ISO 21939-1:2019

A method to calculate and express energy consumption of industrial wastewater treatment for the purpose of water reuse - Part 1: Biological processes

This document sets out the general principles for, and provides guidance on, the quantitative characterization of the energy consumed by industrial biological wastewater treatment systems. It does not aim to characterize the treatment pollutants removal performance or process reliability or any other consideration in the selection of a wastewater treatment system. This document includes the following sub-systems of biological treatment system: - Biological reactors, which might be suspended growth or fixed film processes or a combination thereof, and can include anaerobic, anoxic and/or aerobic tanks and/or zones. - Solid-liquid separation processes such as sedimentation, flotation, or membrane filtration, used for clarification of the water before discharge to downstream processes, which can also involve the return of a the separated solids as sludge back to the biological reactor. - Any pumps, blowers and mixers for water circulation, mixing and air supply in and between the sub-systems listed herein. - Heating or cooling of the water for treatment. This document does not include the following subsystems of the biological treatment system: - Wastewater feed pumps. - Pre-treatment systems, which for the purposes of this document also include preliminary and primary treatment processes, such as but not limited to, screening, sedimentation, dissolved air flotation, chemical oxidation, oil separation. - Post-treatment processes, such as but not limited to, disinfection, desalination, ion exchange, sludge treatment and handling systems. - Site lighting or any energy consumption involved in office operation. - Energy recovery from processes such as anaerobic reactors producing biogas. Filtration processes, which are sometimes part of the biological treatment process and at other times part of the post treatment, are referred to separately within this document.

Titre manque — Partie 1: Titre manque

General Information

• Status: Published
• Publication Date: 23-May-2019
• ICS: 13.060.01 - Water quality in general
• ICS: 13.060.30 - Sewage water
• Technical Committee: ISO/TC 282/SC 4 - Industrial water reuse
• Drafting Committee: ISO/TC 282/SC 4 - Industrial water reuse
• Current Stage: 9093 - International Standard confirmed
• Start Date: 22-Apr-2025
• Completion Date: 13-Dec-2025

Overview

ISO 21939-1:2019 defines a standardized method to calculate and express energy consumption of industrial biological wastewater treatment for the purpose of water reuse. It provides general principles and guidance to quantitatively characterize energy used by biological treatment subsystems (reactors, solid–liquid separation, pumps, blowers, mixers, and heating/cooling). The standard aims to enable consistent comparison and specification of treatment energy performance - it does not cover pollutant removal performance, process reliability, pre‑ or post‑treatment subsystems, feed pumps, site lighting, or energy recovery (e.g., biogas).

Key Topics and Requirements

• Scope of included subsystems: suspended‑growth and fixed‑film biological reactors (anaerobic / anoxic / aerobic), clarifiers/DAF/membrane filtration for solid–liquid separation, circulation/return pumping, aeration/blowers/mixers, and thermal control of process water.
• Normalized Energy Consumption (NEC): the indicator is expressed as kWh per kilogram of net oxidizable mass removed (kWh/kg NOR).
• Net Oxidizable Mass Removed (NOR): NOR is quantified from constituent loads (inlet minus outlet) and includes:
  • COD removed (factor +1.0)
  • TKN removed (factor +1.71)
  • Nitrate and nitrite handled with specified positive/negative multipliers (nitrate ±2.86; nitrite ±1.71) depending on creation or removal
• Measurement & calculation principles:
  • Use average daily flow and representative concentration samples (refer to ISO 5667‑10 for sampling guidance).
  • Use actual average daily power consumption (kW) for equipment included.
  • Concentrations in g/m3 and flow in m3/d; all calculations in metric units.
• Component‑level accounting: the standard details how to allocate energy contribution from aeration, mixing, pumping, sludge recirculation, and solid–liquid separation, and discusses adjustments for temperature, barometric pressure and wastewater type.

Applications and Users

ISO 21939-1:2019 is useful for:

• Plant designers and process engineers specifying energy-performance in tender documents.
• Operators and facility managers benchmarking wastewater treatment energy consumption.
• Consultants and auditors performing life‑cycle cost and energy-efficiency comparisons of biological treatment options.
• Regulators and project owners evaluating energy impacts for industrial water reuse projects.

Practical uses include energy benchmarking, system selection (when comparing energy demand of alternatives), optimizing aeration/mixing strategies, and standardizing reporting in water reuse programmes.

