ISO 9784:2024

Guidelines for biological filtration of secondary effluent for water reuse

This document provides guidance for biological filtration of secondary effluent for water reuse. It specifies general recommendations, process components and technical recommendations of each component, post-treatment process and water reuse applications. This document is applicable to all types of stakeholders involved in implementing biological filtration for advanced treatment of secondary effluent for water reuse.

Lignes directrices relatives à la filtration biologique de l'effluent secondaire pour la réutilisation de l'eau

General Information

• Status: Published
• Publication Date: 27-Oct-2024
• ICS: 13.060.01 - Water quality in general
• Technical Committee: ISO/TC 282/SC 2 - Water reuse in urban areas
• Drafting Committee: ISO/TC 282/SC 2 - Water reuse in urban areas
• Current Stage: 6060 - International Standard published
• Start Date: 28-Oct-2024
• Due Date: 03-Nov-2024
• Completion Date: 28-Oct-2024

Overview

ISO 9784:2024 - Guidelines for biological filtration of secondary effluent for water reuse - provides practical, stakeholder-oriented guidance for using biological filtration to upgrade secondary effluent for reuse. The standard covers general recommendations, process components, technical requirements for each component (biofilter, mixing, backwashing, control), post‑treatment, and water‑reuse applications. It supports the “fit for purpose” approach to reusing treated municipal and similar industrial effluents for nonpotable and industrial uses.

Key technical topics and requirements

• Process components
  • Biofilter unit (BAF / DNF): configuration options (upflow/downflow), influent distribution, graded gravel layer, filter bed, aeration and backwash drainage, outlet and control systems.
  • Mixing unit: dosing and mixing of external carbon sources where heterotrophic denitrification is required.
  • Backwashing unit: pumps and blowers; recommendations on backwash mode and equipment sizing.
  • Controlling unit: on‑site PLC and central control room architectures for centralized or decentralized control.
• Filter media: acceptable materials include graded mineral media (sand, gravel), granular activated carbon, ceramsite, foamed ceramics, engineered plastics, and particulate media containing reduced sulfur/iron for autotrophic or combined denitrification.
• Treatment targets and terminology
  • Removal of organics (carbon), ammonium (NH4+-N), nitrate and total nitrogen via combined BAF and denitrification filter (DNF) configurations.
  • Performance metrics referenced include backwash rate (commonly expressed as m3/(m2·h)) and volumetric nitrogen loading rate (kg N/(m3·d)).
• Monitoring and control
  • Instrumentation, monitoring systems and control logic to ensure stable biological performance, manage backwashing cycles, and protect environmental and operational safety.
• Annexes
  • Annex A: example structure diagrams for BAF and DNF.
  • Annex B: case studies of reuse applications.
  • Annex C: information on autotrophic denitrification.

Practical applications and users

ISO 9784:2024 is intended for:

• Municipal wastewater utilities and operators upgrading secondary effluent for urban reuse (irrigation, cooling, nonpotable supply).
• Water reuse planners, process designers and engineering consultants specifying BAF/DNF systems.
• Equipment manufacturers (media, aeration, backwash, control systems).
• Regulators and asset owners setting technical guidance for advanced treatment trains.

The standard helps select appropriate configurations (single/two‑stage BAF, DNF), choose filter media, size backwash and aeration equipment, and integrate monitoring and post‑treatment to meet reuse objectives.

Related standards

• This document contains no normative references. Terminology draws on ISO 6107:2021 and ISO 6707‑1:2020. For implementation, consult local water reuse regulations and applicable national standards.

Standards Content (Sample)

International
Standard
ISO 9784
First edition
Guidelines for biological filtration of
2024-10
secondary effluent for water reuse
Lignes directrices relatives à la filtration biologique de l'effluent
secondaire pour la réutilisation de l'eau
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 General . 2
4.1 Overall recommendations .2
4.2 Basic process .3
5 Process components. 4
5.1 Biofilter unit .4
5.2 Mixing unit .4
5.3 Backwashing unit .4
5.4 Controlling unit .4
6 Technical recommendations of the biofilter unit . 4
6.1 Filter media .4
6.2 Water and air distributor zone .5
6.3 Configuration of the biofilter unit .5
7 Technical recommendations of the mixing unit . 6
8 Technical recommendations of the backwashing unit . 6
8.1 Backwashing mode .6
8.2 Equipment .7
9 Technical recommendations of the controlling unit . 7
9.1 M onitoring instrumentation .7
9.2 M onitoring system .7
9.3 Control system .7
10 Post-treatment process . 7
11 Water reuse applications . 8
Annex A (informative) Example structure diagrams of BAF and DNF . 9
Annex B (informative) Cases of water reuse application using biological filtration for secondary
effluent treatment .12
Annex C (informative) Autotrophic denitrification.16
Bibliography . 17

