known since more than 40 years, is the most widespread in Europe . It
uses solid and biologically active filter material based on organic substances.
As the waste gas passes through the filter bed, the pollutants are sorbed onto the surface of the filter media
where they are degraded by microorganisms in the liquid film surrounding the media.
The microorganisms involved are usually ubiquitous, i.e. microorganisms occurring everywhere in the
environment. The main representatives belong to different genera of bacteria interacting in a complex way
with each other forming a so-called biocenosis.
A changed substrate supply can lead to changes in the composition of the microbial population without
affecting the performance of the system.
The biofilter is usually combined with an upstream scrubber which is often a bioscrubber (see Figure 3) or a
[28]
chemical scrubber .
Biofilters can be constructed in open or closed design. Open design is commonly used for odour treatment
[6] [1],[7]
applications, e.g. air treatment at wastewater treatment plants, composting plants and agriculture .
In recent years, closed biofilters (see Figure 5) have proven to be more advantageous. Weather effects are
avoided and finer filter material with larger specific surface and higher microbial activity can be used.
Furthermore, the exhaust air can be collected and discharged via a chimney. This is the reason why they are
commonly used in industrial applications.

Key
1 optional nutrients 6 humid gas
2 fresh water 7 wastewater
3 exhaust gas 8 clean gas
4 optional acid M motor
5 control unit
Figure 3 — Scheme of a closed biofilter
4.5.2 General description of the procedure
To describe the flow of gases through porous media, some simplified assumptions shall be made.
First, the gas flow through the media is assumed to be steady-state and isothermal. This means that the flow
velocity at any single point of each streamline in the filter does not vary with time.
For a continuously operating biofilter, this assumption provides
...