Related Standards

• ISO 20670 - Water reuse - Vocabulary (referenced for terms)
• ISO 5667‑10 - Water quality sampling guidance (referenced for representative sampling)

Keywords: ISO 21939-1:2019, energy consumption, industrial wastewater treatment, biological processes, normalized energy consumption, NOR, COD, TKN, aeration, membrane bioreactor, water reuse.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 21939-1
First edition
2019-05
A method to calculate and express
energy consumption of industrial
wastewater treatment for the purpose
of water reuse —
Part 1:
Biological processes
Reference number
©
ISO 2019
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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ii © ISO 2019 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions, symbols and abbreviated terms . 2
3.1 Terms and definitions . 2
3.2 Symbols and abbreviated terms. 2
4 Expression and normalization of energy consumption . 2
4.1 Energy consumption indicator . . 2
4.2 Calculation . 4
5 Components contribution to the energy consumption . 4
5.1 General . 4
5.1.1 Steady state conditions . 4
5.1.2 Consideration of flow rate . 5
5.1.3 Calculation of variable consumptions . 5
5.1.4 Accounting for addition of oxidizing chemicals . 5
5.1.5 Physico-chemical separation pre-treatment processes . 5
5.2 List of system components . 6
5.2.1 General. 6
5.2.2 Aeration and combined mixing and aeration equipment . 6
5.2.3 Mixing . 6
5.2.4 Solid-liquid separation . 6
5.2.5 Internal circulation pumping . 6
5.2.6 Circulation of settled sludge . 6
5.2.7 Other pumping. 6
5.2.8 Air driven pumping and/or mixing . 6
6 Factoring of different process conditions . 7
6.1 General . 7
6.2 Temperature adjustments . 7
6.3 Barometric pressure adjustment . 7
6.4 Type of wastewater . 7
Annex A (informative) References to formulae and calculations . 8
Annex B (informative) Example calculations. 9
Bibliography .12
Foreword
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electrotechnical standardization.
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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
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.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 282, Water reuse, Subcommittee SC 4,
Industrial water reuse.
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 © ISO 2019 – All rights reserved