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
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of 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).
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
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 282, Water reuse, Subcommittee SC 2, Water
reuse in urban areas.
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
Global water consumption has been increasing by approximately 1 % annually since the 1980s, due to
population growth, socio-economic development and changing consumption patterns. It is expected
that global water demand will keep increasing at a similar rate, and that by 2050 the water use will have
[9]
increased by 20 %-30 % compared to the current level . Water reuse is an efficient measure to alleviate
the global water shortage problem and is a common action for many countries in the world. “Fit for purpose”
is a consensus in the field of water reuse, which reduces the demand for fresh water by reusing secondary
effluent in different scenarios, such as urban miscellaneous water, circulating cooling water, process and
product water, etc. Treated secondary effluent acts as an important source of water supply, providing
opportunities to strengthen traditional water supply in some cities, particularly where long-distance
transfers are required to meet water demands. Organics and nitrogen in wastewater are the key indicators
for the management and control of water reuse in various countries.
Biological filtration, as a mainstream technology for wastewater treatment and reuse, is widely used for
advanced treatment of secondary effluent to achieve the effective removal of carbon, nitrogen and solid
substances. However, due to differences in technological, economic, social and environmental conditions,
there is no unified guideline on technical recommendations for biological filtration on a global scale. This
hinders the effective application of biological filtration as a “fit for purpose” practice to treat secondary
effluent.
v
International Standard ISO 9784:2024(en)
Guidelines for biological filtration of secondary effluent for
water reuse
1 Scope
This document provides guidance for biological filtration of secondary effluent for water reuse. It specifies
general recommendations, process components and technical recommendations of each component, post-
treatment process and water reuse applications.
This document is applicable to all types of stakeholders involved in implementing biological filtration for
advanced treatment of secondary effluent for water reuse.
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
biological filtration
wastewater treatment process depending on the physical filtration of the filter media (3.7) filled in the
treatment infrastructure and the biochemical action of the biofilm attached to the filter media
3.2
biofilter
bed of inert filter media (3.7) with large holes through which wastewater is caused to percolate for the
purpose of purification by means of an active biological film (bacteria bed) on the inert filter media
Note 1 to entry: Also known as biological filter.
[SOURCE: ISO 6107:2021, 3.72, modified — The preferred term has been changed from “biological filter” to
“biofilter”; Note 1 to entry has been modified.]
3.3
biological aerated filter
BAF
combination of contact oxidation and filtration by means of artificial aeration, intermittent backwashing
and other measures, aiming to remove organic pollutants, ammonium nitrogen and suspended solids
3.4
backwash rate
flow rate of backwashing water or backwashing air passing through filter bed (3.6) per unit area per unit time
3 2
Note 1 to entry: Backwash rate is generally represented as m /(m ·h).

3.5
denitrification filter
DNF
biofilter (3.2) for the purpose of denitrifying nitrate nitrogen in wastewater
3.6
filter bed
filter that consists of layers of filter media (3.7) arranged over one another so that a liquid flowing through
one filter media does not carry it into the next to clog it
Note 1 to entry: Also known as graded filter.
[SOURCE: ISO 6707-1:2020, 3.3.4.45, modified — The preferred term “graded filter” and the admitted term
“leaching field” have been removed; in the definition, “coarse gravel, coarse sand, and fine sand” has been
changed to “filter media”; Note 1 to entry has been added.]
3.7
filter media
materials used in the filter bed (3.6), such as coarse gravel, fine gravel, ceramsite, coarse sand and fine
sand serving as support for microorganisms’ adhesion and growth, and also with the function of physical
interception and filtration for suspended solids in supplied water
3.8
graded gravel layer
granular material layer which is laid between the water distribution system and the filter bed (3.6), to
prevent the filter media (3.7) from leaking into the water distribution system
3.9
heterotrophic denitrification
process by which bacteria convert nitrate nitrogen into nitrogen and other gases using organic matter (such
as methanol, acetic acid, sodium acetate, etc.) as energy and electron donor
3.10
volumetric nitrogen loading rate
amount of nitrogen (such as ammonium nitrogen and nitrate nitrogen) removed per cubic meter of filter bed
(3.6) per unit time
Note 1 to entry: Volumetric nitrogen loading rate is generally expressed as kg N/(m ·d).
4 General
4.1 Overall recommendations
The general recommendations for applying biological filtration of secondary effluent for water reuse are as
follows.
a) Biological filtration is applicable for the treatment of both raw wastewater and secondary effluent of
municipal wastewater treatment plants. Also, it is suitable for the biological treatment of industrial
wastewater with water quality characteristics similar to those of municipal wastewater.
b) Biological filtration can be applied alone or integrated with other wastewater treatment technologies.
The selection of biological filtration should be based on different considerations of influent water quality
and treatment, along with comprehensive analysis of technical, economic and environmental factors.
c) The construction of biofilters should be based on topographic, meteorological, geological conditions,
operation and environmental safety factors. Measures should be especially taken to avoid freezing,
odour, flies and corrosion.
d) The form and flow direction of the corresponding reactors of the biological filtration systems should be
designed and selected according to different influent water quality and treatment considerations.