Introduction
Biological wastewater treatment is a major process step in many cases of industrial wastewater
treatment for reuse.
In the field of biological wastewater treatment, the energy consumption of the process is commonly
normalized in order to account for difference in flow and concentration when comparing different
treatment options. For example, energy consumption is reported per unit volume of wastewater treated
(e.g. kWh/m ) or per unit of pollutant removed BOD (e.g. kWh/kg BOD ), TN (e.g. kWh/kg TN).
5 5
The disadvantages and limitations of these indicators include:
— Neither of the existing indicators are agreed to and accepted as a standard in the industry;
— They do not provide means to compare nitrifying processes to non-nitrifying processes;
— There is no differentiation between design values and measured values, and there are no standard
methods to obtain the indicator for each case.
Another approach is to report on the results from standard tests in water as oxygen transfer capacity
per unit energy consumed (e.g. kg O /kWh).
The disadvantages and limitations of this type of indicator include:
— It is less convenient for the market to apply to its cases, (for example because not all removal is by
oxidation, some is by metabolic consumption);
— It only considers oxygen transfer equipment or processes able to dissolve oxygen in water, which
does not apply to all biological treatment processes (such as rotating biological contactor (RBC) and
trickling filter (TF)).
These issues are discussed in the literature, which compares results expressed in different ways and
correspondingly provides contradicting indications (see References [4], [5] and [10]).
Therefore this document aims to create a quantitative measure for universal characterization of the
energy consumption for aerobic biological wastewater treatment systems. Such standardization will
benefit engineers in specification of systems and comparison of systems. The need arises especially in
consideration of life cycle cost of a wastewater treatment system.
NOTE 1 Normalized energy consumption of wastewater treatment systems typically decreases with
increasing size of the system, mainly due to higher efficiency of larger electromechanical equipment such as
pumps and blowers. Thus, large wastewater treatment plants will have lower normalized energy consumption
than small wastewater treatment systems. In order to neutralize the influence of plant size in comparison of
energy consumption, a correlation such as published by Silva, C., et al. (see Reference [11]) can be used, regardless
of this document.
NOTE 2 Higher effluent quality requirements are generally associated with higher normalized energy
consumption. Typically, normalized energy consumption will be compared for similarly performing systems.
However, it is possible, for example, to present the normalized energy consumption for different treatment
qualities as such.
NOTE 3 This document quantifies the energy consumption, regardless of treatment efficiency that can vary
for different types of wastewater. Treatment efficiency might typically be characterized by retention time and/or
volumetric loading rate and/or volumetric removal rate or other indicators which are not part of this document.
INTERNATIONAL STANDARD ISO 21939-1:2019(E)
A method to calculate and express energy consumption
of industrial wastewater treatment for the purpose of
water reuse —
Part 1:
Biological processes
1 Scope
This document sets out the general principles for, and provides guidance on, the quantitative
characterization of the energy consumed by industrial biological wastewater treatment systems. It
does not aim to characterize the treatment pollutants removal performance or process reliability or
any other consideration in the selection of a wastewater treatment system.
This document includes the following sub-systems of biological treatment system:
— Biological reactors, which might be suspended growth or fixed film processes or a combination
thereof, and can include anaerobic, anoxic and/or aerobic tanks and/or zones.
— Solid-liquid separation processes such as sedimentation, flotation, or membrane filtration, used for
clarification of the water before discharge to downstream processes, which can also involve the
return of a the separated solids as sludge back to the biological reactor.
— Any pumps, blowers and mixers for water circulation, mixing and air supply in and between the
sub-systems listed herein.
— Heating or cooling of the water for treatment.
This document does not include the following subsystems of the biological treatment system:
— Wastewater feed pumps.
— Pre-treatment systems, which for the purposes of this document also include preliminary and
primary treatment processes, such as but not limited to, screening, sedimentation, dissolved air
flotation, chemical oxidation, oil separation.
— Post-treatment processes, such as but not limited to, disinfection, desalination, ion exchange, sludge
treatment and handling systems.
— Site lighting or any energy consumption involved in office operation.
— Energy recovery from processes such as anaerobic reactors producing biogas.
Filtration processes, which are sometimes part of the biological treatment process and at other times
part of the post treatment, are referred to separately within this document.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 20670, Water reuse — Vocabulary
3 Terms and definitions, symbols and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 20670 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.2 Symbols and abbreviated terms
A O anaerobic-anoxic-oxic wastewater treatment process
Bardenpho a process for biological nutrient removal developed by James Barnard of South Africa
in the 1970’s
BOD bio-chemical oxygen demand (5 days) concentration or load, mg/l or kg/d, respectively
COD chemical oxygen demand concentration or load, mg/l or kg/d, respectively
DAF dissolved air flotation
DO dissolved oxygen
DGF dissolved gas flotation
MBR membrane bio-reactor
MLE modified Ludzack Ettinger process of wastewater treatment
NEC normalized energy consumption, kWh/kg
NO -N net oxidizable mass removed
TN total nitrogen concentration or load, mg/l or kg/d, respectively
UCT University of Cape Town process of wastewater treatment
VIP Virginia Initiative Plant process of wastewater treatment
VFD variable frequency drive
RBC rotating biological contactor
TF trickling filter
TKN total Kjeldahl nitrogen concentration or load, mg/l or kg/d, respectively
VSS volatile suspended solids
4 Expression and normalization of energy consumption
4.1 Energy consumption indicator
Following are details and explanations of values needed for later calculations in 4.2.
2 © ISO 2019 – All rights reserved

4.1.1 Normalized energy consumption shall be expressed in terms of energy per net oxidizable mass
removed.
4.1.2 The net oxidizable mass removed (NOR) shall comprise of
— COD removed,
— TKN removed,
— Nitrate created (or subtraction of nitrate removed).
4.1.3 Constituents included in the indicator shall be factored for their oxygen demand value. Specifically
the following factoring principles shall be applied, based on the references indicated in Annex A and in
References [1], [2] and [5].
— COD removed shall be multiplied by +1,0.
— In design, COD concentration in the sludge shall be calculated on the basis of VSS concentration
multiplied by 1,42 (References [8], [12], Annex A); the VSS concentration in the sludge, taken for
calculations in this document, shall be the same as the value for VSS concentration in the sludge
taken in process design. Process design values for sludge composition can be calculated according
to conventional design procedures (Reference [8], Annex A).
— TKN removed shall be multiplied by +1,71 (corresponding to oxidation to nitrogen gas regardless of
the chemical pathway).
NOTE The fraction of TKN which was not reduced to zero-valence nitrogen is accounted for as nitrate
and nitrite, with the multipliers as specified below.
— In design, TKN concentration in the sludge shall be calculated as 12 % of the VSS, on the basis
described above for COD calculation
...