4.2 Basic process
Possible process flow diagrams for applying biological filtration for carbon and NH -N (ammonium nitrogen)
removal, total nitrogen removal of effluent containing NH -N and nitrate nitrogen removal are shown in
Figure 1, Figure 2 and Figure 3, respectively. Possible structure diagrams of biological aerated filter (BAF)
(one/two stages) and denitrification filter (DNF) are also given in Annex A.
Figure 1 — Possible process flow diagram for applying biological filtration for carbon and NH -N
removal from the secondary effluent
Figure 2 — Possible process flow diagram for applying biological filtration for total nitrogen
removal from the secondary effluent containing NH -N
Figure 3 — Possible process flow diagram for applying biological filtration for nitrate nitrogen
removal from the secondary effluent

5 Process components
5.1 Biofilter unit
The components of the biofilter unit (BAF and DNF) that should be considered are shown in Table 1. Possible
structure diagrams for BAF and DNF are shown in Annex A.
Table 1 — Major components of the biofilter unit
DNF
Components BAF
Upflow Downflow
Influent distribution area   —
Water inlet ditch — — 
  
Water and air distributor
Graded gravel layer   
Filter bed   
Process aeration system  — —
Backwash drainage ditch   
Water outlet system   
  
Automatic control system
5.2 Mixing unit
The purpose of the mixing unit is to mix secondary effluent and external carbon source from the carbon
source dosing tank.
5.3 Backwashing unit
The backwashing unit is mainly comprising backwash water pump and blower.
5.4 Controlling unit
When applying biological filtration, automatic control systems for centralized and decentralized controlling
should be adopted. The control of related equipment is generally set up on-site, by programmable logic
controller (PLC) and central control room.
6 Technical recommendations of the biofilter unit
6.1 Filter media
Filter media can be made of the following materials:
— graded crushed mineral (e.g. quartz sand);
— granular activated carbon;
— mineral beads or granules (e.g. ceramsite);
— particulate matter containing reduced sulfur and iron for autotrophic or combined heterotrophic and
autotrophic denitrification to save external carbon sources;
— foamed beads (e.g. foamed ceramics);
— plastics of regular size and shape (e.g. polypropylene ball);

— plastic sheets or tubes assembled as modules to give a lightweight support media with a voidage of 90 %
or higher.
The characteristics of filter media should be:
— good strength, not easily broken;
— large specific surface area;
— good hydrophilicity;
— no toxic or harmful ingredients should be produced in the treated water.
Examples of type, diameter and density of filter media in water reuse applications using biological filtration
are given in Annex B.
6.2 Water and air distributor zone
Water and air distributor zones are composed of the cover board of the water-collecting ditch, the water and
air distributor, and the water inlet pipe and air intake tube. The cover board of the water-collecting ditch is
made of carbon steel or stainless steel. Air distribution tubes consist of main tubes and branch tubes. The
branch tube for air conditioning is installed above the cover board of the water-collecting ditch. The size of
the water-collecting ditch should be determined according to the volume of the treated water, the amount of
filter water, the amount of backwash water, the flow rate and the thickness of the cover board. The diameters
of the air main intake tube and the branch tube should be considered according to the backwash rate, the
pressure in the tube and the speed of water flow. Water distribution pipes and air distribution tubes should
be made of stainless steel.
6.3 Configuration of the biofilter unit
The set-up of a biological filtration unit involves fixed investment, operational costs, etc. By adopting
the standardized configuration process, a configuration scheme which is applicable for centralized and
decentralized treatment is obtained. The process flow diagram for setting up a biofilter unit is shown in
Figure 4.
The size of the biofilter unit and the number of compartments is related to the volume of the treatment
water, the quality of influent and effluent water and the surface of loading rate.
The selection of filter media should be based on the actual situation and the volume of filter media is linked
with its volumetric nitrogen loading rate and
...